# Converting Disolved Oxygen from micromole/L to millimole/kg

I have dissolved oxygen data that is in units of µmol/liter.

I need to convert it to mmol/kilogram.

I have been told by someone who knows more about chemistry than I do (but only a little bit more) that I need to know the mass of my sample as well (which I don't know).

So only knowing my original DO data (in µmol/L) and the density of SW (assume 1.027 g/cm³), how does one convert the data to mmol/kg?

• Depends a lot on what the oxygen is dissolved in. If it is in water, then the conversion l → kg is easy. – Jan Oct 7 '15 at 7:20
• The O2 is dissolved in seawater. – Vinterwoo Oct 7 '15 at 15:30

## 1 Answer

Oxygen is barely soluble in water, so honestly, treating the saltwater as though it were "pure" saltwater with no dissolved oxygen is roughly equivalent to treating it as heavily oxygenated. The difference is in the fourth or fifth decimal place. Note that this would start to fall apart if you are concerned about the solubility of high pressure oxygen in water, but for small quantities of oxygen (such as the micromoles and millimoles you're using) this is likely not a factor.

The conversion is relatively straightforward. For a solution with concentration $X$:

$$(X\ \mathrm{\mu mol/L}) \times (1\ \mathrm{mmol}/1000\ \mathrm{\mu mol})\times(1\ \mathrm L/1.027\ \mathrm{kg}) = 0.0009737\ X\ \mathrm{mmol/kg}$$

• Thanks for that. So to be clear, the conversion factor is 0.0009737? AKA- If I had an O2 measurement of 23umol/L, I can just multiply by 0.0009737 to get 0.00223951 mmol/kg? – Vinterwoo Oct 7 '15 at 0:10
• @Vinterwoo Yep, that's it, though you'd really only know the first two digits, rounded. (i.e. 0.0022 mmol/kg) – Jason Patterson Oct 7 '15 at 22:46