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Pakistan Journal of Biological Sciences
Year: 2005  |  Volume: 8  |  Issue: 4  |  Page No.: 596 - 598

Colorimetric Determination of Organic Carbon in Soils by Dichromate Digestion in a Microwave Oven

F. Azam and M.H. Sajjad    

Abstract: External heating of soil samples following addition of dichromate is a commonly used method for the determination of organic carbon (C). We have successfully standardized microwave oven as a source of external heating for reliable and quick determination of organic C in soil samples by colorimetric method. Ten soils varying in C content were collected from agricultural fields differing in physico-chemical and other characteristics. The method involves: I) addition of 8 mL of conc. H2SO4 and 5 mL of 2N K2Cr2O7 solution to soil samples containing ca 5 mg C and placed in 100 mL glass beakers, ii) heating the treated samples in a microwave oven with rotating plate, iii) dilution of the contents of the beaker to 50 mL with distilled water, iv) determination of optical density of the samples at 590 nm, v) calculations for C content against a glucose standard (5 mg C mL-1 solution; 1.25 g glucose 100-1 mL) treated like the samples. The newly developed method gives values of C lower than those obtained with the conventional colorimetric method and Walkley-Black method. However, almost perfect correlations existed between the values obtained by the three methods. Regression equations thus developed can conveniently be used for the accurate measurement of organic C in soil samples. Time of heating in microwave oven that varied from 45 to 120 sec did not have a significant bearing on the amount of C determined. Hence, heating for 60 sec can conveniently be adopted for routine analysis. The proposed method is not only quick but highly economical in terms of energy used in heating the samples.

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