~Effect of Varying Ammonia.~--The quant.i.ties of copper and acid were the same as in the series of experiments last noticed. The bulk was 200 c.c.

The results were:--

Dilute ammonia 20.0 c.c. 30.0 c.c. 50.0 c.c. 100.0 c.c.

Cyanide required 20.9 " 21.7 " 22.3 " 24.6 "

~Effect of Varying Acid.~--The quant.i.ties of copper and water were the same as in the last-noticed set of experiments: 30 c.c. of dilute ammonia were used.

Nitric acid 5.0 c.c. 10.0 c.c. 15.0 c.c.

Cyanide required 21.6 " 21.7 " 21.5 "

On adding nitric acid to the solution it combines with a portion of the ammonia to form ammonic nitrate; it will be seen from the last series of experiments that the lessening of the amount of free ammonia will decrease the quant.i.ty of cyanide required; but, on the other hand, the ammonic nitrate which is at the same time formed will increase the amount required; under the conditions of the a.s.say these two effects neutralise each other, and such differences in the quant.i.ty of acid as are likely to occur are unimportant.

~Effect of Varying Ammonic Salts.~--The quant.i.ties of copper, water, and ammonia were the same as in the last mentioned set of experiments, but no nitric acid was used.

Ammonic nitrate added 1 gram 5 grams 10 grams 20 grams Cyanide required 21.2 c.c. 22.1 c.c. 23.1 c.c. 24.1 c.c.

These show that combined ammonia seriously affects the t.i.tration, and that the principle sometimes recommended of neutralising the acid with ammonia, and then adding a constant quant.i.ty of ammonia, is not a good one, because there is then an interference both by the ammonia and by the variable quant.i.ty of ammonic salts.

The same quant.i.ty of combined ammonia has the same effect, whether it is present as sulphate, nitrate, chloride, or acetate, as the following experiments show. Four lots of 20 c.c. of "copper nitrate" were taken, and 20 c.c. of dilute ammonia added to each. These were carefully neutralised with the respective acids, rendered alkaline with 30 c.c.

more of ammonia, cooled, diluted to bulk, and t.i.trated. The results were:--

With sulphuric acid 22.5 c.c. of cyanide " nitric acid 22.6 " "

" hydrochloric acid 22.6 " "

" acetic acid 22.5 " "

~Effect of Foreign Salts.~--Sulphates, nitrates and chlorides of sodium or pota.s.sium have no action, whilst the hydrates, carbonates, bicarbonates, sulphites, and nitrites have an important effect. The interference of ammonic salts has already been shown.

Salts of silver, zinc, and nickel react with cyanide just as copper does, and consequently interfere. Ferrous salts are sure to be absent, and ferric salts yield ferric hydrate with the ammonia, which is not acted on by the cyanide, but, owing to its bulkiness, it settles slowly; this lengthens the time required for t.i.tration, and so modifies the manner of working. _An a.s.say should not be worked with ferric hydrate present, unless the standard contains about the same amount of it._ On mines it is often inconvenient to separate the copper by means of sulphuretted hydrogen; hence it is customary to t.i.trate without previous separation. In this case, instead of standardising the cyanide with electrotype copper, a standard ore should be used. This should be an ore (of the same kind as those being a.s.sayed) in which the copper has been carefully determined.

~Effect of Varying Copper.~--In these experiments 10 c.c. of nitric acid, 30 c.c. of ammonia, and water to 200 c.c. were used.

Copper nitrate present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

Cyanide required 0.7 " 11.2 " 21.7 " 54.5 " 108.1 "

These results show that under the conditions laid down the various causes of disturbance nearly neutralise one another, and the results within a fair range are practically proportional.

