Experiment 7
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Experiment 7: Synthesis and Analysis of Potassium
Oxalatocobaltate(III) Hydrate, K(2x-3)[Co(C2O4)x].yH2O
Introduction (General)
The cobalt(III) hexaquo ion, [Co(H2O)6]3+, is unstable in aqueous solution with respect to
the cobalt(II) hexaquo ion, [Co(H2O)6]2+, as indicated by the high standard reduction potential in
acidic solution:
[Co(H2O)6]3+ + e- → [Co(H2O)6]2+;
Eq = + 1.82 V
Several Co(III) complexes are, however, more stable than their Co(II) analogues, which
are readily oxidized by air. The reduction potentials in these cases are less positive or even
negative.
In this experiment, the oxalatocobaltate(II) complex is made in aqueous solution and oxidized to the oxalatocobaltate(III) complex with lead(IV) oxide. The series of reactions by which
this complex is prepared is summarized in the following UNBALANCED equation:
___CoCO3 (s) + ___H+ (aq) + ___C2O42- (aq) + ___ PbO2 (s) + ___CH3COOH (aq) +
___K+ (aq) + ___ H2O (l)
→
___K(2x-3)[Co(C2O4)x].y H2O (s) + ___ CO2 (g) + ___ Pb2+ (aq) + ___CH3COO- (aq)
Free oxalate ion itself would be oxidized under these conditions but is stable in the complexed form. The Co3+ ion of the product is capable of oxidizing oxalate ions but reaction
between the central metal ion and its own ligands occurs only when catalyzed by light. While
there is a possibility of decomposition to more stable products, the rate of this reaction in the
dark is very slow. The product is said to be "kinetically" stable.
A complete analysis of the product can be carried out, since the metal ions and ligands
can be separated and their molar quantities can be estimated by quantitative reduction and oxidation, respectively.
Introduction
II. Analysis for cobalt by iodimetry
Aqueous solutions of cobalt(III) species are only stable if the Co3+ ion is complexed because it will oxidize water itself:
Co3+ (aq) + e- → Co2+ (aq);
Eq = + 1.81 V
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II. Analysis for cobalt by iodimetry
O2 (g) + 4 H+ + 4 e- →
Eq = + 1.23 V
2 H2O (l);
The insolubility of cobalt(III) oxide in water makes it a weaker oxidizing agent so that it is
stable in contact with basic aqueous solutions:
3 H2O (l) + Co2O3 (s) + 2 e-
→ 2 Co(OH)2 (s) + 2 OH- (aq);
Eq = + 0.14 V
When the cobalt(III) oxide is treated with acidified potassium iodide solution, it instantaneously forms the Co3+ (aq) ion, which rapidly and quantitatively oxidizes the iodide ion to iodine
because of the big difference in Eq values. I2 (aq) is retained in solution as I3- (aq) because of
the excess of I- ion present. The presence of I3- (aq) does not significantly alter the Eq value of
the reduction reaction below:
I2 (aq) + 2 e- → 2 I- (aq);
Eq = 0.54 V
The iodine is then quantitatively reduced back to iodide ion by thiosulfate ion. This reaction a very sharp end point when a starch solution is used as the indicator.
S4O62- (aq) + 2 e- → 2 S2O32- (aq);
Eq = 0.08 V
The overall reaction is therefore the oxidation of thiosulfate to tetrathionate by cobalt(III)
oxide. This use of iodine as an intermediate can be applied with minor modification to the
quantitative analysis of a wide range of powerful oxidizing agents. The main requirement is that
the Eq value for the oxidizing agent be greater than about 0.8 V so that the oxidation of iodide is
essentially quantitative.
COLLECTION containers:
Lead and cobalt.
