[msun]

Hi everyone,

This is somewhat of an urgent issue and so far, my research has not produced anything concrete.

1) I am actually mixing glacial acetic acid (assumed to be pure) into distilled water, and taking measurements from there. Every single time, the pH measurement is lower than the expected value. The results are so off that it cannot be confirmed using the relationship generated by Ka = [H+][AcO-]/[AcOH]. We are getting numbers as low as 2.29 pH for a mixture of 2 mL of glacial acetic acid in 300 mL water, which implies that 60+ mL of acetic acid were added to the initial mixture. My team is out of answers. The pH probe was calibrated using standardized buffers of 3.00, 4.00, 7.00, and 10.00. We tested fresh buffers and distilled water, and had no issues reading them. The glassware is washed, dried, rinsed with distilled water. The chemicals we use are stock. One of my theories is that carbonic acid might be a culprit, but because atmospheric carbon dioxide would not enter the water in such a high quantity, the pH should be relatively unaffected. Besides that, I'm stumped.

2) I have some questions about experimental design, but I wish to resolve issue #1 first. It involves Acetic Acid - Water - Mineral Oil Equilibrium and the best ways to test it.


Both of these subjects have, if any, very little results via searching on the internet. I'm surprised to find that there is not AA-W-MO EQ data anywhere. I hope I can get some help here. Thanks.

[enahs]

http://www.chembuddy.com/?left=pH-calculation&right=pH-acid-base-solution

And in general
http://www.chembuddy.com/?left=pH-calculation&right=toc

Without showing some of your data or calculations and what you actually get, not really sure what to tell you.
I assume you are thoroughly mixing the solution before taking pH, so there is not a region of high acetic acid before complete mixing? And that when taking the pH you are gently swirling the solution, if it is stationary or mixing too fast/vigorous your measurement will be off.

[Borek]

We are getting numbers as low as 2.29 pH for a mixture of 2 mL of glacial acetic acid in 300 mL water, which implies that 60+ mL of acetic acid were added to the initial mixture.

More like about 16 mL (not 60+), but I agree pH around 2.3 doesn't make sense, that is (theoretical) value for 1M solution, while your dilution should yield about 0.12M and pH around (again, theoretical value) 2.85.

Carbonic acid is too weak and it won't survive in so low pH.

Other than that I can think only about two things - some kind of contamination of glacial acetic acid, and problems with pH electrode. After all, you are using it outside of the calibrated range. But these are both rather long shots.