[Corribus]

We are doing some experiments in sodium chloride solution. By ICP-MS it was determined that the NaCl (Aldrich, ACS grade) is contaminated by a sulfur source (in saturated aqueous solution, the concentration is about 7 ppm sulfur). Sulfur (particularly free sulfide) adversely influences our experimental results. Anybody have any clever ways of determining what is the guilty species (i.e., sulfide, sulfate, sulfite etc.)? And more importantly, getting rid of it?

I have bought some of the ultrapure stuff at Aldrich, 99.999% trace metals basis. It's expensive but the concentration of sulfur is only diminished by about half.

[wildfyr]

It seems like the contamination is coming from somewhere else if the 99.999% stuff has 3.5 ppm sulfur... Anyways, you can do a barium sulfate test to see if its sulfate using barium chloride. Sulfate would be my guess as most likley contaminent.

[Corribus]

It's definitely coming from the NaCl, because the impurity roughly scales with the concentration, as determined by ICP-MS.
I am under the impression that the purity percentage is on a trace metals basis (well, that's what it says on the label), which does not include non-metallic contaminants like sulfur. So though it does not contain more than 0.001% by mass metallic impurities (probably by ICP-MS), S is not part of the analysis.

Can you elaborate on the barium sulfate test, and what the approximate sensitivity is?

[wildfyr]

Pick a wavelength on a UV vis (lets go with 450 nm). Then make up several sodium sulfate solutions from 1 ppm to 30 ppm or so. Add an excess of barium chloride to each one, wait 5 minutes, then get the absorption at that wavelength (which is really a turbidity reading of barium sulfate precipitating out). The absorption should increase linearly with sulfate concentration. Then do the test on your sample and if the barium sulfate ppm matches up with your ICP-MS concentration, then sulfate is all of your contaminent. Getting it out...recrystallize from an alcohol perhaps? Thats a shot in the dark.

[Corribus]

This is great information, thanks! Right now I'd settle just for knowing what the impurity is.

[wildfyr]

Just a heads up, above about 10 ppm this test can show sulfite too. barium sulfite is 10x more soluble in water than sodium sulfate.