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Chem 3024 Fall 2003 Iodometric Determination of Copper. Introduction.

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Chem 3024

Fall 2003 Iodometric Determination of Copper


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Introduction

This procedure involves the reaction of cupric ions with iodide to produce iodine, which is titrated with standard sodium thiosulfate. Oxides of nitrogen produced during dissolution of the ore sample are removed by reaction with urea. The reaction between cupric and iodide ions is carried out in an acidic solution with an approximate pH of  3.5. The pH is important to prevent the hydrolysis of the cupric ion which in turn would cause an incomplete reaction between the cupric ion and the iodide.


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Introduction

The solid cuprous iodide (CuI) that is formed during the oxidation of iodide absorbs some of the iodine formed, releasing it only slowly at the end point. This can cause the color change to be gradual at the equivalence point. Potassium thiocyanate (KSCN)  is added to displace the iodine, resulting in a sharper color change and more accurate estimate of the equivalence point.


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Chemical Reactions

Pretreatment:

3 Cu(s) + 8 HNO3 3 Cu(NO3)2(aq) + 2 NO(g) + 4 H2O

  • Since NO is the anhydride of HNO2, the next step involves removal of the nitrous acid by reduction with urea. The products are gaseous.

    2 HNO2 + CO(NH2)2 3 H2O + CO2(g) + 2N2(g)

  • Finally, KI is added which reacts with the Cu+2

    2 Cu 2+ + 4I-12CuI(s) + I2


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Chemical Reactions

Titration: Reduction of iodine with thiosulfate

  • I2 + 2 S2O32- S4062- + 2 I-1


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Procedures - Preparation and standardization of 0.1 M sodium thiosulfate

  • Dissolve about 25 g of Na2S203. 5H2O in 1 L of distilled water that has just been boiled for 5 min and cooled. Transfer to a clean storage bottle that was rinsed with the boiling water and add 1 pellet of NaOH as a preservative.

  • Dip a 2-in. length of pure copper wire (14 or 16 gauge) in dilute nitric acid for 5 seconds, rinse with deionized water, and wipe dry with a piece of filter paper.


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Procedures - Preparation and standardization of 0.1 M sodium thiosulfate

  • Weigh accurately three 0.20- to 0.25-g (0.1mg) samples of wire to and transfer to numbered, 250-mL conical flasks. In the hood, add 5 mL of 6 M HNO3 and warm until the copper is dissolved.

  • Dilute with 25 mL of water, add about 0.5 g of urea, and boil again for 5 min.

    5. Add 6 M NH3 until deep blue Cu(NH3)4 2+ starts to form. A precipitate of Cu(OH)2 may form and redissolve before the deep blue color appears. Carefully add 3M H2SO4dropwise until the deep blue color just disappears, then add 2.0 mL of 85% H3PO4.



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Procedures - Preparation and standardization of 0.1 M sodium thiosulfate

Treat each sample individually from this point!

6. Add about 3 g of KI and titrate the liberated iodine immediately with the 0.1 M thiosulfate solution prepared above continuing the titration until the solution is a very pale yellow.

7. Interrupt the titration to add 5 mL of starch indicator, and then continue the addition of titrant drop wise until the blue color just disappears with the addition of one drop of the titrant.


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Procedures - Preparation and standardization of 0.1 M sodium thiosulfate

8. Add 2 g of KSCN and swirl the flask and contents vigorously for ~ 30 s. Complete the titration with thiosulfate to an end point free of the blue starch-I2 color. Because the precipitate of CuI is present, there should be a distinct change from the blue to a white or cream color at the end point.


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Iodide Blank Test

Because of the possible presence of small amounts of KIO3

(potassium iodate) in the KI (potassium iodide), one performs

a reagent blank test on the KI. Do the following to determine

if the blank correction is necessary, and the size of the

correction.

1. Dissolve 3.00 g  .1 g of the KIthat you are using in your titrations in 75 mL of deionized water. To this solution, add 2 mL of 6M HCl and 5 mL of the starch indicator solution. If a blue color does not develop, then KIO3 is absent and the reagent blank is 0.00 mL.


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Iodide Blank Test

2. If a blue color results, KIO3 is present; titrate the blank as per the instructions for standardization /determination with thiosulfate titrant until the blue color disappears; the volume of thiosulfate required is the “blank correction”. This correction should be small (0.00 - 2.00 mL of titrant) and needs to be subtracted from the volume of thiosulfate required in both the standardization and determination titrations.


