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Toxicity of Zinc

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					        Chemistry Olympiad 2010 –

   Toxicity of Zinc
Authors:
KUAN Timothy        LAM Guy     LAU Ka-yu   LAW Pok-yin   NG Ching-ho

Aaron                           Martin                    Justin

關冠傑                 林 楷         劉加譽         羅博彥           吳楨浩

Mistress-in-Charge:
Ms. TANG Hung-yuk    (鄧紅玉 老師)




        DIOCESAN BOYS’ SCHOOL
TOXICITY OF ZINC



(Affix Group Photo with Miss Tang)




Ms. TANG Hung-yuk
Mistress-in-Charge
Bachelor of Science, Master of Philosophy, Postgraduate Diploma in Education; The Chinese
University of Hong Kong.



KUAN Timothy Aaron
Senior Secondary One Student
Studies Biology, Chemistry and Physics in Diocesan Boys’ School.



LAM Guy
Senior Secondary One Student
Studies Biology, Chemistry and Religious Studies in Diocesan Boys’ School.



LAU Ka-yu Martin
Senior Secondary One Student
Studies Biology, Chemistry and Physics in Diocesan Boys’ School.



LAW Pok-yin
Senior Secondary One Student
Studies Biology, Chemistry and Physics in Diocesan Boys’ School.



NG Ching-ho Justin
Senior Secondary One Student
Studies Chemistry, Geography and Physics in Diocesan Boys’ School.


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Contents
Acknowledgments............................................................................................................ 7
0 Fact Page of Zinc ........................................................................................................... 8
   0.1 Physical Properties of Zinc ............................................................................................... 8

   0.2 Properties of Zinc in terms of Electronic Arrangement................................................... 8

   0.3 Comparison of Zinc and Elements of the Same Group.................................................... 9

1 Abstract ...................................................................................................................... 10
2 Idea Development ....................................................................................................... 11
   2.1 Iron and Rusting ............................................................................................................ 11

      2.1.1 A brief introduction to Iron .................................................................................... 11

      2.1.2 Rusting of Iron ........................................................................................................ 11

      2.1.3 Problems lead by Rusting ....................................................................................... 12

   2.2 Protection of Iron Food Cans......................................................................................... 13

      2.2.1Ways to Prevent Rusting ......................................................................................... 13

      2.2.2 Impractical Methods .............................................................................................. 14

      2.2.3 The use of Metals to Protect Iron from Rusting ..................................................... 15

   2.3 The Selection of Metal for Preventing Rusting.............................................................. 16

      2.3.1 Characteristics of Food Cans .................................................................................. 16

      2.3.2 Problems of Some Sacrificial Protection Metals .................................................... 17

3 Zinc to protect Iron from Rusting ................................................................................. 19
   3.1 As the Final Choice ........................................................................................................ 19

   3.2 Advantages of Zinc ........................................................................................................ 19

   3.3 Disadvantages of Zinc - Toxicity .................................................................................... 19

      3.3.1 Toxicity of Zinc to humans ...................................................................................... 19

      3.3.2 Toxicity of Zinc towards animals ............................................................................ 20

4 Experiment A – Test to Zinc’s Toxicity ........................................................................... 22
   4.1 Introduction ................................................................................................................... 22

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TOXICITY OF ZINC


   4.2 Choice of Enzyme .......................................................................................................... 23

   4.3 Hypothesis ..................................................................................................................... 23

   4.4 Experiment Design......................................................................................................... 23

      4.4.1 Experiment Setup ................................................................................................... 23

      4.4.2 Tested Solution ....................................................................................................... 24

      4.4.3 Steps ....................................................................................................................... 25

      4.4.4 Error Assessment .................................................................................................... 26

   4.5 Experiment Report ........................................................................................................ 27

      4.5.1 Results .................................................................................................................... 28

      4.5.2 Photo Gallery .......................................................................................................... 29

5 Method to Reduce Zinc’s Toxicity ................................................................................. 33
   5.1 How does Zinc’s inhibition work ................................................................................... 33

   5.2 Chelation in Medicine.................................................................................................... 33

   5.3 Selection of Complex Ions ............................................................................................. 34

   5.4 How to form the Complex Ion ....................................................................................... 35

6 Experiment B – Toxicity of Zinc Ligands ........................................................................ 37
   6.1 Aim................................................................................................................................. 37

   6.2 Hypothesis ..................................................................................................................... 37

   6.3 Experiment Design......................................................................................................... 37

      6.3.1 Experiment Setup Plan ........................................................................................... 37

      6.3.2 Use of Solutions ...................................................................................................... 38

      6.3.3 Steps of Experiment ............................................................................................... 39

      6.3.4 Safety Precautions .................................................................................................. 40

      6.3.5 Possible Error Assessment...................................................................................... 40

   6.4 Experiment Report ........................................................................................................ 41

      6.4.1 Raw Results............................................................................................................. 41
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      6.4.2 Analysis ................................................................................................................... 43

      6.4.3 Results Discussion................................................................................................... 44

      6.4.4 Photo Gallery .......................................................................................................... 45

7 Applications ................................................................................................................ 49
   7.1 Railings........................................................................................................................... 49

   7.2 Iron Ships ....................................................................................................................... 49

   7.3 Coinage .......................................................................................................................... 50

   7.4 Cans ............................................................................................................................... 51

8 Socioeconomical Advantages ....................................................................................... 52
   8.1 Economical Advantage .................................................................................................. 52

   8.2 Higher Abundance ......................................................................................................... 52

   8.3 Zinc Recycling ................................................................................................................ 53

References ..................................................................................................................... 55




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TOXICITY OF ZINC




                   “Knowledge grows when shared.”

                             Bhartrihari




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  Acknowledgments


          We would like to thank Mr. Terrence Chang Cheuk-Cheung, headmaster of the
Diocesan Boys’ School for his support in this event. Thanks must also be given to Ms. Tang
Hung-yuk, our mistress-in-charge for her invaluable help during the event.


     We would also like to show our gratitude towards Ms. Wong Yuen-Ting, head of biology
department for her precious advice on use of enzymes. We would also thank Mr. So Tai-ming
for his help during experiments.


