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Experiment 122019 S AvzianovaPotentiometric Titration of the Mixture of two acidsUseful videos:IntroductionPotentiometric titration is a volumetric method in which the potential difference betweentwo electrodes is measured (reference and indicator electrode) as a function of the added reagentvolume. The change in analyte’s concentration can be monitored by measuring the cell potential.In this experiment a mixture of two acids is titrated with a standard base. Because the pH doesnot change abruptly enough at the equivalence points to allow estimation with visual indicators,a potentiometric titration is employed.TheoryThe pH meters operate by measuring the voltage generated between a reference electrodeand an indicator electrode. A reference electrode is an electrode which has a stable and well-reproductable electrode potential. There are several different types of reference electrodes.Most potentiometric methods employ the hydrogen electrode, which is based on the halfcell equation:2H+(aq) + 2e= H2(g)Practical application of the hydrogen electrode is limited by the difficulty in preparingand maintaining it, primarily due to the requirement for gaseous hydrogen H2(g).Two other common reference electrodes are the saturated calomel electrode (SCE):Hg2Cl2(s) + 2e= 2Hg (l) + 2Cl(aq)and the silver-silver chloride electrode (Ag/AgCl):AgCl (s) + e−= Ag(s) + Cl−(aq sat’d)Both the SCE and the Ag/AgCl reference electrodes offer stable half-cell potentials thatdo not change over time.The most common sensing or indicator electrode is the glass-membrane electrode used ina pH meter. This electrode is H+ion sensitive and consists of a glass bulb filled with dilute HCl
into which is inserted a silver or platinum wire. The glass bulb is permeable to H+ions but not toother ions. If the solution into which the electrode is placed is more concentrated than that in thebulb, H+ions will move from the solution into the bulb. The HCl solution inside the bulb willthen have an excess of H+ions and will be positive with respect to the solution being measured.This potential difference across the glass membrane can be measured then compared to thereference voltage and a pH determined for the measured solution.Quite often the reference and indicator electrodes are combined into one probe called acombination probe or electrode. It operates the same way as the two electrodes.Fig. 1. A combination glass electrode or probe for measuring pH.The combination electrode consists of a pH-sensitive glass membrane and an internalAg/AgCl reference electrode in a solution of 0.1 M HCl. The probe also has the sample’sreference electrode, which is a Ag/AgCl electrode in a solution of KCl (which may be saturatedwith KCl or contain a fixed concentration of KCl). A porous wick serves as a salt bridge betweenthe sample and its reference electrode (Fig. 1). Because the typical thickness of an ion-selective