The percentage of sulphur multiplied by 2.5 gives the percentage of sulphuric oxide. Thus a sample of copper sulphate containing 12.85 per cent. of sulphur will contain 12.85 × 2.5 or 32.12 per cent. of sulphuric oxide.
In minerals and metallurgical products, it is common to find the sulphur in both conditions—i.e., as sulphate and sulphide. Generally in these the percentage of sulphur only is wanted; but this will depend entirely on commercial requirements, and not on the fancy of the assayer. Soluble sulphates are determined separately by extracting with small quantities of cold water, so as to avoid the separation of basic sulphates, or, if the sulphides present are not at the same time attacked, by dilute hydrochloric acid. Lead sulphate may be extracted by boiling with ammonic acetate; whilst barium, strontium, and, perhaps, calcium sulphate, will be mainly found in the residue insoluble in acids.
Weigh up from 2 to 5 grams of the material according to the amount of sulphur judged to be present, and dissolve them in the titrating-dish with 1 c.c. of hydrochloric acid and 50 c.c. of water. Add 10 grams of sodic acetate, and 10 c.c. of acetic acid; dilute, boil, and titrate. In the case of ferric salts, half the quantity of acetic acid will be better, as then the ferric iron will be precipitated, and a colourless solution will be left, in which the end reaction is more readily distinguished.
Determined in this way, 5 gram samples of the following salts gave the results indicated below:—
| Salt. | "Barium Chloride" Required. | Sulphuric Oxide. |
| Copper sulphate | 64.25 c.c. | 32.12 % |
| Magnesium sulphate | 65.25 " | 32.62 " |
| Zinc sulphate | 56.25 " | 28.12 " |
| Ferrous sulphate | 58.25 " | 29.12 " |
| Sodium sulphate | 51.25 " | 25.60 " |
Determination of Sulphuric Oxide in Barytes (Heavy spar).—Fuse 2 grams of the powdered mineral with 5 grams of "fusion mixture" for five minutes; and, when cold, extract with water. Filter, acidulate the filtrate with an excess of 10 c.c. of acetic acid, dilute, boil, and titrate. For example, a transparent crystallised sample required 27.0 c.c. of barium chloride, which is equivalent to 13.6 per cent. of sulphur, or 34.0 per cent. of sulphuric oxide. Theory requires 34.3 per cent. of the latter. Since both carbonate of soda and potash are liable to contain sulphates, a blank determination should be made on 5 grams of the "fusion mixture," and the amount found be deducted from that got in the assay.
PRACTICAL EXERCISES.
1. The price of sulphur in an ore being 4-1/2d. per unit in the northern markets, what would be the price of a ton of ore containing 49 per cent. of sulphur? What would be the effect on the price of an error of 0.25 per cent. in the assay?
2. Pyrites carries 50 per cent. of sulphur, and on calcining yields 70 per cent. of its weight of burnt ore. Supposing the burnt ore carries 3.5 per cent. of sulphur, what proportion of the sulphur will have been removed in the calcining?
3. How would blende compare with pyrites as a source of sulphur for sulphuric acid making?