" I introduced the pHmeter into a mix of OA and Glycerol and the
 equipment gave me around pH 4. "

This reading of 4 is meaningless.  The glass electrode in your pH meter is intended to work in water solutions and nothing else.  The reading in glycerin tells you nothing at all about how ionized oxalic acid is in glycerin.  Glass electrodes give fair measurements in some water mixtures where water consists of at least 30% of the total solvent but any less water and readings rapidly become meaningless.  They can also become meaningless in pure water solutions of salts that are very soluble when those salts are at high concentrations.

Dick

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