A Text-book of Assaying: For the Use of Those Connected with Mines Part 36

Time exposed -- 1 hour 1 day 2 days 3 days Permanganate required 19.6 c.c. 19.6 c.c. 19.5 c.c. 19.4 c.c. 19.5 c.c.

Similar solutions boiled required, before boiling, 20 c.c.; after boiling for one hour, replacing the water as it evaporated, 19.3 c.c.; and after evaporation to a paste and redissolving, 17.0 c.c.

~Effect of Varying Temperature.~--Solutions similar to the last were t.i.trated and gave the following results:--

Temperature 15 30 50 70 Permanganate required 19.8 c.c. 19.6 c.c. 19.5 c.c. 19.4 c.c.

~Effect of Varying Bulk.~--As before, 20 c.c. of the iron solution, and 10 c.c. of the dilute acid were diluted to the required volumes and t.i.trated.

Bulk 30 c.c. 100 c.c. 500 c.c. 1000 c.c.

Permanganate required 20.4 " 20.3 " 20.8 " 21.5 "

The variation due to difference in bulk here, although only equal to an excess of 0.7 milligram of iron for each 100 c.c. of dilution, are about three times as great as those observed in a sulphuric acid solution.

~Effect of Free Hydrochloric Acid.~--In these experiments 20 c.c. of the ferrous chloride solution were used with varying quant.i.ties of acid, the bulk of the a.s.say in each case being 100 c.c.

Dilute acid present 5 c.c. 10 c.c. 50 c.c. 100 c.c.

Permanganate required 20.2 " 20.2 " 20.5 " 21.0 "

The last had a very indistinct finis.h.i.+ng point, the brown coloration being very evanescent. The effect of the acid is modified by the presence of alkaline and other sulphates, but not by sulphuric acid.

Repeating the last experiment we got--

Without further addition 21.0 c.c.

With 100 c.c. of dilute sulphuric acid 22.0 "

" 10 grams ammonic sulphate 20.5 "

" 10 " sodic sulphate 20.0 "

" 10 " magnesium sulphate 20.4 "

" 10 " manganese sulphate 20.2 "

The results with these salts, in counteracting the interference of the acid, however, were not a complete success, since the end-reactions were all indistinct, with the exception, perhaps, of that with the manganese sulphate.

~Effect of Varying Amounts of Iron.~--In these experiments the bulk of the a.s.say was 100 c.c., and 10 c.c. of acid were present.

Ferrous chloride used 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.

Permanganate required 1.1 " 10.3 " 20.3 " 50.4 " 100.1 "

In making himself familiar with the permanganate of pota.s.sium t.i.tration, the student should practise by working out a series of experiments similar to the above, varying his conditions one at a time so as to be certain of the cause of any variation in his results. He may then proceed to experiment on the various methods of reduction.

_A solution of ferric chloride_ is made by dissolving 5.01 grams of iron wire in 50 c.c. of hydrochloric acid (sp. g. 1.16), and running from a burette nitric acid diluted with an equal volume of water into the boiling iron solution, until the liquid changes from a black to a reddish-yellow. About 4.5 c.c. of the nitric acid will be required, and the finis.h.i.+ng point is marked by a brisk effervescence. The solution of iron should be contained in an evaporating dish, and boiled briskly, with constant stirring. There should be no excess of nitric acid. Boil down to about half its bulk; then cool, and dilute to one litre with water. Twenty c.c. of this solution diluted to 100 c.c. with water, and acidified with 10 c.c. of dilute hydrochloric acid, should not decolorise any of the permanganate of pota.s.sium solution; this shows the absence of ferrous salts. And 20 c.c. of the same solution, boiled with 20 c.c. of the ferrous sulphate solution, should not decrease the quant.i.ty of "permanganate" required for the t.i.tration of the ferrous sulphate added. In a series of experiments on the various methods of reduction, the following results were got. The modes of working were those already described.

(1) _With Stannous Chloride._--Twenty c.c. of the ferric chloride solution required, after reduction with stannous chloride, 20 c.c. of "permanganate." Fifty c.c. of a solution of ferrous chloride, which required on t.i.tration 49.8 c.c. of "permanganate," required for re-t.i.tration (after subsequent reduction with stannous chloride) 50 c.c.

of the permanganate solution.