~Determination of Copper in Copper Pyrites.~--Weigh up 2 grams of the dried and powdered ore, and place in an evaporating dish about four inches in diameter. Cover with 20 c.c. of nitric acid and put on a hot plate. Evaporate to dryness without further handling. Allow to cool and take up with 30 c.c. of hydrochloric acid, boil, dilute, and transfer to a pint flask, filtering if necessary. Make up the bulk with the washings to about 150 c.c. Precipitate with sulphuretted hydrogen, filter, and wash back the precipitate into the flask. Add 15 c.c. of nitric acid, and boil down rapidly to 10 c.c. Dilute, add 30 c.c. of dilute ammonia, make up to 150 c.c., and cool. For the standard, weigh up 0.5 gram of copper, more or less, according to the quant.i.ty judged to be present in the a.s.say. Dissolve in 20 c.c. of dilute nitric acid, boil off nitrous fumes, add 30 c.c. of dilute ammonia, make up to the same bulk as that of the a.s.say, and cool. t.i.trate the two solutions side by side and as nearly as possible in the same manner.

Since the a.s.say solution is often turbid from the presence of small quant.i.ties of lead and of iron from incomplete washing, and since this slight precipitate is very slow in settling, the standard can hardly be compared strictly with the a.s.say. This can be counteracted by precipitating in both solutions a mixture of ferric and aluminic hydrates, which settles readily and leaves the supernatant liquor clear.

To effect this, boil the nitric acid solutions with 30 c.c. of a solution containing 15 grams each of alum and ferrous sulphate to the litre. In an actual determination 2 grams of the ore were taken and compared with 0.5 gram of copper. The a.s.say required 57.7 c.c. of cyanide and the standard 52.5 c.c.

52.5 : 0.5 :: 57.7 : 0.5495

This on 2 grams of ore = 27.47 per cent.; the same sample by electrolysis gave 27.60 per cent. of copper.

~Determination without Previous Separation.~--Dissolve up 2 grams as before, but, instead of pa.s.sing sulphuretted hydrogen, add 30 c.c. of dilute ammonia, shake well, and cool. Prepare a standard by dissolving 0.5 gram of copper in 1 c.c. of nitric acid, add 0.6 gram of iron in the form of ferric chloride and 20 c.c. of hydrochloric acid, dilute to about 150 c.c., add 30 c.c. of dilute ammonia, and cool. t.i.trate the two solutions side by side. In a determination on the sample last used, 58 c.c. were required for the a.s.say and 53 c.c. for the standard, which indicates 27.3 per cent. of copper.

This method of working is somewhat rough.

IODIDE METHOD.

This is based upon the fact that when pota.s.sic iodide in excess is added to a strong solution of a cupric salt in a faintly acid solution, cuprous iodide is formed and an equivalent of iodine liberated.[56] The iodine is measured by t.i.trating with a solution of sodium hyposulphite,[57] using starch paste as indicator. The iodine is soluble in the excess of pota.s.sium iodide, forming a deep brown solution; the hyposulphite is added until this brown colour is almost removed. Starch paste is then added, and strikes with the remaining iodine a dirty blue colour. The addition of the "hypo" is continued until the blue colour is discharged. The end reaction is sharp; a drop is sufficient to complete it.

As regards the t.i.tration, the process leaves little to be desired; the quant.i.ty of "hypo" required is strictly proportional to the copper present, and ordinary variations in the conditions of working are without effect. The presence of salts of bis.m.u.th masks the end reaction because of the strong colour imparted to the solution by the iodide of bis.m.u.th. Under certain conditions there is a return of the blue colour in the a.s.say solution after the finishing point has apparently been reached, which is a heavy tax on the patience and confidence of the operator. This is specially apt to occur when sodium acetate is present, although it may also be due to excessive dilution.

~The standard "hypo" solution~ is made by dissolving 39.18 grams of the crystallised salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water and diluting to one litre. One hundred c.c. will equal one gram of copper.

The starch solution is made by mixing 1 gram of starch into a thin paste with cold water, pouring it into 200 c.c. of boiling water, and continuing the boiling for a minute or so. The solution must be cold before use, and about 2 c.c. is used for each a.s.say. It should not be added until the bulk of the iodine has been reduced.

