11 Potentiometry

Dr. Heena Rekhi Rekhi

1. Description

Potentiometry is a classical analytical technique with roots before the 20th century. It is based on the measurement of potential of an electrode system. In the presence of multitude of other substances it enables the selective detection of ions. It is one of the electrochemical analytical methods. An electric circuit is used to measure the current and potential created by the flow of charged particles. For complete electrochemical cell from which the negligible current is drawn the EMF is given by:

Ecell = Eind- Eref + Ej

where, Eind, Eref and Ej are the potentials of the indicator electrode, reference electrode and liquid junction potential respectively. The potentiometric methods cuddle two major type of an analysis. One is direct measurement which involves the direct measurement of an electrode potential from which the activity of an active ion may be derived. Other kind is indirect potentiometric method which measures the variation in EMF brought by the addition of a titrant to the sample.

Description of charge transfer process is as follows:

• Movement of electrons from zinc electrode to copper electrode.
• In the solution zinc ions move away from the electrode and sulfate ions move towards.
• Positive ions move towards right and negative towards left in salt bridge.
• On the surface of electrodes electrons are transferred to ions.

5. Zinc dissolves and metallic copper deposits.

Fundamentals of Potentiometry

The electrode potential is developed and given by Nernst equation whenever metal ion immersed in asolution containing its own ions Mn+.

E = E° + (RT/nF) ln aMn+

The value of an electrode potential E can be established by linking the calomel electrode and measuring the EMF of the resultant cell. It can be deduced form the reference electrode. It is also possible to measure the potential directly via direct potentiometer. This involves the use of an electrode of second kind. For example Ag-AgCl electrode formed by coating a silver wire with AgCl. This measures the concentration of chloride ion in the solution. The basic functions of potentiometer are as follows:

Furthermore, the requirements and methodologies used to accomplish the analysis are modified day to day by the researchers working in this field to improve the routine analytical methods. The equipment required for direct potentiometric measurements includes an ion-selective electrode,a reference electrode, and a potential-measuring device. The reference electrode should provide a highly stable potential for an extended period of time. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Ion-selective electrodesare  mainly  membrane-based  devices,consisting of selective ion-conducting materials,Potentiometer which separate the sample from the inside of the electrode. It is necessary to select an appropriate electrode both indicator and reference depending on chemically reacting components in various titrations. Reference electrode is the electrode with a potential which is an independent of concentration and temperature. It must be reversible and obeys the Nernst equation. It gives the stable potential with time and always returns to its original position after the passage. The common reference electrode used in potentiometer is calomel electrode Hg/Hg2Cl2 (satd), KCl (xM) and the half cell reaction is as follows:

Hg2Cl2 (s) + 2e- ↔ 2Hg + 2Cl-

A reference electrode, Eref, is a half-cell having a known potential that remains constant at constant temperature and independent of the composition of the analyte solution. Another is an indicator electrode having a potential that varies with variations in the concentration of an analyte. Metallic indicator electrode and membrane electrodes are types of indicator electrodes.

2. Instrumentation

The measurements of potential are generally employed from the non-electronic, electronic and automatic instruments. Every kind of potentiometric measurement requires a suitable galvanic cell consisting of an indicator electrode and a stable reference electrode. The reference electrode may be immersed directly in the sample solution or brought in contact with it through a salt bridge. The type of an indicator electrodes are either noble metal ions or ion selective electrodes.

The commonly used electrodes systems in potentiometry are described as follows:

Reference electrodes: The reference electrode has a potential that is known accurately, constant and completely insensitive to the composition of an analyte solution. When the demand of current is made upon it, the value of a potential must not depart from its equilibrium value. This kind of an electrode consists of an internal element, salt bridge electrolyte, setting up the fluid junction potential. Potentials are quoted with reference to standard hydrogen electrode termed as a primary reference electrode. In addition, this electrode should be rugged and easy to assemble and maintain a constant potential while passing minimal currents. Reproducibility factor involves the two aspects one is reference electrode should respond according to the Nernst equation and the second is feasibility of establishing an easy and standard method of electrode preparation. For example calomel electrode can be represented schematically as

Hg|Hg2Cl2(satd),KCl(xM)||

Where, x represents the molar concentration of potassium chloride in the solution. It consists of metallic mercury and calomel (mercury chloride) in equilibrium with solution of KCl solution of definite concentration. The most commonly used electrode in saturated calomel electrode (SCE) because of the suppressive effect of saturated KCl solution on liquid junction potentials. Other kind of

electrodes decimolar and molar electrodes are preferred in cases where accurate values of electrode potentials are required. A calomel half cell is represented as:

||Hg2Cl2 (satd), KCl (M) ||Hg

All the electrodes are same differ only in their potassium chloride concentrations, all are saturated with calomel electrode. In the modern electrodes an ion exchange membrane is incorporated in lower part of the electrode which prevents migration of mercury ions to sintered disc and hence to the test solution. Here the main function of this double junction is to prevent the ingress of ions from the test solution which may interfere with the electrode.