     Thanks should also be given to Mr. Ho Wai-keung and Mr. Chun Ka-ming, lab
technicians of the Biology and Chemistry laboratories for their kind help.


     Last, but not least, we are utmost grateful to our classmates and schoolmates who
showed support and help during the whole Olympiad journey.




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TOXICITY OF ZINC




  0 Fact Page of Zinc


     Zinc (symbol: Zn) is a transitional metal element located at the first element of group
12. It’s atomic number is 30, meaning it fills up to 3d104s2 Orbital.




0.1 Physical Properties of Zinc



           Zinc has a melting point of 419.58°C, boiling point of 907°C, gravity of
           7.133g/cm3 (25°C), with a valence of 2. Zinc is a lustrous blue-white
           metal. It is brittle at low temperatures, but becomes malleable at
           100-150°C. It is a fair electrical conductor. Zinc burns in air at high red
           heat, evolving white clouds of zinc oxide.


           (Helmenstine 2007)




0.2 Properties of Zinc in terms of Electronic Arrangement



           Zinc has the electronic structure [Ar] 3d104s2. When it forms ions, it
           always loses the two 4s electrons to give a 2+ ion with the electronic
           structure [Ar] 3d10. The zinc ion has fully filled 3d orbital.


           (Clark 2004)




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0.3 Comparison of Zinc and Elements of the Same Group


            Melting       Boiling       Atomic         M2+ ionic       1st - 3rd Ionisation
Element
            point (°C)    point (°C)    radius (Å)2    radius (Å)      energies (kJ mol-1)

Zn          420           907           1.34           0.74            906, 1733, 3831

Cd          321           765           1.51           0.95            877, 1631, 3644

Hg          -39           357           1.51           1.19            1007, 1809, 3300

Table 0.1




             The elements of group 12 - zinc, cadmium and mercury - differ from
             the transition metal elements (groups 3 to 11) in that they form
             compounds in which their oxidation states are no higher than +2…The
             reason for the difference in chemistry between the alkaline earth
             metals and the elements of group 12 is the arrangement of the
             electrons just below the valence shell…Mercury has many unique
             properties within its group, such as being the only liquid metal of all in
             s.t.p., and some of its chemistry is very different from zinc and
             cadmium.


              (British Broadcasting Company, 2003)




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  1 Abstract


      Iron is a very important metal and used in many aspects. However, it rusts readily. An
effective method to protect iron from rusting is to coat iron with zinc. However, Zinc ions are
toxic by itself and hence unsuitable to be used in metal food cans or other eating
instruments.


      Our aim in this project is to introduce a method such that the toxicity of zinc can be
reduced. With a better way of protecting iron cans, they could be stored for a longer time.


    As from our research, we found when ammonia react with Zinc in a 4:1 ratio, a kind of
complex ion tetraammine Zinc may be formed. With the formation of this complex ion,
Zinc’s toxicity may be reduced.


    Our experiment is separated into 2 parts, with the first part studying the toxicity of zinc
ions towards enzymes and the second part testing if tetraamine Zinc worked in reducing
Zinc’s toxicity.


    Results are encouraging, with 17.6% recovery in 4:1 ratio and up to 41.2% recovery in 9:1
ratio. This indicate that tetraamine Zinc successfully reduce Zinc’s toxicity.


    This kind of ion can be used in various applications such as railings, food cans, drink cans
and other aspects which include iron as the raw material.




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  2 Idea Development

2.1 Iron and Rusting



2.1.1 A brief introduction to Iron


       Iron (symbol: Fe) is the most commonly used metals all around us. It accounts for 95% of
global metal production every year. Just if you don’t believe in statistics, you may consider every
day used things. From iron machines, iron railings to iron robots, iron is beside us almost every
day.


       Iron is cheap, yet it has high strength. It can be used in a vast scale, from large construction
sites to toys for kids. It is also a perfect metal for alloying. Steel, for example, is an alloy that is
formed by iron, chromium, tungsten and other metals.




2.1.2 Rusting of Iron




 Photo 2.1
       However, one major problem of using iron is that iron oxidizes readily. Commonly known as
rusting, such oxidization occurs in the presence of oxygen and water. The equations are shown as
below:

                                      Fe  Fe2+ + 2e-        ---(1)

                                   O2 + 2H2O + 4e-  4OH-       ---(2)



                                     Multiplying equation (1) by 2:

                                      2Fe  2Fe2+ + 4e-      ---(3)


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                                       Combining (2) and (3):

                                  2Fe + O2 + 2H2O  2Fe2+ + 4OH-



                                          Overall Equation:

                                4Fe + 3O2 + 2nH2O  2Fe2O3•nH2O



      Rust is in fact hydrated iron(III) oxide. It is reddish-brown in color. The formation of it could
be tested by Potassium Hexacyanoferrate(III) solution which changed from yellow to blue with
the presence of iron(III) ion. Figure 2.1 shows a rusted iron.




                                                                 Figure 2.1



     Rusting can be further speeded up by placing the iron near electrolytes (i.e. compounds
that does not conduct electricity in solid state but conduct electricity in molten or liquid state)




2.1.3 Problems lead by Rusting


     Rusting can result in lots of problem in both economical aspects and health aspects.



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           “The cost of corrosion was estimated for the individual economic sectors
           discussed above. The total cost due to the impact of corrosion for the
           analyzed sectors was $137.9 billion per year…By estimating the
           percentage of U.S. GDP for the sectors for which corrosion costs were
           determined and by extrapolating the figures to the entire U.S. economy, a
           total cost of corrosion of $276 billion was estimated. This value shows
           that the impact of corrosion is approximately 3.1 percent of the Nation's
           GDP.”


           (Virmani 2002)




     If the iron(III) ion is further ingested inside the body, diseases may be caused, such as heart
failure and liver cancer. This is also known as iron poisoning.




           Large amounts of ingested iron can cause excessive levels of iron in the
           blood. High blood levels of free ferrous iron react with peroxides to
           produce free radicals, which are highly reactive and can damage DNA,
           proteins, lipids, and other cellular components.


           (Wikipedia)




2.2 Protection of Iron Food Cans



2.2.1Ways to Prevent Rusting


     Principles to prevent rusting are based on the following:
           (a)   Avoid the iron object from contacting with air or water.
           (b)   Inhibit the formation of Iron(II) Ion.


