Will sodium thiosulfate react with nitrate and/or nitrite ions I am doing nitrate and nitrite quantification in raw water and treated water using a colorimetric method. The manufacturer of the instrument states that water with chloride will interfere with the test result. I was thinking to inactivate the chloride in the water with sodium thiosulfate. However, I am not sure how is sodium thiosulfate react with the nitrate and nitrite in my sample. Thanks in advanced for any guidance provided! Cheers!
First, typically only high concentrations of chloride will interfere with the colorimetric test...I assume you are using NitraVer or something similar? If you aren't using seawater (i.e., $\ce{Cl-} \gt \pu{100 mg/L}$) then you will be okay. But, let's say you are using seawater. There is an easy solution, simply make your standards in an aqueous matrix containing the same amount of chloride. (I hope you are making standards or have a check standard and are not just blindly trusting the UV-Vis result...)

Second, thiosulfate is a reductant and thus could potentially reduce nitrate and nitrite. However, your most powerful answer will be your own tests - you need to do control experiments. Make some nitrate and nitrite solutions of known concentrations and then mix them with thiosulfate at concentrations you would use. Check the nitrate/nitrite concentrations after mixing and determine if thiosulfate reduced them.