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The Analysis of the Unknown

Your sample is a copper ore sample; it might also

contain some insoluble silica or, if lead is present, lead chloride

which will be removed in step 4 below.

Procedure:

  • Dry the sample at 110 C for 2 hours and cool in a desiccator.

    2. Accurately weigh approximately 9.0 g (to  .1 mg) of the brass sample into a 500-mL Erlenmeyer flask.

    3. Working in the hood, carefully add 50 mL of concentrated HCl and 25 mL of concentrated HNO3 (the mixture of concentrated HCl and HNO3 is known as aqua regia, and will dissolve most metallic ores and alloys.) Mix and carefully warm until the sample dissolves.


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The Analysis of the Unknown

  • When the solution process is complete, add 25 mL of concentrated H2SO4 and boil the mixture until thick white fumes of sulfur trioxide (SO3) are evolved. If insoluble material is present, filter the sample as described in the following. If insoluble materials are not present, go to step 5

    (At this point, if there are still some insoluble materials present, it is probably silica or lead(II) chloride, and you will need to filter the sample before adding bromine water. If your sample needs to be filtered add 50 mL of water and filter the sample through student grade filter paper using a conical glass funnel, collecting the filtrate and washings in a second 500-mL Erlenmeyer flask. This is a quantitative filtration, so that all of the filtrate and washings are retained. Wash the residue on the filter paper with hot water, using several small quantities. If the total volume of combined filtrate and washings exceeds 200 mL, the volume of solution will need to be reduced by careful evaporation of the excess water, back to an approximate volume of 200 mL.)


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The Analysis of the Unknown

  • Allow the solution to cool to room temperature and add 25 mL of bromine water. Mix the solution well and then heat it to boiling to expel the excess, un-reacted bromine; this is evidenced by the loss of the red/brown color of the bromine and the appearance of the blue or bluish green color of the copper (II) ion.

    6. Quantitatively transfer all of the above solution to a 250-mL volumetric flask. Stir to mix the solution well. Dilute to the mark and invert the volumetric flask several times to make sure that the solution homogeneous.


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The Analysis of the Unknown

7. For your first titration, use a 25.00 mL pipet to transfer a sample to a 250-mL Erlenmeyer flask. To this sample add 6 M NH3 until either the deep blue [Cu(NH3)4] 2+ complex ion or a precipitate of brown Fe(OH)3 forms. Carefully add 3M H2SO4 dropwise until the deep blue color just disappears, then add 2.0 mL of 85% H3PO4. Mix this solution well and now treat in like was done in the standardization of the thiosulfate, steps 6-8 in the standardization procedure.

8. If the amount of thiosulfate needed to reach the end point is in the range of 20 - 45 mL, use 25.00 mL aliquots on subsequent titrations. If the volume of thiosulfate is outside this range, adjust the size of the aliquot so that subsequent titrations require a volume within the recommended range of 20 - 45 mL. (P.S. – you might need special sized pipets if this is the case.)


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The Analysis of the Unknown

9. Calculate the mass percent copper present in the brass sample.

  • Repeat the above steps starting in step #7 with an aliquot of appropriate size to obtain at least 3 results whose precision is 4 ppt or better.


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The Report

In your written report, be sure to include each of the following:

  • your sample number

  • the average weight % copper

  • the standard deviation (s) for your determinations

  • the 95% confidence interval (CI95)


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Calculations

From the chemical equations involved at the equivalence point the number of moles of cupric ion = moles of thiosulfate. The w/w% Cu may be found by

% Cu = [S2O32- ] X V (L) X 63.55 X 100

Sample mass X (Aliquot V/250.0)

Where V is the size aliquot you took with a pipet.


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REFERENCES

1. Larry G. Hargis, “Analytical Chemistry: Principles and Techniques,” c1988, Prentice-Hall Inc., pages 588-590.

2. Daniel C. Harris, “Exploring Chemical Analysis, 2e,” c2001, W. H. Freeman & Co, pages 335 –337.

3. D.A. Skoog, D.M. West, and F.J. Holler, “Fundamentals of Analytical Chemistry, 7th e, c1996, Saunder College Publishers, pages 842-3.

______________________

Revised 15-October-2003


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