Experiment 4 by kbQ7NR

VIEWS: 1 PAGES: 9

									                 Effect of Ambient Water Quality Criteria on
                         Free Copper Concentrations
Introduction
Copper is a transition metal that is potentially toxic to aquatic organisms. In aquatic
organisms such as fish and aquatic insects, the Cu2+ ion is absorbed across their gills by
proteins that normally control the balance of Na+ and Ca2+. The presence of Cu2+ in these
“ion channels” blocks the passage of Na+ and Ca2+ and can disrupt cell function in the
organism. (The 2003 Nobel Prize in Chemistry was recently awarded to the scientist who
determined the structure of ion channels.) Regulatory limits for copper in the aquatic en-
vironment are as low as 10 ppb (10 g Cu/L of water; 1.57 x 10-7 moles/L) in some
states, reflecting the low levels that can potentially harm aquatic organisms.

Important Points to Consider:

      Metal ions form complexes.
       A complex is formed when one or more small molecules (such as water or am-
       monia) or ions (such as OH-, Cl-, or CO32-) bind to a metal ion. These small mole-
       cules or ions which bind the metal ion are referred to as ligands. The resulting
       group (called a complex) may be charged or neutral. For example, Cu(NH3)42+ is
       a complex that forms when four ammonia (NH3) molecules bind to a copper(II)
       ion. Because the ammonia molecules are neutral, the complex has an overall
       charge of +2 due to the charge from the copper(II) ion. Depending on the exact
       chemical species present in a natural water system, many different complexes can
       form. When the metal ion is complexed only by water molecules in aqueous so-
       lution, it is often said to be “uncomplexed” (even though, strictly speaking, this is
       not correct), and is referred to as the “free” metal ion. The number of water mole-
       cules often is not specified, and, for example, the “free” copper(II) ion would be
       symbolized as Cu2+(aq).

      Complexes of metals are often less toxic than the “free” metal ion.
       Recent research has shown that the “free” form of the metal ion is usually the
       most toxic, so quantifying the amount of “free” metal ion is especially important
       in predicting the expected toxicity of metals in a water body.

      We must consider the concentration of the small molecules that form complexes.
       The amount of metal complex formed depends on the number of small molecules
       or ions present in solution.

      Complexed Metal + Free Metal = Total Metal
       If there are any metal complexes formed in a water body, the amount of “free”
       metal ion must be less than the total metal, since complexes always decrease the
       amount of “free” metal ion.

      We can use the above information to predict the effects that will occur from the
       presence of metal ions at a specific location.
The Experiment
You have been hired…
Because of the above considerations, the CopperCover Metal Plating Company has hired
the company for which you work (Casey’s Lab of Indentured Servitude, Inc.) to advise it
on its discharges of copper in wastewater. Continuance of their discharge permit is based
on the survival of test organisms, so if the effluent is toxic to those organisms, the
company pays hefty fines or could even lose its permit. CopperCover has several fa-
cilities located in the Middle and Southern Atlantic states, and they have specific ques-
tions at each site about the effects of local receiving waters on the toxicity of copper to
aquatic organisms. Your company has signed a contract to evaluate the level of free cop-
per and complexing agents present at each site. You will also be conducting controlled
experiments to determine if changes in the local water supply will have an impact on the
level of free copper at the various sites. The workers at your company will be divided
into three groups, each group will be assigned to study water from one of the sites de-
scribed below, and to devise tests to gather data to answer some specific questions.

Equipment and Materials Available at Casey’s Lab of Indentured Servitude, Inc.

      pH probe: measures the pH (acidity) of a solution
      Copper Probe: measures the “free” copper concentration of a solution
      Ammonia Probe: measures the “total ammonia nitrogen” (TAN; that is, the sum
       of the concentrations of NH3 + NH4+) in a solution
      Alkalinity Titration: determines alkalinity of a solution
      pH Adjustment Solutions
           o 1.0 M HCl for adjusting solutions to below pH 3
           o 10-2 M HCl for adjusting solutions to the range of pH 3 to 5
           o 10-4 M HCl for adjusting solutions to the range of pH 6 to 7
           o 10-4 M NaOH for adjusting solutions to the range of pH 7 to 9
           o 10-2 M NaOH for adjusting solutions to the range of pH 9 to 12
      Ammonia Adjustment Solution: 0.1 M NH4Cl
      Carbonate Adjustment Solution: 0.1 M NaHCO3

