What is the theoretical background of potentiometry? - Metrohm

1. BASICS OF POTENTIOMETRY1.3. Measuring electrodes1.3.1. pH glass electrodesHow does a pH glass electrode work?The glass membrane of a pH glass electrode consistsof a silicate framework containing lithium ions. Whena glass surface is immersed in an aqueous solutionthen a thin solvated layer (gel layer) is formed on theglass surface in which the glass structure is softer.This applies to both the outside and inside of theglass membrane. As the proton concentration in theinner buffer of the electrode is constant (pH = 7), astationary condition is established on the inner surfaceof the glass membrane. In contrast, if the protonconcentration in the measuring solution changesthen ion exchange will occur in the outer solvatedlayer and cause an alteration in the potential at theglass membrane. Only when this ion exchange hasachieved a stable condition will the potential of theglass electrode also be constant. This means that theresponse time of a glass electrode always depends onthe thickness of the solvated layer. Continuous contactwith aqueous solutions causes the thickness ofthe solvated layer to increase continuously – even ifonly very slowly – which results in longer responsetimes. This is why conditioning the electrode in a suitableelectrolyte is absolutely necessary to ensure aninitial solvated layer condition that is as stationary aspossible so that results can be obtained that are as reproducibleas possible.Figure 3:The silicate skeleton of the glass membrane contains lithium ions,among other things. During the formation of the solvated layerat the glass surface these are partly replaced by protons. If the concentrationof the protons in the solution changes then a new stationarycondition must again be achieved in the solvated layer; thisresults in a change in potential at the glass membrane.Why are there different types of glass forpH electrodes?Different demands are placed on a pH glass electrodedepending on the particular application. Various propertiessuch as response time, thermal resistance,chemical stability, shape, size and electrical propertiesmust all be taken into account in order to have anoptimal electrode available to solve each problem. Inorder to be able to do justice to the numerous applications,different glasses are available with the followingproperties:Table 1: Overview of the different electrode membrane glasses used by Metrohm LtdApplicationpH rangeTemperature rangecontinuousshort-termMembranesurfaceSpecial featuresMembraneresistance(MΩ)U glass(green)0...140...80 °C0...100 °CElectrodeswith largemembranesurfaceFor stronglyalkaline solutions,long-termmeasurementsand measurementsat hightemperatures150...500T glass(blue)0...140...80 °CElectrodes withmedium to largemembranesurface(mini-electrodes)Measurementsin non-aqueoussample solutions40...600M glass(colorless)0...140...60 °CElectrodeswith smallmembranesurface(microelectrodes)Measurementsinsmall-volumesamples300...700Aquatrode glass(colorless)0...130...80 °CLarge surfacesResponds veryquickly, soparticularly suitablefor measurementsin ion-deficient orweakly bufferedsolutions80...200E glass(yellow)0...130...80 °CElectrodes withmedium to largemembranesurfaceQuick response,excellent stabilityin continuoususe50...30076

