As an example:—There was taken 1 gram of a nickel alloy used for coinage. It was dissolved in 10 c.c. of nitric acid, and diluted to 100 c.c. with water. The copper was then precipitated by electrolysis. It weighed 0.734 gram. The solution, after electrolysis, was treated with sulphuretted hydrogen, and the remaining copper was thrown down as sulphide, and estimated colorimetrically. This amounted to 3-1/2 milligrams. The filtrate was evaporated, treated with ammonia, warmed, and filtered. The ferric hydrate was dissolved in dilute acid, and reprecipitated, dried, ignited, and weighed. Its weight was 0.0310 gram. The two filtrates were mixed, and reduced in bulk to about 50 c.c.; a considerable excess of ammonia was added, and the nickel precipitated by electrolysis. It weighed 0.2434 gram. These quantities are equivalent to:
| Copper | 73.75 | per cent. |
| Nickel | 24.34 | " |
| Iron | 2.17 | " |
| ——— | ||
| 100.26 |
VOLUMETRIC DETERMINATION.
An alkaline solution of potassium cyanide, to which a little potassium iodide has been added, can be assayed for its strength in cyanide by titrating with a standard solution of silver nitrate. Nickel interferes with this assay, doing the work of its equivalent of silver; and the quantity of nickel present can be calculated from the amount of its interference in the titration. A volumetric assay for nickel is based on this. It has the disadvantage of all indirect titrations in that it requires two standard solutions. On the other hand it gives good results even under unfavourable conditions, and is applicable in the presence of much zinc. Small quantities of cobalt will count as so much nickel, but larger quantities make the assay unworkable. Some of the other metals—lead for example—have no appreciable effect; but practically the solution demands a preliminary treatment which would result in their removal. Nevertheless it is a very satisfactory method and makes the determination of nickel quick and comparatively easy in most cases.
The standard solution of silver nitrate is made by dissolving 14.48 grams of recrystallised silver nitrate in distilled water and diluting to 1 litre: 100 c.c. of this solution are equivalent to 0.25 gram of nickel.[73]
The standard solution of potassium cyanide should be made so as to be exactly equal to the silver nitrate solution. This can be done as follows: Weigh up 12 grams of good potassium cyanide (95 per cent.), dissolve in water, add 50 c.c. of a 10 per cent. solution of sodium hydrate and dilute to 1 litre. Fill one burette with this and another with the solution of silver nitrate. Run 50 c.c. of the cyanide into a flask; add a few drops of potassium iodide solution and titrate with the standard silver nitrate until there is a distinct permanent yellowish turbidity. The titration is more fully described under Cyanide, p. 165. The cyanide solution will be found rather stronger than the silver nitrate; dilute it so as to get the two solutions of equal value. For example, 51.3 c.c. of silver nitrate may have been required: then add 1.3 c.c. of water to each 50 c.c. of the cyanide solution remaining. If the full 950 c.c. are available, then add to them 24.7 c.c. of water. After mixing, take another 50 c.c. and titrate with the silver nitrate; the two solutions should now be exactly equal. The cyanide solution, being strongly alkaline with soda, keeps very well; but its strength should be checked from time to time by titrating with silver nitrate; should there be any slight inequality in the strengths of the two solutions it is easily allowed for in the calculations.
The titration.—The solution, containing not much more than 0.1 gram of nickel, and free from the interfering metals, must be cooled. It is next neutralised and then made strongly alkaline with a solution of soda (NaHO); an excess of 20 or 30 c.c. suffices. This will produce a precipitate. The cyanide solution is now run in from a burette until the solution clears, after which an excess of about 20 c.c. is added. It is well to use some round number of c.c. to simplify the calculation. Add a few drops of potassium iodide solution, and run in the standard solution of silver nitrate from a burette. This should be done a little at a time, though somewhat rapidly, and with constant shaking, till a permanent yellow precipitate appears. If the addition of the cyanide did not result in a perfectly clear solution, this is because something besides nickel is present. The residue may be filtered off, though with a little practice the finishing-point may be detected with certainty in the presence of a small precipitate. If the student has the slightest doubt about a finish he should run in another 5 c.c. of the cyanide and again finish with silver nitrate. The second result will be the same as the first. For example, if 40 c.c. of cyanide and 30 c.c. of silver nitrate were required at the first titration, then the 45 c.c. of cyanide in the second titration will require 35 c.c. of silver nitrate. The difference between the quantities of the two solutions used in each case will be 10 c.c. It is this difference in the readings of the two burettes which measures the quantity of nickel present. Each c.c. of the difference is equal to .0025 gram of nickel. But if the cyanide solution is not exactly equal in strength to the silver nitrate, the quantity of cyanide used should be calculated to its equivalent in silver nitrate before making the subtraction.
The following experimental results illustrate the accuracy of the assay and the effect upon it of varying conditions. A solution containing 1 gram of nickel sulphate (NiSO4.6H2O) in 100 c.c. was used. By a separate assay the sulphate was found to contain 22.25 per cent. of nickel. For the sake of simplicity the results of the experiments are stated in weights of nickel in grams.
Effect of varying excess of Cyanide Solution.—In each experiment there was 20 c.c. of the nickel solution, equal to .0445 gram of nickel. There were also 10 c.c. of soda solution, 3 or 4 drops of potassium iodide and sufficient water to bring the bulk to 100 c.c. before titrating.