When using any of the “adjustment” solutions, you should add the solution dropwise to
the test solution, mixing thoroughly after each drop. If you are adjusting the pH, you can
keep the pH probe in the solution to monitor this quantity as you add the adjusting solu-
tion.
Research Required by CopperCover

Florence, SC
Their Florence, SC facility discharges water into the Pee Dee River, which drains
swampland in the coastal plain of South Carolina. After passing through the wetlands,
this water has high levels of dissolved organic matter (DOM) and a relatively low pH.
DOM is a complex mixture of substances, many of which have sites that are capable of
binding to a metal ion.

To simulate water discharging into the Pee Dee River:
       1. Measure 10 mL of 10-3 M copper solution into a beaker.
       2. Add 90 mL of Pee Dee River water and mix thoroughly.
       3. Measure the pH.
       4. Measure the amount of Cu2+ in the solution.
(It would probably be a good idea to perform the above procedure with at least two sepa-
rate water samples, to check your reproducibility.)

CopperCover would like to know the following:
    The concentration of DOM in the river will change with seasonal differences in
      rainfall. How will changes in DOM affect the amount of free copper? Set up an
      experiment to determine this using at least three different amounts of DOM.

Devise controlled experiments that can answer these questions. Remember that in a
“controlled” experiment, only one factor should be varied and all others should be kept at
least relatively constant. What variables do you need to keep constant? What do you
think is a reasonable number of trials to perform?

When your group has decided on procedures to gather the data needed to answer the
above questions, be sure to discuss them with your instructor before proceeding.


New Market, VA
At this facility, water is discharged into the South Fork of the Shenandoah River, which
flows out of a limestone-rich area. Limestone is primarily composed of calcium carbon-
ate (CaCO3), which is slightly soluble in water. When it dissolves, it breaks apart into
Ca2+ and CO32- ions. The level of CO32- in water largely controls its alkalinity. Alkalinity
is the ability of a solution to neutralize acid. Carbonate neutralizes acid by the following
set of chemical reactions:

               H+(aq) + CO32-(aq)  HCO3-(aq)

               H+(aq) + HCO3-(aq)  H2CO3 (aq)  H2O (l) + CO2 (g)

The measured alkalinity is the sum of all materials in the water that can accept H+. In
waters originating from areas with limestone, the dominant solutes that can accept pro-
tons are CO32- and HCO3- (hydrogen carbonate ion).
To simulate water discharging into the Shenandoah River:
       1. Measure 10 mL of 10-3 M copper solution into a beaker.
       2. Add 90 mL of Shenandoah River water and mix thoroughly.
       3. Measure the pH.
       4. Measure the amount of Cu2+ in the solution.
       5. Measure the alkalinity of the solution.
(It would probably be a good idea to perform the above procedure with at least two sepa-
rate water samples, to check your reproducibility.)

CopperCover would like to know the following:
    CO3 acts as a natural buffer, maintaining high pH levels and preventing drastic
      changes in pH. Because of this property, it is hard to determine if the free copper
      concentration in the water is due to the presence of CO3 or is caused by a higher
      pH. Will changes in pH affect the amount of free copper in the Shenendoah
      River? Set up an experiment to determine this using at least three different pH
      levels.


Devise a controlled experiment that can answer this question. Remember that in a “con-
trolled” experiment, only one factor should be varied and all others should be kept at least
relatively constant. In this case, there are two variables which could potentially affect the
concentration of “free” copper. What are they? Which should be kept constant? What
do you think is a reasonable number of trials to perform?

When your group has decided on procedures to gather the data needed to answer the
above questions, be sure to discuss them with your instructor before proceeding.


Princess Anne, MD
This CopperCover plant is located on a tributary of the Wicomico River, downstream
from a large poultry farming operation. The poultry farm stores used poultry litter in a
mound on its property. After heavy rains, runoff from the poultry farm sometimes brings
large amounts of ammonia (NH3; produced in poultry excrement) into the river.

To simulate water discharging into the Wicomico River:
       1. Measure 10 mL of 10-3 M copper solution into a beaker.
       2. Add 90 mL of Wicomico River water and mix thoroughly.
       3. Measure the pH.
       4. Measure the amount of Cu2+ in the solution.
       5. Measure the ammonia concentration of the solution.
(It would probably be a good idea to perform the above procedure with at least two sepa-
rate water samples, to check your reproducibility.)

