The iodide determination was attained using a combined iodide ion selective electrode due to its selectivity and simple incorporation in the flow system. Furthermore, the use of potentiometric detection is not influenced by the sample colour or turbidity. The SAM was implemented in-line by the sequential aspiration of the different standard solution and sample.
Iodate was measured indirectly through the determination of nitrite in a sequence of two reactions : 1 oxidation of hydroxylamine by iodate with production of nitrite and iodide; and 2 Griess reaction for the determination of the formed nitrite: diazotisation of sulphanilamide by nitrite in acidic medium and the coupling of the intermediate with N-(1 - naphthyl)-ethylenediamine hydrochloride(N1NED) to produce a coloured azo dye possible of being determined by spectrophotometry. To improve the oxidation of hydroxylamine by iodate, which is a relatively slow reaction, the mixture of the reagent and sample was enhanced by using a mixing chamber, in a flow batch approach. A flow-batch system corresponds to the incorporation of a mixing chamber into the flow system, this way combining the advantages of batch and flow analysis, namely improved mixing between reagents and sample in a reproducible way. By choosing this approach, not only a more efficient mixing is achieved, but the possibility of implementing stop periods to increase the reaction time was enabled. As the iodate concentration was calculated through the nitrite determination, based on the Griess reaction, the nitrite content of the samples was also assessed using a previously described method. In the end , the nitrite content of the sample was subtracted from the assessed value that corresponded to the iodate plus nitrite in order to calculate the iodate concentration in the sample. As previously mentioned, to achieve the expected low levels of iodate and to minimize matrix interference, SAM was used and performed in-line. But in this case, fully exploring the flow-batch approach, SAM was accomplished using a single standard solution. With the mixing chamber placed in a side port of the selection valve, additions of different volumes of the standard solution combined with different volumes of deionized water(in line preparation of the standard solutions) were sent to the mixing chamber and added to the same sample volume for the in line standard addition procedure.