What is the theoretical background of potentiometry? - Metrohm

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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
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What is the theoretical background of potentiometry? - Metrohm

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