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        Ways induced to prevent rust formation include the following:
             1.     Painting, greasing, plastic coating to form a protective layer.
             2.     Cathodic protection by means of supplying electron from an external power
                  source.
             3.     Electroplating of metal to separate it from air or sacrificial protection of a more
                  reactive metal to prevent iron ion formation.




2.2.2 Impractical Methods


        Food cans are used to place or store food. Since so, the agents used on the food can to
prevent rusting must not contain any toxic materials.


  2.2.2.1 Problems of Method One:
        Many greasing or painting materials are toxic by it. Hence, it could not be used on food
cans.




                                                                                        Figure 2.2


        Some plastic materials (for example polystyrene) are not toxic. However, the cost of making
such plastic is rather high. Furthermore, many of these plastic are thermoplastics (as shown in
figure 2.2). They will change its shape in a high temperature (about 100oC). Therefore, such
containers would not be able to store hot food or drinks.


  2.2.2.2 Problems of Method Two:
        Meanwhile, it is practically impossible to place an electricity supply attached with the can


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until usage. Besides, it is a wastage of electricity, which is neither environmental-friendly nor
economically possible.


2.2.3 The use of Metals to Protect Iron from Rusting


  2.2.3.1 Introduction to the Electrochemical Series
     There are two kinds of using metal for protecting iron from rusting. This is determined by
the electrochemical series as of below:

                                       equilibrium                 E° (volts)


                                                                     -3.03

                                                                     -2.92

                                                                     -2.87

                                                                     -2.71

                                                                     -2.37

                                                                     -1.66

                                                                     -0.76

                                                                     -0.44

                                                                     -0.13

                                                                       0

                                                                    +0.34

                                                                    +0.80

                                                                    +1.50

Table 2.1

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      Table 2.1 shows the electrochemical series of major metal ions. The charge is compared
with standard hydrogen electrode. The higher it is in the series (e.g. lithium) indicate it is a
stronger reducing agent while the lower it is (e.g. gold) it is a stronger oxidizing agent.


      We compared the table with iron. Metals labeled blue (the lowest five) mean it has less
tendency to form ion when compared to iron while metals labeled in red (the top seven) mean it
has a higher tendency to form ion.



  2.2.3.2 Different Abilities to Prevent Rusting
      As of table 2.1, lithium, potassium, calcium, sodium, magnesium, aluminium and zinc are
more reactive than iron. They adopt sacrificial protection. Under this method, corrosion still
occurs but it is the more reactive metal that is being corroded, not the iron to form its ion.
      An example is shown below:

                                          Mg  Mg2+ + 2e-

                                          Fe  No Reaction



      For metals less reactive than iron (tin, lead, copper, silver and lead), they can also protect
iron from rusting, but only when it is completely plated on iron layer. Once the iron layer is
scratched, iron itself would become the “sacrificed” metal, and corrosion would occur even
faster.


2.3 The Selection of Metal for Preventing Rusting



2.3.1 Characteristics of Food Cans


      Cans require a long-term storage period. Hence, the protection method must be durable.
As of Section 2.2.3.2, once the metal layer of a less reactive metal (compared to iron) is scratched,
the iron would corrode even faster than before. Hence, such method cannot be used.


      Metals using sacrificial protection to protect iron are a more preferable method. However,
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there are some problems of such metals which limited the choice.




2.3.2 Problems of Some Sacrificial Protection Metals


  2.3.2.1 Potassium, Calcium and Sodium

     The above listed are extremely reactive metals. They react with water vigorously already.

The equations are shown as below:

                                     2K + 2H2O  2KOH + H2

                                   2Na + 2H2O  2NaOH + H2

                                    Ca + 2H2O  Ca(OH)2 + H2




                                           Figure 2.3



     Figure 2.3 shows a piece of potassium put into water, a vigorous reaction occurred. Besides,

the three solution obtained are all soluble hydroxide solutions that are alkaline. These strong

solutions may cause a problem as contents (drinks) in the cans normally contain carbonate acid

(H2CO3).




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   2.3.2.2 Magnesium
          Magnesium is less reactive than Sodium. However, it would also react with hot water to
form Magnesium Hydroxide. Although this is insoluble (not alkaline), it is still a base and might
react with the carbonic acid (H2CO3) inside many carbonated drinks.

                                        Mg + H2O  Mg(OH)2 + H2

                                   Mg(OH)2 + H2CO3  2H2O + MgCO3



   2.3.2.3 Aluminum
          Aluminum might be a correct choice; However, the cost for extraction of aluminum is high.
Aluminum also forms a hard oxide layer (Al2O3) by itself. This may affect the protection ability
itself.


          The following study (dated January 2008) showed a forecast of aluminium costs:



               Analysts forecast (in the year 2008) that the aluminium price will hit
               $4,000 a tonne in the next couple of years. The metal was trading at
               $2,900 a tonne on the London Metal Exchange last week, up 50 per cent in
               two years. Power costs have risen by about 50 per cent for the industry in
               the past five years and are expected to rise further as oil and gas prices
               continue to rise.


               (Haruni, 2008)




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  3 Zinc to protect Iron from Rusting

3.1 As the Final Choice


     After discussion of different methods of rusting prevention on Section 2, Zinc is the
remaining choice. This section discusses on the advantages of zinc. Disadvantages will be
discussed on the next section.


3.2 Advantages of Zinc


     Zinc is a more reactive metal in iron, even if there is scratch on zinc; the iron object will not
corrode immediately. So the iron can be preserved for a longer time when compared to plating of
a less reactive metal.


     Secondly, Zinc would not react with water. Though it reacts with steam to from a white
powder, Zinc Oxide. Yet such reactions seldom occur in normal conditions.


     Furthermore, Zinc galvanizing has the following advantages:

           (Galvanized Zinc has) high strength, formability, light weight, ,corrosion
           resistance, aesthetics, recyclability ,low cost

           (International Zinc Association)




3.3 Disadvantages of Zinc - Toxicity



3.3.1 Toxicity of Zinc to humans


     Zinc ion is toxic when consumed. The following are previous studies regarding Zinc’s toxicity
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to human beings.

