A colleague and I are developing a transgenic bacterial strain that contains an arsenite oxidase gene. The bacteria can oxidize arsenite into arsenate autotrophically. We need a method to measure the concentration of arsenite and arsenate over time in a water sample. First we thought of using a pH meter, but we need to measure the arsenite and arsenate concentration over a 48 hour period, and we are worried that the pH meter wouldn't hold up. Could there possibly be a chemical that reacts with arsenite to change the color, and we could use a spectrophotometer or a colorimeter. If there are any ideas for measuring concentrations of arsenite (around 5 mM), please let me know.
Speciation analysis is very challenging because you would like to separate the species first before detection. In order times electrochemical detection was done by now ion chromatography is the way to go for arsenic speciation. You will need ion chromatography system with inductively coupled plasma with mass spectrometry. Have a look at this paper: Determination of Anionic, Neutral, and Cationic Species of Arsenic by Ion Chromatography with ICPMS Detection in Environmental Samples, Anal. Chem., 1998, 70 (17), pp 3649–3655 The good thing about Analytical Chemistry papers is that most of the methods are reliable.