To standardise the "hypo," weigh up 0.3 or 0.4 gram of pure copper, dissolve in 5 c.c. of dilute nitric acid, boil off nitrous fumes, and dilute with an equal bulk of cold water. Add "soda" solution until a permanent precipitate is obtained, and then 1 c.c. of acetic acid. This should yield a clear solution. Fill an ordinary burette with the "hypo."

Add 3 grams of pota.s.sium iodide crystals to the copper solution, and, when these are dissolved, dilute to 100 c.c. with water. Run in the "hypo" solution rather quickly until the brown colour is nearly discharged--_i.e._, to within 3 or 4 c.c. of the finish. Add 2 c.c. of the starch solution, and continue the addition of the "hypo" a few drops at a time until the tint suddenly changes to a cream colour. The blue colour must not return on standing three or four minutes. Calculate the standard in the usual way.

In a.s.saying ores, the copper is dissolved and separated with sulphuretted hydrogen as in the other processes, but the sulphide should be washed more completely to ensure the absence of iron salts.

The following experiments show the effect of variation in the conditions of the a.s.say. Use a solution of copper sulphate containing 39.38 grams of copper sulphate crystals (CuSO_{4}.5H_{2}O) in the litre. 100 c.c.

equal 1.00 gram of copper.

~Effect of Varying Temperature.~--The a.s.say after the addition of the pota.s.sic iodide must be kept cold, else iodine may be volatilised.

~Effect of Varying Pota.s.sium Iodide.~--In various descriptions of the process the amount of iodide required is variously stated at from "a few crystals" to as much as 10 grams. The proportion required by theory for 1 gram of copper is a little over 5 grams: an excess, however, is required to keep the liberated iodine in solution. On economic grounds this excess should not be extravagant; if the student uses 10 parts of the iodide for each part of copper in the a.s.say he will have sufficient.

In the experiments there were used 20 c.c. of the copper sulphate, with varying amounts of pota.s.sic iodide, and the following results were got:--

Pota.s.sic iodide added 1.5 gram 3 grams 5 grams "Hypo" required 20.0 c.c. 20.0 c.c. 20.0 c.c.

In these the iodide was added direct to the solution containing the copper, which was afterwards diluted to 100 c.c. and t.i.trated. In another series the iodide was added after the dilution to 100 c.c., and the results were:--

Pota.s.sic iodide added 1.5 gram 3 grams 5 grams 10 grams "Hypo" required 20.0 c.c. 20.1 c.c. 20.0 c.c. 20.0 c.c.

~Effect of Varying Bulk.~--In these experiments, 20 c.c. of copper sulphate were taken, 3 grams of pota.s.sic iodide added, and also water to the required bulk.

Bulk 20.0 c.c. 100.0 c.c. 200.0 c.c. 500.0 c.c.

"Hypo" required 20.0 " 20.0 " 20.0 " 19.9 "

In the last of these experiments the colour was discharged at 18 c.c., but gradually returned until 19.9 c.c. had been run in. It will be seen that considerable variation in bulk does not interfere.

~Effect of Acetic Acid.~--These experiments were like the last series mentioned, but the bulk was 100 c.c., and varying amounts of acetic acid were added.

Acetic acid added 0 c.c. 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c.

"Hypo" required 20.0 " 20.1 " 20.1 " 20.0 " 20.2 "

Acetic acid, then, does not interfere to any serious extent.

~Effect of Varying Sodium Acetate.~--These experiments were like those last mentioned, but without acetic acid, and with varying amounts of sodium acetate.

Sodium acetate added 0 gram 1 gram 2 grams 5 grams 10 grams "Hypo" required 20.0 c.c. 20.0 c.c. 20.2 c.c. 19.3 c.c. 18.2 c.c.

In the 5 grams experiment, when the finishing point had been apparently reached the colour slowly returned; but as the results generally on t.i.trating were not satisfactory a repet.i.tion of the experiment was made with the addition of 5 c.c. of acetic acid, which gave an equally bad result.