What is the theoretical background of potentiometry? - Metrohm

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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
1. BASICS OF POTENTIOMETRYCleaning and care of diaphragmsTable 11: Recommended ways of cleaning diaphragmsDiaphragm typeGeneralFixed ground jointSeparable ground jointCapillaryType of contaminationPrecipitates of silver halidesand silver sulfidesProteins, polypeptidesSuspensions, solids, resins,glues, oils, fatsAll types of contaminationAll types of contaminationElectrolyte flow interruptedCleaningImmerse diaphragm for several hours ina solution of 7% thiourea in 0.1 mol/L HCl.Immerse diaphragm for several hours ina solution of 5% pepsin in 0.1 mol/L HCl.Clean electrode with suitable solventAspirate off reference electrolyte andimmerse electrode inthe corresponding cleaning solution.Loosen the ground-joint sleeve(using hot water if necessary)and clean according tothe type of contamination.Apply slight counterpressure toelectrolyte refilling opening1.4.4. Reference electrolytes and bridgeelectrolytesThe reference or bridge electrolyte is in galvanic contactwith the sample solution via the diaphragm. Thesample solution and electrolyte form a phase boundarywith different ion concentrations on each side.This difference in concentration causes diffusion ofthe ions to the other side and, because of the differention mobilities, a so-called diffusion potential occurs.In order to achieve a high degree of measuringaccuracy the electrolyte composition must be selectedso that any diffusion potentials formed are as negligibleas possible; this is to a large extent achieved bythe use of c(KCl) = 3 mol/L. On the one hand the ionicmobilities of K + and Cl – are practically the same, onthe other hand the ionic concentration in the samplesolution is normally negligibly low in comparison toc(KCl) = 3 mol/L. This is why the equal-transferenceKCl electrolyte is used as standard in all combinedMetrohm electrodes and reference electrodes. However,certain media require the use of other electrolytecompositions in order to suppress effects thatoccur in addition to the diffusion potential.Table 12: Alternatives to the standard reference electrolyte c(KCl) = 3 mol/LMediumSilver ionsNon-aqueousIon-deficient waterProteins/polypeptidesSemi-solid substancesSurfactants (proteins)Problem withthe standard electrolyte c(KCl) = 3 mol/LReaction with Cl – with precipitationof AgCl ➝ slow responsePrecipitation of KCl, solutions and electrolyteimmiscible ➝ unsteady signalContamination of the medium by salt ➝driftPrecipitation of the proteins with KCl andAgCl ➝ zero point shift/reduced slopeContamination of diaphragm ➝zero point shift/slow responseAdsorption on diaphragm ➝zero point shift/ reduced slopeAlternative electrolyteKNO 3 saturated (or Titrodefor more or less constant pH value)2 mol/L LiCl in ethanolor LiCl saturated in ethanolKCl solution of lower concentrationIdrolyte 1Solid electrolyte in combinationwith pinhole diaphragmPorolyte 21 IIdrolyte is a glycerol-based electrolyte whose chloride ion activity corresponds to that of a KCl solution with c(KCl) = 3 mol/L. This meansthat the latter can also be readily replaced by Idrolyte. Idrolyte is excellent for use with solutions containing proteins and aqueous solutionswith an organic fraction.2 Porolyte is a KCl solution that has been gelled by polymerization and is used in electrodes with a capillary diaphragm (Porotrode).Table 13: Electrolyte flow rates and viscositiesElectrolyteViscosity(25 °C) (cP)Fiow rate μL/h(10 cm water column)c(KCl) = 3 mol/LKNO 3 saturatedIdrolytePorolyte~1~18...101200...1500Ceramic pinStandard electrode10...25Microelectrode5...1510...25––Separableground jointØ 10 mm:20...100Ø 5 mm:5...30Ø 10 mm:20...100Ø 5 mm:5...30––Fixed groundjoint5...30–––Ceramiccapillary–––5...30Plied Ptwire––3...25–88
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