§ 50. =Detection.=--Both ammonium and hydric sulphides blacken silver and filter-paper moistened with acetate of lead solution. To test for hydric sulphide in air a known quantity may be aspirated through a little solution of lead acetate. To estimate the quantity a decinormal solution of iodine in potassium iodide[62] solution is used, and its exact strength determined by d.n. sodic hyposulphite solution[63]; the hyposulphite is run in from a burette into a known volume, _e.g._, 50 c.c., of the d.n. iodine solution, until the yellow colour is almost gone; then a drop or two of fresh starch solution is added and the hyposulphite run in carefully, drop by drop, until the blue colour of the starch disappears. If now a known volume of air is drawn through 50 c.c. of the d.n. iodine solution, the reaction I₂ + SH₂ = 2HI + S will take place, and for every 127 parts of iodine which have been converted into hydriodic acid 17 parts by weight of SH₂ will be necessary; hence on titrating the 50 c.c. of d.n. iodine solution, through which air containing SH₂ has been passed, less hyposulphite will be used than on the previous occasion, each c.c. of the hyposulphite solution being equal to 1·11 c.c. or to 1·7 mgrm. of SH₂. [62] 12·7 grms. of iodine, 16·6 grms. of potassium iodide, dissolved in a litre of water. [63] 24·8 grms. of sodic hyposulphite, dissolved in a litre of water. PART IV.--ACIDS AND ALKALIES. SULPHURIC ACID--HYDROCHLORIC ACID--NITRIC ACID--ACETIC ACID--AMMONIA--POTASH--SODA--NEUTRAL SODIUM, POTASSIUM, AND AMMONIUM SALTS. I.--Sulphuric Acid. § 51. Sulphuric acid (hydric sulphate, oil of vitriol, H₂SO₄) occurs in commerce in varying degrees of strength or dilution; the strong sulphuric acid of the manufacturer, containing 100 per cent. of real acid (H₂SO₄), has a specific gravity of 1·850. The ordinary brown acid of commerce, coloured by organic matter and holding in solution metallic impurities, chiefly lead and arsenic, has a specific gravity of about 1·750; and contains 67·95 of anhydrous SO₃ = 85·42 of hydric sulphate. There are also weaker acids used in commerce, particularly in manufactories in which sulphuric acid is made, for special purposes without rectification. The British Pharmacopœia sulphuric acid is directed to be of 1·843 specific gravity, which corresponds to 78·6 per cent. sulphuric anhydride, or 98·8 per cent. of hydric sulphate. The dilute sulphuric acid of the pharmacopœia should have a specific gravity of 1·094, and is usually said to correspond to 10·14 per cent. of anhydrous sulphuric acid; but, if Ure’s Tables are correct, such equals 11·37 per cent.