          Although consequences of zinc deficiency have been recognized for many
          years, it is only recently that attention has been directed to the potential
          consequences of excessive zinc intake…Zinc is considered to be relatively
          nontoxic, particularly if taken orally. However, manifestations of overt
          toxicity symptoms (nausea, vomiting, epigastric pain, lethargy, and
          fatigue) will occur with extremely high zinc intakes.


          (Fosmire 1990 )




          Inhaling large amounts of zinc (as zinc dust or fumes from smelting or
          welding) can cause a specific short-term disease called metal fume fever,
          which is generally reversible once exposure to zinc ceases.


          (Draggan 2008)




3.3.2 Toxicity of Zinc towards animals


     Other than harming human beings, Zinc would also affect animals such as rats and parrots,
previous studies are quoted as follows:




          Zinc poisoning is very common among pet parrots as it is normally caused
          by ingestion of toxic zinc that is used to galvanize parts of the parrots
          metal cage and almost all of its toys as a means to prevent rusting.


          (Pet Parrots 101)




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Eating food containing very large amounts of zinc for several months
caused many health effects in rats, mice, and ferrets, including anemia
and injury to the pancreas and kidney. Rats that ate very large amounts of
zinc became infertile.


(Draggan 2008)




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  4 Experiment A – Test to Zinc’s Toxicity

4.1 Introduction


     Enzymes are biological substances made of amino acids. They act as catalysts inside body
for body reactions. Activation energy of such reactions would be lowered to make them happen
more ready. Animals would not be able to live since reaction would not have taken place or taken
place in an extremely slow rate.




                                                              Figure 4.1


     Figure 4.1 shows a systematic model of an enzyme. In which an substrate is placed into an
active site. And so the reaction may take place. This may either be anabolic (building up
substrates together) or catabolic (separating already-formed compounds into smaller bits).


     Referring to the lock and key hypothesis, introduced by German chemist Hermann Emil
Fischer, that one active site would only be able to fit in one kind of substances. That is, once the
active site is destroyed, the reaction held by enzyme shall no longer work.


     Though Zinc’s Toxicity towards enzymes are not studied before, we believe that Zinc would
act as a denaturing agent towards enzymes based on the following points



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     1.   Like many heavy metals (such as copper and lead), Zinc ion shall also be able to
          denature enzymes.
     2.   By considering elements of the same group, mercury also shows denaturing
          properties towards enzymes.


4.2 Choice of Enzyme


     Among various types of enzymes, our selection of enzymes is Catalase. Catalase catalyses
hydrogen peroxide to form oxygen gas and water.


                                               Catalase
                                  2H2O2                       H2O + H2



     We chose Catalase for the following reasons:
     1.   Hydrogen Peroxide is a toxic substance to living organisms. But Catalase holds an
          catabolic reaction which transforms it into water and hydrogen which are non-toxic to
          living organisms.
     2.   Catalase is a common enzyme which is nearly found in all living organisms.
     3.   The working temperature of Catalase is around 22-25⁰C, which is easier to control.



4.3 Hypothesis

     Our hypothesis is that Zinc ions will denature Catalase such that it will not react with
hydrogen peroxide solution to from oxygen gas and water.




4.4 Experiment Design



4.4.1 Experiment Setup


     In the reaction of catalase (as shown in section 4.2), hydrogen gas and water will be
released. In our experiment, hydrogen gas is to be detected to show whether the catalase
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TOXICITY OF ZINC


solution worked in a certain condition (further discussed in section 4.4.2).


     Therefore, we have made the following considerations to eliminate some technical
problems.
             (a)   The whole set up has to be water tight to prevent hydrogen to be released
                   before measuring.
             (b)   The test tube for measuring results (labeled in green in figure 4.1) will be filled
                   with water in order to detect hydrogen gas (which will displace water when
                   formed).




Figure 4.2


4.4.2 Tested Solution


  4.4.2.1 Solutions used

     The following solutions are involved in the experiment
      Name of Solution                    Chemical Formula               Concentration / Molarity
     Hydrogen Peroxide                           H2O2                                1M
             Catalase                              /                        2% (Conc. By Mass)
         Zinc Nitrate                          Zn(NO3)2                              1M

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          Zinc Chloride                            ZnCl                                1M
          Distilled Water                          H2O                                 Nil
Table 4.1

  4.4.2.2 Contents of the Test Tubes

     The tested solution includes the following:
             (a)     Water + Hydrogen Peroxide + Catalase, hereby denoted as Test Tube A
             (b)     Zinc Nitrate Solution + Hydrogen Peroxide + Catalase, hereby denoted as Test
                     Tube B
             (c)     Zinc Chloride Solution + Hydrogen Peroxide + Catalase, hereby denoted as Test
                     Tube C
             (d)     2 Test tube of each kind is used to prevent possible error


  4.4.2.3 Reasons of Testing
             (a)     Test Tube A acts as a control experiment to show how oxygen gas and water is
                     produced under normal conditions.
             (b)     Test Tube B involves Zinc Nitrate Solution which will provide Zinc ion and Nitrate
                     ion in which Zinc ion is expected to denature the enzyme.
             (c)     Test Tube C involves Zinc Chloride Solution which will provide Zinc ion and
                     Chloride ion in which Zinc ion is expected to denature the enzyme.
             (d)     Both Test Tube B and C involves Zinc cation and another anion. This is to avoid
                     the misconception that it is the anion as the denaturing agent.
             (e)     Solutions containing Zinc ions are used in this experiment. This is because Zinc
                     requires a long time to be ionized in water. Therefore, “ready-made” Zinc ion
                     solutions are used.

