[tarahans]

Fellow scientists,

I am a microbiologist with a problem.  I am exposing Escherichia coli cells to copper sulfate pentahydrate in order to induce the viable but nonculturable state of my bacteria.  However, it has proven to be very difficult for me to enumerate my cells on agar plates because I am not absolutely sure that I am removing all the copper from solution.  In other words, cells that should only be exposed to copper for 30 minutes might be exposed for 24 hours.  I have tried searching for a chemical that would precipitate the copper, like sodium hydroxide or ammonia, but all options would change the pH too drastically for my cells to survive.  The other option I have is to use sodium citrate as a chelating agent.  But, I don't know what proportion I would need in relation to my copper (20 mM).  Does anyone have another suggestion for me or perhaps an answer to my sodium citrate math problem?

Thank you,

Tara Hans
Poultry Science, Texas A&M University

[Borek]

First of all - what is the maximum acceptable concentration of copper? No matter what complexing agent you will use, some copper will be left, amount of copper left will depend on the complexing equilibrium - which in turn depend on the complexing agent concentration and usually on pH.

Second: what is acceptable pH of your samples?

Citrate reacts with copper 1:1 (in fact there are several different complexes created, but let's keep things simple), log K = 5.9, so amount of citrate added must be at least in the same range of 20 mM, but details will depend on the pH.