Abstract
A method is proposed to determine the chemical oxygen demand(COD) of sludge, sediment, and soil. It is based on the following reactions of the organic substance with iodate: IO3-+6H++5e→(1/2)I2+3H2O and CkHmOn→kCO2+{4k-(2n-m)}e. Weigh a sample(3 meq as oxygen consumed) and put it into a tall beaker (50ml). To this add 5meq of KIO3, weighed accurately and 10 ml of strong phosphoric acid prepared by heating H3PO4 (85.0%) up to 250°C. Mix intimately these reagents with the sample and heat the mixture on a hot plate (350°C).When the liberation of iodine sets in, stir the contents of the tall beaker and continue the heating till the temperature of the solution reaches 250°C. Transfer the syrupy solution to a beaker, boil to expel all of iodine liberated and make the volume 150ml with water. Determine the excess iodate by iodometry using 0.2M thiosulfate solution and calculate the COD values as follows : COD (meq/g)=a(1-q/p)/S, where a:KIO3(meq) used, p: volume of 0.2M thiosulfate solution(ml) equivalent to KIO3 used, q: volume of 0.2M thiosulfate solution(ml) required in the above titration, and S: weight of sample taken(g). Oxalic-, citric-, and tartaric acids, and saccharides react quantitatively with iodate and also many other compounds examined react nearly quantitatively with iodate.COD values of sludge, sediment, and soil are determined with (23) % standard deviation for 10meq/ g to 160meq/g of COD. The values obtained by the present method include the oxygen consumed for oxidizing -NH2 to N2 (or N2O), and chloride to chlorine. Acetates, benzene derivatives, fatty acids, and high polymers result in the slightly lower COD values than the actual values, because they are volatile or decompose only slightly.
