New column bad results

[Andy Alpert]

Mix equal molar quantities of H3PO4 and NaH2PO4. This will give a pH around 3.1. Then, add NaOH solution dropwise until you get to pH 3.5.

[danko]

Phosphate wouldn't be feasable at pH 3.5 (pKa1 phosphate = ca. 2.2)
In that case acetate would be my suggestion.

Best Regards

[Andy Alpert]

Right; I meant 2.1, not 3.1. As for buffering at 3.5, formate's pKa is 3.7 while acetate's is 4.7. It would probably be better to start with unbuffered formic acid in water and add NaOH or NH4OH until you get to pH 3.5.