(2) _With Sulphuretted Hydrogen._--Two experiments with this gas, using in each 20 c.c. of the ferric chloride solution, and 10 c.c. of hydrochloric acid, required (after reduction) 20.2 c.c. and 20.1 c.c. of "permanganate." Repeating the experiments by pa.s.sing the gas through a nearly boiling solution, but in other respects working in the same way, 21.3 c.c. and 21.6 c.c. of the permanganate solution were required. The sulphur was not filtered off in any of these. In another experiment, in which 50 c.c. of the ferrous sulphate solution were t.i.trated with "permanganate," 48 c.c. of the latter were required. The t.i.trated solution was next reduced with sulphuretted hydrogen, brought to the same bulk as before, and again t.i.trated; 47.9 c.c. of the permanganate of pota.s.sium solution were required.

(3) _With Sodium Sulphite._--Twenty c.c. of the ferric chloride solution, reduced with sodium sulphite, required 19.9 c.c. of "permanganate." In one experiment 50 c.c. of the ferrous sulphate solution were t.i.trated with "permanganate"; 49.3 c.c. of the last-mentioned solution were required. The t.i.trated solution was reduced with sodium sulphite, and again t.i.trated; it required 49.2 c.c. of the permanganate of pota.s.sium solution.

(4) _With Zinc._--Twenty c.c. of the ferric chloride solution, reduced with zinc and t.i.trated, required 20.8 c.c. of "permanganate." Fifty c.c.

of a solution of ferrous sulphate which required 49.7 c.c. of "permanganate," required for re-t.i.tration, after reduction with zinc, 49.7 c.c.

The student should next practise the t.i.tration with b.i.+.c.hromate, which is more especially valuable in the estimation of hydrochloric acid solutions. The following experiments are on the same plan as those already given. In each experiment (except when otherwise stated) there were present 20 c.c. of the ferrous chloride solution, and 10 c.c. of dilute hydrochloric acid, and the bulk was 300 c.c.

~Effect of Varying Temperature.~--The quant.i.ties of the b.i.+.c.hromate of pota.s.sium solution required were as follows:--

Temperature 15 30 70 100 b.i.+.c.hromate required 20.2 c.c. 20.3 c.c. 20.3 c.c. 20.4 c.c.

~Effect of Varying Bulk.~--

Bulk 50 c.c. 100 c.c. 200 c.c. 500 c.c. 1000 c.c.

b.i.+.c.hromate required 20.4 " 20.4 " 20.4 " 20.5 " 20.8 "

~Effect of Varying Acid.~--In these, variable quant.i.ties of dilute hydrochloric acid were used.

Acid present 10 c.c. 50 c.c. 100 c.c.

b.i.+.c.hromate required 20.3 " 20.3 " 20.2 "

~Effect of Foreign Salts.~--The effect of the addition of 10 grams of crystallized zinc sulphate was to decrease the quant.i.ty of "b.i.+.c.hromate"

required from 20.3 c.c. to 20.1 c.c., but the colour produced with the test-drop was very slight at 18.5 c.c., and with incautious work the finis.h.i.+ng point might have been taken anywhere between these extremes.

Zinc should not be used as a reducing agent preliminary to a "b.i.+.c.hromate" t.i.tration. Ten grams of ammonic sulphate had the effect of rendering the finis.h.i.+ng point faint for about 0.5 c.c. before the t.i.tration was finished, but there was no doubt about the finis.h.i.+ng point when allowed to stand for a minute. The student should note that a t.i.tration is not completed if a colour is developed on standing for five or ten minutes. Ten grams of sodic sulphate had no effect; 20.3 c.c.

were required.

~Effect of Varying Iron.~--The results are proportional, as will be seen from the following details:--

Ferrous chloride present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

b.i.+.c.hromate required 1.0 " 10.2 " 20.3 " 51.0 " 102.3 "

The student may now apply these t.i.trations to actual a.s.says of minerals.

