I prepared 20% wt PEG solution to concentrate protein. 10 mM, pH 7.4 phosphate buffer was used to dissolve PEG. But the final 20% PEG-phosphate buffer became pH 10 (roughly, from color of universal indicator). Do anyone knows that how PEGs affect pH? And if I want to maintain pH of the PEG-phosphate buffer to be 7.4, how to achieve it?
Thanks in advance.
You should probably use an actual pH meter to measure your solution as high concentrations of PEG can change the hydrogen-bond accepting (HBA) basicity and hydrogen-bond donating (HBD) acidity of aqueous solutions and may give odd results with dyes or pH paper. Phosphate buffers should work fine or you could use a suitable Goods buffer like HEPES for pH 7.4.
If, after taking Edward's advice and measuring the pH with a pH meter, it turns out that the pH really is too high, you may be able to fix the problem by increasing the buffer concentration. Alternatively you can dialyze the PEG solution against a large volume of the buffer to remove whatever basic substance is in it.
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