Hi Bibi, of course insitu ...

Hi Bibi, of course insitu measurements are practical for having direct outcomes, but I saw great variations in results from As kits. We normally double check the insitu results in the lab by using stabilization techniques by means of a small resin cylinder and preserve the water sample until analysis in the laboratory. The preservation procedure aims to maintain the distributions of natural species and their concentrations. The As stability problems can be overcome when the separation of As(V) from As(III) is carried out on site using a chloride-form strong base anion exchange resin. The pH of each sample has to be measured prior to As speciation and has to be in the range of < 8. In this pH range, As(III)is completely protonated as H3AsO3, and As(V) is present in ionic forms (i.e., H2AsO4– and HAsO42-). Therefore, the resin retains the As(V) and allows As(III) (i.e., H3AsO3) to pass through the resin column. Best regards, Jaap van Nes