We are trying to determin the pH of acetic acid solutions in the range of 95 to 75 vol%. The pH meter shows strongly different values for 75 (pH = 0.5) and 95 (pH = -0.5) vol% from the calculated ones. With an indicator we observe pH values around 1.5 for both concentrations. Can someone help us with the question, why the pH meter shows such small and different pH values for these high concentrations? The pH meter was calibrated with commercial buffer solutions.
Your pH electrode may not be suitable for measuring such concentrated acetic acid solutions because the pH may be outside its working range. Also, the highly concentrated acetic acid may not be suitable for accurate pH measurement because of the low water concentration, leading to little dissocation of the acid.
Finally, accurate measurement of pH requires calibration with standard buffers straddling the pH range. Did you calibrate the pH electrode with a standard buffer having negative pH?
Thank you for your answer. We did not use a buffer for negative pH. The low water concetration may be the major problem in our opinion, but shouldn't the pH then be higher (more basic) than the calculated one? Or can the high concentration and the fact, that we technicaly have water in acetic acid and not actic acid diluted in water, lead to a higher proton activity?
The technical aspects of pH electrodes are surprisingly complicated. It may be worthwhile to contact the manufacturer for advice for your application.
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