1 DETERMINATION OF COPPER AND NICKEL BY COMPLEXOMETRIC TITRATION

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					                                                                                  Date of Revision: 9/6/2003


    DETERMINATION OF COPPER AND NICKEL BY COMPLEXOMETRIC TITRATION


INTRODUCTION:

         Monel metal is an alloy of nickel and copper which is highly resistant to corrosion; it is
commonly used in applications involving exposure to acids. Depending on the specific application,
small quantities (less than 1% by mass) of other metals, like iron and manganese, may also be included
in the alloy. You will be given a quantity of Monel metal as a powder and asked to determine the mass
percentage of nickel and copper present in the powder. In this experiment, you will dissolve the
powder sample and a portion of this solution will be analyzed by complexometric titration. The
remaining portion of the solution will be saved for Experiment #4; in this experiment, the nickel and
copper present in small replicate volumes are separated using ion-exchange chromatography, then the
amount of nickel is determined based on a gravimetric analysis. The copper recovered from the
separation will be analyzed by iodometric titration in Experiment #5.


PRINCIPLE:

        The use of ethylenediaminetetraacetic acid (EDTA) as a titrant for the determination of metal
ion concentrations is a common analytical method. The completely deprotonated (dissociated) EDTA
molecule is capable of forming up to 6 separate coordination bonds with a single metal ion; this is
accomplished by donation of six separate lone pairs of electrons on the dissociated EDTA molecule to
empty orbitals existing on the metal ion. The resulting product of this reaction is a metal-chelate
complex with one-to-one stoichiometry; the reaction is described as:
                                         Y4- + Mn+ ----> MYn-4

where Y represents the completely dissociated EDTA molecule and M represents the metal ion.
         The reaction is usually rapid and, under the correct conditions, quantitative (conversion of
99.9% of the metal ions into a metal-chelate complex is achieved at the equivalence point). The
condition of greatest concern is the pH of the solution containing the metal ion. Since EDTA is a weak,
polyprotic acid and can only react effectively with a metal ion when it is completely dissociated, the pH
of the metal ion solution directly affects whether the titration can be performed quantitatively. This is
reflected in the conditional formation constant for the titration; a conditional formation constant for an
EDTA titration is the product of the intrinsic formation constant (the equilibrium constant for the
condition where all EDTA is completely dissociated) and the fraction of EDTA which is completely
dissociated. This fraction is determined by the pH of the solution containing the metal ion. (For a more
detailed of conditional formation constants see your text book.)
         Another condition which influences the formation constant is the presence of species (e.g,


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ligands) in solution which can form complexes with the analyte ions. If the concentration of a ligand is
large or the formation constant for its reaction with the analyte is large, the titration reaction will no
longer be quantitative. This phenomenon is often exploited by analysts to determine the concentration
of two or more metal ions in the same solution. As an example, consider a solution which contains both
Cu2+ and Ni2+, both of which you wish to quantify. The first step in this determination is to titrate the
solution with a standardized solution of EDTA; this result will give you the combined concentrations of
the Cu and Ni ([Cu2+] + [Ni2+]). The next step is to determine the Ni concentration only; this can be
performed by adding thiosulfate (S2O32-) to the solution prior to titration with EDTA. The thiosulfate
will selectively bind Cu and prevent it from reacting with EDTA. This titration will then give the
concentration of Ni only. Lastly, the concentration of Cu can be calculated by subtracting the
concentration of Ni from the combined concentration obtained in the first step.
         In this experiment, you will be determining the concentration of copper and nickel in the sample
of Monel metal. The procedure for this determination consists of 1) preparing a solution from a Monel
metal powder sample, 2) preparing 0.015 M EDTA and 0.015 M primary standard calcium solutions,
3) standardizing the EDTA solution using the primary standard calcium solution, 4) titrating the sample
solution with the EDTA solution to determine the combined concentrations of both Ni and Cu and 5)
repeating the titration after adding Na2S2O3 solution to the sample solution to determine the
concentration of Ni only.

A note about indicators: Another complicating factor in this analysis is the choice of an indicator; the
indicators for EDTA titrations are chemically similar to EDTA itself. They are polyprotic acids which
can form complexes with the analyte and these complexes impart a color to the solution.

                                  HxInd + M W M(Ind) + x H+
                                             (Color #1)

Ideally, only a small fraction (> 0.1%) of the metal is complexed by the indicator, so that a quantitative
amount of the metal remains free to react with EDTA as it is added during the progress of the titration;
once sufficient EDTA has been added to reach the equivalence point, the first excess drops of EDTA
react with the metal-indicator complex to liberate the indicator and produce a color change.

