Leather ashes, analysis

Estimation of Chromium.—In the analysis of chromites or of other substances containing chromium, such as leather ashes, pigments, etc., it is first necessary to obtain a solution. This is done by finely powdering the substance and heating it with a suitable flux in a crucible, preferably of nickel. Many fluxes have been employed, usually caustic alkali or alkali carbonates, but the one in most common use at present is sodium peroxide, whereby the chromium compound is rapidly converted to a chromate. - Excess of alkali is removed by boiling with ammonium carbonate, which also precipitates any iron present. The filtrate is then acidified with dilute sulphuric acid and the chromium estimated either by gravimetric or volumetric methods. 

The ash of true leather tanned with tannin consists essentially of calcium carbonate with traces of iron and of phosphates. Coloured leathers may contain metals from the mordants used (tin, copper, iron, chromium, aluminium) tin may also be introduced as stannous chloride used for bleaching. Small quantities of silicates (talc, kaolin) may be employed in the treatment of the leather. Finally, other mineral matters (barium, magnesium and lead salts and sodium chloride) may have been added as filling to increase the weight. Complete quantitative analysis of the ash is rarely necessary, but determination of its calcium content is sometimes required, this being made by the ordinary methods. 

Chromium analysis can be performed on the ash sample from leather or can be perfonned on the leather directly. ASTM D2807 is a method for testing chromium by leather digestion (ASTM D 2807-1998). In this method, weigh 1 g of leather to the nearest 0.0001 g and cut it into small pieces 0.5 cm in diameter. Transfer the specimen to a 250 ml flask and add sequentially 20 ml of concentrated nitric acid (HNOj), 15 ml of perchloric acid (HCIO ) and 10 ml of sulphuric acid (H. SOj. 

Total ash content in leather can be measured by the standard test methods ASTM D2617-96 or by thermogravimetric analysis (TGA). 

Several methods have been used for the characterization of refuse. Some of these have been derived from the techniques employed for the characterization of solid fuels such as the proximate and ultimate analyses. The reader will recall from the previous section that by proximate analysis we mean the moisture, volatile, fixed carbon, and ash content of the refuse. The ultimate analysis gives the actual chemical composition of the refuse. Still another classification which is frequently employed provides a breakdown of the refuse components according to their origin, viz., glass, leather, paper, etc. 

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