1. The student will perform an iodometric determination.
2. The student will pretreat the sample.
3. The starch indicator will be used.
4. The known and unknown will be treated alike.
Standard: Weigh out four samples of 0.2000 g of pure copper shot (or copper wire) into 250 Erlenmeyer flasks. Add 10 to 15 mL of concentrated (16 M) nitric acid and heat the mixture gently to decompose and dissolve the sample (use a hot plate). Cover with a watch glass. When all of the sample appears to have dissolved add 10 Lof concentrated (18 M) sulfuric acid and evaporate the solution over a hot plate, swirling the solution continuously (it bumps) until dense white fumes of sulfur trioxide are evolved. (Use a hood.) Cool the solution to room temperature. Cautious add, 1 mL at a time with gently swirling after each addition, a total of 20 mL of d.i. Use care to avoid having the concentrated sulfuric acid spatter as it is diluted. Swirl each of the flasks containing the cooled sulfuric acid-copper (II) solutions and add concentrated (15 M) aqueous ammonia until the first permanent deep blue copper(II) ammine color is obtained. (may be 12 to 15 mL).
* FROM THIS POINT TO THE END OF THE PROCEDURE, TREAT EACH SAMPLE SOLUTION INDIVIDUALLY. TREAT THE AMMONIA SOLUTION OF COPPER(II) ACCORDING TO THE FOLLOWING PROCEDURES: (THIS MUST BE DONE ON THE DAY YOU PLAN TO TITRATE)
Slowly add 3 M sulfuric acid until the blue copper-ammine color just disappears; then add 1 mL of 3 M sulfuric acid in excess. Cool the solution. Next add 4 gm of solid potassium (KI) to the sample solution, swirl the solution to dissolve the solid, and immediately titrate the mixture with standard 0.1 M sodium thiosulfate solution. Continue the titration until the color of triiodide becomes indistinct (pale-yellow) Add 5 mL of starch solution and titrate slowly until the starch-iodine color has become faint. Then dissolve 2 gm of solid potassium thiocyanate (KCNS) in the solution, and titrate the mixture to the complete disappearance of the starch-iodine color, the later not returning for at least several minutes (it may take only one drop!).
NOTE: It is not always easy to ascertain when the starch should be added because the copper(I) iodide is brownish-colored owing to absorbed triiodide. Sometimes it is helpful to let the CuI precipitate settle and to observe the color of the supernatant solution. When the KCNS is added, the starch-iodine color should intensify because of the release of adsorbed triiodide. If the starch-iodine color reappears too soon, iron (III) may be slowly oxidizing the iodide, in which case more phosphoric acid or solid ammonium hydrogen fluoride should be added in the analysis of the remaining samples.
Unknown: Weigh out accurately, into 250 mL Erlenmeyer flasks 1.5 g samples of the copper-containing sample. Add 10 to 15 mL of concentrated (16 M) Nitric acid and heat the mixture gently over a hot plate to decompose and dissolve the sample. Continue the heating until any residue, if one does remain, is white or only light gray. If the sample is unusually slow to dissolve, add 5 milliliters of concentrated (12 M) HCl and continue the heating as required. When all of the sample appears to have dissolved (except for a residue of whitish silica or metastannic acid), add 10 mL of concentrated (18 M) sulfuric acid and evaporate the solution over a hot plate, swirling the solution continuously, (it bumps) until dense white fumes of sulfur trioxide are evolved. (Use the hood). Cool the solution to room temperature. Cautiously add, 1 mL at a time with gentle swirling after each addition, a total of 20 mL of d.i. Use care to avoid having the concentrated sulfuric acid spatter as it is diluted. Swirl each of the flasks containing the cooled sulfuric acid-copper(II) solutions and add concentrated (15 M) aqueous ammonia until the first permanent deep blue copper(II) ammine color is obtained (may be 12 to 15 mL). Follow the same procedure given on the preceding page following "*From this point".
Preparation of 0.1 M Thiosulfate Solution:
Weigh out approximately 25 grams of reagent grade sodium thiosulfate pentahydrate (Na2S2O3.5H2O) and 100 mg NaHCO3. Add 1 liter of freshly boiled d.i. Store in a glass bottle which has been rinsed with boiled d.i.
Preparation of Starch Solution:
Dissolve 1 gram of soluble starch in about 5 mL of cold d.i. Slowly add the starch suspension to 95 mL of rapidly boiling water. Boil until the solution clears. Cool to room temperature. NOTE: Starch solutions often do not keep. Make it the day you will need it (if you also will need it the following period try to keep it--if it has a mold growth then it must be discarded).
NOTE: The initial sample of copper oxide should be dried for one hour at 100o C.
CALCULATION: Calculate the percent of copper present in the sample after calculating the molarity of the sodium thiosulfate from the known copper titrations. Include a computer print-out for the calculation.
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