Liquid-membrane ion-selective electrode using nicotinate

Three ion-selective electrodes for nicotinic acid are described.  A solid-membrane electrode with Cu nicotinate as sensor had a linear concn. range of 5 × 10-4-5 × 10-2M, a response time <50 s, and was also sensitive to Cu ions.  The liq.-membrane electrode based on molten cetyl nicotinate as sensor had a linear concn. range of 10-4-0.5M, response time ∼60 s, gave a graph slope of 25 mV/decade concn., but required a working temp. of 50°.  The liq.-membrane electrode based on the anion exchanger trimethylcetylammonium nicotinate (I) in decan-1-ol gave the best performance.  Linear response was shown at 5 × 10-4-1.0M, response time «30 s, and a slope of 59 mV/decade nicotinate activity was obsd. at >10-2M nicotinate.  The results are reproducible and sufficiently selective for titrn. of nicotinate.  Nicotinamide in pharmaceutical and biol. products was detd. by dissolving the sample in CH2Cl2, passing a current of NOCl to sep. the nicotinic acid, filtering off the acid, neutralizing to pH ∼7 with aq. NaOH, and measuring the potential of the I electrode vs. SCE as a function of the vol. of 0.1N H2SO4 added.  The electrode may also be used as a differential detector in the ion-exchange chromatog. of pyridinemonocarboxylate isomers.