Preparation of Complex
Chemicals required
Oxalic acid dihydrate
2.5 g
Potassium oxalate monohydrate
7.4 g
Cobalt(II) carbonate
2.2 g
(labelled as cobaltous carbonate)
Lead(IV) oxide
2.6 g
(labelled as lead dioxide)
8.75 mol.L-1 acetic acid solution
95% ethanol
5 mL
50 mL
(labelled as 50% acetic acid)
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Synthesis Procedure
Completely dissolve 2.5 g of oxalic acid dihydrate in 50 mL of hot deionized water in a
250 mL beaker and then dissolve 7.4 g of potassium oxalate monohydrate in this solution. Add
2.2 g of cobalt(II) carbonate in small portions, allowing effervescence to subside between each
addition. Warm the solution (avoid boiling!!) until all CoCO3 has reacted and a purple solution is
obtained. Look up through the bottom of the beaker to ensure that no pale purple powder has
collected. If any is observed, stir well until it disappears. Gravity filter the mixture to remove
any dark solid residue before continuing. The filtrate obtained should have a volume of about
50 mL and a temperature of 35–40°C. If water has been lost through evaporation, add deionized water to increase the volume of solution to about 50 mL. Once this is done, measure the
temperature of the solution and heat or cool as necessary to obtain the desired temperature.
Add 2.6 g of lead(IV) oxide, PbO2, to the solution above while stirring it vigorously. Once
the PbO2 is added, place the beaker into a bowl or beaker containing water at a temperature of
about 40°C. Continue vigorous stirring using a magnetic stirrer and a stirring bar and add 5 mL
of an 8.75 mol.L-1 acetic acid solution from a separatory funnel very slowly, a few drops at a
time, over a period of 30 minutes. Be sure to keep the temperature of the water in the bowl
at about 40°C during the entire acid solution addition. Oxidation takes place during the addition and the resultant solution should be deep green. Vigorous stirring is essential to allow the
solid lead oxide to react.
Filter off excess PbO2 using suction filtration with a Buchner funnel. Dispose of the collected solid and the filter paper in a lead collection container. Transfer the filtrate to a 250 mL
beaker and to it add 50 mL of 95% ethanol. Once the ethanol has been added, stir the mixture
for ~15 minutes using a stirring bar and magnetic stirrer. Break up any lumps that form with a
stirring rod. Cool the filtrate in an ice bath and once crystallization is complete, decant much of
the filtrate solution into a Buchner funnel to separate the crystals from the filtrate. Transfer the
green crystals from the beaker to the Buchner funnel using a stirring rod and the remaining filtrate. Wash them with 95% ethanol and apply suction to remove most of the liquid remaining in
the solid. Spread the green solid out on a watch glass and let it air dry in the dark. Discard the
filtrate in a cobalt collection container. Record the mass of your product and calculate a percent yield value. Submit your product for grading when you submit your report.
If the product is contaminated with pink material (cobalt(II) oxalate), consult a demonstrator. Use the following method to purify your product only if instructed to do so!
Experiment 7
Synthesis Procedure
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(continued)
Dissolve the green crystals in not more than 30 mL of deionized water at room temperature and filter off the pink solid oxalate. Now add 50 mL of ethanol again, cool in an ice bath,
and collect the product as before. As mentioned above, the compound is unstable with respect
to internal oxidation-reduction, especially in aqueous solution and in daylight. Therefore, solutions must not be allowed to stand longer than necessary and the compound should be kept in
the dark as much as possible.
Analysis of Potassium Oxalatocobaltate(III) Hydrate
On treatment with base, the cobalt is precipitated quantitatively as the insoluble
cobalt(III) oxide, Co2O3, while the ligands remain in solution as free oxalate ions. The two parts
can thereby be separated quantitatively by filtration and analyzed separately. Parts I and II
should be completed in the same lab period.
NOTE: Ensure that the procedure described below (Parts I., II. and III.) is carried
through for each 0.8 g sample separately; clearly identify the precipitate, filtrate and beaker
belonging to each sample. Label carefully!!
Analysis Procedure
I.
Decomposition of the oxalatocobaltate(III) anion
Weigh accurately by difference, on an analytical balance, two samples, each of about
0.8 g, of dry product previously synthesized. Perform the rest of the procedure outlined below
for each sample. Dissolve the sample in about 50 mL of deionized water in a labelled beaker
(e.g. 1, 2, etc.) and add 8 mL of 3 mol.L-1 sodium hydroxide to the solution. Heat the solution
and stir thoroughly using a magnetic stirring-hot plate and a stirring bar for a few minutes to help
ensure the complete precipitation of black cobalt(III) oxide. Remove the beaker from the heat
and allow the black solid to settle. The solution must be colourless (i.e. not green) when the oxide has settled.
