Inhibited Monoethylene glycol

[Oxidation]

Does anyone have any experience of measuring the pH of a water glycol solution (90% by wt glycol) at 40 Degree C.
When using litmus paper I tend to get a reading 1,5 to 2 points lower than using a pH electrode.
One more question, is MEG stable when in contact with standard motor gas or will it degrade? If it does degrade what is the mechanism.

 

[bchoate]

oxidation

It is almost impossible to achieve an accurate pH measurement of nearly pure ethylene glycol. pH meters are susceptible to errors and instability when used in organic liquids. A simplistic concept of pH is that of a dimensionless measure of the hydrogen ion concentration in solution. The hydrogen ion comes from dissociation. Since most pure organic materials do not dissociate, pH is not an applicable method. With water in the solution, dissociation does occur and any pH reading is likely some measure of the water and its dissolved materials. The higher the MEG concentration the less accurate the pH measurement. pH papers respond over a 2 pH unit band dependent upon the pKa of the dye used in the paper.

Degradation. Is the MEG un-inhibited? MEG degrades by auto-oxidation in the presence of oxygen. Degradation is accelerated by temperature and metal ions in solution. MEG is oxidized to one of 5 different organic acids. These acids attack certain metals in the system. Minimizing contact with air will minimize degradation. Inhibitors protect metals from attack, retard degradation, and neutralize (buffers) acids formed by degradation. The organic acids are not useable by pH methods for determining the health of the MEG solution. They are weak acids that dissociate very little.

Organic hydrocarbons should not contribute to the degradation of the MEG.

 

[Oxidation]

bchoate:
Thanks for the reply. I have heard that by diluting the sample of the solution with water that a more reliable pH reading can be obtained, as I am not a Chemical Engineer I have no idea if this is infact true. However, is there some other method that could be used to detect the "quality" of corrosion protection if not pH.

 

[MAeng11]

you might want to purchase this ACS publication on pH measurements in MEG + Water to help you.

http://pubs.acs.org/doi/abs/10.1021/ie061305a

I have no personal experience to give you tips.

sorry,
MAeng11