                              M(Ind) + Y + x H+ W MY + HxInd
                                                      (Color #2)

To achieve this ideal behavior, the formation constant should be large enough to prevent reaction
between the EDTA and the metal-indicator complex before the equivalence point is reached, yet
significantly smaller than the formation constant for the metal-EDTA complex so that the EDTA may
displace the indicator from its complex with the metal after only adding a drop or two of excess EDTA
solution.
         In this analysis, the standardization of the EDTA titrant solution against the Ca solution is
carried out at pH = 10 and uses the indicator Erichrome Black T, which is added as a solid powder (1
g Erichrome Black T in 100 g NaCl). The initial solution color is wine-red and the endpoint is marked

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by a sharp color change to a deep blue.
         The titration of the sample solution for both Ni and Cu content is carried out at the same pH,
but uses Murexide as an indicator, since both Ni and Cu form complexes with Erichrome Black T
which are too stable to be decomposed by EDTA. Additionally, the pH 10 buffer (a NH3/NH4+
solution) must be added carefully, so that excess NH3 does not interfere with the formation of the
complexes between Murexide and the analyte metals. The initial color is yellow to greenish-yellow and
the endpoint is marked by a change to a violet color. This change is slightly less sharp than the
Ca/EDTA titration using Erichrome Black T, with an orange color being seen just prior to the endpoint.
Once this orange color appears, the titration should be performed very slowly to avoid overshooting the
endpoint volume.
         The titration of the sample solution for Ni only is carried out at a lower pH (pH = 8) and with a
high concentration of S2O32-, so that Cu does not form a stable complex with the EDTA or Murexide.
Unfortunately, this lower pH reduces the stability constant for the Ni-Murexide complex as well; as a
result the titration does not produce a sharp endpoint, but starts out as orangish-yellow and progresses
gradually through a series of peachy-orange colors until a reddish-violet color is reached at the
endpoint. Since no sharp color change is observed, the best strategy for obtaining a reproducible
endpoint is to have a titration solution at the endpoint color for comparison while carrying out each
individual titration.

PROCEDURE:

Preparation of the sample

1) Obtain from your instructor a sample of Monel metal powder in a weighing bottle; dry it for at least
one hour at 160o C, then store it in a desiccator until ready to weigh it.

2) Weigh approximately 1.1 g of your Monel metal powder into a 250 mL beaker. Record this mass
to 0.0001 g.

3) Slowly add 10 mL concentrated HNO3 to the powder using an eye dropper; the initial addition will
cause a great deal of effervescence as some of the Ni metal in the powder reacts with the acid to form
H2 gas. After the entire volume of HNO3 has been added, place a watch glass over the beaker and
heat the solution using a hot plate. Make certain to position the hot plate under the hood enclosure at
your bench station to prevent any fumes from escaping into the lab.

4) After no more orangish-red NO2 fumes are evolved and no dark solid remains at the bottom of the
beaker, remove the watch glass and allow the HNO3 to evaporate down to a volume of 2-3 mL.
Remove the beaker from the hot plate and allow it to cool to room temperature.

5) Add 20 mL concentrated HCl to the solution, again keeping the beaker under the hood enclosure.
Heat the solution on the hot plate until all liquid has been evaporated. Frequently monitor the beaker
during this process so that you can remove the beaker from the hot plate as soon as the last bit of liquid

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has evaporated; this will prevent possible cracking of the beaker from heating after there is no more
liquid. Allow the beaker and its contents to cool to room temperature.

6) Add 10 mL concentrated HCl to the residue in the beaker and gently heat to dissolve it. After
dissolution is complete, again allow the liquid in the beaker to evaporate completely, then place the
beaker in an oven for one hour at 100oC- 110oC.

7) After removing the beaker from the oven, allow it to cool to room temperature, then add ca. 20 mL
9 M HCl to it. Place a watch glass over the beaker and heat it on a hot plate until all the residue has
dissolved. Cool the solution, then quantitatively transfer it to a 50 mL volumetric flask, using small
portions of 9 M HCl to rinse out the beaker. After the transfer is complete, dilute the solution to the
mark on the flask using 9 M HCl.


Preparation of the reagent solutions

1) The buffer solutions and 5% Na2S2O3 solution are provided.

2) To prepare 0.015 M EDTA dissolve 5.5 g analytical reagent grade disodium dihydrogen EDTA salt
in sufficient deionized water to make one liter of solution. (The one-liter volume may be measured out
using a graduated cylinder, since the EDTA solution will be standardized.). The dissolution is kinetically
slow and may take several minutes. In case turbidity develops after dissolution is complete, add NaOH
until the solution appears clear. (Do NOT add more NaOH than needed.)

3) To prepare a 0.015 M standard calcium solution, weigh out accurately 0.35-0.40 g primary
standard CaCO3 into a small beaker. Add approximately 50 mL deionized water to the beaker, then
dropwise add 3 M HCl to the solution until the CaCO3 dissolves and no more bubbling is observed.
(No more than 3 mL of acid should be required.) Quantitatively transfer the solution to a 250 mL
volumetric flask by washing the beaker with several small portions of deionized water after the initial
transfer is made; these portions are transferred to the volumetric flask. Dilute the solution to the mark
and mix it thoroughly.