The following examples will ill.u.s.trate the mode of working and of calculating the results:--

~Determination of Iron in Chalybite.~--Weigh up 1 gram of the dry powdered ore, and dissolve in 10 c.c. of dilute sulphuric acid and an equal volume of water with the aid of heat. Avoid evaporating to dryness. Dilute and t.i.trate. The result will give the percentage of iron existing in the ore in the ferrous state. Some ferric iron may be present. If it is wished to determine this also, add (in dissolving another portion) 10 c.c. of dilute hydrochloric acid to the sulphuric acid already ordered, and reduce the resulting solution before t.i.trating. By dissolving and t.i.trating (without previous reduction) one has a measure of the ferrous iron present; by dissolving, reducing, and then t.i.trating, one can measure the total iron; and as the iron exists in only two conditions, the total iron, less the ferrous iron, is the measure of the ferric iron.

~Determination of Iron in Brown or Red Ores or Magnet.i.te.~--Weigh up 0.5 gram of the ore (powdered and dried at 100 C.), and dissolve in from 10 to 20 c.c. of strong hydrochloric acid, boiling until all is dissolved, or until no coloured particles are left. Dilute, reduce, and t.i.trate.

~Determination of Iron in Pyrites.~--Weigh up 1 gram of the dry powdered ore, and place in a beaker. Cover with 10 c.c. of strong sulphuric acid, mix well by shaking, and place on the hot plate without further handling for an hour or so until the action has ceased. _Allow to cool_, and dilute to 100 c.c. Warm until solution is complete. Reduce and t.i.trate.

~Determination of Iron in Substances Insoluble in Acids.~--Weigh up 1 gram of the ore, mix with 5 or 6 grams of carbonate of soda and 0.5 gram of nitre by rubbing in a small mortar, and transfer to a platinum crucible. Clean out the mortar by rubbing up another gram or so of soda, and add this to the contents of the crucible as a cover. Fuse till tranquil. Cool. Extract with water. If the ore carries much silica, evaporate to dryness with hydrochloric acid to separate it. Re-dissolve in hydrochloric acid, and separate the iron by precipitating with ammonia and filtering. If only a small quant.i.ty of silica is present, the aqueous extract of the "melt" must be filtered, and the insoluble residue washed and dissolved in dilute hydrochloric acid. Reduce and t.i.trate.

A convenient method of at once separating iron from a solution and reducing it, is to add ammonia, pa.s.s sulphuretted hydrogen through it, filter, and dissolve the precipitate in dilute sulphuric acid. The solution, when boiled free from sulphuretted hydrogen, is ready for t.i.trating.

STANNOUS CHLORIDE PROCESS.

The colour imparted to hot hydrochloric acid solutions by a trace of a ferric compound is so strong, and the reducing action of stannous chloride is so rapid, that a method of t.i.tration is based upon the quant.i.ty of a standard solution of stannous chloride required to completely decolorise a solution containing ferric iron. This method is more especially adapted for the a.s.say of liquors containing much ferric iron and of those oxidised ores which are completely soluble in hydrochloric acid. It must be remembered, however, that it only measures the ferric iron present, and when (as is generally the case) the total iron is wanted, it is well to calcine the weighed portion of ore previous to solution in order to get the whole of the iron into the higher state of oxidation, since many ores which are generally supposed to contain only ferric iron carry a considerable percentage of ferrous.

_The stannous chloride solution_ is made by dissolving 20 grams of the commercial salt (SnCl_{2}.2H_{2}O) in 100 c.c. of water with the help of 20 c.c. of dilute hydrochloric acid, and diluting to a litre. The solution may be slightly opalescent, but should show no signs of a precipitate. The strength of this is about equivalent to 1 gram of iron for each 100 c.c. of the solution, but it is apt to lessen on standing, taking up oxygen from the air, forming stannic chloride. A larger proportion of hydrochloric acid than is ordered above would remove the opalescence, but at the same time increase this tendency to atmospheric oxidation, as the following experiments show. The stannous chloride solution (20 c.c.) was mixed with varying amounts of strong hydrochloric acid (sp. g. 1.16), diluted to 100 c.c., and exposed in open beakers for varying lengths of time; and the residual stannous chloride measured by t.i.tration with permanganate. The quant.i.ties required were as follows:--

Time Exposed. 50 per cent. Acid. 10 per cent. Acid. 1 per cent. Acid.

1 hour 33.2 c.c. 34.4 c.c. 34.5 c.c.