4.4.3 Steps

     1.      Add the Solutions to 2cm3 of 0.01% Catalase solution (Percentage by mass)
     2.      Wait for 5 minutes for denaturing of enzymes
     3.      Add the Solutions to Hydrogen Peroxide, immediately connect it to the transferring
             tube.
     4.      Dip the tube under the funnel which is already filled with water
     5.      If oxygen gas is produced, gas bubbles shall float upward and displace water.
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     6.   Measure the water displaced (if any)




4.4.4 Error Assessment


  4.4.4.1 Source of Errors
     Despite of the apparent result of the above experiment, there are several possible factors
that could have caused errors and affect the result of the experiment. These errors are listed
below:
     1.   Contamination
          (a)   Contamination caused by the unclean utensils and apparatus of the experiment.
                Beakers, test tubes, droppers may contain chemicals that exists as impurities in
                the experiment that may cause the denaturation of the catalase enzymes and
                affect the speed of the catabolic reaction.
     2.   Purity
          (a)   The purity of Hydrogen peroxide; the hydrogen peroxide solution may contain
                impurities that lead to the alteration of rate of the catabolic reaction of the
                experiment or cause other unwanted reactions.
          (b)   The purity of zinc nitrate and chloride solutions; impurities that exist in the
                solution may lead to the alteration of rate of the catabolic reaction of the
                experiment.
          (c)   The purity of the distilled water; the distilled water may be contaminated when
                being transfer throughout the experiment. This may lead to the alteration of rate
                of catabolic reaction.
     3.   Temperature
          (a)   Temperature is a key to the reaction of enzymes. With the change of
                temperature, enzymes may be inactivated or denatured. This may lead to
                difference of results if the temperature is different in different test tubes.
     4.   pH Value
          (a)   Difference in pH value may also lead to change in behavior of the enzyme.
                However, since there is no acidic or alkaline substances involved in this
                experiment, this is not a major concern.
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                                                                        DIOCESANS BOYS’ SCHOOL




  4.4.4.2 Means to Reduce Errors
     Though several errors may exist in the experiment, however, their existence would only
cause minor fluctuation of the experimental result. Furthermore, more than one test tube (of the
same kind) has been used throughout the whole experiment.


     Regarding to temperature, all the tests are performed at room temperature pressure. Room
temperature pressure mean 298K (25°C) and at 760mm Hg (1 atmospheric pressure).


     Due to the great comparison of the result for the three test samples, it can still be
concluded that zinc ions causes denaturation of enzymes provided with water acting as the
medium for the reaction.


4.5 Experiment Report


     Details of the experiment are enclosed as follows:


     Date: 9th February, 2010
     Time: 3:35pm – 4:45pm
     Duration: 1 Hour and 10 Minutes
     Location: Chemistry Laboratory, Diocesan Boys’ School
     Attendance:     Kwan Timothy Aaron
                     Lam Guy
                     Lau Ka-yu Martin
                     Ng Ching-ho Justin
     Absence with Apology: Law Pok-yin




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4.5.1 Results


                              Test Tube A               Test Tube B             Test Tube C
    Time waited:               5 Minutes                5 Minutes                5 Minutes
  Water Displaced:               43mm                      0mm                     0mm
Other Observations:         Exothermic and                     Nil                  Nil
                             Effervescence.
Table 4.2


     Table 4.2 is a summary of the experimental results. Test tube A which consists of water,
catalase and hydrogen peroxide solution showed quite a fast reaction. Within 20 seconds, gas
bubbles are produced (effervescence) and pumped into the inverted test tube. The reaction
stopped 30 seconds later. Heat is also detected in the experiment.


     Regarding Test tube B & C where each of them contained a Zinc ion solution instead of
water in Test tube A, there are no observable changes after the reaction. This indicates that the
reaction of catalase is stopped. I.e. Catalase is denatured in such environment. A positive result
that zinc ion inhibits catalase is shown by this experiment.




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4.5.2 Photo Gallery


  4.5.2.1 Result of Test Tube A




                                                                  Figure 4.3a



Figure 4.3 Testing solution with test tube A, the control experiment.
Figure 4.3a A magnified image of test tube A, with the gas bubbles shown.




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       TOXICITY OF ZINC




                                                        43mm




       Figure 4.4 Amount of water displaced after reaction completed.


           4.5.2.2 Results of Test Tubes B & C




Zinc ion
solution
                                                                    Catalase
                                                                    solution



       Figure 4.5 Showing a test tube containing Catalase solution and Zinc ion solution. They will be
       mixed then placed for 5 minutes for denaturation to occur.




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                                                                    Hydrogen
                                                                    Peroxide
Zinc Ion and
Catalase




Figure 4.6 Showing a test tube containing catalase solution and zinc ion solution (placed for 5
minutes already) and another with hydrogen peroxide solution. They are then mixed and placed
in the setup to detect whether the enzymes are denatured.




Figure 4.7 Test tube containing Zinc Chloride solution, Catalase solution and hydrogen peroxide
solution. No gas is displaced indicating there is no reaction by catalase. i.e. denatured.




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Figure 4.8 Different solutions placed on a test tube rack.




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5 Method to Reduce Zinc’s Toxicity

5.1 How does Zinc’s inhibition work


     In enzymatic reaction, the substrate fits exactly into its enzyme. As discussed in Section 4.1,
once the active site of an enzyme is destroyed or changed shape, it would not work anymore.


     Many heavy metal, including Zinc in this case, has the ability to react with parts of the
enzyme such that its active site will be changed. This is shown in Figure 5.1 below.




                                                                                  Figure 5.1




5.2 Chelation in Medicine


     Before considering Zinc alone, we first consider methods used in medicine for treating
other diseases.


     If some heavy metals are absorbed into the body, chelation therapy is a method to treat
patients with metal poisoning. Chelation therapy is a method to form complex ions with the

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TOXICITY OF ZINC


poisonous metal so that it would become non-toxic to human beings.




          Chelation therapy is administering a man-made amino acid called EDTA
          into the veins. (EDTA is an abbreviation for ethylenediamine tetraacetic
          acid. It’s marketed under several names, including Edetate, Disodium,
          Endrate, and Sodium Versenate.) EDTA is most often used in cases of heavy
          metal poisoning (lead or mercury). That’s because it can latch onto or bind
          these metals, creating a compound that can be excreted in the urine.


          (American Heart Association)




     However, the chelation therapy is a perfect method. Problems are stated as below:

          (The use of) Chelation therapy to enhance the elimination of metal
          already absorbed. It must be remembered. However, that most chelating
          agents are toxic and should only be used when their benefits are likely to
          outweight the risks of their use.


