Chapter XXXIX: Section V: Colour Measurement

_Colour Measurement._--It is recommended that the method used by English chemists, namely, measuring with Lovibond’s Tintometer (as described by Professor Procter and Dr. Parker, Journ. Soc. Chem. Ind., 1895, 125), shall be used, and the results stated in units of red, yellow and black. The measurement may be made on the solution used for analysis, but must be calculated to one containing 0·5 per cent. of tanning matter, in a centimetre cell.

ANALYSIS OF USED LIQUORS.

It was decided at Liège, 1901, and Leeds, 1902,[196] that the “Shake Method” with chromed hide-powder, of the American Association of Official Agricultural Chemists, 1901 (A.O.A.C.), should be employed in the detannisation of used tanning liquors, as with these the filter method is apt to give too high results owing to the amount of non-volatile acids which they contain. The method of the A.O.A.C. is given in Appendix C.

[196] Procter and Blockey quoted experiments at the Leeds Conference,
proving that gallic acid and some other non-tanning substances were
largely absorbed by the hide-powder filter, though probably not
permanently retained by leather; while the error, though still
considerable, was much less when the chromed hide-power shake method
was employed. Where only gallotannic and gallic acids are present, as
in the case of sumach and commercial gallotannic acids, the most
accurate quantitative estimation is probably that by the Löwenthal
method carried out as described L.I.L.B., p. 123, but considerable
skill is required in its execution.

ANALYSIS OF SPENT TANS.

It was decided at Leeds, 1902, that spent tans must be analysed like fresh tanning materials; but where the prescribed strength of solution cannot otherwise be obtained it is permissible to concentrate the entire solution by evaporation. It is advisable, where suitable apparatus is available, to concentrate _in vacuo_; but failing this, an ordinary flask may be used, in the neck of which a funnel is placed.