# HSO4- ion equilibrium, which side more acidic

I did an experiment in involving the reaction $\ce{HSO4- + H2O -> H3O+ + SO4^{2-}}$. An acid/base indicator was added to the solution. When more $\ce{SO4^{2-}}$ was added to the solution, it turned pinkish red indicating that the acidity has increased, pH lowered. more $\ce{SO4^{2-}}$ should mean a shift to the left, producing more $\ce{HSO4-}$ and $\ce{H2O}$. Based on Ka values, I thought the right side of the equation should be the one to show more acidity and lower pH, but this doesn't seem to be the case. Can someone explain this?

• You are correct. Adding more $\ce{SO4^{2-}}$ should shift the reaction to the left, and raise the pH. I'd suspect that the $\ce{SO4^{2-}}$ wasn't pure and had some $\ce{HSO4^{-}}$. – MaxW Mar 3 '16 at 21:22
• The indicator was thymol blue. I changed from a purple blue to a pink when I put the SO4 in. Am I correct in that the color change was opposite what should be expected? – Caesium-133 Mar 3 '16 at 21:55
• Thymol blue is blue at pH > 9. I don't know what you have added to what, but it might have been $\ce{HSO4-}$ to a previously alkaline solution. – aventurin Mar 3 '16 at 22:24