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 110 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.

Ferrocyanide Determination.-The most reliable method is probably that of determining the iron and calculating to ferrocyanide.

It must of course be assumed that all the iron found occurs in that form, but as ferricyanides are most unlikely to be present in ordinary circumstances the assumption is a fairly safe one.

As in the case of the copper determination (see page 33) especial care must be taken to decompose the cyanogen compounds, either by evaporating with nitric and sulphuric acids till white fumes are given off or by evaporating twice with nitric acid to dryness and then with strong sulphuric to white fumes; and as before, if dealing with a large volume of original solution

it is better to begin by removing the lime with sodium carbonate or potassium oxalate. The solution resulting from the evaporation is cooled, diluted and boiled to dissolve anhydrous salts. It is then cooled again, neutralized with ammonia in slight excess, and heated to boiling. The ferric hydroxide formed is filtered out and washed on the filter.

Potassium Sulphocyanate Method for Estimating the Iron.-If the quantity of iron is very small it is best estimated colorimetrically by at once dissolving the precipitate in hydrochloric acid and carefully washing the filter paper. A standard solution of ferric iron is prepared by dissolving 0.1 gram of C.P. iron wire in nitrohydrochloric acid. Dilute the solution thus obtained and precipitate the iron with ammonia. Filter out the ferric hydroxide and wash thoroughly and then re-dissolve in dilute hydrochloric acid and add distilled water to make up to 1 litre. The test is made similarly to that described for the colorimetric estimation of copper. Two colorless glass cylinders marked at 100 cc are taken. In one of them is placed 1 or 2 cc of hydrochloric acid and 10 or 15 cc of a solution of potassium or sodium sulphocyanate of such a strength as to have an excess of the latter after running in the standard iron. Fill up nearly to the mark with distilled water leaving space for addition of the standard iron. Into the other cylinder pour a similar quantity of hydrochloric acid and sulphocyanate solution and a measured amount of the solution to be tested filling up to the mark with distilled water. The formation of a blood-red color will indicate the presence of sulphocyanate. Standard iron solution is then added from a burette to the first cylinder until the color corresponds with that of the solution being tested. Should the first cylinder lack much of being filled to the mark by this time a little more water may be added and if necessary a few more drops of the iron solution. Here as in the copper estimation it is well to enclose the cylinders 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 compared by looking down vertically through the column of liquid. Care should be taken not to have so much iron present in the test

as to make the color too deep for comparison. When adding standard solution from the burette it is important that the contents of the cylinder be well mixed after each addition, and for this purpose it is better to turn the liquid out into a beaker each time.

Reduction Method for Estimating the Iron.-Should the amount of iron obtained from the cyanide solutions be large enough to be determined by reduction and direct titration, the procedure may be as follows: onto the filter paper containing the washed precipitate of ferric hydroxide pour some dilute sulphuric acid to dissolve it, taking care to wash the

FIG. 3.-Apparatus for Making Ferrous Iron Solution.

paper free from iron. Clennell1 recommends boiling this solution with clean aluminium turnings until a drop of the liquid no longer imparts a red tint to a drop of potassium sulphocyanate solution. The flask is then cooled rapidly without removing the aluminium and titrated with standard potassium permanganate. Evolution of hydrogen must have entirely ceased before titration.

The decinormal permanganate solution is made by dissolving 3.156 grams of the pure dry salt in distilled water and making up to 1 litre.

1 cc of this solution

=

0.0056 gram Fe or 0.0369 gram K1Fe (CN)6 (potassium ferrocyanide).

If the permanganate is pure and dry the solution made up as above will be almost exactly decinormal, but it may be standardized against an iron solution, if desired.

1 Personal communication to the writer.

Fit a boiling flask with a rubber stopper through which passes a short length of glass tube to act as a steam vent, or better still, a bent tube long enough for its free end to pass below the surface of some water in a beaker. Remove the stopper and pour in about 100 cc of dilute sulphuric acid and a few grams of sodium. carbonate to expel the air by formation of carbonic acid. Then place in the flask 0.1 gram of pure soft iron wire previously cleaned with scouring paper, and allow to dissolve. 0.1 gram of such iron wire may be considered to contain 0.0996 gram of pure iron. Cool the solution rapidly and titrate with the permanganate solution until a faint rose color remains permanent. With a strictly decinormal solution of permanganate 17.85 cc should be required to oxidize 0.1 gram of iron, or 17.78 cc for the 0.1 gram of wire taken.

Sulphocyanate Determination.-The method usually given for detection of sulphocyanate is to acidify and add a ferric salt, a blood-red color indicating the presence of KCNS. Silver and copper, however, interfere, and if present in sufficient amount may entirely prevent the formation of the characteristic color, so they must be removed before any determination can be made. The silver may be conveniently precipitated by shaking up a portion of the solution in a bottle with a little aluminium dust. There is usually sufficient caustic soda in working solutions to effect this reaction, but if not a tiny splinter of caustic soda may be added. In order to find out when precipitation is complete a little of the solution may be filtered and added to a flask containing dilute sodium sulphide; if any discoloration ensues agitation should be continued, or if necessary more aluminium dust added. If not, the solution may be filtered, and the test proceeded with.

If, after removing the silver, copper is still present, the writer has found that it may be conveniently eliminated by acidifying in presence of an excess of potassium ferrocyanide, the copper being precipitated as insoluble ferrocyanide and the excess of potassium ferrocyanide being removed as prussian blue on the addition of an excess of a ferric salt.