Standardization

1) To determine the concentration of the EDTA solution accurately, pipet a 25.00 mL aliquot of the
primary standard Ca solution into a 250 mL Erlenmeyer flask followed by 5.0 mL of the pH 10 buffer
and a small quantity of Erichrome Black T indicator powder; swirl the solution to dissolve the powder.
The solution should appear wine red.

2) Fill a 50 mL buret with the EDTA solution, then titrate the prepared Ca solution with the EDTA
solution until the color changes to pure blue. (No wine red color should remain.)

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3) Calculate the concentration of the EDTA to 0.00001 M based on the primary standard [Ca].

4) Repeat the titration two more times; the concentrations calculated should agree within +/- 0.00003
M of the first value. If they do, average the three and use this average as the standard concentration of
the EDTA; if they do not agree, repeat the titration two more times, then average all the results using the
Q-test to reject any outliers.

Determination of the total copper and nickel concentration

1) Pipet 1.00 mL of your sample solution into a clean 250 mL Erlenmeyer flask and dilute with 50 mL
of deionized water.

2) Carefully add 3 M NaOH solution dropwise while swirling until the analyte solution becomes
colored lightly bluish-green and cloudy.

3) Carefully add 3 M HCl solution dropwise while swirling until the cloudiness just disappears.

4) Carefully add pH 10 buffer dropwise while swirling: after the first few drops, the solution should gain
a light bluish color and it should become cloudy; continue adding the buffer solution dropwise with
swirling until the cloudiness has just disappeared. (Do not add the buffer solution beyond this point.)

5) Add a small quantity of Murexide indicator powder and swirl to dissolve; the solution should appear
yellow or greenish-yellow.

6) Titrate with the EDTA solution until the color changes to orange, then slowly add the EDTA
dropwise until the color changes to violet. Keep this solution for comparison and repeat the titration
two more times.

7) Using the EDTA concentration from the standardization, calculate the total concentration of Cu and
Ni ([Cu] + [Ni]) in your sample solution to 0.0001 M for each of the titrations. If the concentrations
calculated agree within +/- 0.0005 M, average the three and use this average as the total concentration
of Cu and Ni ([Cu] + [Ni]); if they do not agree, repeat the titration two more times, then average all
the results using the Q-test to reject any outliers.


Determination of nickel concentration only

          By masking Cu in your sample, you can selectively titrate only Ni using the EDTA titrant. To
mask Cu, you will add the thiosulfate ion and use a lower pH of 8. Since the color change during this
titration is gradual, it is helpful to prepare a titration solution which is at the endpoint color to use as a
source of comparison.

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1) To prepare this solution, pipet 2.00 mL of your sample solution into a clean 250 mL Erlenmeyer flask
and dilute with 30 mL deionized water.

2) Carefully add 3 M NaOH solution dropwise while swirling until the analyte solution becomes colored
lightly bluish-green and cloudy.

3) Carefully add 3 M HCl solution dropwise while swirling until the cloudiness just disappears.

4) Add 10 mL 5% Na2S2O3 solution and swirl.

5) Add 20 mL of pH 8 buffer and swirl.

6) Add a small quantity of Murexide indicator powder and swirl to dissolve; the solution should appear
orangish-yellow.

7) Titrate the analyte solution with the EDTA solution, noting the gradual changes in the solution color.
Once the solution color has become reddish-violet with no observable remnant of orange or yellow, stop
the titration and note the volume. Add 0.5 mL more of the EDTA solution to see if any noticeable color
change occurs. Repeat this addition process until no further color change is observed. When no further
color change is observed, note the volume and use this solution for comparison in subsequent titrations
to help determine when the endpoint has been reached.

8) Repeat the preparation steps with a new 2 mL volume of the sample solution, but this time titrate
more slowly to avoid overshooting the endpoint volume. Constantly compare your current titration
solution to the one which has been taken to the endpoint. When the two solutions appear to be the same
color, record the volume of titrant used. To check the validity of the endpoint, add a few more drops of
EDTA and check to see if there has been any noticeable change in the color. Repeat the titration with
four additional 2 mL volumes of your sample solution. When all 5 volumes have been obtained, use the
Q test to reject any volume which qualifies as an outlier.




DETERMINATION:

       For the determination of Ni, calculate [Ni] for your sample solution based on the average
volume from the last series of titrations, then calculate the %Ni by mass in your original Monel metal
sample. To determine [Cu], subtract the average value for [Ni] from the average of the total Cu and Ni
concentration. Use [Cu] to calculate the %Cu by mass in your Monel metal sample.




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