Filter the mixture into a 250 mL Erlenmeyer flask through a piece of fluted ashless filter
paper (Whatman #40) supported by a glass funnel. Wash the beaker that held the mixture with
25 mL of hot deionized water to remove as much of the remaining solid as possible. This wash
mixture should be poured into the fluted filter paper. Next wash the precipitate with 25 mL of hot
deionized water. The filtrate collected in the 250 mL Erlenmeyer flask should be colourless, not
green. Retain the filter papers containing the precipitates and the beakers (with adherent
Experiment 7
Analysis Procedure
I.
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(continued)
Decomposition of the oxalatocobaltate(III) anion
fragments of oxide) for part II. The filtrates should be kept for part III.
II.
Analysis for cobalt by iodimetry
Carefully remove the filter paper containing the precipitate from the funnel, fold it and put
it into a glass-stoppered 500 mL reagent bottle. Wash out the beaker in which the decomposition was carried out with a solution containing 1 g of potassium iodide, 5 mL of 3 mol.L-1 hydrochloric acid and 20 mL of deionized water (this will dissolve any remaining cobalt(III) oxide).
Transfer this solution quantitatively to the appropriate stoppered bottle. Shake the stoppered
bottle vigorously, without losing any of the solution, until the mixture is a dark brown colour.
Leave to stand for about 30 minutes before titrating.
Slowly titrate the iodine with standardized 0.05 mol.L-1 sodium thiosulfate until the dark
brown iodine colour of the mixture lightens to pale tan (Do not confuse this colour with that of
the pale pink Co2+ (aq) ion which is also present! It plays no further part in the reactions.).
The solution should appear yellow at this point. Now dilute the mixture to a volume about 200
mL with deionized water and add sufficient starch solution until the mixture becomes deep blue
or olive green. About 2 mL of starch solution should be needed. Continue dropwise addition of
thiosulfate solution until the blue or green colour vanishes, leaving only the pink colour of Co2+
(aq). This endpoint is extremely sharp.
Repeat this procedure with the second Co2O3 sample. When both titrations are complete, decant the resulting solutions into a large beaker and then carefully pour the solution into
the cobalt collection container. Place the wet filter paper in the beaker into a garbage container.
Retrieve the stirring bars and return them to the stock room.
III.
Analysis for oxalate using potassium permanganate
If you are unsure of how to use a volumetric flask to prepare a solution, consult an
instructor. When the filtrate and washings obtained in the decomposition of oxalatocobaltate(III) (part I.) have cooled to room temperature, transfer each solution quantitatively, using a
stirring rod or funnel and a water bottle, to a separate labelled 250.00 mL volumetric flask. Add
sufficient deionized water to fill each flask to the mark. Once the bottom of the meniscus is resting on top of the mark, stopper and then invert each flask twenty to thirty times to ensure that
the resulting solutions are well mixed.
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Analysis Procedure
III.
Winter 2013
(continued)
Analysis for oxalate using potassium permanganate
Pour a small volume of one of the solutions into a clean, dry 250 mL beaker and then
pipette a 50.00 mL aliquot of this solution (50 mL pipettes are available at the stockroom)
into a clean 250 mL Erlenmeyer flask. To this flask add 25 mL of 3 mol.L-1 sulfuric acid, heat
nearly to boiling and titrate with the supplied standardized 0.02 mol.L-1 potassium permanganate. The flasks should be too hot to touch, i.e. above 60°C, when titrating. At the endpoint, a
faint pinkish tint of permanganate will persist. Repeat until two titres within 0.05 mL are obtained for the first solution analyzed. Then repeat the entire procedure for the second solution.
Calculations
Record all of your calculated results in the table included at the end of this lab
outline. Submit the completed table as part of your lab report. Include one complete set
of sample calculations as part of your lab report.
1.
Write balanced reaction equations showing the stoichiometry for the reactions
between Co3+ and I- and between I2 and S2O32-. Produce these equations using the half reactions provided in the introduction to this experiment. Combine
these two equations into one overall equation. Deduce the stoichiometric relationship between Co3+ and S2O32-. Using this relationship, calculate the mass
percent of cobalt for each sample analyzed. Also report an average value for
mass percent of Co.
