Docstoc

AP Chemistry - Download as DOC

Document Sample
AP Chemistry - Download as DOC Powered By Docstoc
					AT Chemistry

                 Gravimetric Determination of Nickel(II) Ion
Introduction

       Gravimetric analysis is a standard classical method for determining the amount of
a given component present in many solutions, or solid-unknown samples. The method
involves precipitating the component of interest from the unknown by means of some
added reagent. From the mass of the precipitate, the percentage of the unknown
component in the original sample may be calculated.

        Generally, the reagent causing precipitation is chosen to be as specific as possible
for the component of interest. This process is intended to remove and mass only the
particular component of interest from the unknown sample. The precipitating agent must
also be chosen carefully so that it completely precipitates the component under study.
The resulting precipitate must have an extremely low solubility, must be of known
composition, and must be chemically stable. Handbooks of chemical data list suggested
precipitating agents for routine gravimetric analysis of many unknowns. One of the most
common labs of this type involves dissolving a weighed sample of a Group 1 or Group 2
chloride salt in water and precipitating the chloride with excess aqueous silver nitrate.
The silver chloride precipitate is then filtered, dried and massed. The precipitate must be
kept out of light because photons of light can cause the silver ion in the precipitate to be
reduced to silver metal.

        A complete gravimetric analysis includes a series of distinct steps. First, a
precise, known amount of original unknown sample must be taken for analysis. If the
unknown is a solid, an appropriately sized portion is taken and massed as precisely as
permitted by the balances available (typically to the nearest 0.001 or 0.0001 g). If the
unknown is a solution, an appropriately sized aliquot (a known fraction) is taken with a
pipette. The sample of unknown taken for the analysis must be large enough that
precision may be maintained at a high level during the analysis but not so large that the
amount of precipitate generated cannot be handled easily.

       Next, the unknown sample must be brought into solution (if it is not already
dissolved). For some solid samples, such as metal ores, this may involve heating with
acid to effect the dissolving. Frequently, the pH of the solution of unknown must be
adjusted before precipitation can take place; this is usually done with concentrated buffer
systems to maintain the pH constant throughout analysis.

        The reagent that causes the precipitation is then added to the sample. The
precipitating agent is added slowly, and in fairly dilute concentration, to allow large,
easily filterable crystals of precipitate to form. The precipitate formed is allowed to stand
for an extended period, perhaps at an elevated temperature, to allow the crystals of
precipitate to grow as large as possible. This waiting period is called digestion of the
precipitate.
         The precipitate must then be filtered to remove it from the liquid. Although
filtration could be accomplished with an ordinary gravity funnel and filter paper, this
would probably be very slow. The precipitates produced in gravimetric analysis are often
very finely divided and would tend to clog the pores of the filter paper. Specialized
sintered glass crucible funnels have been prepared for routine gravimetric analysis.
Rather than filter paper, such funnels have a frit plate constructed of several layers of
very fine compressed glass fibers that act to hold back the particles of precipitate. Such
glass funnels can use suction to speed up the removal of liquid from a precipitate, can be
cleaned easily before and after use, and are not affected by reagents in solution.

        Once the precipitate has been transferred to the sintered glass funnel, it must be
washed to remove any adhering ions. As the precipitate forms, certain of the other
species present in the mixture may have been absorbed on the surface of the crystals.
The liquid chosen for washing the crystals of precipitate is usually chosen so as to
chemically remove such adhering ions. The washing of the precipitate must be
performed carefully to prevent peptization of the precipitate. The wash solution must not
redissolve the precipitate or break up the crystals to the point where they might be lost
through the pores of the filter.

        After the precipitate has been filtered and washed, it must be dried. This is
usually accomplished in an oven whose temperature is rigorously controlled at 110°C.
The oven must be hot enough to boil off water adhering to the crystals, but cannot be so
hot that it might decompose the crystals. Some precipitating reagents are organic in
nature and cannot stand very strong heating.

        Finally, the dried precipitate is massed. From the mass and composition of the
precipitate, the mass of the component of interest in the original sample is determined.
Generally, gravimetric analyses are done in triplicate (or even quadruplicate) as a check
on the determination. The precision expected in a good gravimetric analysis is very high,
and if there is any major deviation in the analysis, it is likely due to some source of error
either in the procedure or by the operator.

