The New Method For Determining CODcr With Silver-Free Catalyst By Microwave Digestion Method

Chemical oxygen demand expressed as CODCr, is one of the important indexes of ourcountry implement total emission control. Determination of CODCrvalue of the state standardmethod is potassium dichromate reflux method, although the precision and accuracy of thismethod are high; but there are complicated to operate, long analysis time, large power andhuge water consumption, the need for a large number of expensive silver sulfate reagent andother shortcomings. Therefore, the potassium dichromate method is more applicable toarbitration analysis, not suited to large quantities of sample analysis and can not adapt toincreasingly heavy environmental pollution monitoring needs; particularly it can not timelyfeedback treatment effect in the sewage treatment plant.This paper has studied a new method for determining CODCrwith silver-free catalyst bymicrowave digestion method and discussed the digestion conditions on the detection results.This experiment using Manganese sulfate, Nickel sulfate, Cupric sulfate, Magnesium sulfate,Ferric sulfate as catalyst respectively, have determined CODCrby the microwave fastdigestion. While compared the obtained CODCrvalue with that by the national standardmethod, determined the optimal catalyst and dosage and digestion time. Comparative resultsshowed that: relative errors of the determination of the results with standard backflow are lessthan5%, standard deviation is2.828mg/L, and the coefficient of variation is0.704%. whenthe volume of the sample is5mL, the volume of0.20mol/L K₂Cr₂O₇solution is5mL,Manganese sulfate catalyst amount of catalyst is3mg, the microwave digestion time is3min,the results are best. And compared to the national standard method the microwave digestionmethod is saving at least4Yuan money when only one water sample testing.The results haveshown that the method is quick，accurate，reliable and saving reagents and electrical energy，and can be considered as alternative method for replacing the conventional reflux methodused in CODcr determination. It is also really suitable where large numbers of CODCrdetermination have to be carried out daily.The cycling steps of silver sulfate catalyst in waste water: waste containing silver werecollected in the laboratory, then waste of silver ions was converted to silver chloride precipitation by adding an appropriate amount of sodium chloride. Precipitate after furtherprocessed into a more pure silver chloride was added concentrated sulfuric acid and washeated until all dissolved. After cooling, silver sulfate was obtained by dilution with water.Recovery of silver sulfate and commercially available silver sulfate （Analytical reagent） issimilar by determination of CODCrin water, this shows that the recovered catalyst can bereused in the laboratory. Results have shown that this method which is short, simple operationprocess has greatly reduced the cost and avoided the secondary pollution; recovery rate is upto70%or more. This method is suitable for use in the laboratory.