The work was easier with the more alkaline solutions. The titration in the presence of zinc is comparatively easy, but, in learning it, it is well to have a burette with cyanide so that if a titration be overdone it can be brought back by the addition of 1 or 2 c.c. more cyanide and the finish repeated; a quarter of an hour's work in this way will ensure confidence in the method.
Effect of other substances.—It was found that an alkaline cyanate, sulphocyanate, ferrocyanide, nitrite, borate, silicate or carbonate has no effect. The ferricyanide had a small influence and, as might be expected, hyposulphite is fatal to the assay. The addition of salts of lead and cadmium was without effect. On the other hand, nickel produces its full effect; and the quantity of nickel added can be calculated with accuracy from the extent of its interference with the titration.
Assay of commercial cyanide of potassium.—Break off 20 or 30 grams of the cyanide in clean fresh pieces, weigh accurately to the nearest centigram. Dissolve in water containing a little sodium hydroxide; transfer to a 2-litre flask: dilute to 2 litres; add a few grams of white lead; shake up and allow to settle. Run 50 c.c. of the clear liquor from a burette into an 8 oz. flask; add 2 or 3 c.c. of soda solution and 3 drops of potassium iodide. Titrate with the standard solution of silver nitrate. The percentage may be calculated by multiplying the number of c.c. used by 40 (50 c.c. is one fortieth of the 2 litres) and dividing by the weight of commercial cyanide originally taken.
Alkalinity of commercial potassium cyanide and of cyanide solutions.—Hydrocyanic acid like carbonic acid has no action on methyl-orange;[43] hence the alkaline cyanides may be titrated with "normal acid" as easily as the carbonates or hydrates. 100 c.c. of normal acid will neutralise 6.5 grams of pure potassium cyanide.[44] A solution of commercial cyanide prepared as for the assay last described, but best without the addition of white lead, may be used for the test. Take 50 c.c. of it; tint faintly yellow with methyl-orange and titrate with normal acid till the liquor acquires a permanent reddish tint. In the case of the purer samples of cyanide the quantity of acid used will correspond exactly with that required to neutralise the actual quantity of cyanide present as determined by the assay with nitrate of silver. The less pure samples will show an excess of alkalinity because of the presence of sodium carbonate or of potassium carbonate.
In comparing the alkalinity and cyanide strength of a solution the simplest plan is to take 65 c.c. of the solution and titrate with normal acid; for in this case each c.c. of normal acid corresponds to .1 per cent. of potassium cyanide. In systematic assays of this kind, the alkalinity would no doubt be generally in excess of that required by the cyanide present: there would be no inconvenience in recording such excess in terms of potassium cyanide.
Determination of the acidity of an ore.—Most ores have the power of destroying more or less of the alkalinity of a cyanide solution and in a proportionate degree of damaging its efficiency. An assay is needed to determine how much lime or soda must be added for each ton of ore in order to counteract this. Whether this acidity should be reported in terms of the lime or of the soda required to neutralise it will depend on which of these reagents is to be used in the actual practice. Again, if the ore is washed with water before treating with cyanide on the large scale, then the assay should be made of the acidity of the ore after a similar washing.
The standard solutions of acid and alkali used for this determination may be one-fifth normal. 200 c.c. of the normal solution should be diluted to 1 litre in each case, 1 c.c. of the resulting solutions would be equivalent to 8 milligrams of soda (NaHO) or 5.6 milligrams of lime, CaO. It must be remembered this refers to the pure bases in each case. Suppose it is desired to report as so many lbs. of lime to the short ton (2000 lbs.) of ore. Since 1 c.c. of the standard solution is equivalent to 5.6 milligrams of lime, if we take 2000 times this weight of ore (i.e. 11,200 milligrams or 11.2 grams) for the assay, each c.c. of standard solution will be equivalent to 1 lb. of lime to the short ton.[45]
Total acidity.—Weigh out 11.2 grams of the ore, place them in a four-inch evaporating dish and measure on to it from a burette 10 or 20 c.c. of the standard solution of soda. Stir the soda solution into the ore and allow to stand for 15 or 20 minutes with occasional stirring. Stir up with 30 or 40 c.c. of water, float a piece of litmus paper on the liquid and titrate with the standard solution of acid. If the ore is strictly neutral the quantity of "acid" required to redden the litmus will be the same as the quantity of "soda" originally used. If the ore is acid, less acid will be used. For example, if 10 c.c. of soda were used and only 7 c.c. of acid were required, the ore will have done the work of the remaining 3 c.c. of acid. And the ton of ore will require 3 lbs. of lime to neutralise its acidity.
Acidity after washing.—Take 11.2 grams of the ore; wash thoroughly with water and immediately treat the residue, without drying, exactly as just described.
Examination of cyanide solutions for metals, &c.—Take a measured quantity of the solution, say 20 c.c.[46] and evaporate in a small dish with, say, half a c.c. of strong sulphuric acid. Evaporate at first, on a water-bath in a well ventilated place, but finish off with a naked Bunsen flame, using a high temperature at the end in order to completely decompose the more refractory double cyanides. Allow to cool; moisten with strong hydrochloric acid; warm with a little water and test for the metals in the solution by the ordinary methods. Since the quantities of the metals likely to be present may be given in milligrams the work must be carefully performed. It may be worth while to determine the proportions of lime and magnesia as well as those of the metals proper.