1. BASICS OF POTENTIOMETRYconditions (z = 1, T = 298.15 K) the Nernst potentialU N is equal to 59.16 mV. For other temperatures it canbe corrected in the Nernst equation by using Table 2.Modern pH meters automatically take the temperaturedependency of the Nernst potential into accountif a temperature sensor is connected. In principle,within the context of GLP/ISO recording and documentationof the temperature is required for all measurements.However, it must be remembered that a pH meter canonly correct the temperature behavior of the electrodeand never that of the solution to be measured.For correct pH measurements it is essential that thepH is measured at the temperature at which thesample was taken. For example, sodium hydroxidec(NaOH) = 0.001 mol/L at 0 °C has a pH of 11.94, at50 °C it is pH = 10.26 and only at 25 °C is it pH =11.00. This change in pH is caused by the dependencyof the ionic product of water on the temperature.In some conventional electrodes the temperature sensoris not located in the immediate vicinity of themembrane, i.e. in the electrode foot. This means thatit cannot measure the temperature of the solutioncorrectly and that the pH compensation will be incorrectas the temperature and pH are not measuredat the same location. In modern pH electrodes thetemperature sensor should be located within theelectrode in the immediate vicinity of the glass membrane.This is the only way in which an accurate pHmeasurement is possible. If the sensor is located outsidethe membrane then problems when cleaning theelectrode could easily occur. The temperature sensorshould also satisfy the requirements of DIN/IEC 751for class B temperature sensors at the very minimum.Figure 6:Isothermal intersection pointHow to store a pH glass electrode?The swelling of the glass surface is indispensable forthe use of glass as membrane for pH glass electrodes;without this solvated layer, no pH measurementwould be possible. Glasses for pH glass electrodes areoptimized in such a way that only protons can penetrateinto the glass membrane. However, because ofthe very slow but steady swelling of the glass, it isunavoidable that also other ions penetrate into theglass, e.g. sodium and potassium ions. At higher concentrations,these lead to the so-called alkali error ofthe glass electrode. This means that the measuredvalue is falsified at comparatively low proton concentrations.If the glass electrode is stored for a very longtime in a strong solution of potassium or sodium, thisleads to prolonged response times of the glass membranesince the protons must expulse the «addedions» from the solvated layer.Table 2:Dependency of the Nernst potential U N on the temperatureTemperature T Slope U N Temperature T Slope U N(°C) (mV) (°C) (mV)0 54.20 50 64.125 55.19 55 65.1110 56.18 60 66.1015 57.17 65 67.0920 58.16 70 68.0825 59.16 75 69.0730 60.15 80 70.0735 61.14 85 71.0637 61.54 90 72.0540 62.13 95 73.0445 63.12 100 74.03Figure 7:Cross-section of a pH glass membrane. If several kinds of cationsare present in the measuring solution, these compete for the freespaces in the solvated layer. Especially potassium and sodium canpenetrate into the glass membrane and prolong the response time78

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

Calibrating a metal electrodeRedox-buffer solutions (6.2306.020) are used forquickly checking metal or redox electrodes. As thepotential measured in a redox buffer solution is insensitiveto the electrode's surface condition, contaminationof the metal electrode is often not recognized.For this reason redox-buffer solutions are rather moresuitable for checking the reference electrode. If thepotential is displaced then the metal electrode iscontaminated, the redox buffer partly oxidized or thefunctioning of the reference electrode is affected.Under no circumstances should the indicated potentialbe set to the theoretical value.If measurements are made in weakly redox-bufferedsolutions then a suitable pretreatment of the metalelectrode is recommended to adapt the surface conditionas much as possible to the measurement conditions(abrasive pretreatment: carefully clean theelectrode with abrasive paste, reductive or oxidativepretreatment: switch electrode as anode or cathode;see also electrode data sheet). The reference electrodecan either be checked against a second referenceelectrode that has already been checked inbuffer solutions 4 and 7 (response behavior and referencepotential) or by using the redox buffer.In the literature the so-called standard redox potentialsE 0 can usually be foundCl 2 (g) + 2e - → 2 Cl -Fe 3+ + e - → Fe 2+Cd 2+ + 2e - → Cd 2-E 0 = +1.359 VE 0 = +0.771 VE 0 = -0.403 VThe zero point of these systems is defined (arbitrarily)with the standard hydrogen electrode (SHE) which isassigned a standard potential of 0 mV. If electrons arereleased by a redox system to the SHE then this isreduced and the redox pair receives a negative sign;if electrons are accepted then the SHE is oxidized andthe result is a redox potential with a positive sign. Thestandard hydrogen reference electrode is difficult tohandle. The specifications of the SHE stipulate that aplatinized platinum wire must be used; this is locatedin a stream of hydrogen gas at a partial hydrogenpressure of 1.0 bar, and that the activity of the hydrogenions in the solution in which the platinizedplatinum wire is immersed is to be exactly 1.00 mol/L.The normal alternative is the Ag/AgCl/KCl referenceelectrode, which has a potential E 0 = +0.2076 V atc(KCl) = 3 mol/L and T = 25 °C. The Metrohm redoxstandard (6.2306.020) can be used for checking separateand combined metal electrodes. Platinum andgold electrodes together with the Ag/AgCl/KCl referenceelectrode (c(KCl) = 3 mol/L and T = 20 °C)produce a potential of +250 ± 5 mV.Table 5: Measuring data for 6.2306.020 redox standard as a function of the temperatureTemp. (°C) 10 20 25 30 40 50 60 70mV ± 5 +265 +250 +243 +236 +221 +207 +183 +178pH ± 0.05 7.06 7.02 7.00 6.99 6.98 6.97 6.97 6.98If instead of an Ag/AgCl/KCl reference electrodec(KCl) = 3 mol/L an Ag/AgCl/KCl reference electrodec(KCl) = sat. is used for the measurement then at25 °C a correction of +10 mV must be applied; if themeasurement is made using an Hg/Hg 2 Cl 2 /KCl calomelreference electrode, which for toxicological reasonsis no longer available from Metrohm, the correctionto be applied is -37 mV.The Titrodes are checked by a standard titration as nosuitable calibration or buffer solutions are available.For example, the certified ion standard c(NaCl) = 0.1mol/L (6.2301.010) can be titrated with a silver nitratestandard solution.81

