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