# How to acurately measure pH in heterogeneous solution?

Following my experiment, I adjust pH of $\ce{Ca3PO4}$ from acid to base (about 11)with ammonia solution by using pH meter for precipitating $\ce{Ca3PO4}$. I waited and found pH wasn't constant. In addition, I tried to wash a pH probe and be placed in the solution again but pH was changed from last value even if I didn't add more acid or base.

• Please explain the procedure better. Of course the pH is not constant while you are still precipitating. What do you mean by "you washed a probe and placed in solution"? I assume by "a probe" you mean "sample"? Or do you mean the pH probe? – Karl Jan 7 '18 at 14:52
• Please don't just accept the first answer that turns up. – Karl Jan 7 '18 at 14:54
• @Karl It’s a pH probe. I think solid which is on a pH probe affect measured pH so I cleaned a pH probe. – I. Kor Jan 8 '18 at 17:18
• The article ("the") makes a huge difference to the meaning in English. It's NOT just decoration. – Karl Jan 8 '18 at 19:12
• Why do you call it a "heterogeneous" solution? – Karl Jan 8 '18 at 19:16

The $\mathrm{pK}_{a3}$ of phosphoric acid is 12.319 according to Wikipedia. So at a pH of 11 most of the phosphate species would be in the form of $\ce{HPO4^{2-}}$. Thus as $\ce{Ca3(PO4)2}$ precipitates the solution will become more acidic. So an unanswered question here is how fast would equilibrium be established in the solution. Normally once precipitation starts, then equilibrium is established fairly quickly.
Also the solution is basic enough that $\ce{CO2}$ from the atmosphere will be forming carbonate species which would also slowly make the solution more acidic. (not sure how much drift in pH that you are observing...).