水質汚濁指標に関する研究(第2報)酸性高温過マンガン酸法によるCOD_Mn測定における硝酸銀による塩素イオンのマスキングとその特異現象について

抄録

Masking effect on chemical oxygen demand (COD) values to chloride ions by silver salts was investigated by using semimicro acidic high-temperature permanganate method. Chloride ions react with permanganate, but the reaction was incomplete and the COD showed larger values after 60 min heating than 30 min. In the blank test, oxygen demand by chloride ions was masked with AgNO₃ added near to the equivalent amount. Chemical oxygen demand of glucose and p-hydroxybenzoic acid was almost unaffected with increasing addition of silver salts when chloride ion was absent, and these substances were used as examples of organic substances. When chloride ions were masked with AgNO₈ in the presence of glucose or p-hydroxybenzoic acid, the decrease of COD values was recognized at the near equivalent point of Cl^- and silver. As a reason of the decrease, NO^-₃ ion, increased by the addition of AgNO₃, affected the end point of titration with 0.025_N KMnO₄ in the presence of AgCl precipitate and produced a difficulty in recognition of pale reddish color of KMnO₄. This decreasing effect on COD values was completely diminished by the further addition of definite amount of silver salts, but moderate amount of excess AgNO₃ necessary for COD determination in the presence of chloride ions will be investigated in the following experiments.