Fig. 18.
Lime-water and lime-liquors may be titrated as above, with sulphuric or hydrochloric acid and methyl orange; but in the latter case ammonia (and if soda ash or "Inoffensive" is used, soda and potash also), and the lime salts of weak organic acids will be estimated with it. It is difficult to get a sharp end-reaction in old liquors from the organic acids (caproic, amidocaproic, &c.) present. To determine the ammonia, 50-100 c.c. of the liquor may be distilled in a small retort or flask, and the escaping NH3 collected in a U-tube or "nitrogen bulb" ([Fig. 17]), containing 20-50 c.c. of normal acid, which is afterwards titrated back with sodic carbonate and methyl orange. Kathreiner employs the arrangement shown in [Fig. 18]. 30 c.c. of the liquor to be examined is placed in a shallow vessel on a piece of ground-glass, and 10 c.c. of normal acid in a second cup, which is supported over the other by a glass or wire triangle. The whole is covered with a small bell-glass, of which the edges are smeared with, vaseline. At the end of 24 hours, all the ammonia will have been absorbed by the acid, which is titrated back. The lime-liquor sample should be drawn after well plunging the lime, and rapidly filtered into a flask from a funnel covered with a clock-glass.
Determination of Gelatin and Coriin in Lime-liquors.—This cannot be done directly, though considerable quantities of dissolved hide-substance are precipitated on acidification of the liquor with hydrochloric acid and saturation with common salt. If the liquor be neutralised with hydrochloric acid, and evaporated to dryness on the water-bath, nitrogen may be determined in the residue by combustion, and the hide-substance calculated from it (compare [p. 108]). This method is serviceable in determining the amount of hide dissolved by different solutions, or under different conditions.
The total solids of lime-liquors are estimated by evaporating 20-30 c.c. in a porcelain crucible at 212° F. (100° C.). The organic matter is then found by igniting and determining loss (using ammonia nitrate if necessary to complete the combustion of the carbon). The ash is mostly lime carbonate. Soda, potash, and other bases may be determined in it by the usual methods, if required.
ESTIMATION OF SULPHUR AS SULPHIDE IN SODIUM SULPHIDE, &c.
32·6 grm. of chemically pure zinc is dissolved in dilute sulphuric or hydrochloric acid. This is readily accomplished in a flask, if a piece of platinum foil, or a few drops of platinic chloride are added to form a galvanic couple with the zinc. After solution, sufficient ammonia is added to redissolve the precipitate at first formed,[I] and the whole is made up to 1 litre. Each c.c. = 0·016 grm. sulphur or 0·242 grm. of sodic sulphide. This solution is added drop by drop from a burette to the solution of sulphide, and forms a white precipitate of zincic sulphide. The end of the reaction is known by placing a drop (with a glass rod) side by side on a piece of white filter paper, with a drop of solution of lead acetate. So long as sulphide remains in solution, it will form a black margin of lead sulphide where the drops touch. The drops must not be placed too close, as the solid zinc sulphide is always darkened if it comes in contact with lead acetate. It must be noted that tank-waste liquors, and many other sulphur solutions, contain polysulphides which are estimated by zinc, but which do not unhair, at any rate in an unaltered state.
[I] If any brown residue remains, the zinc is contaminated with iron.
CHEMICAL EXAMINATION OF LEATHER.
