Error sources and interferences

The known amount of liberated iodine then is titrated with thiosulphate solution. The standardization of the thiosulphate solution is exactly the same as the last step in the determination of oxygen and thus any subjective error in recognizing the endpoint of the titration or any other bias affecting the indication of the endpoint is compensated for. 

The above equations indicate that 1 mole of iodate produces 3 moles of iodine (I2), an amount consumed by exactly 6 moles of thiosulphate. In order to ensure quantitative coproportionating and complex formation, iodide and hydrogen ions are added in excess. 

The method is suitable to determine oxygen in seawater of all salinities as long as no H2S is present. 

After acidification of the fixed samples, the iodide solution is sensitive to photochemical oxidation (Carpenter, 1965 Carritt and Carpenter, 1966), especially if the solution already contains iodine. Therefore, any exposure of the acidified iodide solution to sunlight or other UV light soiu-ces should be avoided. The liberated iodine should be titrated without delay. 

Iodine, even bound to iodide as l3, is bmely soluble in water and has a significant vapor pressure. Again, speed of titration is essential. 

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