Fig. 19.
Estimation of Grease.—To determine oil and grease, a weighed quantity (5-10 grm.) of the leather in fine shavings or raspings is exhausted with petroleum-ether (gasoline) in a fat-extraction apparatus, of which a convenient form is represented in [Fig. 19]. The leather is placed in the upper vessel, of which the lower opening is loosely plugged with cotton-wool, and the petroleum-ether in the flask, which is gently heated in a water-bath. The petroleum-ether boils and condenses in the inclined condenser through the casing of which a stream of cold water is passed, whence it drops back into the flask through the material to be exhausted. When the exhaustion is complete (when a drop of petroleum-ether from the leather leaves no grease when allowed to evaporate on a clean glass), the upper part of the apparatus is removed, and the ether is distilled off. If the flask has been previously weighed, it is maintained in an air-bath at 212°-248° F. (100°-120° C.) for some hours, allowed to cool, and weighed, when the gain of weight is the grease and oil. Paraffin would also be extracted and reckoned, and probably traces of resin if present. Ordinary ethylic ether cannot be used, since tannins and many of their products are soluble in it. Probably carbon disulphide might be substituted. Care must be taken to avoid explosion, as the vapours of petroleum are very combustible. The residue left in the percolator may be examined for matters soluble in water, by extracting again with hot distilled water, or for resins (and phlobaphenes) by extraction with alcohol.
Estimation of matters soluble in water.—This is important both to detect weighting, and to draw conclusions as to the materials used in tanning. Fine raspings or shavings may be exhausted with warm water in a percolator, or roughly a weighed piece (20 grm.) of leather, air-dry, may be well kneaded and worked in 100 c.c. of warm water in a basin. 50 c.c. of this may be evaporated to dryness in a light basin over the water-bath (or under a paper hood on a steam boiler), and the gain of weight will give the amount dissolved from 10 grm. This is more accurate and quicker than redrying the leather and weighing loss. The residue will contain tannins and their products, often in considerable quantities, and may be examined by the table of reactions, [p. 112], though these are as yet very imperfect. It will also contain glucose, dextrin, and soluble salts, if these have been used to give weight and firmness. The absolute proof of weighting with glucose or dextrin is difficult, since tanning materials naturally contain these and analogous principles. The residue may be powdered and exhausted with cold water, and the tannins and colouring matter removed by shaking with magnesia ([p. 108]) or lead carbonate. Fehling's solution[J] is then added and the mixture is heated nearly to boiling. A rapidly formed and considerable precipitate of red cuprous oxide indicates weighting with glucose or dextrin. Leather extracts, however, invariably reduce Fehling's solution more or less, and a conclusion can only be drawn after some experience and comparative tests. Gallotannic acid and pyrogallol reduce it when heated, but not cane sugar or gum arabic. If a solution of cane sugar be heated to 68° C. for 1/4 hour with 10 per cent. of fuming hydrochloric acid, it is "inverted," and then after neutralising the acid with potash or soda, will reduce Fehling's solution when heated.
[J] 4 grm. cryst. cupric sulphate are dissolved in 20 c.c. of water; and 16 grm. of neutral potassic tartrate and 13 grm. of fused sodic hydrate are dissolved in 60 c.c. The two are mixed, made up to 100 c.c., and boiled for some minutes. It should always be tested before use by boiling a portion, which should remain perfectly clear.
The soluble mineral salts are detected by igniting the residue left after evaporation of a separate portion in a porcelain crucible.[K] From unweighted leather, the quantity is very small. The ash is exhausted with a few c.c. of distilled water, which will dissolve most sulphates and chlorides, which may be detected in small portions of the solution by baric chloride and silver nitrate respectively. Baric chloride and lead acetate are precipitated by a drop of sulphuric acid, and the latter is blackened with ammonic or sodic sulphide. Lime is precipitated by addition of ammonic chloride, ammonia, and ammonic oxalate; magnesia by the subsequent addition of sodic phosphate (see [p. 109]). The carbonates in the insoluble part (mostly derived from salts of organic acids) may be taken up by dilute hydrochloric acid and tested separately, or the acid may be used at first. Any residue undissolved by the acid is probably lead chloride, and will be dissolved by hot water.
[K] A platinum crucible must not be used for fear of its destruction by lead, unless this metal has been proved absent.
Estimation of ash.—The leather in small pieces (either after or before extraction with water) is incinerated in a porcelain crucible. The ash is extracted with hydrochloric acid. The insoluble portion may contain barium sulphate (barytes), lead sulphate, sand, clay, &c. For further examination, ordinary chemical text-books must be consulted. Any large amount of ash indicates weighting. Müntz found only about 0·5 per cent. of ash from bark-tanned leather.
Determination of hide substance.—It is sometimes of interest to determine the proportion of dry hide-substance in a sample of leather, but there is no known means of doing this directly. If, however, the leather be dried, finely powdered by rasping, and the nitrogen determined by combustion, either with soda-lime (Will and Varrentrapp's method), or with copper oxide (Dumas), the hide-substance may be calculated, since tannin contains no nitrogen. Müntz found unhaired skin dried at 230° F. (110° C.) to contain 51·43 per cent. of nitrogen (compare also [p. 20]).
DETERMINATION OF FREE ACIDS IN TAN-LIQUOR.
The lime-water method mentioned on [p. 172] is, from its simplicity, well suited for daily use in the tannery as a control method for ordinary working; but where it is necessary to make very exact estimations, or to determine the various acids separately, it is not so satisfactory as one recently published by Kohnstein and Simand (Dingl. Polyt. Jour., 1885, cclvi. 38).