1. BASICS OF POTENTIOMETRYTroubleshootingTable 6: Problems encountered when measuring with metal electrodesElectrodeAgPt/AuSource of errorElectrode poisonssuch as S 2– , I - , Br -Fats or oilsWeakly redoxbufferedsolutionCOD determinationEffectsPassivation ofAg layer ➝slow responseIsolating layer ➝slow response,incorrect potentialAdsorbed ionson the surface(e.g. oxides) ➝slow responseDeactivation of PtCleaningCleaning withabrasivesCleaning withsolventAbrasive, oxidative(for oxidizingsolutions) orreducing (forreducing solutions)pretreatmentAlternativesUse of Au or PtUse of Au1.3.3. Ion-selective electrodesHow does an ion-selective electrode work?An ion-selective electrode (ISE) can selectively recognizean ion in a mixture of ions in a solution. There arevarious types of ion-selective electrodes, the mostcommonly used ones are:Glass membranewithCrystal membranePolymer membraneframework of silicate glassinterstitial sites for H + andNa +crystal lattice containingdefined gaps for the ion to bemeasuredpolymer membranecontaining a molecule(= ionophore) that only bindsthe ion to be measuredIn contrast to metal electrodes, an ISE does not measurea redox potential. If the ion to be measured iscontained in the sample solution then this ion canpenetrate the membrane. This alters the electrochemicalproperties of the membrane and causes achange in potential. One hundred percent selectivityfor exactly one type of ion is only possible on rareoccasions. Most ion-selective electrodes have «only»a particular sensitivity for a special type of ion, butalso often react with ions with similar chemical propertiesor a similar structure (see Table 7). This is whythe cross-sensitivity to other ions that may be containedin the sample solution must always be takeninto consideration when selecting an ISE. One of thebest-known examples of such a cross-sensitivity is theso-called alkali error of pH glass electrodes. Withsome types of glass the linear range does not extendthroughout the whole pH range from 0 to 14 and athigh pH values a departure from linear behavior canbe observed. The reason for this is that at very lowH + -concentrations any alkali ions present in the solution(possibly released from the walls of the vessel)will falsify the measured value. Unfortunately thereare only a very few ion-selective electrodes that havea linear range similar to that of pH glass electrodes.The use of an ISE is normally restricted to a concentrationrange of 6 to 8 powers of ten. If an ISE is usedfor a measurement right at the limit of the linearrange then the Nernst equation (Eq. (5), Section 1.2.)must be extended by the contribution made by theparticular interfering ion for the evaluation of themeasured potential:U = U 0 + 2.303 * R * T * log (a M +K S*a S )z * F(Nikolsky equation) (10)K S is the so-called selectivity coefficient of the ionselectiveelectrode for interfering ion S. This is a factorthat describes the influence of the interfering ion inrelationship to the ion to be measured. These selectivitycoefficients are known for the most importantinterfering ions for an ISE and therefore a simpleestimation can be made as to whether an interferingion contained in the sample solution will influencethe measured value or not.82

