Leather Chemists' Pocket-book: A Short Compendium of Analytical Methods by Henry Richardson Procter Edmund Stiasny Harold Brumwell

Author:Henry Richardson Procter, Edmund Stiasny , Harold Brumwell
Language: eng
Format: epub, pdf
Publisher: Spon & Chamberlain
Published: 1912-03-25T05:00:00+00:00

Estimation of Colour,

136. The estimation of colour in extracts is frequently demanded, though it is mostly valuable as a numerical basis of contract, and gives little information

112 LEATHER CHEMISTS' POCKET-BOOK

as to the colour to be expected in tanning, unless the extracts are of the same make and character.

137. Tintometer. —^The analytical solution containing 3*5~4'5 gr^- of tannin per litre is matched in a I cm. cell by the coloured glasses of the instrument, and the results are calculated from the analysed strength to a 5 grm. per litre solution of tanning matter by simple rule-of-three. The results are not theoretically correct, nor in accordance with the I.A.L.T.C. prescription, which demands the use of an accurate o • 5 per cent, solution ; but it is found that colour-changes rapidly occur in keeping the liquors, and the method of calculation had been so generally adopted before its error was discovered that it is not easy to alter it. An alternative colorimetric method introduced by Procter has been recommended provisionally by the I.A.L.T.C, and may very probably be made compulsory at the next conference.

138. Ppoetep's Colorimetric Method.* —Depends, like the tintometer, on comparison with Lovibond's standard graduated colour-glasses, but instead of determining the glasses to match a standard strength of liquor, the strength of liquor is determined which will match a definite colour-standard in glasses. This has the advantage that strengths are directly proportionate, while colours are not; an extract of double the actual colour of another allowing accurately half the strength, while the tintometer readings are much less

♦ Journ. Soc. Chem. Ind. 1910, 663; Colleg. 1910, 292, 459.

than twice as great. The tints red and yellow are also much more easily understood as percentages of a total standard than as varying indeterminate amounts, and the measurement, as it does not in the first instance depend on analysis, can be made at once on the anal)rtical liquor, and with a suitable instrument is much more rapid and exact.

Probably the best instrument for the purpose is the small Schmidt and Haensch colorimeter with Lummer-Brodhun prism ;* but any accurate colorimeter with variable depth of liquid will serve, and the author has constructed a cheap form with mirrors which answers well, and will probably be put on the market shortly. As a standard colour the sum of 10 Lovibond units of red or yellow is adopted, and should be matched to tenths, though for rough work units only may be used, with a great economy of glasses. The colorimeter (with water in the cylinder) is first carefully adjusted so that the two fields are absolutely equal in light. If necessary, colourless glasses may be added to one side or the other, but the last adjustment is best made by shifting the source of light (preferably an incandescent gas-bmner, about 6 inches from the opal glass reflector of the colorimeter) slightly to one side or the other. The solution to be measured is now substituted for water in the cylinder, and colour glasses (say, 7 * 5 yellow and 2 • 5 red) placed on

* The Figs.

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