Preliminary Study On The Reliability Of The Determination Of Chemical Oxygen Demand In Coastal Water

Chemical Oxygen Demand（COD） is one of the important comprehensive indicators to evaluate the degree of water pollution as well as an important item in water-quality monitoring. The K2Cr2O7 method(COD_Cr)is widely used to the determination of river water pollution and the KMnO4 method（CODMn） is applicable to the determination of marine water pollution. The coastal zone with the intensive human activities, is the transition zone between ocean and land. With the rapid development of industry and agriculture, the water body have been polluted greatly in the coastal area. However, there is no uniform method for accurate determination of COD. Therefore, the research on the measurement method of COD in the polluted coastal zone will contributed to truly reflect the water pollution situation, and make objective evaluation to the water quality of water body, which is of great significance in the treatment of water pollution.In the present dissertation, the influence of bromide ion and salinity on COD measurements were studied. Besides, we choose the three seagoing rivers in Yantai city as the study areas that are affected seriously by human activities. The differences between different determination methods of COD are systematically studied to find out the problems in the present analytic method of COD. The main conclusions came as follows:（1） Bromine ion has a great influence on the determination of COD_Cr and acid CODMn, while it has no obvious effect on the determination of alkaline CODMn. When the true value of COD_Cr is constant, the bromine ions have good linear relationship with COD_Cr and acid CODMn, respectively, and almost no linear relationship with the alkaline CODMn. Keeping the same true value of COD_Cr, the contribution of bromine ion per unit mass to COD_Cr is gradually increased with an decrease in concentration of Bromine ion. The contribution of bromine ion per unit mass to acidic CODMn was not affected by the true value of acidic COD_Cr.（2） The COD_Cr true value can be obtained by measuring the concentration of bromide ion and the COD_Cr measured value. When the concentration of bromide ion is greater than 800 mg/L, the water sample should be diluted before measurement. Although the concentration of bromide ion has less effect on alkaline CODMn, the alkaline CODMn true value cannot be obtained by measuring the alkaline CODMn measured value and the concentration of bromide ion.（3） The salinity has great effect on the COD_Cr and acidic CODMn and the situation is opposite for the alkaline CODMn.（4） The geographical location is not suitable to be used as the selection criterion for the determination method of COD. Instead, it is a reasonable choice to take the salinity of 5 PSU as the selection criterion for the determination method of COD.（5） It is not feasible to evaluate the CODMn value by COD_Cr value in coastal waters. Although there is a relationship between CODMn and COD_Cr, the CODMn value estimated by COD_Cr value cannot provide an situation of water pollution.