Mercury mercuric oxide electrode: It consists of mercury pool in contact with a solution of NaOH or KOH saturated with HgO.

All the calomel electrodes are subjected problems arising from accelerated disproportionation of mercury (I) to mercury (II) at higher temperatures.

The silver-silver chloride electrode: An electrode is immersed in a solution that is saturated in both potassium chloride and silver chloride

Ag|AgCl (satd), KCl (satd)||

It consists of a silver wire or a silver plated platinum wire coated electrolytically with a thin layer of AgCl, dipping into a KCl solution of known concentration. Ion exchange membranes and double junctions are used to prevent the clogging of sintered disc in Ag-AgCl electrode. The main part of drawback of this kind of an electrode includes not applicable for the determination of proteins, sulphide, bromide and iodide. The strong reducing agents should also be avoided because they can reduce the Ag+ ions to Ag metal at liquid junction potential.

Indicator electrodes: The potential depends on the activity of a particular ionic species which is desired to quantify in an indicator electrodes. For anions it may take the form of a gas electrode. This responds rapidly and reproducibly to the changes in concentration of an analyte ion. These are of various types as described below:

Metallic indicator electrode: Metallic indicators are classified further into electrodes of first kind, electrodes of second kind and an inert redox electrode. First kind is a pure electrode that is direct equilibrium with its cations in the solution. The equilibrium between the metal and a cation is represented as:

Thus silver electrode can serve as an indicator electrode of the second kind for chloride ion. Note that the sign of the logarithmic term for an electrode of this kind is opposite to that for an electrode of the first kind.

An inert conductor responds to the redox systems only. To monitor the redox systems platinum, gold, palladium and carbon can be used. The potential of platinum electrode immersed in solution written as follows

It is a convenient electrode used for the titrations involving standard cerium (IV) solutions.

3. Potentiometric titrations

The measurement of an analyte can be done by the titration in which the potential of an indicator electrode is measured as a function of the volume of titrant added. This is known as potentiometric titration. Sudden change in potential in the plots of emf against volume of titrating solution reveals the end point. A typical potentiometric curve is represented as below:

Procedure of Potentiometric titration

A suitable reference and an indicator electrode are required. The solution to be titrated is contained in a beaker and a titrant is added from the burette into a titration vessel. The moisture is removed from the solution by passing inert gas. Till end point the amount of titrant is added successively to the container containing the solution. The potential is noted after each addition. After each addition a sufficient time should be allowed to reach constant potential before the next increment. The value of potential or emf is measured by the pH meter or with an automated recording device.

Features of titration curve

The very steep part of the graph is the region in which the equivalence point is found. The point at which the slope becomes greatest is defined as the equivalence point. To determine this graphically, plot the graph manually, using gridlines to simulate graph paper. The main features of titration curve are as described below:

oxidizing and reducing agent must have large difference. A reversible redox system may be represented as:

Oxidant + ne- = Reductant

The Electrochemical series enlists a number of systems according to decreasing standard reduction potentials at 25°C. The most powerful oxidizing agents lie at the top and the most powerful reducing agents are present at the bottom of electrochemical series. The oxidizing agent is generally placed in a burette. The titration curve is characterized by the sudden change of potential at the equivalence point. These may be used in procedures such as monitoring of cyanides, manufacturing bleach compounds for paper industries. It is also of a great use in pharmaceutical drugs, alkaloids, in agriculture, water treatment, sewage treatment also has a wide applicability of these redox titrations. For example KMnO4 in the presence of dilute H2SO4.

