Any ideas about an instrumental method for hydroxyl values?

[MarkZ]

The method currenty in use is old-fashioned wet chemistry and is touchy. The method goes like this...

Your synthetic goal is to make the diester of a fatty acid and propylene glycol, say. To monitor the reaction, you take an aliquot of the batch, add an excess of a mixture of acetic anhydride dissolved in pyridine, heat for 30 minutes to make the acetic acid ester of any unreacted hyroxyls, add water to convert the excess acetic anhydride to acetic acid, and then titrate with sodium hydroxide solution to a phenophthalein end point.

Any suggestions for an easily made UV chromophore? How about a low boiling derivative that will go through a GC? Any other ways, not based on HPLC or GC?

[Consumer Products Guy]

Oh, yes, very familiar with hydroxyl value determinations, both with acetic and with phthalic anhydride. Also with pressure bottle reaction as opposed to refluxing.

I would rather use capillary GC to monitor the reaction, nonpolar column such as DB-1 or DB-5, dissolve sample in DMF or pyridine, take aliquot and add BSTFA, mix, inject. If you want to see the intact diester as well, maybe a specialty high-temperature capillary, like Restek MXT-5.

[MarkZ]

Sorry, Cosumer Guy, I don't know BSTFA

[Markus Laeubli, Metrohm]

Dear MarkZ

Your method seems to be pretty old. Anyway I assume that finally nothing goes arround titration. But of course automated potentiometric titration is preferred.

ASTM Standards:
ASTM E222 - 10 Standard Test Methods for Hydroxyl Groups Using Acetic Anhydride Acetylation
ASTM E1899 – 08 Standard Test Method for Hydroxyl Groups Using Reaction with p-Toluenesulfonyl Isocyanate (TSI) and Potentiometric Titration with Tetrabutylammonium Hydroxide

[Consumer Products Guy]

Sorry, Consumer Guy, I don't know BSTFA

BSTFA is a common reagent for making trimethylsilylderivatives of molecules with active hydrogens. There are several related derivatitizing agents, Search for gas chromatography derivatizing agents or similar.

Sorry, Consumer Guy, I don't know BSTFA

Maybe you should !!!

[krickos]

Hi

Markz, if you search this forum, even this subsection you will find some pointers to improve the general test, one of the most important ones is that a reagents including anhydride need to be fresh/freshly prepared (moist sensitive) and blanks runned side-by-side.

Then there is another thing to be considered, if you follow a specific process, it should be easy as CPG indicated to use a GC procedure with dervatisation, you could look at the Ph Eur monograph for sodium stearyl fumarate as an example if I recall right it catches stearyl alcohol among other things.

For "natural products" like some oils it may be more difficult to find a reagent that fits all potential variants (different sources etcetera) and the "old fashinoned method" might be good enough, but thats somewhat outside my area so hopefully someone else can chip in and expand.