Determining the pH of an organic solvent

Our work is faced with the issue of determining the charge of compounds that are dissolved in chloroform. One could realistically determine the charge of the relevant moieties knowing the $$\mathrm{p}K_\mathrm{a}$$ of those groups. However,

• How would you determine or calculate the $$\mathrm{pH}$$ of an organic solvent?

An alternative way of asking this question is how would one calculate the concentration of hydronium ions or free protons in a nonaqueous solution.

• I think a good first step to answering this question would be to identify exactly what ion is formed by the addition of protons to chloroform. Hydromium ions can't exist without water (is the system open to atmospheric water vapor?). Maybe one or more of the Cl atoms on the solvent gets protonated. – Pat Apr 27 '12 at 14:08

I'm not sure why you're wanting to exert so much effort to find the charge of compounds.

If you really want to know the charge, determine it more directly. Electrophoresis can allow you to do so. A charged species is put in a capillary tube that is in between a cathode an an anode. When a charged species is subjected to an electric field, it will experience a force proportional to its charge and the field strength.

Drag forces present in the solvent are also a consideration in this setup and they depend on the "size" of your ion as well as viscosity of your solvent. One also has to worry about Joule heating, as an increase in temperature will cause poor reproducibility. Picking a suitable detector is fun, but hopefully you can get by with the common UV-vis detector.

So if you can find a suitable setup, know the radius of your ion and know the time between injection of your sample and its arrival at a detector, you can at least determine its effective charge.

• Are you suggesting that I should calculate the pI of my solvent and then backtrack that overall charge? – bobthejoe May 9 '12 at 22:14
• No. I'm suggesting that you measure its electrophoretic mobility, $\mu_{ion}= \frac{q}{f}$. Once you know the friction coefficient, $f$, then you can determine its charge, $q$. – Chris May 10 '12 at 1:20