Last week, as a preliminary test before titrating an unknown diamine sample, I conducted a titration of meta-phenylenediamine in water, DMSO, DMF, methanol, and ethanol using 0.1M HCl as the titrant. I used bromocresol green as the indicator.
However, I observed that the indicator changed from blue to yellow before adding the expected amount of HCl.
I suspect that this occurred because one of the NH₂ groups was protonated to NH₃⁺, which reduced the basicity of the other NH₂ group, thereby preventing proper titration.
Could you provide any advice regarding the titration of diamines?
Try to measure the pH during titration, make a simple curve in Excel as pH against volume of HCl, and see if you have two regions with rapid pH change - they will correspond to the two protonations. Just continue to add HCl until no more pH change is observed
I think Peter has the right idea. I would just add that is is possible that the color change of your indicator could be affected by the co-solvents. Therefore a pH measurement would give more information. There is no doubt that the second amine would be weaker than the first. Most books will list two pKbs for these molecules.
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