        In this experiment the percentage of nickel in a sample will be determined
gravimetrically by precipitation with the organic reagent dimethylglyoxime (DMG).
(Dimethylglyoxime is a bidentate ligand). Nickel is often added in small amount during
the production of steel; the precipitation analysis with DMG is especially useful in this
situation. Dimethylglyoxime is a complexing agent, forming a characteristic bright red
coordination compound with nickel ion. In addition to the gravimetric determination of
nickel, DMG is also often used as a spot test to detect the presence of nickel in a sample
on a qualitative basis. DMG does precipitate a few other metal ions (for example,
platinum), but the bright red color of the Ni(II)/DMG precipitate is very difficult to
confuse.

        The precipitate produced by DMG with nickel(II) ion contains two DMG species
complexed per nickel ion and consists of 20.32% nickel by weight. The precipitate is
very fluffy and has a very low density, which makes it somewhat difficult to handle in



                                                                                           2
any great quantity. For this reason, the practical use of DMG in nickel analysis is
restricted to samples in which the percent of nickel is rather small. As an organic
reagent, DMG is not very soluble in water and is provided as a solution in alcohol. The
presence of volatile alcohol, combined with the nature of the Ni/DMG precipitate itself,
causes the precipitate to creep up the sides of the funnel used for its filtration. Caution
must be exercised while transferring the precipitate to the filter funnel to prevent its loss.
Because DMG is a weak acid, the precipitation of nickel ion is somewhat sensitive to pH.
Before precipitation, the solution is buffered at basic pH with ammonia.

Safety Precautions

              Safety goggles must be worn at all times.
              Wear disposable gloves during the procedure – nickel(II) compounds are
               toxic.
              The nickel/dimethylglyoxime precipitate will stain skin if it is spilled.

Prelab Assignment

   1. What is meant by the term gravimetric factor?

   2. A sample containing nickel was massed out and treated with dimethylglyoxime.
      The nickel/DMG precipitate (20.32% nickel by mass) was then massed.
      Calculate the percentage of nickel in the original sample, given the following data
      and include the uncertainty:

                       Mass of nickel sample taken:           0.512 g + 0.002 g
                       Mass of Ni/DMG precipitate:            1.311 g + 0.002 g

   3. In the introduction dimethylglyoxime is described as a bidentate ligand. Use your
      textbook to write a definition of this term See if you can find the structural
      formula and formula mass of this compound.

   4. Outline a flowchart for the procedure in this experiment.

   5. In this lab you will be vacuum filtering the Ni-DMG precipitate using an aspirator
      attached to a faucet. Explain how this works and identify the physics principle
      involved.

Reagents and materials:

nickel sample                          buchner funnel              rubber policeman
1% dimethylglyoxime solution           suction filter apparatus    distilled water
6M NH3 (dropper bottles)               oven                       milliigram balance
tartaric acid                          600-mL beaker              thermometer
pH paper                               analytical balance         stirring rod
                                       weighing paper             beaker tongs


                                                                                            3
Procedure

Part I Preparation and Precipitation of the Nickel Samples

   1. Obtain an unknown nickel sample and record the identification number in your
      data table.

   2. Obtain a 600-mL beaker and label with your initials. Make sure the beaker has no
      cracks in the glass. Mass out between .4 and 0.5-g sample of the unknown nickel
      sample, using weighing paper, and transfer to the beaker. Make the mass
      determination to at least the nearest milligram, and do not go above 0.5 g total
      mass of sample. Record the mass of sample. Add approximately 150 mL of
      distilled water to the beaker.

   3. Mass out approximately 0.2 g of tartaric acid using the weighing boat and the
      decigram balance, and add one to the beaker. Stir to dissolve the acid. (The
      tartaric acid forms soluble, stable metal complexes with any other metal ions that
      might be present in a real sample and will prevent these other metals from
      precipitating with the nickel in this determination).

   4. Adjust the pH of the two samples to between 8 and 9 by adding 6M ammonia
      solution drop by drop while stirring. This should be done in the hood. The
      correct way to test the pH is to wet the end of a stirring rod with the solution and
      touch the end of the rod to the pH paper.

   5. Now heat the sample to approximately 80°C using a hot plate. Then remove the
      beaker from the hot plate using beaker tongs.

