[157] Wissenschaftliche Beilage des ‘Ledermarkt,’ 1901, p. 107.
Fig. 67.
American Milk-shaker.
The “shake-method” adopted by the American Association of Official Agricultural Chemists, possesses some advantages, especially in the analysis of used liquors which, from the acids they contain, are apt to give somewhat too high results by the filter method (see [App. B], [page 480]). It has the further advantage of being much less dependent than the filter-method on the quality of the hide-powder employed. It has therefore been accepted by the International Association as permissive for all tanning materials, and as compulsory for used liquors (see [App. A]), and must therefore be briefly explained. It can be carried out successfully with somewhat inferior hide-powders to those required for the filter, but generally gives results 1 or 2 per cent. lower in tannins than the latter. A special shaking machine must be employed, capable of thoroughly agitating a mixture of hide-powder and the liquor to be analysed; and if many analyses have to be done, it is convenient that it should be driven by power, as otherwise the work becomes somewhat laborious. A machine called a “milk-shaker,” [Fig. 67], employed in the mixing of summer drinks, is generally used. The quantity of powder required for the analyses to be made (about 8 grm. of ordinary air-dried powder for each determination, with say 5 grm. added), is stirred in a large beaker with 25 times its weight of distilled water, and allowed to soak for 24 hours, 1·5 per cent. of chrome-alum previously dissolved in water being added at the beginning of the operation, and 1·5 per cent. more not less than 6 hours before its end. The powder is then washed by squeezing through linen, and the washing is continued till the wash-water no longer gives a precipitate with barium chloride; and is then well squeezed out in linen, preferably with the aid of a press. The damp squeezed powder is now roughly weighed, to determine approximately what quantity it is necessary to take, to give 7·5 grm. of the original dry powder to each estimation (air-dried powder contains about 15 per cent. of moisture), and a portion is accurately weighed in a basin, and dried, first on the water-bath, and then in the drying oven, to determine its moisture by loss. The approximate amount of powder required for each determination—if possible a round number of grams—is now weighed into as many bottles of about 300 c.c. capacity as determinations are to be made, 100 c.c. of the filtered liquors, prepared as before described, are introduced into each bottle, and the bottles are then each shaken for 10 minutes (Mr. Alsop states that in his experience 5 minutes is sufficient). The contents of the bottles are now filtered through funnels, the stems of which are plugged with pure cotton-wool, and the liquor is returned till a clear filtrate is obtained, of which 50 c.c. is evaporated as in the International method. It is now necessary to accurately correct the residue obtained, for the amount of water carried in by the wet powder. The loss of weight of the powder which has been dried, divided by its wet weight, gives the water contained in each gram of wet powder, and this multiplied by the weight of wet powder added to the liquor, gives the weight in grams (or volume in c.c.) of water which has been added to each 100 c.c. of liquor. Consequently, if the residues found be multiplied by this weight plus 100, and the product divided by 100, the weight will be obtained which should have been given by 50 c.c. of undiluted but detannised liquor; and from this the non-tannins are calculated exactly as in the case of the residues from the filter process. Of course, in practice, a factor is found, by which it is simply necessary to multiply all the residues, to correct them to undiluted weight. The process sounds somewhat complicated, but in reality, where a large number of determinations have to be made, is quite as quick, if not quicker than the filter method; which it is quite possible it may ultimately supersede, as much attention is being devoted to its improvement.
Having determined the tanning, and soluble non-tanning matters of the materials, it remains to determine the moisture, and the insoluble which make up the whole. To determine moisture, a quantity, not exceeding two or three grams of dry solid materials, or half a gram of moist or liquid extracts, is weighed into a basin, and dried in the same way as has been described for the residues, only that a considerably longer time will be required before constancy is attained. The object of employing so small a quantity of liquid extracts is to abridge this time, and the consequent oxidation, as much as possible, as the extract soon forms a hard skin on the exterior, which renders further drying very tedious. It is advantageous to add a little alcohol to liquid and semi-liquid extracts, and so dilute them that by inclining the basin they can be distributed in a thin layer over its sides, while at the same time the alcohol facilitates the evaporation of the water. The weight of the dried residue in the basin is the “total solids,” while the loss is the “water”; and these can be converted into percentages by multiplying by 100 and dividing by the weight of substance originally taken. An alternate method, which is frequently convenient with extracts, is to pipette off 50 c.c. (in duplicate) of the dissolved and well-mixed extract-solution before filtration, and dry exactly in the same way as for “total soluble.” The sum of the two residues in milligrams, divided by the weight of extract taken for analysis, gives the “total solids”; subtracting this from 100 leaves the “water,” while the difference between the “total solids” and the “total soluble” is the percentage of insoluble matter. Two further points must be noted. If the total solids are determined by the first method, and the total soluble in the ordinary way, in an extract which contains no insoluble matter, it frequently happens that they differ by 0·1 or 0·2 per cent., owing either to the difficulty of driving off the whole of the water, or to slight oxidation of the total soluble residue. On the other hand, if the second method is adopted, a small amount of “insoluble” is invariably found, even in perfectly soluble extracts, which is due to the absorption of tannin or colouring matter by the filter paper. On the correction of this error, see Collegium, 1902, pp. 145-158, and [App. A], [p. 477].
As the value of a tanning material often depends very much on the paleness of its colour, it has become customary to specify in contracts the intensity of colour of a solution of it containing one-half per cent. of tanning matter (as measured by the I.A.L.T.C. method of analysis), in a glass cell of one centimeter thick, by comparison with standard coloured glasses in the tintometer. On the method of making the measurement see L.I.L.B., p. 131.
Note.—All the apparatus named in this chapter can be obtained of Messrs. Reynolds and Branson, Commercial Street, Leeds; or of Messrs. Portway, Jamaica Road, S.E.; and of most other dealers in chemical apparatus.
CHAPTER XXI.
GRINDING OF TANNING MATERIALS.
Before the tannin they contain can be extracted, most materials require to be ground, almost the only exceptions to this rule being divi-divi and algarobilla, in which the tannin is very loosely contained. Extracts, whether solid or liquid, merely require to be dissolved in water or liquor, in which they are, for all practical purposes, perfectly soluble. With the less soluble extracts it is generally preferable to dissolve at a temperature of 50° to 60° C. with vigorous stirring.