Analysis of tanning materials

Analysis of vegetable tannins is used to determine the tannin content in the extract solution from the raw or spent materials. The tannin analysis method set up by the American Leather Chemists Association (ALCA, 1954a) is based on the absorption of materials from the extract by hide protein. It is not based on chemical analysis of a true tannin molecule. 

Filter a testing solution prepared from tannin extracts from the raw or spent materials on a standard filter paper. Pipette 50 ml of the filtrate to a preweighed beaker with stopper. Dry the testing solution overnight in a forced-air oven at 100 2 °C. Cool the beaker with the stopper in a desiccator. Weigh the beaker containing the solid residue. The solid residue converted to each litre of the filtrate is defined as the total soluble solids of the testing solution (g 1 ). 

Dry the hide powders in an oven for 16 h and cool in a desiccator. Add 1 ml of 3% chrome alum CrK(SO )2 solution for each gram of air-dried hide powders at 25 2 °C for 2 h. Wash the pretanned hide powders thoroughly with water at 25 2 °C. Filter the suspended hide powders and squeeze the powders to obtain about 75% of the moisture in the powders. Weigh 50 g of the wet hide powders (containing approximately 75% moisture), equivalent to 12.5 0.3 g of dried powders, and add them to a 200 ml volumetric flask. Fill the flask with testing solution to the 200 ml mark. Close the bottle and keep the solution at 25 2 °C for 15 min. Filter the solution immediately into a beaker containing 2 g of kaolin. 

Chrome tanning is the most important tanning method in leather production. The effluents from the tannery house contain a considerable amount of chromium. A limit to the chromium discharge is mandated by pollution regulations in almost every country. It is necessary to recycle chrome tanning materials from the effluents. The most common way of recovering the spent chromium salts is by precipitation (Thorstensen, 1993). The pH of the effluents may be raised to the precipitation point of the chromium salts, which precipitate as a hydrated chromium oxide. 

No matter what the source of chromium tannages, either from raw materials or from recycled solutions, care must be taken to maintain the quality of the leather. This can be only achieved by constant chemical analysis and chemical control. The chromium content in the solution can be tested by the following two methods, titration and atomic absorption analysis  

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Analysis of tanning materials may comprise qualitative analysis for detecting and distinguishing them and quantitative analysis for the estimation of the proportions of the different components, especially of the tannin. 

The principal methods for the quantitative analysis of tanning materials are given below. 

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