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

1.4.2. The Metrosensor «Long Life» referencesystemMost electrodes are equipped with the silver/silverchloride reference system. The solubility product ofsilver chloride in water is very small (10 -10 mol/L). Inthe concentrated, chloride-containing solution of thereference electrolyte soluble complexes of the series(AgCl 2 ) - , (AgCl 3 ) 2- , (AgCl 4 ) 3- are formed. This meansthat the reference system poses several problems.Outside the electrode the chloride concentration isfrequently lower and the complexed silver chlorideprecipitates in the region surrounding the diaphragm(«liquid junction»). The result: precipitated silver chlorideblocks the diaphragm, the response time of thepH electrode increases and with time the electrodebecomes inactive. A further problem is presented bythe dependency of the solubility product of AgCl onthe temperature. If the electrode is used at a differenttemperature then the equilibrium that determines thepotential of the reference electrode must be reestablished.The larger the surface with solid AgCl in relationshipto the electrolyte volume, the shorter thetime required. The «Long Life» reference system preventshigh concentrations of complexed AgCl fromoccurring in the outer electrolyte, as the silver chloridereservoir is connected with the outer electrolyteby a highly effective diffusion barrier. The concentrationof the silver complex in the reference electrolyteremains low. Even after one year the concentrationof silver chloride in the outer electrolyte has onlyreached 5% of the saturation value.Figure 9:Conventional Ag/AgCl/KClsystem. The chloride concentrationoutside the electrodeis usually lower than in theelectrolyte chamber. Thesoluble silver chloride complexesprecipitate out inthe region surrounding thediaphragm and may block it.Figure 10:The Metrohm «Long Life»reference system. Thedissolved AgCl is retainedin the AgCl cartridge andcan no longer block thediaphragm.The advantages of the «Long Life» referencesystems at a glance:• Long working life of the electrode• Rapid response to changes in pH• Rapid response to temperature changes• Greatly reduced reaction between Ag + and samplesolution, e.g. if this latter contains sulfide ionsBlocking the diaphragm by crystallized AgCl alsoaffects the electrolyte flow. If the «Long Life» referencesystem is used then the flow of the KCl solutionthrough the diaphragm into deionized water onlydecreases slightly. In a conventional electrode theflow is reduced by more than 90% in only 10 days.As in the «Long Life» reference system the silver chlorideis present in a smaller volume of potassium chloridesolution, the thermodynamic equilibrium betweensilver, silver chloride (solid) and silver chloride(dissolved) is established very quickly and the potentialof the reference electrode becomes stable after avery short time.1.4.3. DiaphragmsFaulty measurements, unstable measured values andvery long response times usually have their source inthe «liquid junction» between the sample solutionand the reference electrode. The diffusion, streamingand Donnan potentials that occur there – which arenormally known together as the diaphragm potential– have various causes and can result in a very incorrectmeasured value. The measuring error mayassume vast proportions if measurements are madeunder the following conditions:• with a blocked, virtually impermeable diaphragm,• in ion-deficient solutions with an unsuitablediaphragm,• in strong acids and bases with an unsuitablediaphragm,• in colloidal solutions.In all such cases errors may occur that cannot betolerated. This is why the following questions must bein the foreground whenever an electrode and thereforethe optimal type of diaphragm are to be selected:• Does the reference electrolyte react with thesample solution to form a precipitate in thediaphragm?• Does the electrolyte flow alter the compositionof the sample solution in an unacceptable way?• Is there a risk of depositing sample solutioncomponents on the diaphragm?• Is the chemical resistance assured?85

