What is the theoretical background of potentiometry? - Metrohm

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Direct measurement or standard addition?The question often arises as to which determinationmethod is most suitable for a particular sample. Inprinciple there are three different ways of carryingout an ion measurement with ion-selective electrodes:Direct measurementDirect measurement is chiefly of benefit with highsample throughputs or with a known sample solutionof a simple composition. The ion-selectiveelectrode is calibrated with special standard solutionsof the ion to be measured before the measurementitself in a similar way to the calibrationof a pH glass electrode and can then be used forseveral determinations in series.Standard additionStandard addition is recommended whenever adetermination only needs to be carried out occasionallyor when the composition of the sample isunknown. Defined volumes of a standard solutionof the ion to be measured are added to thesample solution in several steps. The concentrationin the original solution can then be calculatedfrom the initial potential and the individual potentialsteps after the addition of the standard. Theadvantage of standard addition is that the ISE iscalibrated directly in the sample solution, whicheliminates all matrix effects.Sample additionSimilar to standard addition, with the differencethat defined volumes of the sample solution areadded to a defined amount of an ion standard.Modern ion meters such as the 781 pH/Ion Meterfrom Metrohm can carry out these addition methodsautomatically. The addition of the standard or samplesolution is automatically controlled from the ion meter– by pressing a single key – and evaluated by usingthe Nikolsky equation.ISA and TISAB – when and why?The activity coefficient of an ion (Section 1.2.) is afunction of the total electrolyte content. For this reasoncare must be taken that ion-selective measurementsare always carried out in solutions with approximatelythe same ionic strength. In order to achievethis, the so-called ISA solutions (Ionic Strength Adjustor)or TISAB solutions (Total Ionic Strength AdjustmentBuffer) should be added to the sample solution(see Table 7). These are chemically inert and havesuch a high ionic strength that the ionic strength ofthe sample solution can be neglected after their addition.Table 7: Interfering ions and recommended ISA and TISAB solutions for ion-selective electrodesIonAg +Br –Ca 2+Cd 2+Cl –CN –Cu 2+F –I –K +Na +Na ++NH 4–NO 3Pb 2+S 2–SCN –MembranematerialCrystalCrystalPolymerCrystalCrystalCrystalCrystalCrystalCrystalPolymerGlassPolymerGas membranePolymerCrystalCrystalCrystalpH range 12...80...142...122...120...1410...142...125...70...142.5...115...93...12112.5…14...72...122...10ISAor TISAB 2c(KNO 3 ) = 2 mol/Lc(KNO 3 ) = 2 mol/Lc(KCl) = 1 mol/Lc(KNO 3 ) = 5 mol/Lc(KNO 3 ) = 2 mol/Lc(NaOH) = 0.1 mol/Lc(KNO 3 ) = 1 mol/LNaCl/glacial acetic acid/CDTAc(KNO 3 ) = 2 mol/Lc(NaCl) = 0.1…1 mol/LC(TRIS) = 1 mol/Lc(CaCl 2 ) = 1 mol/L–c((NH 4 ) 2 SO 4 ) = 2 mol/Lc(NaClO 4·H 2 O) = 1 mol/Lc(NaOH) = 2 mol/Lc(KNO 3 ) = 1 mol/LMost importantinterfering ions 3Hg 2+ , proteinsHg 2+ ,Cl – , I – ,S 2– , CN –Pb 2+ , Fe 2+ , Zn 2+ , Cu 2+ , Mg 2+Ag + , Hg 2+ , Cu 2+Hg 2+ , Br – , I – ,S 2– , S 2 O 32–,CN –Cl – , Br – , I – ,Ag + , Hg 2+ , S 2–OH –Hg 2+ , S 2– , S 2 O 32–,TRIS + , NH 4+ Cs + , H +H + ,Li + , K + , Ag +SCN – , K + , lipophilic ions–Cl – , Br – , NO 2– , OAC –Ag + , Hg 2+ , Cu 2+Hg 2+ , proteinsCl – , Br – , I – , S 2– , S 2 O 32–,CN –Remarks1) The given pH rangeonly applies to ionselectiveelectrodesfrom Metrohm Ltd.2) Alternatives or moredetailed compositionscan be foundin the manual «IonSelective Electrodes(ISE)», order number8.109.14763) More detailedinformation aboutinterfering ions andother interferencescan be found inthe manual «IonSelective Electrodes(ISE)», order number8.109.147683
1. BASICS OF POTENTIOMETRYTroubleshootingTable 8: Possible sources of interference and remedies for ion-selective electrodesElectrodeIon-selective crystalmembraneIon-selective polymermembraneSource of interferenceDissolution processes,oxidation processesElectrode poisonsDissolution processesEffectsRough surface ➝slow response,poor detection limitsFormation of more sparingly solublesalts on the electrode surface thanwith the ion to be measured ➝ zeropoint shift, reduced linearity rangeDiffusion into the membrane ordissolution of membrane componentsCleaningPolish withpolishing clothPolish withpolishing cloth,mask interfering ionEliminate interferingcomponentsNH 3 sensorVolatile bases (amines)SurfactantsElectrolyte becomes contaminated ➝displacement of calibration line,limited linearityMembrane becomes wetted ➝slow responseChange electrolyteReplace membrane1.4. Reference electrodesReference electrodes are usually electrodes of thesecond kind. In this type of electrode a metal electrodeis in contact with a sparingly soluble salt of thesame metal. The potential depends only on the solubilityof the salt. As a first approximation, electrodesof the second kind do not themselves react with thesolution and therefore supply a constant potential.The most frequently used reference electrode is thesilver/silver chloride reference electrode (Ag/AgCl/KCl). The calomel electrode (Hg/Hg 2 Cl 2 /KCl), whichwas formerly widely used, is hardly used at all todayas mercury and its salts are extremely toxic and all theapplications can also be carried out with the silver/silver chloride reference electrode. The standardhydrogen electrode SHE is also an electrode of the secondkind. It is only used for calibration purposes.Some titrations offer the possibility of using pH glasselectrodes as reference electrodes. Even if protons aretransferred during the titration it is usually still possibleto make an accurate determination of the endpoint.1.4.1. Silver/silver chloride reference electrodeThe reference element of the silver/silver chloride referenceelectrode is the silver/silver chloride/potassiumchloride solution system: Ag/AgCl/KCl. The referenceelectrode is usually filled with c(KCl) = 3 mol/Lor saturated KCl solution. Tables 9 and 10 show thepotentials of the reference electrode as a function ofthe reference electrolyte and temperature. Each ofthese values has been measured against the standardhydrogen electrode under isothermal conditions.Table 9: Standard redox potentials of the silver/silver chloride reference electrode as a function of the temperature and concentrationTemp. (°C) 0 +10 +20 +25 +30 +40 +50 +60 +70 +80 +90 +95E 0 (mV) with +224.2 +217.4 +210.5 +207.0 +203.4 +196.1 +188.4 +180.3 +172.1 +163.1 +153.3 +148.1c(KCl) = 3 mol/LE 0 (mV) with +220.5 +211.5 +201.9 +197.0 +191.9 +181.4 +170.7 +159.8 +148.8 +137.8 +126.9 +121.5c(KCl) = sat.Table 10: Standard redox potentials of the silver/silver chloride reference electrode as a function of the concentrationc(KCl) / mol/L (25 °C) 0.1 1.0 3.0 3.5 sat.E 0 (mV) +291.6 +236.3 +207.0 +203.7 +197.084
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What is the theoretical background of potentiometry? - Metrohm

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