how does the addition of methanol to either a buffered or unbuffered alkaline (pH 8.0) mobile phase alter the pH of the mixture. Is it enough to cause column damage or is this just a urban legend created by column vendors to explain occasional problems associated with so-called pH stable columns.

Thanks
Randy

The pH change depends on the nature of the buffer.

Check Neumaier´s links in "Strategies for HPLC development" of March 1, also the numerous discussions on buffers.

An acidic buffer, such as phosphate would increase in pH, depending how much MeOH you have added. Different acids may increase to a different extent! Cationic buffers (bases), such as e.g. Triethylamine most likely decrease in pH with amount MeoH in mobile phase.

Whether it could damage your column depends alot on what column you use (C18 I guess and if so which manufacturer), on the buffer etc.................

Some hints:

Journal of Chromatography A, Volume 797, Issues 1-2, 27 February 1998, Pages 111-120
J. J. Kirkland, M. A. van Straten and H. A. Claessens

Journal of Chromatography A, Volume 762, Issues 1-2, 21 February 1997, Pages 97-112
J. J. Kirkland, J. W. Henderson, J. J. DeStefano, M. A. van Straten and H. A. Claessens

Journal of Chromatography A, Volume 728, Issues 1-2, 29 March 1996, Pages 259-270
H. A. Claessens, M. A. van Straten and J. J. Kirkland

About pH in aqueous organic: REVIEW
Roses M., Bosch E., Journal of Chromatography A, Volume 982 (2002) Pages 1-30


Regards,

36CII

Check out LCGC November and December, and references there in, for detailed answer to this question. If the buffer is made from a neutral acid, for example acetic, or an anionic acid such as phosphate, the addition of methanol will make the solution more basic as a result of lowering the pKa of these buffer acids.

However, if the buffer is prepared from a cationic acid, for example ammonium, the addition of methanol will result in a more acidic solution at methanol concentrations up to about 80%.

Up to about 50% methanol there is very little correction to be made between methanol/water and water, so one can calibrate a pH probe with aqueous buffers and directly measure a pH that closely approximates -log(hydrogen ion activity) in methanol/water solution with less than 50% methanol, or for that matter less than about 50% acetonitrile. So, you can do these experiments yourself and see what happens.

See latest J. Chromatogr.vol 988 p 309 for some data on this topic.

Kirkland erroneously concluded that phosphate buffers were unexpectedly aggressive at attacking silica when used to prepare high pH buffers. They postulated some sort of complexing mechanism of silica to explain attack. That phosphate is bad at high pH and amines good has become part of the LC folklore.

A better explanation is that they measured the pH of their buffers before methanol addition and did not account for the fact that the phosphate and carbonate buffer would become more basic upon adding methanol. The even greater(10X) attack rate of silica by their carbonate buffer would not be explained by complexation of silica hypothesis.

A pH measurement of the buffers after methanol addition correlates with silica attack rates.

While I have not done the experiment, I expect one would find that at an identical pH, measured in the actual modified mobile pahse, phosphate and amine buffers would have about the same effect on silica dissolution. in other words, silica dissolution is affected by pH and not the composition of the buffer.

As you can see, the story is not trivial. In addition, nobody should believe that at pH 7.99 a C18 column is infinitely stable, and at pH 8.01 it will die within a few hours. The decline with pH is rather gradual. We did some studies at elevated temperature that demonstrate this over the pH range from 4 to 12. In addition, the temperature plays the usual role. A packing that is stable at pH 7.5 for 6 months at room temperature may show a column collapse at 50 to 60 degrees within some three weeks.
I personally do not recommend to operate a classical C18 column at pH 8 or above.

Hi Bill,

Just wanted to say thanks for writing a very useful series of articles. It is much easier to point our chemists to these articles than the collection of your posts here that I had been using.