Potentiometry

 Chapter 13

 Potentiometry 

Introduction 

• Different electrode systems are used in combination to measure potential or pH (hydrogen ion concentration) of a solution. A pair of electrodes is commonly required in measurement. One electrode acts as an indicator electrode, while the other as reference electrode. In order to understand this mechanism, let us consider the concept of electrode potential and its measurements.
• Electrode Potential: When a metal rod (electrode) is immersed in a solution of its own ions a potential is established between the metal rod and its ions in solution, e.g., zinc rod dipped in zinc sulphate solution. The potential difference is given by reaction.

M"* ne            M

           This potential is expressed by Nernst equation as

Where, R = gas constant, T = absolute temperature, F = Faraday, n = valency of ions, in = natural log to base 10, a M"* = activity of ions and Egg = constant dependent of metal ions.

Constructions and Working of Reference Electrodes 

• Normal Hydrogen Electrode (NHE): This consists of a glass tube (A) which has holes at its bottom. Inside this tube is another glass tube (D) with a platinum or copper wire with a platinum foil (B). The surface of the platinum foil is coated electrically with platinum black. Pure hydrogen gas at one atmosphere pressure is passed through the opening C of glass tube A and it escapes through small holes at the bottom of electrode.

Indicator Electrodes 

• In order to measure potential of a solution, an appropriate electrode is needed. This electrode measures potential when connected to a suitable reference electrode. An electrode which is used to measure potential or pH of a solution is called an indicator electrode. There are various types of indicator electrodes in use, of which some are described below.
• Hydrogen electrode: We have seen the construction of a normal hydrogen electrode. This electrode is similar to the NHE; instead of using hydrogen gas at one atmospheric pressure and dipping the platinum wire of it in a solution of acid whose pH is to be determined, one can construct a hydrogen electrode. Since this electrode is responsive to hydrogen ion concentration, it can be used for pH measurement. Platinum of the electrode does not take part in the electrochemical reaction, but only acts as the site for the transfer of electrons. The potential of the hydrogen electrode is given by:

Potentiometers 

• A linear relationship exists between pH of a solution at a given temperature with potential E, employing a suitable indicator electrode and reference electrode.

Thus,         E = K — 0.0591 pH or

AE/A pH = — 0.0591

• In instruments, the calibration in millivolts is converted into pH units by dividing by 0.0591. Resistance values of wire are so chosen that the divisor is 100. In commercial instruments (pH meters) there are auxiliary compensating devices in their electrical circuits. Most instruments for e.g. incorporates, a temperature compensating device in the form of a variable manually adjutancy resistance or a thermostat for automatic model.

Potentiometric Titrations

• Potentiometric titrations are performed for solutions which show changes in potential or pH by addition of a reagent or Titrant. In this technique, the absolute value of potential with respect to standard half-cell is not required to be known. The changes occurring during the course of addition of Titrant are sufficient. The equivalence point of reaction is shown by sudden change in potential on a plot of e.m.f. readings against the volume of titrant being added.

• Potentiometric titrations are performed for solutions which show changes in potential or pH by addition of a reagent or Titrant. In this technique, the absolute value of potential with respect to standard half cell is not required to be known. The changes occurring during the course of addition of Titrant are sufficient. The equivalence point of reaction is shown by sudden change in potential on a plot of e.m.f. readings against the volume of titrant being added.

Methods of Detecting End Point

• There are different methods adopted to locate the end point. The critical problem in location of end point is to know the point at which the quantities of reacting species are present in equivalent amounts at the equivalence point. It is generally found graphically. There are three methods used for this purpose.

• In the second method, difference of e.m.f. (E) or pH for the volume of Titrant added (AE or ApH) is plotted as ordinate versus the volume of Titrant (V) added as abscissa (Fig. 13.5 (b)). From the shape of graph, it becomes clear that maximum change in e.m.f. (E) or pH occurs at the equivalence point. The end point can be readily recorded by drawing perpendicular from the peak of the graph on volume axis (abscissa).

Type of Reactions in Potentiometric Titrations 

• Various types of titrations involving different chemical reactions are possible to be covered and followed using potentiometer with appropriate electrode systems. Some such chemical reactions are given below:

• Neutralization reactions: In this type of reaction acid can be titrated against alkali and vice-versa. The indicator electrodes used may be hydrogen, glass or antimony and calomel as a reference electrode. The accuracy or end point depends upon the magnitude in the change of e.m.f. (E) in neighborhood of equivalence point. This depends upon the concentration (amount) and the strength of acid and alkali used. Less accurate results occur when acid or alkali are very weak (K < 10-*) and dilute. Dibasic, tribasic and polybasic acids can be titrated with alkali to intermediate or full end point. This is possible provided that the dissociation constant for each stage in the titration is sufficiently apart.

•  Redox reactions: Many Redox titrations are possible using potentiometer provided no heating or cooling in the chemical reaction is involved. Indicator electrode most commonly employed is platinum. The potential of the indicator electrode is a function of the ratio of oxidized and reduced forms of an ion. The potential of indicator electrode is given by the expression.

• Where Eg is the standard oxidation potential (or reduced potential) of the system. The equivalence point is indicated by sudden inflection in the titration curve. From the standard values of reduction potential and the observed values of potential, the ratios of concentrations of oxidation / reduction can be determined.
•  Precipitation reactions: Potentiometric titrations are possible for certain types of reactions involving precipitations. The solubility product of the almost insoluble material formed during a precipitation reaction determines the ionic concentration at the equivalence point. The indicator electrode must rapidly come into equilibrium with one of the ions.

Dead Stop - End Point Method 

• Determination of moisture or water by Karl Fischer reagent method is most commonly carried out by dead stop end point technique. This method can be adopted for many chemical reactions. The principle of this method is: In a uniformly stirred solution of an analyte when two small but similar platinum electrodes are dipped and a small potential Lon Selective Electrodes.
• In the titration of iodine against thiosulphate, when two platinum electrodes are immersed in the iodine solution and connected to the battery appreciable current flows through cell (12 + 2e = 21). The amount of oxidized form reduced at cathode is equal to that formed by oxidation of the reduced form at anode.