Neutralization titrations: Neutralization titrations are performed with standard solutions of strong acids or strong bases. A standard solution is a reagent of exactly known concentration. These are useful where visual end point detection is not permitted by the presence of coloured species or turbidity. The strength of an acid and base will depict the equivalence point. The standard solution used in the titration. Two basic methods are used to determine the concentration of standard solutions. One is direct method, a carefully weighed quantity of a primary standard is dissolved and diluted to an exactly known volume in a volumetric flask. In the second method, the solution is standardized by titrating a weighed quantity of a primary and secondary standard. The hydronium ion concentrations are widely different for various indicators. A list of common acid base indicators is described below:

Precipitation Titrations: The titrations based upon the formation of slightly soluble precipitate are termed as precipitation titration. This titration is very important because it is perfect method for the determination of halogens and metal ions. The magnitude of change in potential at the equivalence point depends on solubility of the substance being precipitated. The titration curve is influenced by Ksp value. When the value of Ksp is small the titration curve is perfect. The indicator used in titration of chloride ion is chromate ion in which the end point is being signaled by the appearance of red silver chromate. It may be represented as follows:

Hg| Hg2Cl2(s) |KNO3(F) | Chloride sample solution |Ag

The KNO3 salt bridge is introduced between the sample solution and saturated calomel electrode so that KCl solution doesnot diffuse into the sample.

Complexometric Titrations: The formation of complex results from the interaction of a sparingly soluble precipitate with excess of a reagent. For example when silver nitrate is treated with potassium cyanide, the initially produced AgCN dissolves in excess KCN. The reaction continues till all the cyanide ions have been converted into complex ion, the increase in concentration means the gradual increase in free silver ions. It leads to increase in potential which determines the equivalence point. But if addition continued even after this point emf changes very slowly and AgCN is precipitated.

Differential titrations: In order to increase the accuracy attainable in the titration of acids and bases, it is desirable to have precise methods for evaluating the purity of the several substances that are commonly employed as acidimetric and alkalimetric standards. The technique requires two identical electrodes, one of which is well shielded from the bulk of the solution and other is contained in side arm test tube. Due to the restricted access by the addition of a titrant the composition around the shielded electrode hardly alters. The small difference in solution composition gives rise to potential difference between the electrodes. At the end point the value of potential difference is maximum. The elimination of reference electrode and salt bridge makes it more fruitful titration.

Automatics titrations: The Automatic titrator is provided with two-point auto calibration and standardization. The instrument is capable of displaying pH and mV of the sample, with temperature compensation. The Automatic titrator can accept a variety of electrodes to cater to various applications in different fields. This kind of instrumentation is ideal for performing the multiple analysis in which the fundamental analytical procedure remains fixed as in quality control situation. The entire titration can be performed automatically by the titrators equipped with microcomputers, digital convertors and using dedicated software.

4. Real life applications

Potentiometry represents a powerful technique with a myriad of different applications in biology, physics as well as chemistry. It was also used for the respiratory gas analysis in the hospitals consisting of respired gas samples from the patients undergoing anesthesia. It can also be used for the genomic studies with potential applications for clinical medicine. Due to its speed and sensitivity, potentiometry has played a pivotal role in space related applications, drug discovery, geologic research for petroleum composition measurements,carbon dating and some other research endeavors. Three most interesting uses of potentiometry in real life are as follows:

Food industry: Titration is a method or the process of determining the concentration of a dissolved substance in terms of the smallest amount of a reagent of known concentration required to bring about a given effect in reaction with a known volume of the test solution.

Paper manufacturing: Atmospheric emissions from the pulp and paper manufacturing industry can be determined by the potentiometric measurements.

Pharmaceutical industry: To determine the pH of given chemical agents and also in the detection of end point in potentiometry titration of certain drugs like amoxicillin, propranolol.

Example: Determination of pH of blood for diagnosis of acidosis or alkalosis

Example: Determination of NO3, NO2 in meat preservatives.

Example: To determine the NaCl content in meat, fish, dairy products and fruit juice

Example: To determine the Ca content in dairy products, beer, wines and brewing solutions

Example: To determine the K content in fruit juices, beer and wines

Bibliography:

• D.A. Skoog, F. J. Holler and T.A. Nieman, Principles of Instrumental Analysis, 5th edition Orlando, FL: Harcourt Brace College Publishers, 1998.
• H. Jeffery, J. Bassett, J. Mendham and R. C. Denney, Vogel’s textbook of quantitative chemical analysis fifth edition, John Wiley and sons, Newyork.
• Douglas A Skoog, Donald M, West Holler Thomson, Fundamentals of Analytical Chemistry,8th Edition.
• Galen W. Ewing, Instrumental Methods of Chemical Analysis.
• Kaur, Instrumental methods of chemical analysis, A pragati edition, Arihant electric press, Meerut.
• R.S. Drago, Physical methods in inorganic chemistry, Reinhold publishing corp., Newyork, 1965.
• Hoffmann, V. Stroobant, Mass spectrometry Principle and its applications, Third edition, John Wiley and sons, 2007