What is the theoretical background of potentiometry? - Metrohm

Fixed ground-joint diaphragms have a uniform andreproducible electrolyte flow and are therefore particularlysuitable for use with sample changers.Separable ground-joint diaphragms are easy to cleanand therefore particularly suitable for applicationswhere contamination of the diaphragm cannot beprevented. The electrolyte flow may reach up to 100μL/h and is normally considerably higher than theamount of electrolyte flowing from a ceramic diaphragm.The ring-shaped geometry and the small polarsurface of the ground-joint diaphragm have afavorable effect on the measurement. The increasedelectrolyte flow influences the sample solution morethan if a ceramic pin diaphragm was to be used, thereference electrolyte normally needs refilling on adaily basis during long-term measurements..Capillary diaphragmsIn pH measurements in critical samples the very smallpores of conventional ceramic diaphragms are easilyblocked. The concept that has been realized in thePorotrode (6.0235.100), with two capillaries and aflow rate of 15...25 μL/h ensures unhindered contactbetween the reference electrolyte and the samplesolution (liquid/liquid phase boundary), while the twocapillaries of the Porotrode are practically insensitiveto contamination. The reference electrode is filledwith Porolyte, which has been specially developed forthis electrode. The constant flow of Porolyte ensuresthat the potential is established quickly and reproducibly.The flow rate and therefore the refilling intervalsare comparable to those of conventional electrodes.Extra maintenance work is not necessary. ThePorotrode can be stored in the «usual» referenceelectrolyte c(KCl) = 3 mol/L instead of in Porolyte.Measurements in problematic samples can be carriedout easily and reproducibly thanks to the conceptthat has been realized in the Porotrode. The pH ofsamples containing protein, such as milk and beer,can now be determined without any diaphragm problems.In contrast to traditional pH electrodes the Porotrodemeasures correctly even at high surfactantconcentrations.Pinhole diaphragmsMeasuring the pH in semi-solid samples such ascheese, meat and fruit places special demands on anelectrode. Proteins, fats and carbohydrates and othersemi-solid substances in foodstuffs tend to block thefine pores of the ceramic diaphragms used in mostpH electrodes, as such substances adhere extremelywell to the fine-pore ceramic surface. With the developmentof the spearhead electrode (6.0226.100)and the polymer electrolytes this problem has beenelegantly eliminated: two pinhole diaphragms takeover the function of the «liquid junction» betweenthe sample and the reference electrode. The polymerelectrolyte adjacent to the openings, which is spikedwith potassium chloride and thickened, is to a largeextent insensitive to contamination by media containingproteins and fats. This insensitivity to contamination,the efficient protection of the reference electrodeagainst the penetration of electrode poisonsand the optimized inner buffer of the measuringelectrode ensure that the new spearhead electrodehas an outstanding long-term behavior: even whenused in difficult media the electrode zero point retainsits long-term stability. The use of polymer electrolytesmeans that refilling a liquid reference electrolyte is nolonger necessary.Plied platinum wireIn combination with the reference electrolyte Idrolyte,which contains glycerol, the plied-platinum-wire diaphragmis outstandingly suitable for applications inbiological media. The precipitation of proteins is suppressedby using an electrolyte with a low KCl content.The multi-capillary system (channels betweenthe platinum wires) reduces contamination effectsand the electrically conductive platinum reduces theresponse time and the diaphragm resistance. However,cross-sensitivity may occur in strongly redoxbufferedsolutions.87