The effect of nitrite on the spectrophotometric determination of iodate in seawater

A widely used method for determining iodate in seawater involves reaction of the IO₃^? with excess I^?, under acid conditions, to form I₂ which is measured colorimetrically. It is known that when the acidification is by mineral acid, nitrite can interfere with the method by oxidising I^? to I₂ in an analogous way to IO₃^?. Some workers have used sulphamic acid for acidification since it is reported to destroy nitrite in solution.We have investigated the extent of nitrite interference in the colorimetric determination of iodate using both mineral acid and sulphamic acid for acidifying the reaction mixture. In seawater the extent of the interference is equivalent to 0.212 μM 1^?1 IO₃^? per μM 1^?1 NO₂^? with sulphuric acid, and 0.128 μM 1^?1 IO₃^? per μM 1^?1 NO₂^? with sulphamic acid. This means that for a seawater containing 0.31 μM 1^?1 (39 μg 1^?1) IO₃^?, the presence of 0.5 μM 1^?1 NO₂^? will lead to an error of 25% in the iodate determined colorimetrically in methods using sulphuric acid, and 15% in methods in which sulphamic acid is used.