IODOMETRIC DETERMINATION OF COPPER IN BRASS BACKGROUND. This experiment illustrates the analytical method involving the iodine-iodide couple:

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1 #7. Iodometric Determination of Copper 53 EXPERIMENT 7. IODOMETRIC DETERMINATION OF COPPER IN BRASS BACKGROUND This experiment illustrates the analytical method involving the iodine-iodide couple: I 2 + 2e- <===========> 2 I - E = 0.53 V (1) This couple is important because it has a standard electrode potential that permits the analytical use of iodine as an oxidant for substances of lower electrode potential and iodide as a reductant for substances of higher potential. Because its electrode potential is little affected by either ph change or complexing agents, this couple can be used in conjunction with half-reactions that change potential with ph or with the addition of auxiliary reagents. Analytically useful applications include those in which solutions of iodine are used to titrate reduced materials directly and those in which oxidizing agents are determined through oxidation of iodide to iodine. In the latter the iodine formed is titrated with a standard solution of sodium thiosulfate, Na 2 SO 3. Iodometric Determination of Copper In this experiment the copper in a brass sample is determined by a method involving the liberation of iodine. The sample is dissolved in nitric acid, and the solution boiled to remove most of the nitrogen oxides formed during the metal oxidation. The residual nitrogen oxides are eliminated by the addition of urea. Complete removal of nitrogen oxides is necessary to prevent iodide oxidation. Iron present in most brasses, also causes iodide oxidation. This interference is eliminated by the addition of fluoride, which forms a stable complex with iron(iii). Next the ph is adjusted to 3.5 to 4.5, an excess of potassium iodide is added, and the iodine formed is titrated with sodium thiosulfate. Although the titration of iodine is the only titration of significance for which thiosulfate is used as a standard solution, it is an important one. Other applications of thiosulfate are few. One reason is that oxidizing agents stronger than iodine oxidize it to a mixture of higher oxidation states of sulfur; another is that ions of transition metals such as copper decrease the stability of sodium thiosulfate solutions by catalytic air oxidation. Acids also make thiosulfate

2 54 #7. Iodometric Determination of Copper solutions unstable by promoting disproportionation to sulfite and elemental sulfur: HS 2 O > HSO 3 + S (2) Addition of a small amount of sodium carbonate to make the thiosulfate solution alkaline prevents this decomposition. Water used for the preparation of standard thiosulfate solutions may be boiled to destroy sulfur bacteria, which find these solutions an attractive medium for growth. Alternatively, a bacterial agent such as a mercury(ii) salt or chloroform may be added. Any of several primary standards, including iodine, potassium chromate, potassium iodate, and electrolytic copper metal may be used to standardize thiosulfate solutions. For this experiment copper is chosen because it is the material being determined and is readily available in primary standard quality as electrical wire. The principal reactions involved in iodometric copper analysis are (a) dissolution of sample in dilute nitric acid, or 3 Cu + 8 HNO > 3 Cu NO + 4 H 2 O + 6 NO3- (3) Cu + 4 HNO > Cu NO H 2 O + 2 NO3- (4) (b) removal of residual nitrogen oxides by addition of urea, and 2 NO + O 2 <===========> 2 NO 2 (5) 2 NO 2 + H 2 O <============> HNO 2 + HNO 3 (6) 2 HNO 2 + (NH 2 ) 2 CO > 2 N 2 + CO H 2 O (7) (c) neutralization of remaining acid with sodium hydroxide, followed by ph adjustment; (d) complexation of iron(iii) with fluoride, Fe 3+ + F - <==========> [FeF] 2+ (8)

