Measurement of hexavalent chromium in leather by colorimetric methods

Colorimetric methods of measurement forms an important basis for analysis of a variety of compounds and metallic species. Some of the more important advantages of using colorimetric methods include low cost, limited need for sophisticated instruments or personnel.

Determination of hexavalent chromium (Cr(yi)) by diphenylcarbazide(DPC) is a very common method. Chromium (VI) is carcinogenic and its determination is very important. Chromium (VI) occurs in chromiimi-tanned leather samples and there are regulations on permissible limits. The current official procedure (called the IUC-18 procedure) is based on DPC and is generally inapplicable to leather samples that produce a colored extract. The customary DPC method as well as several modifications thereof have been studied in a variety of leather samples with a view to improve sensitivity of the procedure. A pre-bleaching procedure was developed. This allowed reliable analysis of a majority (but not all) of the samples studied as compared to results obtained with ICPOES of cation exchanged samples as the reference method. Also, the sensitivity of the DPC method has been greatly improved upon by use of a liquid core waveguide based flow through absorbance detector.

As a necessary component to the above efforts, the development of an optical fiber coupled Light Emitting Diode (LED) light source based flow-through optical absorbance detector was carried out. The LED source is readily changeable. Optical fibers are used to carry light from the electronics/display unit to a reflective flow-through cell and back. The cell can thus be located remotely from the electronics unit and the umbilical connection is not susceptible to electrical noise. The noise level of this detector with LEDs of different emission maxima were observed to be in the range of 3-20 µAU under actual use conditions, with a maximum short term drift of 4 µAU/min after the initial warm up period. When the analyte absorbance is well matched with the source emission characteristics, the detector response is linear with concentration over at least two orders of magnitude. The liquid flow path through the cell is linear with a large exit aperture such that bubbles are not trapped in the optical path. The optical arrangement is such that the incident light crosses the liquid flow orthogonally, and is reflected back by a rear mirror to the receiver fiber. This arrangement reduces the refractive index sensitivity by an order of magnitude relative to conventional Z-path flow cells.