Research On The Method For The Rapid Determination Of Chemical Oxygen Demand

Chemical oxygen demand(COD) is a comprehensive evaluation index of water pollution, and one of the index of total quantity control pollutants in our country. The common quantitative determination methods include potassium dichromate method, potassium permanganate method, and fast digestion spectrophotometric method,etc. As national standard method, the determination result of potassium dichromate method is accurate, and its reproducibility is good. However with a long and complicated operation, and requiring the use of highly toxic chemicals, which cause serious secondary pollution, potassium dichromate method is mainly used for the determination of serious polluted industrial wastewater. Potassium permanganate method is mainly used for determination of domestic sewage. The advantages of fast digestion spectrophotometric method include low chemicals use, short analysis time, and being suitable for batch testing, so it can meet requirements of real-time and on-line monitoring of industrial wastewater. The paper is based on the research of fast digestion spectrophotometric method,establish a fast and accurate method for the determination of COD. The main contents include the following aspects:(1)The research is based on fast digestion spectrophotometric method, potassium acid phthalate is used as standard material for the determination of COD,by the use of self-prepared oxidation agent and catalyst, the digestion instrument used is HH-6, establish a fast and accurate method for the determination of COD.(2) The optimum experimental conditions determined by the use of single factor experiment and orthogonal experiment is as following: the amount of potassium dichromate(the oxidizing agent, concentration is 0.250 mol/L) is 1.00 mL, the amount of concentrated sulfuric acid is 4.0mL, the amount of silver sulfate(the catalyst) is 0.04 g, digestion temperature is 165℃; digestion time is 10 min. The importance of the four factors obtained by the orthogonal experiment of range analysis is: the amount of silver sulfate is the most, followed by the amount of potassium dichromate, the effect of digestion time and temperature on the experiment is relatively small. The potassium acid phthalate with theoretical concentration of COD of 100 mg / L re-determined under the optimal experimental conditions, and the actual determined value is 98.9mg/L, so the relative error is 1.1%.(3) Under the optimum determination conditions, self-prepared digestion reagent is used to determine the concentration of standard code samples and standard solutions with different concentrations. All the relative standard deviations of determined values are less than 3%, being within range of 0.43%-2.89%. The relative error is in the range of-1.47%-1.26%.(4) Waste water of different sampling sites in Anshan Iron and Steel Company is analyzed by using self-prepared digestion reagent. The concentration of COD is in the range of 35.4 mg/L-722 mg/L. The calculated relative standard deviation is controlled within the range of 0.686%-2.97%.(5) Fast digestion spectrophotometric method by using self-prepared digestion reagent is compared with the National Standard Method(GB11914-89. Waterquality-Determination of the chemical oxygen demand-Dichromate method) in the paper. By comparative analysis reference solution of different concentration,unknown samples and the wastewater samples, and by using F-test method and t-test method, it is verified that there is no significant difference between the two methods.(6) Experimental study on the adjustment of COD determination less than 50mg/L. For water samples with COD values less than 50mg/L,the determination of the National Standard Method is about 1.20 to 1.13 times of the fast digestion spectrophotometric method by using self-prepared digestion reagent. As proved by results of statistics and inspection, there is no significant difference between the fast digestion spectrophotometric method for correcting the measured value(the determination of COD is measured value times 1.20) and the National Standard Method.The innovation of this paper is as following: quite a lot of tests we did prove that we can get the same result by using fast digestion in 10 minutes and by using standard method in 2 hours; spectrophotometric method, instead of titration, can be used for the determination of COD; self-prepared digestion solution can replace the special oxidizing agent and catalytic agent which are used for HH-6 COD Analyzer. The method, which saves time and reduces analysis cost, can be used for fast batch determination of COD in waste water accurately.