3 #7. Iodometric Determination of Copper 55 (e) addition of excess potassium iodide, and (f) titration of iodine with thiosulfate, 2 Cu I - <===============> Cu 2 I 2 + I 2 (9) I S 2 O 3 2- <===========> 2 I - + S 4 O 6 2- (10) Precautions must be taken to avoid side reactions. For instance, iodine will slowly oxidize tetrathionate to sulfate, especially at high ph values. At low ph sulfurous acid may be formed by thiosulfate (equation (2). Another source of error in strong acid solution is air oxidation of iodide: O H I - <============> 2 I H 2 O (11) This is called oxygen error. These side reactions may be avoided by carrying out the thiosulfate - iodine reaction in the ph range 2-5. The volatility of iodine creates problems. Iodine loss can be minimized by keeping the temperature low, titrating promptly, and adding excess iodine to stabilize the iodine in solution as triiodide: I 2 + I - <==========> I 3 - K eq = 7.1 x 10 2 (12) Starch gives an intense blue color with triiodide that serves as a specific indicator. The triiodide ion appears to be just the right size to enter the helical structure of starch and thereby form a colored complex. The color of free iodine, red-brown in high and yellow in low concentrations, also can be used as an indication of its presence, but it is not so easily seen as that of the complex with starch. In acidic solutions starch tends to undergo decomposition that is accelerated by high concentrations of iodine. Therefore, addition of starch is best delayed until near the equivalence point of the titration. Most strong oxidizing agents can be determined iodometrically. An excess of iodide is added, and iodine is produced in an amount equivalent to that of the oxidant present in the sample. The liberated iodine is titrated with thiosulfate.

4 56 #7. Iodometric Determination of Copper PROCEDURE (Median Time 3.9 hours) Reagent List: Unknown Sample Sodium thiosulfate 5H 2 O - prepare 0.1M soln. Na 2 CO 3 HNO 3 conc. 16M copper turnings >99% pure Urea - 4% soln. needed sodium hydroxide pellets - 2.5M soln. needed ammonium acid fluoride(nh 4 HF 2 ) potassium iodide Starch -.2% soln indicator Standardization of 0.1 M Na 2 S 2 O 3 Solution Dissolve 25 g of Na 2 S 2 O 3.5H 2 O and 0.1g of Na 2 CO 3 in a liter of distilled water. N.B. This solution may be saved for Experiment 8. Weigh accurately 0.2 g samples of clean copper turnings into 200 ml conical flasks. 1 Add 10 ml of 6 M HNO 3 to each in a fume hood. When dissolution is complete, boil the solution for a short time to remove most of the nitrogen oxides. Add 10 ml of water and 5 ml of 4 % urea solution, and boil again for about one minute. Allow the solution to cool. When ready to titrate, add 30 ml of water and then add 2.5 M NaOH until a slight permanent precipitate of Cu(OH) 2 is obtained. This will require about 15 to 20 ml of NaOH, depending on the amount of HNO 3 present. Add 1 to 2 g of ammonium acid fluoride, NH 4 HF 2, and swirl until dissolved. Cool, add approx. 3 g of KI, and titrate immediately to near the end point with Na 2 S 2 O 3. When the solution has become pale yellow or buff, add about 5 ml of fresh starch solution and titrate to the first complete disappearance of blue. 2 PROCEDURE FOR THE SAMPLE Do not dry the unknown. Determine the total weight of the unknown. Transfer the unknown to a 250 ml volumetric flask. Dissolve the unknown with 10 ml of 6 M HNO 3. Dilute to 250 ml. Pipet 25 ml into a 250 ml erlenmeyer. Add urea and boil as described for the standards. When ready to titrate, neutralize with sodium hydroxide and dilute to 50 ml with water. Add about 1 g of ammonium acid

5 #7. Iodometric Determination of Copper 57 fluoride and swirl until dissolved. Add approx. 3g of KI and complete the determination as in the standardization. Calculate the molarity of the Na 2 S 2 O 3 solution. One mole of copper requires 1 mole of thiosulfate for titration. Calculate and report the percentage of copper in the sample. NOTES 1. Trace impurities markedly increase the resistance of copper wire. For electrical use they are removed by electrolytic refining to a level well below a part per thousand. Copper produced for electrical wiring is therefore an excellent primary standard. The thin coat of oxide sometimes present on the wire can be removed by polishing with fine emery cloth, followed by wiping with clean toweling. 2. To avoid etching of flasks by HF, empty and rinse immediately after completing each titration.