GC method with H3PO4 and KH2PO4 in the sample solution

[Brian1706]

I am sorry for the confusion, but there is a reason I cannot give out every detail. I thought it was clear that I get water soluble alcohols from the hydrolysis of my esters, but I understand that it was not that clear after all. At least now you know...

You are right about the excess of the phosphoric acid. Up until now, it had to be pH 2-3 in order to get the free acids from the hydrolised esters and not the potassium salts of them. But of course I could use 2 different sample preparations with no excess of H3PO4 for the GC analysis of the alcohols...

[Don_Hilton]

With the limited information - the use of a large slug of glass wool is the best I can come up with. Do note that KOH is bad for GC columns, but will accumulate on a glass wool plug. KOH will corrode glass and make for very active sites, so change that liner often. So the glass wool plug an need to understand how many injections the liner will last is needed - and that is regardless of wether you quench the reaction or not. Also with strong pH - you can get some chemistry going - perhaps confounding your results as to what is going on the flask. After studying some derivitization condions, we put some of our sample into a syringe followed by a slug of MSTFA. We made the injection and found that the sample was completely derivitized in the inlet. Very short mixing time, but low viscosity and high temperature for the reagents.

The design of an alternative prep would depend on the analytes and potential interferences. With the information you have given, I have attempted to toss some ideas to give you some different directions to think about so that you might have a more robust analysis.

[Peter Apps]

I am sorry for the confusion, but there is a reason I cannot give out every detail. I thought it was clear that I get water soluble alcohols from the hydrolysis of my esters, but I understand that it was not that clear after all. At least now you know...

You are right about the excess of the phosphoric acid. Up until now, it had to be pH 2-3 in order to get the free acids from the hydrolised esters and not the potassium salts of them. But of course I could use 2 different sample preparations with no excess of H3PO4 for the GC analysis of the alcohols...

I can understand your not being able to release all the details of the process, but ethylene glycol is a lot more water soluble than monohydric alcohols, so that particular detail would have been very helpful.

So. without going into specifics; you do a base-catalysed hydrolysis in butanol to produce alcohols and acids from esters and then you lower the pH to get hte acids in free form?

Have you thought about analysing the acids rather than the alcohols ? At a pH of 2 many organic acids will partition quite happily into an organic solvent.

Peter

[Brian1706]

Thanks a lot for all your suggestions

You are right about the procedure.
For the acids I already have a HPLC method for which I need the acids in free form. But for my purpose that is not enough, I need to analyze both the acids and the alcohols.
But as you mentioned, instead of giving an excess of H3PO4 to the solution I could only neutralize it (or buffer it at pH 6-7) so that I only get the phosphates which will precipitate from the solution. So, I would have 2 sample preparations with getting to pH 2 for the acids and HPLC and getting to pH 7 for the alcohols and GC...

[Peter Apps]

Sounds like a plan - please let us know how it goes.

Peter