Macrocycle based membrane sensors for the determination of cobalt(II) ions

Poly(vinyl chloride) (PVC) based membranes of macrocycles, 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene diperchlorate (I), 3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene diperchlorate (II) and 5,10,15,20-tetraphenylporphyrin (III) with sodium tetraphenylborate (STB) as an anion excluder and dibutyl phthalate (DBP), dioctyl phthalate (DOP), dibutyl butylphosphonate (DBBP) and 1-chloronaphthalene (CN) as plasticizing solvent mediators were prepared and investigated as Co2+-selective electrodes, The best performance was observed with the membrane having the composition III-PVC-STB-DBP in the ratio 3:8:1:8, which works well over a wide concentration range (8.0 x 10(-)6)-1.0 x 10(-1) mol l(-1)) with a Nernstian slope of 29.0 mV per decade of activity between pH 2.8 and 7.3, This electrode shows a fast response time of 20 s and was used over a period of 4 months with good reproducibility (s = 0.4 mV), The selectivity coefficient values of the order of 0.01 for mono-, di- and trivalent cations indicate excellent selectivity for Co2+ over a large number of cations. Anions such as Cl- and SO42- do not interfere and the electrode also works satisfactorily in a partially non-aqueous medium, The sensor has been used as an indicator electrode in the potentiometric titration of Co2+ with EDTA.