The pH in organic solutions is irrelevant; it should be always measured in aqueous solutions, You can report as an apparent pH however. Any organic solvent will interfere with pH measurements, i.e., H+ concentration will be different at the electrode interface.

You should remember that pH is a function of proton activity, not concentration. Imagine two infinitely diluted solutions of acid in different solvents (aqueous and mixed aqueous organic). Due to different interactions with medium (solvation) the free energies of proton in such solutions are also different. This means the transition from aqueous solution to non-aqueous results in the change of proton activity and therefore pH. When you deal with concentrated solutions of acid the additional factor is the change of its dissociation degree or ion pairs formation, this leads to increasing pH in less polar media when compared with aqueous solution. One more factor is the problem of pH measurement using glass electrode. Due to unknown value of junction potential at the interface of mixed solvent and aqueous solvent in reference electrode the measured pH value is considered as instrumental, not true.

Dear Islam,

The decisive factor for behavior of the acids when transferring between different media is the extent of charge delocalization in the anion. Generally, the dissociation of an anion in water is larger than in organic solvent.

For more details and pka values of acids in different solvents, please see the following link:

https://ams.ut.ee/acidity-of-strong-acids-pka-in-water-and-organic-solvents-3/

Hoping this will be helpful,

Rafik