Part 19 (1/2)

The sulphocyanate solution is standardised by placing 50 c.c. of the silver nitrate solution in a flask with 20 c.c. of dilute nitric acid, diluting to 100 c.c. with water, and running in the sulphocyanate until the greater part of the silver is precipitated; then adding 2 c.c. of the ferric indicator, and continuing the t.i.tration until a reddish-brown colour is developed, and remains permanent after shaking continuously.

The a.s.say is similarly performed, the silver being used in the state of a nitric acid solution.

The effect of variations in the conditions of the a.s.say may be seen from the following experiments, in which 20 c.c. of standard silver nitrate were used:--

~Effect of Varying Temperature~:--

Temperature 10 C. 30 C. 70 C. 100 C.

Sulphocyanate reqd. 19.6 c.c. 19.3 c.c. 19.0 c.c. 18.6 c.c.

~Effect of Varying Nitric Acid~:--Varying nitric acid has no effect, except that with a fairly acid solution the finis.h.i.+ng point is somewhat sharper.

Nitric acid added 5 c.c. 10 c.c. 20 c.c. 50 c.c.

Sulphocyanate reqd. 19.6 c.c. 19.5 c.c. 19.6 c.c. 19.6 c.c.

~Effect of Varying Bulk~:--

Bulk 50 c.c. 100 c.c. 200 c.c. 300 c.c.

Sulphocyanate reqd. 19.5 c.c. 19.6 c.c. 19.6 c.c. 19.7 c.c.

~Effect of Varying Ammonic Nitrate~:--

Ammonic nitrate 0 gram 1 gram 5 grams 10 grams Sulphocyanate reqd. 19.6 c.c. 19.6 c.c. 19.7 c.c. 19.9 c.c.

~Effect of Varying Silver~:--

Silver added 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.

Sulphocyanate reqd. 1.0 c.c. 9.70 c.c. 19.6 c.c. 49.4 c.c. 99.0 c.c.

This method is valuable for determining silver in salts, alloys, and solutions, where no more than an ordinary degree of accuracy is demanded. It is easy, and applicable under most of the usual conditions.

Its greatest disadvantage is the brown coloration produced by the sulphocyanate when the a.s.say is nearly, but not quite, finished; and the slowness with which this is removed on shaking up with the precipitate.

This is worse with large quant.i.ties of precipitate, and if about 1 gram of silver is present, it gives an indefiniteness to the finish which lowers the precision of the process to about 1 in 500; this is useless for the a.s.says of bullion. One writer states that this inconvenience is due to portions of liquid being entangled in the precipitate, but it appears much more likely to be due to the action of the precipitate itself. In attempting to apply the process to the a.s.say of bullion by working it on the principle of a Gay-Lussac a.s.say, it was found that a very considerable excess of silver was required to complete the reaction. In these experiments 100 c.c. of ”sulphocyanate” (very accurately measured) was run into the solution containing the weighed portion of bullion (fine silver) and, after shaking the solution, was filtered. In the filtrate the remaining silver, if there should be any, was determined by the ordinary t.i.tration, but with ”sulphocyanate” of one-tenth the strength. This final t.i.tration was quite satisfactory. The amount of silver precipitated by the first 100 c.c., however, varied with the quant.i.ty of silver present as in the following series.[17]

Silver present. Silver precipitated.

1.1342 gram. 1.1322 gram.

1.1375 ” 1.1335 ”

1.1405 ” 1.1351 ”

1.1484 ” 1.1379 ”

These, of course, preclude a method of the kind aimed at, and at the same time emphasise the importance of uniformity of work in the ordinary process. In the determination of chlorides in sea-water, Dittmar used a combined method: precipitating the bulk of the silver as chloride, and after filtering, determining the small excess of silver by sulphocyanate. This modification answers admirably when applied to the a.s.say of bullion. In the ordinary Gay-Lussac method, the precipitation of the bulk of the silver by the 100 c.c. of salt solution leaves nothing to be desired, either as to ease in working or accuracy of result; the silver precipitate settles quickly, and leaves a clear liquor admirably fitted for the determination of the few milligrams of silver remaining in solution. But the method of determining this residual silver by adding successive small quant.i.ties of salt so long as they continue to give a precipitate is unsatisfactory, and, judged on its own merits apart from the rest of the process, could hardly escape condemnation. It is clumsy in practice, for the continued adding of small portions of salt solution is laborious and becomes impossible with more than a few milligrams of silver in solution. The proposed modification is simple; having precipitated the silver with the 100 c.c.

of salt solution, as described under Gay-Lussac's method (page 120), shake till the liquor clears, and filter into a flask, was.h.i.+ng with a little distilled water. Add 2 c.c. of ”ferric indicator” to the filtrate and t.i.trate with a standard ”sulphocyanate solution” made by diluting the ordinary standard solution to such an extent that 100 c.c. after diluting shall be equivalent to 0.1 gram of silver.[18] Calculate the weight of silver found by ”sulphocyanate” and add it to the weight which 100 c.c. of the salt solution will precipitate.

An advantage of this modification is that an excess of 15 milligrams may be determined as easily and exactly as 5. In standardising the salt solution, then, weigh up, say 1.0150 gram of pure silver, dissolve and t.i.trate. Suppose 13.5 c.c. of ”sulphocyanate” required; then these are equivalent to .0135 gram of silver, (100 c.c. = .1); the silver precipitated by the salt is 1.0150-.0135--_i.e._, 1.0015 gram, which is the standard.

~Application of the Method to a.s.says for a.r.s.enic.~--If silver nitrate be added to a neutral solution of an a.r.s.enate of one of the alkali metals, silver a.r.s.enate (Ag_{3}AsO_{4}), is thrown down as a dark-red precipitate. If, after adding excess of silver nitrate to insure a complete precipitation, the a.r.s.enate of silver be filtered off, the weight of the a.r.s.enic could be estimated from the weight of silver a.r.s.enate formed. But this may be done much more conveniently by dissolving the precipitate in nitric acid, and t.i.trating with sulphocyanate; the silver found will be to the a.r.s.enic present as 324 (1083) is to 75.