Hi there! As I understand it, it is standard practice to acidify filtered water samples down to less than pH 2 before introduction to ICP (for metals analysis). The main rationale for this is repeatedly stated to be "keeping metals of interest in solution". I am wondering, however, how useful the dissolved metals concentrations that result from these ICP analyses really are, considering that the analysis conditions are obviously far different from those of ambient waters (which are not at a pH of 2!). Wouldn't this result in a major overestimation of the concentration of dissolved metals in the ambient environment?
Hope to get a conversation going about this. Thank you!
G'day Eleanor,
It's all about keeping the metals in solution, that are already in solution, in the sample you took, rather than losing them to adhering to the container walls or perhaps precipitating, hence underestimating the result.
I see your point that acidifying the sample is in effect a very mild digestion, so some metals may be brought into solution, but even analysing by ICP will measure more of the metal analytes than say a colourimeteric method.
It's great to ask and discuss these questions in understanding what happens when a sample is taken, whether to analyse dissolved or total metals, and which analytical method to use to help answer the questions you're asking.
Cheers,
Adam
Adam has made important points. I would add only that the international operational definition of 'in solution' is 'passing 0.45 microns'. So if you want to make comparisons that must be done. What more realistic measures you make is up to you.
If the aim is to ‘keep metals in solution’ by acidification then it will do a lot more than that. As you correctly state, it will likely be a measure of TOTAL metal, as you are effectively reversing metal ion hydrolysis reactions that have already occurred in water to generate insoluble metal complexes. Water testing is not my field, but it would seem logical to measure water pre and post filtration to give total and dissolved values.
Thank you for your input gentlemen! Yes, it is always a question of how closely we can capture ambient conditions with any analytical technique. It does seem that there is a rather marked divide between the operational and chemical definitions of "dissolved". And of course, this does not even address the further question of what is actually bioavailable at any given time.
Sampling
Polyethylene bottes have to use for all water samples .All samples for trace analysis must filtrate through membrane filters (0.45-μm micropore) and conserved with HNO3 with concentration from 0. 5 % to 2 % in accordance
with ISO 5667-1, 1980 - Water quality — Sampling —standard requirements
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