          (Souhami and Moxham 1997)




5.3 Selection of Complex Ions


     There are two common complex ions of zinc, namely tetraammine zinc ion and tetracyano
zinc ion. Some of the properties of them are listed below:



     1.   Tetraammine Zinc Ion

          Zn2+ + 4NH3        (Zn[NH3]4)2+
          Equilibrium constant (Log Kst): 9.6




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     2.    Tetracyano Zinc Ion

           Zn2+ + 4CN-        (Zn[CN]4)2-
           Equilibrium constant (Log Kst): 16.7


     The equilibrium constant, denoted Log Kst is a measurement of the tendency of formation
of reacted products. The higher the equilibrium constant indicates the higher tendency to form
the product (i.e. equilibrium shifts to right).


     Both Zinc ions are highly stable. However, for Tetracyano Zinc Ion, its constituent ion
include cyanide ion (CN-) which is toxic by itself. If not all the cyanide ion is reacted, it might
cause a problem itself. Therefore, such ion is not suggested.



           Cyanide inhibits mitochondrial cytochrome oxidase and hence blocks
           electron transport, resulting in decreased oxidative metabolism and
           oxygen utilization. Lactic acidosis occurs as a consequence of anaerobic
           metabolism. The oxygen metabolism at the cell level is grossly hampered.


           (Manbir Online)




     For Tetraammine Zinc ion, ammonia involved would not do harm on human. Even ammonia
is in excess, it will not have any other reactions. Besides, the equilibrium constant is still high
despite it is around 107 times less stable than Tetracyano Zinc Ion. Therefore, such ion is selected
to reduce Zinc’s toxic behavior.




5.4 How to form the Complex Ion


                                    Zn2+ + 4NH3         (Zn[NH3]4)2+


     We may first consider the raw materials of such reaction. Zinc ion and Ammonia is the
constituent materials of the reaction. Zinc ion will be provided in the can, but Ammonia ion is
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TOXICITY OF ZINC


not involved in normal cans.


     The main, and most common, way to obtain Ammonia is from Ammonia solution itself.
Ammonia exists as gas, but has a high solubility in water.

                                             Water
                                   NH3 (g)              NH3 (aq)


     However, Ammonia compound may ionize in water to form ammonium ion as well as
hydroxide ion.


                                     NH3 + H2O  NH4+ + OH-


     Ammonia solution does not ionize readily; around 99% of ammonia still exists as ammonia
molecule rather than ammonium ion.


     Meanwhile Zinc will react with the hydroxide ion formed as below:


                                      Zn2+ + 2OH-  Zn(OH)2


     Zinc hydroxide will be the product of such reaction. Since all hydroxides are insoluble
(except Ammonia Hydroxide, Potassium Hydroxide and Sodium Hydroxide), a precipitate is
formed in the solution.


     However, when ammonia is in excess, Zinc hydroxide will re-react with ammonia to form
the following:


                           Zn2+ + 2OH- + 4NH3        (Zn[NH3]4)2+ + 2OH-


     In this case, the hydroxide ion is given up and our desired ligand structure, Tetraammine
Zinc is formed.



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  6 Experiment B – Toxicity of Zinc Ligands



6.1 Aim


     The aim of this experiment is to test rather Tetraammine Zinc ion, a ligand of Zinc, would
cause toxicity to Catalase, an enzyme.




6.2 Hypothesis


     The hypothesis is that Zinc will denature enzymes but Tetraammine Zinc, the ligand, would
do no harm to Catalase, and hence, doesn’t affect human or bodies of different living organisms.




6.3 Experiment Design



6.3.1 Experiment Setup Plan


     Similar to experiment A, the setup will be aimed to detect whether gas is produced.
However, differently, in this experiment, we aimed to measure amount of gas produced more
accurately. Therefore, instead of measuring “water displaced”, a gas syringe will be used.


     If Catalase worked in such condition hydrogen gas would be produce. This gas will be
transmitted through the delivery tube and the gas syringe will move, and hence enabling us to
measure volume of gas.


     A detailed plan is shown at Figure 6.1




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                           Delivery Tube




                                                          Test Tube
                           Gas Syringe




Figure 6.1




6.3.2 Use of Solutions


     The following solutions were used in the experiment:


      Name of Solution                Chemical Formula                Concentration / Molarity
     Hydrogen Peroxide                          H2O2                             1M
             Catalase                             /                     2% (Conc. By Mass)
          Zinc Nitrate                     Zn(NO3)2                              1M
            Ammonia                              NH3                             1M
         Distilled Water                         H2O                             Nil
Table 6.1


     These solution are separated as follows:
Test Tube       Ammonia     Zinc           Ratio of      Distilled       Catalase      Hydrogen
Reference       (NH3)       Nitrate        NH3 to        Water                         Peroxide
Number                      (Zn[NO3]2)     Zn(NO3)2
A               Nil         Nil            Nil           10 cm3          2 cm3         2 cm3


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B              Nil             1 cm3          Nil            9 cm3         2 cm3         2 cm3
C              1 cm3           1   cm3        1:1            8 cm3         2 cm3         2 cm3
D              2 cm3           1 cm3          2:1            7 cm3         2 cm3         2 cm3
E              3 cm3           1 cm3          3:1            6 cm3         2 cm3         2 cm3
F              4 cm3           1 cm3          4:1            5 cm3         2 cm3         2 cm3
G              8 cm3           1 cm3          8:1            1 cm3         2 cm3         2 cm3
H              9 cm3           1 cm3          9:1            0 cm3         2 cm3         2 cm3
Table 6.2
Remarks:
                 (a) Test tube A acts as an control. In this case, neither Zinc Nitrate nor Ammonia
                       is involved. This is an normal reaction.
                 (b) Test tube B acts as a comparison with Zinc-denatured Enzyme (as proved in
                       Experiment A). No Ammonia is involved.
                 (c) Test tube C, D and E each contained a certain amount of Ammonia, yet not in
                       4 : 1 ratio for the formation of Tetraammine Zinc. These solutions may both
                       contain Zinc Hydroxide as well as Tetraammine Zinc ion. This is used to
                       compare the differences.
                 (d) Test tube F contain Ammonia and Zinc Nitrate in the ratio of 4 : 1. This is just
                       enough for the formation of Tetraammine Zinc ion.
                 (e) Test tube G and H has excess ammonia. All the Zinc should have reacted. Yet
                       this is used to check whether the alkaline environment would in return cause
                       a damage to the enzyme


6.3.3 Steps of Experiment


     1.     Mix the solutions without adding hydrogen peroxide.
     2.     Prepare the gas syringe (reset it to 0)
     3.     Add the solutions with hydrogen peroxide. Immediately measure the value of it. Wait
            for a period of 30seconds.




