The nitrite formed may react with iodide to form additional molecular iodine. This sequence of reactions is well known (Kolthoff et al., 1957). If oxygen is freely available so that these cyclic reactions are allowed to run their course, 1 mol of nitrite may give rise to a theoretical maximum of 0.5 mol of apparent oxygen. In the Winkler reactions scheme, nitrite does not react with iodide until the sample is acidified by the addition of sulfuric acid to dissolve Mn(OH)3 as in reaction (3). However, by that time, the oxygen present initially in the sample has already been sequestered as Mn(OH)3 and is no longer available for its reaction with nitric oxide (reaction 6). However, oxygen may be re-introduced in to the sample as the oxygen dissolved in the added sulfuric acid and as atmospheric oxygen that may dissolve into the sample during the addition of the acid and the subsequent transfer of the sample for the measurement of its absorbance in the spectrophotometric protocol (Pai et al., 1993) or the subsequent titration in the titrimetric protocol (Strickland and Parson, 1972). The amount of oxygen thus introduced, which becomes available for its reaction with nitric oxide, is likely to vary from analyst to analyst and from sample to sample as the handling of the sample is not stringently controlled in this part of the protocol. As a result, the ratio between the apparent oxygen formed from the presence of nitrite and the concentration of nitrite in the sample may be anywhere between 0.25 and 0.50 mol mol−1. In samples of deionized water, artificial seawater and natural surface seawater that had been equilibrated with air in the laboratory and contained known amounts of added nitrite, the concentration of apparent oxygen increased linearly with increasing concentrations of added nitrite with slopes ranging between 0.29 and 0.44 mol mol−1. The average was 0.35±0.07 mol mol−1 (Table 1). Thus, the presence of nitrite does lead to an over-estimation in the determination of oxygen. The range of the slopes fell well within the range that may be attributed to the interference of nitrite according to