A standard way to measure isocyanate (NCO) content in polymer is to first react with dibutylamine then back titrate the unreacted base with a strong acid. My Standard of Procedures says to dissolve sample in toluene, add dibutylamine, add isopropanol then back-titrate. What is the purpose of using isopropanol?
EDIT 1: See https://www.kyoto-kem.com/en/pdf/industry/PlasticRubber/ETII-98003.pdf for a procedure similar to the one I'm using. I am looking for the purpose of isopropanol present in large quantities, when there's already a solvent for the sample, a reagent in excess, for which there is also a (back-)titrant.
Dear Ed Yu, please read carefully the following RG thread. My Regards
https://www.researchgate.net/post/How-to-do-back-titration-of-NCO-prepolymer-using-di-n-butylamine
Abdelkader BOUAZIZ
Thank you for replying but I did not find the answer to my question. In fact I read that thread and the pdf prior to asking this question and I read it again. The thread addressed these topics: why back-titrate, how to tell if reaction completed, what is a sample procedure look like. However, I asked specifically why an alcohol is used in the procedure, which is actually not found in the accompanying pdf.
This is similar to the procedure I'm using https://www.kyoto-kem.com/en/pdf/industry/PlasticRubber/ETII-98003.pdf. Here you can see 100 mL of alcohol is used per test. Please help me ascertain its purpose in order to minimalize chemical wastes if possible. Thank you!
Sometimes, acid groups are generated during titration. These will promote further : polymerization, crosslinking, and visvosity build up. To avoid this, monoalcohols (and othe chemicals) are used to block the generated acid. My Regards
The formed ureas with butyl amine have a strong influence on viscosity or might even be solid. The addition of isopropanol suppresses this. Br Hans
I agree Hans Haberle. The fomed ureas are more polar than the isocyanates and for this reason they are less soluble in toluene. Isopropanol keeps them in solution and avoids precipitation.
Hi
The purpose of the isopropanol is to act as a co-solvent so the acid titrated in will dissolve in the reactant/toluene mix. A way round this is to use tetrahydrofuran instead of toluene. This will dissolve the isocyanate and di-butyl-amine and bo missable with the mineral acid titrant. In this method you don't need the isopropyl alcohol
Acid titrant is in water solution. If you have a toluene mixture, water is not soluble in the mixture, therefore you do not have the acid (usually hydrochloric) homogeneously dissolved. Moreover if you use bromophenol as indicator in a not polar medium, it doesn't dissolve.
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