Hello,

I have followed the typical protocol for a 200 mM sodium orthovanadate solution for use as a phosphatase inhibitor 100x stock:

1.  Dissolve Na orthovanadate in ddH2O

2.  Measure pH; it will be above 10.  Bring pH down to 10.0 with HCl

3.  Solution will turn yellow upon HCl addition; boil until clear again

4.  Cool down on ice to RT

5.  Measure pH; it will have increased to above 10.0 again

6.  Repeat steps 2-5 until solution no longer turns yellow upon HCl addition, and pH stabilizes at 10

The problem I am having is that after I adjust the pH to 10.0 and boil my solution, while it does transition from yellow to clear, the pH does not increase.  In fact, it usually drops down from 10.0 to about 9.5.  

At that point, I'm not sure what to do.  Adding HCl will cause another color change, but of course, it will also drop the pH further.  Should I first increase the pH with NaOH back up to something like 11, perhaps, and then bring it back down with HCl?

On every protocol I read, boiling is supposed to increase the pH, but that is not what happens for me, and I would like to know if anyone else has encountered this.

Thank you!