NOTE:
2.
mass percent cobalt = (mass of Co3+ in sample / mass of sample) x 100%
Write a balanced equation for the reaction between oxalate ion and permanganate ion. Calculate the mass percent of oxalate for each sample analyzed using an average volume of potassium permanganate for each sample. (HINT:
Remember you only analyzed a portion of the oxalate solution). Report an average mass percent of oxalate.
3.
Calculate the mole ratio of oxalate to cobalt, x, for each sample of product analyzed and determine an average value of x.
NOTE: x should be an integer value! ( x = moles of oxalate / moles Co3+)
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Calculations (continued)
4.
Determine the number of moles of product analyzed using the integer value of x
from 3.. Use this number of moles to calculate the molar mass of the product.
5.
Determine the number of water molecules in one formula unit of product, i.e. y.
This can be calculated as follows:
y = amount of product’s molar mass due to water / molar mass of water
Determine an average value of y.
6.
In your report include a balanced equation for each of the following, using your
values of x and y where required:
i)
the preparation of the oxalatocobaltate(II) complex
ii)
the oxidation of the oxalatocobaltate(II) complex to form the oxalatocobalt-
ate(III) complex
iii)
the formation of the product from its constituent parts
NOTE 1: The equations for ii) and iii) can be combined into one equation.
NOTE 2: Have an instructor check your values of x and y before you begin 7.!!!!!!!
7.
Using the equations from 6. above, produce a balanced equation for the overall
reaction. Use the coefficients from your equation to balance the equation given
on the first page of this lab outline. Make use of this balanced equation to calculate the theoretical yield of product produced by all reactants except water (assume it to be in excess) and from these calculations determine the identity of the
limiting reagent. Calculate the percent yield of product you collected based on
the limiting reagent using the chemical formula found in Palmer or your formula
if it is similar.
Questions
1.
Write balanced equations for all the reactions involved in the analysis for cobalt.
2.
Identify a primary standard which can be used to standardize a potassium permanganate solution and show the balanced reaction equation for the standardization. Ensure you provide a reference to the book(s) you use to answer this
question.
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Questions
3.
What is the name of the shape of the complex anion, oxalatocobaltate(III)?
Draw the structure of the complex anion showing all atoms in the ion. What
kind of isomers, if any, are possible for this structure? Draw a diagram showing
all isomers that can form.
References
1.
An analytical chemistry textbook (e.g. D.C. Harris. Quantitative Chemical Ana
lysis, any edition or W.E. Harris and B. Kratochvil. An Introduction to Chemical
Analysis).
2.
F.A. Cotton, G. Wilkinson and P.L. Gaus. Basic Inorganic Chemistry, third edition.
3.
N.N. Greenwood and A. Earnshaw, Chemistry of the Elements.
4.
J.E. Huheey. Inorganic Chemistry, second edition.
5.
W.G. Palmer. Experimental Inorganic Chemistry, pp. 550–551.
6.
G. Rayner-Canham. Descriptive Inorganic Chemistry, first or second edition.
7.
C.E. Housecroft and A.G. Sharpe. Inorganic Chemistry, second edition.
8.
G. Svehla. Vogel's Qualitative Inorganic Analysis, sixth or seventh edition.
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NOTE: Submit this sheet with your lab report!!!
Experiment 7 - Analysis of Potassium Oxaltocobaltate(III) Hydrate
Summary of data and results
Data
Sample 1
Mass of sample
Sample 2
g
g
Analysis for cobalt
Concentration of S2O32- solution
mol/L
Titration volume of S2O32- solution
mL
mL
Moles of S2O32- reacted
mol
mol
Moles of Co3+ reacted
mol
mol
%
%
% of Co3+ in sample
average % of Co3+ in sample
Value of x & analysis for C2O4
%
2-
Concentration of MnO4- solution
Average titration volume of MnO4- solution
mol/L
mL
mL
moles MnO4- reacted
mol
mol
moles C2O42- in 50.00 mL aliquot
mol
mol
moles C2O42- in 250.00 mL solution
mol
mol
%
%
% C2O42- in sample
average % C2O42- in sample
%
value of x
(moles C2O42- / moles Co3+)
Average value of x
Value of y
moles K(2x-3)[Co(C2O4)x].yH2O
moles C2O42- / average x)
molar mass
K(2x-3)[Co(C2O4)x].yH2O
value of y
average value of y
mol
mol
g / mol
g / mol