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6.3.4 Safety Precautions


     1.   Ammonia is an alkaline solution. It also produces a pungent small. There should not
          be direct smelling of the solutions.
     2.   The reaction between Catalase and hydrogen peroxide can be vigorous. Special care
          must be taken in the joints of the set up (i.e. the opening and ending of delivery tube)
          or else it may break during the experiment.




6.3.5 Possible Error Assessment


     1.   Contamination
          (a)   Contamination caused by the unclean utensils and apparatus of the experiment.
                Beakers, test tubes, droppers may contain chemicals that exists as impurities in
                the experiment that may cause the denaturation of the catalase enzymes and
                affect the speed of the catabolic reaction.
     2.   Purity
          (a)   The purity of Hydrogen peroxide; the hydrogen peroxide solution may contain
                impurities that lead to the alteration of rate of the catabolic reaction of the
                experiment or cause other unwanted reactions.
          (b)   The purity of zinc nitrate and chloride solutions; impurities that exist in the
                solution may lead to the alteration of rate of the catabolic reaction of the
                experiment.
          (c)   The purity of the distilled water; the distilled water may be contaminated when
                being transfer throughout the experiment. This may lead to the alteration of rate
                of catabolic reaction.
     3.   Temperature
          (a)   Temperature is a key to the reaction of enzymes. With the change of
                temperature, enzymes may be inactivated or denatured. This may lead to
                difference of results if the temperature is different in different test tubes.
     4.   pH Value

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            (a)    Difference in pH value may also lead to change in behavior of the enzyme.
                   However, since there is no acidic or alkaline substances involved in this
                   experiment, this is not a major concern.
      5.    Measurement Errors
            (a)    There may be measurement errors in the solutions. Hence, a smaller beaker is
                   used for measuring smaller volumes. However, a large measuring cylinder is still
                   required to measure larger volumes (for example 10cm3)
            (b)    The reading of the gas syringe may vary due to reading error. Furthermore, air
                   leakage may due to a certain error.




6.4 Experiment Report


      Date: 2nd March, 2010
      Time: 3:35pm – 5:05pm
      Duration: 1 Hour and 30 Minutes
      Location: Chemistry Laboratory, Diocesan Boys’ School
      Attendance:         Lam Guy
                          Lau Ka-yu Martin
                          Ng Ching-ho Justin
      Absence with Apology:         Kuan Timothy Aaron
                                    Law Pok-yin


6.4.1 Raw Results


    6.4.1.1 Data
      Table 6.3 shows the raw result of the test
Test Tube:           Gas Produced (cm3)
A                    17
B                    No Reaction Detected
C                    No Reaction Detected
D                    Reaction Detected. <1

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E                   Reaction Detected. <1
F                   3
G                   6
H                   7
Table 6.3
    6.4.1.2 Observations
      Observations of our experiment are also listed as below:
            (a)   Test tube A is a colorless solution. After the addition of hydrogen peroxide,
                  effervescence occurred and gas is displaced (as of Table 6.3)
            (b)   Test tube B is also a colorless solution. After the addition of hydrogen peroxide,
                  there is no observable change.
            (c)   Test tube C showed some white gelatinous precipitate. After the addition of
                  hydrogen peroxide, there is no observable change.
            (d)   Test tube D showed more white gelatinous precipitate. After the addition of
                  hydrogen peroxide, there is some reaction, yet extremely slow that it is difficult
                  to detect.
            (e)   Test tube E showed white gelatinous precipitate around the same as test tube D.
                  After the addition of hydrogen peroxide, there is some reaction, yet extremely
                  slow that it is difficult to detect.
            (f)   Test tube F showed white gelatinous precipitate, but less than test tube E. With
                  the addition of hydrogen peroxide, a moderate reaction occurred.
            (g)   Test tube G remained a small amount of precipitate. With the addition of
                  hydrogen peroxide, a smaller effervescence (compared to (a)) occurred.
            (h)   Test tube H is similar to test tube G. A slightly larger effervescence occurred.




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                                                                                       White
                                                                                       gelatinous
                                                                                       precipitate




                                                                               Figure 6.2


6.4.2 Analysis


     Table 6.4 is a summary of our analysis after considering the percentages from Test tube A
(the control)
Test tube   Ratio of NH3 to        Gas Displaced           Difference from        Percentage of Test
            Zn(NO3)2               (cm3)                   test tube A (cm3)      tube A (Percentage
                                                                                  of Recovery)
A           0 (control)            17                      Nil (control)          Nil (control)
B           0:1                    0                       17                     100%
C           1:1                    0                       17                     100%
D           2:1                    <1 (assume max.)        16                     5.88%
E           3:1                    <1 (assume max.)        16                     5.88%
F           4:1                    3                       14                     17.6%
G           8:1                    6                       11                     35.3%
H           9:1                    7                       10                     41.2%
Table 6.4
Note: All figures are rounded up to 3 significant figure


     The following graph is produced according to data of Table 6.4



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TOXICITY OF ZINC




Graph 6.1
Note: The ratio of 5:1 and 6:1 is not available since it is not so significant.


     X-axis: The Change in ratio of Ammonia and Zinc ion
     Y-axis: The percentage of recovery of Enzyme.




     From graph 6.1, a positive relationship between amount of ammonia compound added and
the rate of recovery.


     At 0 (no ammonia added), there was no reaction taken place. However, as the amount of
ammonia is added, reaction rate also increased. At the point Ammonia-to-Zinc ratio 1:4, the rate
of reaction increased the most (from 5.33% to 17.6%).


     This further increased to the maximum of 41.7% when Ammonia-to-Zinc ratio reached 9:1.




