What is the theoretical background of potentiometry? - Metrohm

More documents

Recommendations

Info

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
1. BASICS OF POTENTIOMETRYTable 4: Possible sources of error and their remedies for pH glass electrodesSource of errorHF-containing solutionsHigh pH value andhigh alkali contentHigh temperaturesMeasurements atlow temperaturesDry storageReaction of a solutioncomponent with the glassEffectsEtching and dissolutionof the glass membrane ➝corrosion potential duringthe measurement/shortworking lifeIncreased alkali error ➝pH too lowRapid rise in membraneresistance by aging ➝increased polarizabilityand driftHigh membraneresistance ➝polarization effectsZero point driftSlow response, zero pointshift, slope reductionCleaningStore in waterovernightAlternativesUse of the Sb electrodeUse of electrodeswith U glassUse of electrodeswith U glassUse of electrodes withT glass and Idrolyteas reference electrolyteStore in storage solution6.2323.000 orreference electrolyteTry other glass typesNon-aqueous mediaDeposition of solids onmembrane surfaceReduced sensitivitySlow response, zero pointshift, slope reductionStore in waterSolventor strong acidsT glass/non-aqueouselectrolyte solutionElectrostatic chargingDeposition of proteins onmembrane surfaceSlow responseSlow response, zero pointshift, slope reductionNo dab-drying ofthe electrode5% pepsin in0.1 mol/L HClGrounding ofmeasuring instrumentPossible sources of error and care information for diaphragm problems are given in Section 1.4. for reference electrodes.1.3.2. Metal electrodesHow does a metal electrode work?Metal electrodes have an exposed metal surface. Ifions of this metal are contained in the sample solutionthen an equilibrium is formed at the metal surfacethat depends on the concentration of the metalions in the solution (see «Theory of the electricaldouble layer» in electrochemistry textbooks). Metalions are accepted by the metal surface and simultaneouslyreleased into the solution.(8)Me Me n+ +n * e – E 0 =...This concentration-dependent equilibrium is characterizedby a corresponding potential E 0 (Galvani potential),e.g. the Ag/Ag + equilibrium at a silver surfacehas a value of E 0 = 0.7999 V (25°C). If the samplesolution does not contain any ions of the correspondingmetal then metal electrodes can still form aGalvani potential if a redox reaction occurs in thesample solution.(9)S ox + n * e –S redThe electrode surface is inert to the redox reaction.No metal ions are released from the metal; in thiscase the metal surface only acts as a catalyst for theelectrons. As gold and platinum electrodes are to alarge extent chemically inert, they are used for themeasurement of redox potentials. Silver electrodesare only used as indicator electrodes for titrations.80
Page 3: 1. BASICS OF POTENTIOMETRY1.3. Meas
Page 8 and 9: Calibrating a metal electrodeRedox-
Page 10 and 11: Direct measurement or standard addi
Page 12 and 13: 1.4.2. The Metrosensor «Long Life
Page 14 and 15: Fixed ground-joint diaphragms have

What is the theoretical background of potentiometry? - Metrohm

Create successful ePaper yourself

Delete template?

Save as template?