Micro-volume flow titration and screening the dissociation constants (pK(a)) of weak acids

A micro-volume, monosegmented flow analysis system has been developed for t itration of weak acids and for determining dissociation constants (pK(a)) i n the range 3-6, using pH indicators and spectroscopic detection. The syste m consists of a piston pump, a peristaltic pump, a detector and a 250 mum i .d. reaction silica capillary. The analytical procedure for determining the acid concentration involves the sequential introduction of air, solutions of phenolphthalein indicator, acid sample, NaOH, and finally air again, usi ng the piston pump. The various plugs disperse axially into each other in t he reaction capillary when pumping with the peristaltic pump is started. Th e acid sample will then be partially neutralised and the appearance of the dramatic pH change across the combined Liquid segment, bracketed by the air segments, is registered by measuring the indicator absorbance at 552 nm. I n the trials reported here, the acid concentration range was 35-100 mM, the sample volume less than 1 mul, the time needed less than 2 min, and the R. S.D. values obtained were in the range 0-2%. Two screening methods for the determination of pK(a) values were also developed. The first of these consu mes less than 200 nl of the sample, but requires prior knowledge of the mol ar sample concentration. In this case, an indicator mixture, exhibiting a l inear absorbance change as a function of pH at 598 nm is used instead of ph enolphthalein. The second approach, used when no information about the acid concentration is available, involves both indicator solutions and is based on determining the pH at the half-neutralisation point. The sample volume needed, in this case, is less than 1 mul. (C) 2001 Elsevier Science B.V. Al l rights reserved.