Flow-injection analysis wit potentiometric detection for the speciation of fluoride and calcium Original Research Article

Three flow injection systems with potentiometric detection have been developed for speciation of fluoride using a fluoride ion-selective electrode with a single flow-through detector. Among systems with (i) splitting of the sample zone and confluence point, (ii) flow reversal and (iii) resampling, the latter was most suitable. Its application was demonstrated for a study of the effectiveness of releasing fluoride from complexes with Al(III). Several ligands were examined as decomplexing agents such as EDTA, cyclohexane-1,2-diamine-N,N,N′,N′tetraacetic acid (DCTA), citrate and 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (Tiron). In flow injection measurements, only Tiron was found suitable for decomposition of Al(III)–fluoride complexes. In stopped-flow measurements, it was observed that with use of citrate or DCTA about a 10 min delay was required to release fluoride from Al(III) complexes. A flow-injection system with resampling was also utilised for speciation of calcium with ion-selective electrode in ligand solutions and in natural waters.