What is the theoretical background of potentiometry? - Metrohm

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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
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What is the theoretical background of potentiometry? - Metrohm

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