I am going to make a HEPES A lysis buffer (50 mM HEPES, 500 μM ZnCl2) but as you know, ZnCl2 is an acidic solution and I need to increase the pH from 5 to 7.0. When I want to adjust it with NaOH, the white precipitate is going to be seen. What is the appropriate solution for making an adjustment for HEPES A (or ZnCl2)?
Check out this paper; it discusses also the dissolution of zinc salts in water.
It is rather complex.
Article The biological inorganic chemistry of zinc ions
If you try to adjust the pH after adding the ZnCl2, local, transient excess of NaOH will lead to precipitation of Zn hydroxide, which may be difficult to redissolve at pH 7. I would adjust the pH of HEPES to 7 without the ZnCl2 and add the Zn subsequently. Given that HEPES is in 100-fold excess, ZnCl2 is unikely to affect the pH significantly. This you can check easily by measuring the pH again after addin ZnCl2.
Pierre Béguin thank you for your answer. I've tried adjusting HEPES before adding ZnCl2, however; the initial pH was 7. Then I added a small amount of NaOH and the pH reached to 9.3. After that I added ZnCl2 and as you mentioned no precipitations were made, although the pH eventually reached 5.5 after adding ZnCl2! So if I add NaOH again, the precipitate will be form.
Try stirring your solution overnight.
The Zn(OH)2 may re-dissolve.
There is something impossible in what you report. Adding 0.5 mM (=500 µM) of ZnCl2 cannot possibly bring down to 5.5 the pH of a 50 mM HEPES solution buffered to pH 7. If you have a solution with 50 mM HEPES/HEPES-Na buffered at pH 7.0, adding 500 µM of any acid cannot make the pH change by more than 0.01 units. Check this with any pH calculator available on the web. Are you sure that your HEPES is 50 mM and not 50 µM ?
Achim Recktenwald You're right! and thank a lot for your help. I had a mistake and I weighed ZnCl2 for 0.5 M instead of 0.5 mM.
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