Heavy water is orange
  July 2, 2024


If all water in the ocean was replaced with heavy water, what color would it have?

Long story short

Large bodies of clean water have a bluish color. This is caused by increased absorption of red part of the spectrum through incitement of vibrational modes of water molecules. In heavy water, hydrogen atoms are replaced with deuterium, which is heavier. This decreases the frequency of vibrational modes of the molecules and shifts entire absorption spectrum to towards the infrared.

Figure 1: Transmitted light spectra for heavy and normal water. Data from this paper, visuals by colleague.

Here is a simple demonstration. Two 4 m pipes were filled, one with distilled water and one with heavy water. Ends were closed with acrylic and sealed with silicone (they still leaked a bit though, not ideal when you leak 2 dollars per milliliter). I picked some typical cold LED floodlight, so spectrum might be a bit different than sunlight. Results below.

Figure 2: Test setup and images from inside the pipes.

So why isn’t it colorless?

As with the problem with sky color this simple explanation is just a part of the story. This took me way too long to realize, and I thought our heavy water was contaminated. The absorption spectrum of heavy water looks pretty flat, doesn’t it? It does not look like it corresponds to orange light. We wanted to do some spectroscopic tests on the pipes but we could not get our cheap usb spectrometer to catch the light through the pipe, and I didn’t want to borrow some serious instrument for a little side project. Either way, I don’t have any spectra and I’m not willing to recreate the setup, risking more leaks.
To obtain an approximation of the color our eyes see when exposed to given spectrum of light we need to map spectrum based on relative sensitivities of cone cells in human eyes. I found a ready-made script to generate html codes of perceived colors. I passed through it a blackbody radiation for 5800 K (sun approximation) and a spectrum of the LED floodlight that I quickly extracted using WebPlotDigitizer. We can then compare them with real life:

Figure 3: Comparison of obtained colors with photos from the camera.

The blue generated by the script is a bit more vivid than what we see in the demonstration. I assume it is the problem with the spectrum of the LED light source and representation of the color by camera. The qualitative conclusion though is that heavy water is orange, while normal water is blue.

But why does everyone say colorless?

While I would prefer to have full analysis with experimental spectra and theoretical models of photoabsorption, the ”personal website form factor” gives me some lax in terms of scientific rigor. And I did want to post it, since no place on the internet mentioned the fact that heavy water is orange.
When searching for the answer first thing that turns up is this discussion. While referenced paper claims D₂O is colorless, their website admits that they didn’t actually perform the experiment with it, instead leaving the pipe empty.

Full heavy water story

Isn’t that weird that some rando on the internet has thousands of moneys worth of heavy water? As you may have already guessed, it’s not mine. Our Institute has several liters of the stuff, sitting around since Pons and Fleischmann affair, as they wanted to get on the action. Just goes to show how excited people were about cold fusion back then. Reportedly they did detect some neutrons (anecdotal), but no calorimetric results were replicated.
It is quite plausible that over 30 years some impurities leeched from the plastic container it was kept in. We just assumed that, but since then I convinced myself that the orange color is its intrinsic property, so it’s not that bad.
It seems to me like quite a popularizable subject. I was surprised that no one talked about it, especially since working with large volumes of heavy water is not that rare. Things like CANDU reactors or SNO experiment used tons of the stuff. I actually got excited when I remembered a famous photo with interesting orange hue. I reached out to SNO, but they explained to me that the sphere was empty when that picture was taken. Unfortunately, they could not confirm our observation about the color of heavy water, although it seems like there wasn’t a good access to the vessel once it was filled.