CopperCover would like to know the following:

          Since the amount of NH3 entering the water is not constant, it would be
           beneficial to know the affects of differing NH3 concentrations. Does the
            amount of NH3 in the water affect the amount of free copper? Set up an
            experiment to determine this using at least three different amounts of NH3.

Design controlled experiments that can answer these questions. Remember that in a
“controlled” experiment, only one factor should be varied and all others should be kept at
least relatively constant. What variables do you need to keep constant? What do you
think is a reasonable number of trials to perform?

When your group has decided on procedures to gather the data needed to answer the
above questions, be sure to discuss them with your instructor before proceeding.


Questions

1. Which of the three sites is likely to have the most trouble with toxicity to aquatic
organisms? Briefly explain how you determined this.

2. Which water source had the most complexed copper? Briefly explain how you deter-
mined this.

3. Were there any general trends between the amount of free copper and the pH of the
solutions that were analyzed? If so, what were they?

4. Answer each of the bulleted questions that were asked for the three different site
locations. Justify your answers using the data obtained from the laboratory. Discuss how
the data led you to your conclusions.
Appendices: Specific Experimental Procedures

Operation of the pH Probe

The pH probe allows you to directly measure the pH of a solution. The probe must be
calibrated, so two standard solutions are provided for this purpose (pH 4 and 7).

NOTE:
   The pH probe must always be in a solution. Do not allow the probe to be exposed
      to the air for long periods of time.
   Always rinse the probe with deionized water between different solutions so that
      they do not become contaminated.

Use the following procedure to calibrate the pH probe.
       1. Press the “Mode” key until the display indicates pH mode.
       2. Press the “Setup” key twice, then hit “Enter” to clear existing calibrations.
       3. Rinse the probe thoroughly with DI water from a squirt bottle, then place the
       probe in the pH 7 solution. Stir the solution gently with the probe.
       4. Press the “std” button.
       5. Press the “std” button again.
       6. When the stable icon appears, the meter should indicate “measure” mode in the
       upper left corner of the display.
       7. Repeat steps 3 through 6 for the pH 4 standard.

After calibration, the probe is ready to measure the pH of your solutions.
       1. Place solution in a beaker.
       2. Rinse the probe thoroughly with DI water from a squirt bottle, then place the
       probe in the solution and swirl gently.
       3. Record the pH when the reading becomes stable.
       4. Rinse the pH probe well with deionized water, then repeat these steps for an-
       other solution.
       5. When finished with all solutions, rinse the pH probe and place it into the stor-
       age solution.
Operation of the Copper Ion-Selective Electrode

The copper ion-selective electrode is used to determine the amount of “free” copper in a
solution. This form of copper is most indicative of the potential toxicity of a solution.

NOTE: Always rinse the probe with deionized water between different solutions so that
they do not become contaminated.

The probe must be calibrated. Use the standards that are provided (10-3 M Cu2+, 10-4 M
Cu2+, 10-5 M Cu2+, 10-6 M Cu2+) to develop a calibration curve for the copper probe.

Use the following procedure for all standard solutions and samples that are to be analyzed
for free copper.

1. Measure 100 mL of the solution to be analyzed into a beaker.

2. Add 2 mL of 5 M NaNO3.

3. Place a stir bar in the beaker, set it on the stir plate, and start the stirring motor to give
a gentle stirring action.

4. Using a squirt bottle of deionized water, rinse the copper probe, then place it in the
beaker. Do not let the stir bar hit the tip of the probe, which could damage the probe!

5. Wait until the reading changes by less than 1 mV in 1 minute. At this point, the
measurement is considered stable, so record the response (in mV).

6. Remove the copper probe, rinse with DI water, then repeat steps 1-5 for additional
standards and samples. Make sure that the glassware is cleaned between each analysis.

7. After the last solution has been measured, rinse the probe and place into the storage
container. The copper probe should not be left in the air for long periods of time.

