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How Pure is my Heavy Water?

I have a bottle of heavy water; how pure is it?

Sometime in the distant past, I acquired a partially used bottle of heavy water. It’s been in storage since I got it; I’ve not really done anything with it. The bottle claims to be 99% pure, but it is also cautioned that the liquid inside is hygroscopic and that the bottle should be refilled with an inert atmosphere. I have no idea if this has ever been done. I don’t think I’ve ever opened the bottle myself.

A thread on Mastodon mentioned heavy water, and I realised I had a sample. I thought it might be interesting to see how pure my sample was. This short article documents how I did this.

A hand holding a brown bottle with a red cap and a white and red label The label reads Aldrich Deuterium oxide 99 atom percent D
A hand holding a brown bottle with a red cap and a white and red label The label reads Aldrich Deuterium oxide 99 atom percent D

A hand holding a brown bottle with a red cap and a white label. The label reads 20.03; BP 101.4 MP 3.8 C. D 1.12 Hydroscopic store under inert gas
A hand holding a brown bottle with a red cap and a white label. The label reads 20.03; BP 101.4 MP 3.8 C. D 1.12 Hydroscopic store under inert gas

What I know

I know that the densities of normal pure water and heavy water are 0.9982g/mL and 1.1050g/mL, respectively, at 20℃. Their relative densities are 1.107.

The lab where the good balance lives is kept at 20℃, fortunately.

The display part of METTLER TOLEDO many digit balance. It is reading all zeros.
The display part of METTLER TOLEDO many digit balance. It is reading all zeros.

Method

A micropipette is used to take samples of pure and heavy water to weigh them. The pipette will ensure a constant volume so the mass of the liquid can be used to determine the density. My pipette was set up for 6µL, so that’s the volume I used.

Results

I took 6µL of pure 18MΩ water and weighed it. I did this three times and got an average mass of 4.783mg. If my pipette were perfectly calibrated, I’d expect 5.989mg.

I then took three 6µL samples of the heavy water, replacing the air in the bottle with argon after sampling. My average mass for these samples was 5.379mg.

The ratio of the two masses is 1.124. A bit greater than the expected 1.107, but enough to tell me that my heavy water is still pure. It hasn’t pulled in any significant amount of light water; if it had, the ratio would be lower, as the mixture of light and heavy water would have a lower density.

Ways to improve the result.

It turns out that weighing 6µL of water is pretty difficult. It wants to evaporate as you measure it. I didn’t want to use much of my heavy water, so I only took small samples. I think I could improve the accuracy of the result if I took a larger sample. I could also have taken more samples to improve the statistics.