# Why the pH of xanthan gum in aqueous solution is 7 (neutral) when it has glucuronic and pyruvic acids in its chemical structure?

Since xanthan gum has both glucuronic and pyruvic acids in its chemical structure, it should release $\ce{H+}$ ions upon solvation in an aqueous solution and make it acidic. However, the meausured $\ce{pH}$ is $7$.

Pyruvic and glucuronic acids are both pretty strong acids (pK's of respectively 2.5 and 3.28) but that does not mean that the polymer of xanthan gum will be. Those protons are still there but they may be more tightly bound in the polymer than in the free acids. To find out try titrating a solution with strong base. Eventually you should start prying some of those protons off and you should see an inflection point. OTOH as there are so many carboxylic protons in the polymer the inflection points would probably smear and not be visible but you should see some buffering above some pH if my hypothesis is correct.

I should make clear that this is purely speculation on my part but it sounds like an interesting experiment.

• If this is the case (protons are tightly bound in the polymer), does it mean that xanthan gum is not negatively charged (since it does not release protons to become negatively charged)? – Rheologist Ph.D. Jan 22 '18 at 5:31
• As another comment here points out the answer is no - at least not at netural or acidic pH. Raise the pH and the protons come off leaving a net negative charge – A. J. deLange Jan 22 '18 at 16:23

Xanthan gum disperses difficultly in water; the dispersion process can be speeded up by raising the pH to about 10-11. This implies that acidic groups on the polymer are not much ionized, but addition of alkali puts a lot of negative charge on the polymer, helping it to unfold.

Measuring pH of gums, goos, slimes and muds is difficult, especially when the concentration of protons or hydroxide is low. The more xanthan gum you put into water, the goopier it gets; 1% is quite goopy. A measurement of 7 could be +/- 0.25 at least.