Data Analysis
In order to generate the calibration curve, you must graph the response (in mV) versus the
logarithm of the concentration. Ex. log 10-3 = -3. This is required to get a straight line
for the calibration. Because of this, when you analyze samples, the result generated will
be the logarithm of the concentration, not the concentration itself. To get the actual
concentration of Cu2+, use a scientific calculator to take the antilogarithm.
        1. Enter the value of the log concentration into the calculator.
        2. Press the 10x key. (Note: your calculator may use a different notation for this.
        Be sure that you are performing this operation correctly.)
        3. The result is the concentration.

Record the slope, intercept and R2 for the calibration curve.                   Determine the
concentrations of all the unknowns you prepared.
Operation of the Ammonia Ion-Selective Electrode

The ammonia ion-selective electrode is used to determine the “total ammonia nitrogen”
(TAN; that is, the sum of the concentrations of NH3 + NH4+) in a solution. When you
analyze a sample, you will add a strong base (which produces OH-) that will increase the
pH. This assures that any NH4+ present in the sample will be converted to NH3 by the
chemical reaction:
                      NH4+(aq) + OH-(aq)  NH3(aq) + H2O(aq)

NOTE: Always rinse the probe with deionized water between different solutions so that
they do not become contaminated.

The probe must be calibrated. Use the standards that are provided (10-2 M NH4Cl, 10-3 M
NH4Cl, 10-4 M NH4Cl) to develop a calibration curve for the ammonia probe.

Use the following procedure for all solutions that are to be analyzed for ammonia.

1. Measure 100 mL of the solution to be analyzed into a beaker.

2. Add 2 mL of 5 M NaOH. NOTE: NaOH is caustic and should be immediately
rinsed if it comes in contact with your skin.

3. Place a stir bar in the beaker, set on the stir plate, and start the stirring motor to give a
gentle stirring action.

4. Using a squirt bottle of deionized water, rinse the ammonia probe then place it in the
beaker. Do not let the stir bar hit the tip of the probe, which could damage the probe!

5. Wait until the reading changes by less than 1 mV in 1 minute. At this point, the mea-
surement is considered stable, so record the response (in mV).

6. Repeat steps 1-5 for additional standards and samples. Make sure that glassware is
cleaned between each analysis.

7. After the last solution has been measured, rinse the probe and place into the storage
container. The ammonia probe should not be left in the air for long periods of time.

Data Analysis
In order to generate the calibration curve, you must graph the response (in mV) versus the
logarithm of the concentration. Ex. log 10-3 = -3. This is required to get a straight line
for the calibration. Because of this, when you analyze samples, the result generated will
be the logarithm of the concentration, not the concentration itself. To get the actual
concentration of NH3, use a scientific calculator to take the antilogarithm.
        1. Enter the value of the log concentration into the calculator.
        2. Press the 10x key. (Note: your calculator may use a different notation for this.
        Be sure that you are performing this operation correctly.)
        3. The result is the concentration.
Record the slope, intercept and R2 for the calibration curve. Determine the concentra-
tions of all the unknowns you prepared.



Determination of the Alkalinity of a Water Sample

Alkalinity is a measure of the “acid neutralizing capacity” of a water sample. In natural
water samples, it is primarily due to the presence of carbonate (CO32-) in the sample.
Measure alkalinity using the following procedure.

1. Fill a buret to near the 0.0 mark with 0.1 M HCl. (Record its exact concentration; it
may not be exactly 0.1 M.) Don’t forget to remove any air from the tip of the buret by
opening the stopcock to full flow for several seconds.

2. Record the initial volume reading on the buret (in mL).

3. Place 100 mL of sample in a beaker. Add 3 drops of the alkalinity indicator. The
color should be blue.

4. Slowly add acid from the buret to the sample. Notice that as the acid hits the water, a
yellow or green color momentarily appears. Swirl or stir to mix well so that only one
color is present.

5. As the yellow/green color lingers longer in the beaker, slow the addition of HCl to one
drop at a time. At the first moment that the color stays yellow throughout the entire
solution, stop adding acid and record the volume reading on the buret.

Use the following formula to calculate the alkalinity (in terms of mg CaCO3/L) for your
sample.
                     alkalinity (mg CaCO3/L) = 50000 * CA * VA
                                                          VS

Where:         CA is the concentration of the acid in the buret in moles/L
               VA is the volume of acid delivered (final reading – initial reading)
               VS is the volume of the sample in the beaker
               (Note that VA and VS must be in the same units.)

								
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