1. BASICS OF POTENTIOMETRY• Can physical parameters such as flow, pressure ortemperature cause measuring errors?• Does the process allow cleaning/maintenance ofthe electrode at certain intervals?• Is a short response time and/or high reproducibilitynecessary?The time required for cleaning and maintenance canusually be considerably reduced if the correct choiceof electrode is made. The most frequent cause ofmeasuring problems is contamination of the diaphragm.This is why with pH electrodes the chief attentionis paid to the diaphragm during maintenancewith the pH membrane being of secondary importance.Each unnecessary etching process acceleratesthe aging of the pH electrode. If existing means cannotbe used to determine whether the indicator electrodeor the reference electrode requires cleaning/regeneration, then it is usually best to treat thereference electrode. Various types of diaphragm areavailable to satisfy the diverse requirements. These requirementshave already been taken into considerationfor the electrode recommendations in the applicationlists on pages 6 and 7.remain negligibly small. These properties are particularlyimportant for a SET titration to a defined pHor potential value. For example: the determination ofthe carbonate alkalinity by a SET titration to pH = 5.4according to ISO 9963-2 is a widely used method inthe routine analysis of drinking water. During a titrationit is not possible to dispense with stirring, i.e.with an incorrectly measured pH or potential at thestart of the titration an incorrect endpoint is the inevitableresult. Figures 11 and 12 clearly show the differencebetween the Aquatrode Plus (6.0253.100),which was specially developed for this application,and a conventional pH glass electrode with ceramicpin diaphragm.Ceramic pin diaphragmsCeramic pin diaphragms are the most frequently useddiaphragms. They are primarily suitable for clear,aqueous sample solutions. They normally have porediameters of up to 1 μm with a length and diametereach of about 1 mm. This results in an electrolyteflow rate of up to 25 μL/h, depending on the conditionof the diaphragm. This means that the referenceelectrolyte only requires refilling at long intervals; thisis why electrodes with ceramic pin diaphragms areparticularly suitable for long-term measurements. Onthe other hand, because of their small pores andlarge polar surface (>>500 mm 2 ), ceramic diaphragmstend to become blocked and therefore should not beused in solutions containing precipitates. An importantadvance with regard to the prevention of diaphragmblockages by silver chloride and silver sulfidehas been achieved by the introduction of the «LongLife» reference system (see Section «The Metrosensor’Long Life’ reference system»).Ground-joint diaphragms with fixed orseparable ground jointGround-joint diaphragms with fixed or separableground joint are used in ion-deficient media, amongothers, as they produce a steady signal that is almostindependent of sample flow conditions. The risk ofblockage by silver chloride or by precipitates formedin the sample solution is relatively low because of thelarge surface area. Streaming potentials, which mayoccur in measurements in flowing or stirred solutions,Figure 11:Measured pH of a solution with c(Na 2 CO 3 ) = 0.14 mmol/L. Evenunder vigorous stirring the Aquatrode Plus deviates by only approx.0.05 pH units (corresponding to approx. 3 mV) from the unstirredvalue, in contrast the pH glass electrode with ceramic pin diaphragmdeviates by approx. 0.2 pH units.Figure 12:Endpoint volumes of a SET titration of a solution with c(Na 2 CO 3 ) =0.14 mmol/L with the titrant c(H 2 SO 4 ) = 0.035 mol/L to pH 5.4. Theendpoints of the Aquatrode Plus are virtually independentof the stirring speed. At higher stirring speeds the deviation fromthe theoretical value of the pH electrode with ceramic diaphragmamounts to approx. 5%.86

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