What is the theoretical background of potentiometry? - Metrohm

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