   6. Then slowly and with constant stirring add 25 mL of 1% dimethylglyoxime
      solution. An intensely red, fluffy precipitate should form at this point. If no
      precipitate forms, chances are the pH of your solution has not been correctly
      adjusted. Allow the precipitate to settle until a layer of clear liquid (called the
      supernatant) is visible at the top of the beaker. This will take about 10-15 minutes
      (you may initially observe a clear layer above the precipitate). To ensure that
      precipitation is complete, add 2-3 drops of 1% DMG to the clear layer. If no
      additional red precipitate forms, then you can assume that all of the nickel has
      precipitated. If additional precipitate does form at this place, add 2-3 mL
      additional 1% DMG and allow the precipitate to settle again. Then test the
      supernatant liquid with another drop of 1% DMG. Then allow the sample to cool
      to room temperature.




                                                                                        4
   Part II Filtering the Nickel/Dimethylglyoxime Precipitate

   7. Mass the Buchner filtering funnel with the filter paper to the nearest milligram.

   8. Set up the Buchner filtering funnel on a suction filtration apparatus. See
      demonstration.      Turn on the suction, and first wet the filter paper completely
      using distilled water. Then slowly begin pouring the supernatant liquid from your
      sample into the funnel. Do this gradually; typically the supernatant contains at
      least a small amount of the nickel precipitate. When most of the liquid has been
      transferred, gradually begin transferring the red precipitate. DO NOT FILL THE
      FUNNEL MORE THAN HALF FULL AT ANY TIME. The Ni/DMG
      precipitate has a habit of creeping up the sides of the filtering funnel.   If the
      precipitate begins to creep too far up the sides of the funnel, stop transferring
      precipitate, and allow the suction to remove moisture from the funnel.

   9. When the bulk of the precipitate has been transferred from the beaker to the
      funnel, use small portions of distilled water (the washing solution) and a rubber
      policeman to transfer any remaining particles from the sides of the beaker. When
      all the precipitate has been transferred, continue suction for another two minutes
      to help dry it.

   10. When done filtering break the vacuum seal and turn the water off. Transfer
       the funnel using tongs to the numbered beaker, and return the funnel to the oven.
       When the precipitates have completely dried, mass the funnels.

   11. Obtain data for a second sample from another group and calculate the mean %
       nickel based on two samples.


Calculations (show calculations in lab report following the data table and include the
uncertainties)

The Ni/DMG precipitate is known to contain 20.32% nickel by mass.

   1. The mass of the nickel present in a given amount of precipitate is then

       (mass of precipitate) X (0.2032)

   2. The percent nickel in the original sample is then

       (mass of nickel/mass of sample taken) X 100




                                                                                          5
Data

                                    Sample 1

Identification Number               __________

Mass of sample taken originally     __________ + __________

Mass of empty funnel                __________ + __________

Mass of funnel with precipitate     __________ + __________

Mass of precipitate                 __________ + __________

Mass of Ni in precipitate           __________ + __________

% nickel in original sample         __________ + __________



                                    Sample 2

Identification Number               __________

Mass of sample taken originally     __________ + __________

Mass of empty funnel                __________ + __________

Mass of funnel with precipitate     __________ + __________

Mass of precipitate                 __________ + __________

Mass of Ni in precipitate           __________ + __________

% nickel in original sample         __________ + __________



Mean % nickel           __________ + ________ %

Appearance of nickel solution       ______________________________

Appearance of nickel precipitate    ______________________________




                                                                     6
Questions

   1. The percentage of the nickel in the Ni/DMG precipitate is 20.32% Using the
      atomic weight of nickel and the formula weight of dimethylglyoxime, derive this
      percentage.

   2. It was mentioned that the Ni/DMG precipitate had a tendency to creep up the
      sides of the funnel. If this is not monitored, the precipitate can crawl out of the
      funnel. What error in the % Ni determined for a sample would this introduce?

   3. During the experiment, it was necessary to test the nickel sample for complete
      precipitation of nickel by adding a drop of DMG to the supernatant liquid. What
      error would be introduced into the % Ni determined for a sample if not all the
      nickel had been precipitated?

   4. The DMG is provided as a solution in alcohol. Why is alcohol, rather than water,
      used as a solvent?

   5. In a different type of gravimetric analysis, an impure sample of table salt
      weighing 0.5332 g was dissolved in water and treated with excess silver nitrate
      solution. The resulting silver chloride precipitate weighed 1.044 g after filtering
      and drying.

      a) Why must the AgCl precipitate be stored in the dark? Specifically, how will
         the mass of the AgCl differ from the precipitate dried in the dark versus
         exposed to light while drying? Explain.
      b) Determine the percentage of NaCl in the impure sample.




References

This experiment was taken from Experimental Chemistry by James F. Hall, published by
D.C. Heath & Company, 1989.




                                                                                       7

				
DOCUMENT INFO