2450
Anal. Chem. 1987, 59,2450-2456
Performance of Flowing and Quiescent Free-Diffusion Junctions in Potentiometric Measurements at Low Ionic Strengths Terence R. Harbinson a n d William Davison*
The Freshwater Biological Association, The Ferry House, Ambleside, Cumbria LA22 OLP, United Kingdom
Reference electrodes having freediffusion llquld junctlons (FDJs) created In qulescenl and flowing sdutlons d I O 4 mol kg-‘ HCI were tested agalnst a previously evaluated “reference” FDJ. Junctions formed in 0.5 and 1 mm bore Perspex T pleces performed as well as those formed in cyllnders (wlthln 0.1 mV, 0.002 pH) and were Independent of solution dynamlco and configuratbnal varlatlons. Rather than cyllndrkai symmetry, the prknary prerequisite for an accurate stable ilqukl junction was that the two solutions should meet at a sharply defined stable boundary which Is not subjected to Irregular convectfve Influences. Junction material, bore size, and the ionic strength of slmple electrolytes had little effect on junction performance. Although alternattve deslgns of reference electrodes with renewable FDJs performed less well, they were precise to 0.01 pH. It Is suggested that the operational definition of pH should be modifled to state expllcltly that measurements made with FDJs conforming to m e slmple deslgn crlterla wlil provlde a good estimate of PH.
Potentiometric measurements with ion-selective electrodes require the use of a reference electrode to provide a constant reference potential. The most commonly used reference electrode elements, based on silver-silver chloride (Ag/AgCl) and calomel (Hg/HgzClz), give rise to few problems. Use of a bridge electrolyte between the reference element and the sample solution ensures a constant potential at the solid interface but introduces other uncertainties associated with the liquid junction formed between the bridge electrolyte and the sample solution. If the liquid junction potentials are the same in calibration and test solutions, they cancel one another; but if there is a difference in the two potentials, known as the residual liquid junction potential, an error is introduced into the potentiometric determination. When the pH of dilute solutions are measured (
