I made a buffer solution dissolving 79 g of NaH2PO4.2H2O in 1000 mL DI water. Then tried to adjust the solution pH to 6.0 using HCl or NaOH. Initial pH was below 6, so I added NaOH into the solution. It started increasing the pH gradually, and solution pH came to 5.3. Then I added more NaOH (drop by drop and stirred thoroughly for a good amount of time) but pH was constant at 5.3 for quite some time. Then suddenly it jumped to 7.02. After that, I added HCl (drop by drop and stirred thoroughly) to decrease the pH, but nothing happened. pH was constant at 7.02 for a long time, then suddenly dropped to 5.3. Tried several times but pH jumps between 5.3 to 7.02 and vice versa. I can't seem to find any pH level between these two values. What should I do?
You can prepare a sodium phosphate buffer of the desired pH by mixing various volume ratios of equimolar solutions of Na2HPO4 and NaH2PO4. Here is a site to help you.
https://www.aatbio.com/resources/buffer-preparations-and-recipes/phosphate-buffer-ph-5-8-to-7-4
By the way, it sounds like there may be a problem with your pH meter if you are getting sudden jumps. You may need to replace or recondition the electrode, or there may be a bad electrical connection. Or maybe you are not mixing the solution sufficiently. You should use continuous strong stirring with a magnetic stirrer.
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