What is the theoretical background of potentiometry? - Metrohm

One of the most used electrolytes for pH measurementis c(KCl) = 3 mol/L, since the aequitransferentKCl causes only a very small diffusion potential at thediaphragm and is also economical. Normally a combinedpH glass electrode is stored in c(KCl) = 3 mol/Lonly for this reason, as one wants to have it ready forimmediate use without conditioning the diaphragm.However, on a long-term basis the storage in KClaffects the glass, since it leads to ever longer responsetimes. For the membrane glass, storage in distilledwater would be optimal, but then the diaphragmwould have to be conditioned for several hours. Thepatented storage solution for combined pH glasselectrodes (6.2323.000) solves exactly this problem. Ifa combined pH glass electrode is kept in this solution,the glass membrane remains unchanged regardingresponse time and alkali error. Moreover, if one usesc(KCl) = 3 mol/L as the reference electrolyte, the optimizedcomposition of the storage solution keeps thepH glass electrode ready for measurement. Conditioningbefore the measurement is not necessary, nomatter for how long the electrode has been stored.Figure 8:pH measurement in c(NaHCO 3 ) = 0.05 mmol/L. A glass of theAquatrode stored in the storage solution shows a substantiallyshorter response time than an electrode glass of the same typestored during the same period in KCl.Table 3: The correct storage of pH glass electrodesElectrode resp. reference electrolyteSeparate pH glass electrodeCombined pH glass electrode with c(KCl) = 3 mol/LCombined pH glass electrode with anotherreference electrolyte (Idrolyte, Porolyte, non aqueous)Gel (spearhead electrode)StorageDistilled water6.2323.000 storage solutionIn the respective reference electrolytePorolyte or c(KCl) = 3 mol/LTroubleshootingThe cause of most problems is not to be found in themeasuring electrode and its glass membrane, butrather in the reference electrode, as much more criticaldiaphragm problems can occur there. To avoid incorrectmeasurements and to increase the workinglife, attention must still be paid to the following possiblesources of error:79