6.4.3 Results Discussion


     This experiment proved that Tetraammine Zinc compound worked to reduce the toxicity of

     44                                  CHEMISTRY OLYMPIAD 2010
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Zinc ions. The maximum percentage of recovery is 41.7%, which is a rather satisfactory result.


     Enzymes are reusable. They will not take part in the reaction, therefore though it is not a
100% recovery, 41.7% already mean a significant help to living organisms. Furthermore, livers of
animals may produce enzymes from proteins, therefore, the shortage of such enzymes would not
be long.


     However, we still take note that the rate of recovery is low, and hence not recommended
for food direct eating. Rather, if a small amount is eaten, the Zinc can be used as essential
minerals in human bodies, hence, no toxicity will be caused.




6.4.4 Photo Gallery


  6.4.4.1 Experiment Shots




Figure 6.3 Justin working hard on preparing the solutions




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TOXICITY OF ZINC




Figure 6.4 Mixing of a certain solution


  6.4.4.2 Experimental Results




Figure 6.5 Test Tube A




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Figure 6.6 Test Tube F




Figure 6.7 Test Tube H




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       TOXICITY OF ZINC




Gas bubbles




       Figure 6.8 Effervescence




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  7 Applications

7.1 Railings




                                            Figure 7.1 Railings exposed to the environment



     Railings can be another application of Tetraammine Zinc compound. Since railings need to
stand for a long time in outdoor areas, using Tetraammine Zinc coating will be much better than
purely iron or with a tin coating.


     If birds or other living organisms unfortunately come into contact parts of the coating, it
would not cause harm to them.




7.2 Iron Ships


     Tetraammine Zinc may be plated on iron ships to prevent rusting to occur. Then if sea birds
or fishes consumed parts of the coating, it would not cause a huge problem to their bodies.




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TOXICITY OF ZINC




Figure 7.2 A Rusted Ship




7.3 Coinage


  Zinc is the primary metal used in making American one cent coins since 1982. With the new
treatment to de-toxic the zinc metal, it can be used more extensively without causing any
problems to human health.




  Figure 7.3 A USA Penny



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7.4 Cans


     Iron is a major constituent in cans. Using Zinc to coat cans is a practical way. However,
ammonia has a pungent smell which may make the food not attractive. Therefore, we do not
suggest using tetraammine Zinc on food cans.


     Drink cans, however, may use Zinc as a coating. This is because we only need to inject a
small straw into it. We can open up a small hole for drinking, then the pungent smell would not
come out.




                                                             Straw




             Figure 7.4 A Coca-Cola Can with a straw inserted inside.




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TOXICITY OF ZINC




  8 Socio-economical Advantages



8.1 Economical Advantage


       We compare Zinc’s price with Tin, another main material used to coat iron. We take the
figures as of 4th March, 2010. Prices are presented on Table 8.1:


Price (USD):     Current                     3 Month                     15 Month
Zinc             1.0387                      1.0503                      1.0691

Tin              7.9628                      8.0059                      8.0399

Table 8.1


       Currently, the price of 1 pound of zinc is around US$1 while that of 1 pound of tin is around
US$8. With the sharp decrease in production cost of the can, the price of its products can be
lowered as well.




8.2 Higher Abundance




Graph 8.1

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     Zinc has a higher abundance. As from Graph 8.1, Zinc is around Logx= 3 higher than Tin. i.e.
It is 1000 times more abundant than tin.


     Zinc reserves around the world are expected to last for at least another 100 years. The
discovery of inhibiting the toxicity of zinc metal can promote the use of such metals, which at the
mean time means that the demand for other metals will decrease. This will extend the
availability of other more scarce metals such as tin and gold.




8.3 Zinc Recycling


     Zinc can be a sustainable element and used zinc parts can undergo recycling. The
International Zinc Association has the following description about Zinc recycling:



          Zinc coated steels are easily collected and treated in existing process
          streams. The Electric Arc Furnace (EAF) is the most widely used process for
          recycling zinc-coated steel. The high temperatures cause zinc - which is
          volatile at high temperatures - to leave the furnace along with other gases.
          The gas stream is treated and the zinc collected in the dust, of which zinc
          (18-35%) and iron are the main constituents. These dusts undergo an
          enrichment process in a rotary kiln, known as a Waelz kiln. This leads to
          the production of zinc oxide, which in turn becomes a raw material for the
          production of zinc metal. Several new technologies are in use or under
          development for processing EAF dusts and the valuable metals they
          contain.


          (International Zinc Association)


     The process of EAF is presented as of figure 8.2




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TOXICITY OF ZINC




                                             Figure 8.2




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    References


    1. Photo Credits

Photobucket.com Users:

     Anderson1100

     Kirbinster

     Tkons

     Nny2nk33s

     Wc3355

Books:

     HKDSE Chemistry – A Modern View; by E. Cheng, J. Chow, Y.F. Chow, A. Kai, K.K. Lai and W.H.

      Wong; 2009

     HKDSE Biology – A Modern Approach; by W.K. Chan, K.K. Ng, D. Sy, Y.C. Fung, F.K. Ngan and

      Dr. Jeffrey R. Day; 2009

Youtube.com Users:

     Evansp12




    2. Study Reports, Data and Facts Citation

     American Heart Association; Q&A about Chelation Therapy; Year Unknown


                                 CHEMISTRY OLYMPIAD 2010                             55
TOXICITY OF ZINC



    British Broadcasting Company; Chemistry of the Group 12 Elements; 2003


    Clark, Jin ;Chemguide; http://www.chemguide.co.uk/; 2004


    Draggan, Sidney; Public Health Statement for Zinc; 2008


    Fosmire, G.J.; Penn State University; 1990


    Haruni, Ron; Wall Street Pit; 2008


    Helmenstine, Anne Marie Ph.D ; About.com; 2007


    London Metal Exchange


    Manbir Online; http://manbir-online.com


    Pet Parrots 101.com; Year Unknown


    Souhami, RL, MD and Moxham, J, MD ;Textbook of Medicine; Published by Churchill


     Livingstone; 1997


    Virmani, Paul; Corrosion costs and preventive strategies in the United States;; 2002


    Wikipedia; Various authors; http://en.wikipedia.org


    International Zinc Association; http://zincworld.org




    56                                CHEMISTRY OLYMPIAD 2010

				
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