The strangest thing about ultrapure water, to my mind, is that it's blue. Well and truly blue, like you'd see in a cartoon. Blue in a way that ordinary water or even ultrapure heavy water aren't. (Supposedly, for heavy water. You need a decent amount of the stuff to really be sure of the color with your own eyes, and I've never actually seen that much heavy water, let alone ultrapure heavy water.)

You'll not find this on Wikipedia, so it surprised us a bit when we first saw it. But it's in the literature if you know where to look. Or we could just trust our eyes and our resistivity probes... neither were lying.

Is the material itself blue, or is it structural color?

I ask because I’m aware (as far as I believe) that hydrogen gas is colorless, and the solid hydrogens I’ve seen certainly aren’t blue.

Though structural color is a rabbit hole I’ve had to go down recently for an application (where the component materials have no color), so it wouldn’t surprise me that some materials at ridiculous levels of purity get interpreted as certain colors only because of the interference of light through the material.

I don't think structural color happens in (things that are exclusively) liquids? As far as I know it's the O-H bond, mediated by the other O-H bond being nearby, that's responsible for the color.

Do you have links to textual literature?

I am guessing its blue for reasons similar to why glacial ice (not melts, thats rock flour) is blue. For glacial ice, there are no air bubbles to mess with the selective transmission of blue light.

I don't have anything handy, sorry. This was a while ago.

But from memory you are exactly right. The color only shows up when the scattering length is long and there's nothing else to dominate the inherent color. Both conditions are rather hard to meet simultaneously.

I’d love to see a picture. Any links you know of?

Lots of them out there, but this one gets the color right (at least, it matches my memory...): https://cerncourier.com/a/daya-bay-releases-new-results/

The water pools were a pretty clean white. So, yes, it really is that blue!

Wow. Thanks. That is really blue.

I googled around, but couldn’t find any real results.

Really interesting article. I wonder if there’s any possibility of creating ultrapure water from ultrapure streams of hydrogen and oxygen - but maybe it’s harder to filter the component gases than just filtering the water itself.

My guess is you’d still have the same issues with getting ultra pure gases. Also it might be energy intensive.