Study On The Determination Of Chemical Oxygen Demand (COD) By Aerometry

Chemical Oxygen Demand (COD) is the most important synthetical index whenevaluate the organic pollute of waste water, also the primary index of the water andwaste water's testing in our country. Especially measuring on line has the great socialmeaning and practically value.At present, the classic method for determination of COD is open reflux, that isthe standard method for its virtues of good result,better adaptability and repeatability .The GCOD type COD auto determination apparatus works on principle of open refluxproduced by SANXIN environment-protected company. Aiming at the fact that thejudgement of the titration end-point is unstable, this paper makes detailed analyticalstudy on the errors and finds that the main cause is the sensitivity of photoelectriccolorimetry. This paper researches the change of the light intension when the titrationgets to the end-point in different colorimetric wavelength. The best range ofphotoelectric colorimetric wavelength is decided then. The result indicates that thestability of judging the titration end-point is improved distinctly when the selectedwavelength in this paper is used,which increases the measuring precision effectively.All the while, people make a lot of discuss aiming at the standard method'sdisadvantages: too long the overall process,the second pollute,difficulty ofexamining the batch sample and so on, and also bring forward a lot of new ways todetermine the COD. According to these disadvantages, this paper brings forward anew method :CO2- aerometry to determine COD by testing the volume of CO2produced by the oxidation reaction in airtight condition. The advantages of thismethod are as follows: the theory is very simple, the manipulation is convenient, thetime of determination is shorter than the standard method, determination on line isconvenient, and it can avoid the errors produced by the gas volatilization. When it isused in the condition of high pressure, the chlorine ion may soon avoid to be oxidizedbecause it can reach the gas-liquid balance very soon. So it can reduced thedisturbance of the chlorine ion effectively. iAccording to this theory, the paper designs a suit of equipment and analyses theerrors which effect the determination of COD value: the temperature and again boiloffof the condensate, then the formula about the equal temperature T1,the roomtemperature T0,and the volume of CO2 is educed to count COD value. The formulaeduced has eliminated the effect of temperature. At the same time, the paper adoptsthe pressure-sensor measure technique to weak the effect produced by the condensate'again boiloff. The paper determines the standard samples of two kind of concentrationat last, which validates the method practically.