To assay for copper contained in a cyanide solution, the first essential step of the method is to completely decompose all the cyanogen compounds. For this purpose Clennell recommends evaporating 100 cc of the solution to be tested with 5 cc of nitric and 10 cc of sulphuric acid until dense white fumes are freely given off. Sharwood prefers to evaporate 100 cc of the solution to dryness twice with nitric acid and then dissolve the residue in dilute sulphuric. If in either case a yellowish flocculent precipitate remains undissolved an incomplete decomposition of cyanogen compounds should be suspected. In some instances the writer has found it necessary, after evaporating to dryness twice with nitric acid, to add strong sulphuric and evaporate to white fumes in order to complete the decomposition of the cyanides.
When a large quantity of solution is taken for evaporation the lime separates out as calcium sulphate at the finish so it is better in such cases before starting the analysis to precipitate the lime with sodium carbonate or potassium oxalate and filter it out. When decomposition is complete, whatever method be used, the assay is diluted with water and boiled until all anhydrous salts are dissolved. Sometimes a slight insoluble residue is left which may be filtered out, washing the filter paper with hot water and adding the washings to the filtrate. Next place in the solution a cylinder of thin sheet aluminium and boil until all the copper is precipitated. If hydrogen is not freely given off add more sulphuric acid. When the action is sufficiently brisk the copper deposited on the aluminium will be detached and form spongy masses, leaving the aluminium surface clean. When the copper is precipitated remove the aluminium cylinder and wash it with distilled water. Should any copper adhere to the surface it may be detached with a rubber-tipped glass rod and washed into the beaker. Decant the solution carefully and wash the spongy copper several times by decantation with hot water, avoiding exposure to the air so as to minimize the risk of oxidation.
The copper is now dissolved in 5 to 10 cc of nitric acid, and boiled until all brown fumes are expelled, and the determination made by any of the standard methods, such as electrolysis, titration with cyanide, or the potassium iodide method.
The Cyanide Measurement Method for Copper
Unless a high degree of accuracy is required the cyanide method may be made to give quite satisfactory results.
For this test a standard copper solution is prepared by dissolving 10 grams of pure copper foil in 100 cc of nitric acid, boiling to expel brown fumes, and diluting to 1 litre, 1 cc of this solution will contain 0.01 gram of copper and 0.1 cc of nitric acid.
If silver was present in the original cyanide solution some or all of it will have passed into the copper nitrate solution, and should be removed by adding a drop or two of hydrochloric acid, filtering, and washing the precipitate. The solution is then neutralized with sodium carbonate in slight excess, and about 1 cc of ammonia added, or sufficient to give a clear blue solution, which is put in a beaker of suitable size. Next, put in a similar beaker a measured amount of the standard copper solution, approximating in copper content the quantity of copper contained in the solution to be tested, and accompanied by about the same amount of nitric acid. Add a drop or two of hydrochloric acid (if that was added for precipitating silver, as described), and neutralize with sodium carbonate. Then add 1 cc of ammonia, or the same quantity as was added to the other beaker, and distilled water to make an equal volume in the two beakers.
The standard and the assay (which should both be at the same temperature), are then titrated with a solution of potassium or sodium, cyanide of 0.5 to 2% strength according to circumstances, until only a faint violet tint remains.
The burette reading obtained in testing the standard copper indicates the copper value of each cc of cyanide solution under the conditions of the test, and from this the copper content of the assay is calculated.
Colorimetric Estimation of Copper
Where copper is present in very small amounts it is usually best estimated colorimetrically. For this purpose the standard copper solution already described may be diluted to 1/10 its strength so that 1 cc will contain 0.001 gram of copper instead of 0.01 gram.
The cyanide solution is evaporated with acids as for the regular copper determination and after diluting, boiling, and cooling, a few cc of ammonia is added or sufficient to neutralize the acid and produce the characteristic blue color. When small quantities of iron are present it may be necessary to heat the resulting solution to assist the precipitation of the ferric hydroxide which should be then filtered out, but it is better to avoid heating if possible after addition of the ammonia as this sometimes tends to cause reduction of intensity in the blue, color especially where only traces of copper are present. Even in the cold the blue color tends to fade on standing so no time should be lost in proceeding with the test.
Method of Procedure
Two colorless glass cylinders marked at 100 cc are taken. Into one is poured the blue ammonia-copper solution to be tested, and distilled water added up to the mark. Into the other cylinder is poured a few cc of ammonia and distilled water up to within a short distance of the mark, space being left for the addition of the standard copper solution. The latter is then run into the cylinder drop by drop until the color matches that of the solution in the first cylinder. The number of cc of standard copper solution run in is then read off the burette and the copper content of the test calculated therefrom. In matching the color it is of great assistance to have the cylinders enclosed in a box open at the bottom, in such a way that the light may be reflected upward from a sheet of white paper and the color judged and compared by looking down vertically through the column of liquid.