Electrochemical methods of analysis

1. Electrochemical methods of analysis Electrochemical methods of analysis are physico-chemical methods, which deal with electrical quantity or property of a solution of the substance to be analyzed qualitatively or quantitatively. Advantages of electrochemical methods of analysis Shorten time required for analysis. Can be used when the classical methods of analysis can't be applied, e.g. in case of colored, turbid, very dilute solutions and when there is no suitable indicator.

2. Classification of electrochemical methods Conductometry: these methods depend on movement of ions in an electric field, leading to conductance of electricity without occurrence of redox reaction at the electrode surface (i.e no electron transfer). Potentiometry and polarography: these methods depend on measuring voltage or current between two electrodes, where redox reactions take place at electrodes surfaces (i.e electron transfer occurs at electrodes surfaces)

3. Part I: Conductometry Conductance (G) of a solution is a reciprocal of its resistance i.e G = 1/R Factors affecting conductance 1) Nature of ions Velocity of ions  charge 2) Temperature Conductance is increased by increase of temperature, as viscosity and hydration are decreased. An increase of temperature by 1°C is accompanied by 2% increase in conductance, therefore conductometric determinations must be carried out under thermostatically controlled conditions or using special device which continuously calibrates for the temperature variations.

4. 3) Concentration of ions Conductance has a direct relation with concentration of ions. i.e conductance is directly proportional to the number of ions. 4) The size of the electrodes. Conductometric measurements are usually carried out in a conductance cell, which consists of two parallel sheets of platinum as inert electrodes. The platinum (pt) electrodes must be platinised, i.e covered by pt. black (finely divided pt.) Conductance cell

5. The conductance of electrolyte (G) is directly proportional to surface area of electrode (A) and inversely proportional to the distance between the two electrodes (L). G = K . A/L Specific conductance (K) It is the conductance when L is unity (1cm) and A is unity (1cm2). or it is the conductance of a cubic centimeter of liquid (1cm3)

6. Instrument used in conductometric determinations • To carry out a conductometricmeasuement, it is necessary to measure the resistance (R) of the solution • The instrument consist of two parts • conductance cell, into which the solution to be determined is added. • conductivity bridge. (Kohlrausch bridge), which is formed of • Wheatstone bridge • An oscillator (to produce A.C. from D.C).

7. Applications 1) Direct conductometry: This method is used in industry for checking purity of distilled water or other chemicals and also for determination of some physical constants e.g ionisation constant. In this method, a calibration curve is constructed by plotting the conductonce of a series prepared from extra pure grade of the substance to be analyed versus concentration. The conductance of the sample is measured and then its concentration is obtained from the calibration curve.

8. 2) Indirect conductometry (Conductometric titratlons) A conductometric titration involves measurement of the conductance after successive addition of titrant. The end point is obtained from a plot of conductance against ml. titrant. The most important advantages of this method are: It can be used for determination of turbid and highly coloured solutions. It can be used for determination of very dilute solutions. It can be used, when reaction is not complete and when there is no suitable indicator e.g during weak acid, weak base titration.

9. Precautions to be considered during conductometric titrations • The titrant used must be at least 10 times concentrated as the solution to be titrated e.g titration of 0.01 N HC1 should be against 0.1 N NaOH. By this way, we can minimize decrease in conductance due to dilution. • Avoid the presence of extraneous ions, which will increase initial conductance, i.e change in conductance during titration cann't be accurately observed, e.g in case of addition of buffer to the titrated solution. • The method is not suitable for detection of end point of redox reactions as there is no electron transfer at electrode surface during conductometric determinations.

10. Part II: Potentiometry • Introduction • Potentiometry is a method of analysis concerned with the determination of an ion by dipping a suitable sensor in its solution (indicator electrode). The potential of the indicator electrode is measured relative to reference electrode possessing constant potential the concentration of the ion is determined using Nernest equation. • When a rod of metal is dipped in a solution of its ions, according to the nature of metal, it may have: • 1-Tendency to lose electrons and converted to its ions. In this case the metal has high solution pressure, e.g. Zn°, Fe°, Co°, Ni°. • 2-Tendency of metal ion to accept electrons and converted to element, i.e the metal has high ionic pressure, e.g Cu°, Hg°, Ag°.

11. When metal of high solution pressure is immersed in solution of its ions e.g. Zn°/ Zn2+, the following equilibrium will be established. • Zn° Zn2+ + 2e • EMF produced ( electrode potential) has a negative (-ve) sign. • When metal of ionic pressure is immersed in solution of its ions e.g. Cu°/Cu2+, the following equilibrium established. • Cuo Cu2+ + 2e • EMF produced (electrode potential) has a positive (+ve) sign.

12. Calculation of electrode potential Electrode potential can be calculated from Nernest equation 0.059 E25°C= Eo + Log [Mn+] i.e E250C [Mn+] n Where: E25°C = Electrode potential at 25°C Eo = Standard electrode potential n = Number of electrons gained or lost [Mn+] = Molar concentration of metal ion. From Nernest equation, E25oC is a function of ionic concentration. When ionic concentration is 1 molar, E25°C = Eo ; (i.e standard electrode potential)

13. Standard electrode potential It is the EMF produced when metal is immersed in 1 M solution of its ions. Eo is a quantitative measure of readiness of metal to loose electrons or gaining of electrons by non-metal giving its ions, or it is the quantitative measure of tendency of element to pass into ionic state. N.B. The sign of the potential is similar to the charge on the metal electrode. The potential of single electrode cann't be measured directly, but measured against reference electrode (standard electrode which has known and fixed potential) through electrochemical cell.

14. Electrochemical Cells Galvanic Cell a) Voltaic Cell: Zno/Zn2+// Cu°/ Cu2+ It consists of 2 electrodes (each electrode is considered as half cell), one of them is zinc electrode (Zn°/ Zn2+) and me other is copper electrode (Cu°/ Cu2+). The two electrodes are joined together by liquid junction, known as salt bridge. Salt bridge permitt the passage of electric current between the solutions present in the electrodes

15. b) Concentration cell In this type of galvanic cell, each half cell contains the same metal and its ions, but the concenteration of metal ion is different, this will permitt the passage of electric current, producing EMF of the cell.

16. Liquid junction potential Sometimes potential is developed between the two boundaries of junction at the two ends of salt bridge, this potential is known as liquid junction potential. Liquid junction potential is produced due to the difference in the rates of migration of both cations and anions of the salt bridge, which leads to unequal distribution of charges at the ends of salt bridge, thus producing a potential. • To reduce liquid junction potential, we have to: • Choose the electrolyte of salt bridge, that its cations and anions have nearly the same mobility, so that, they move by the same rate, leading to equal distribution of charges. • e.g KC1 or KNO3 (K+ = 73.5 , Cl- = 76.3 , NO‑3 =71.5). • The use of high concentration of electrolyte in salt bridge to reduce difference in rates of migration of ions.

17. Electrolytic Cell In this type, we apply external EMF, which is transformed to chemical energy. Voltaic cell can be converted to electrolytic cell if we apply sufficiently large potential from external source that its output opposing that of galvanic cell, where electrode reactions are reversed.

18. Classification of electrodes: A) Classification of electrodes according to the function Electrodes can be classified according to the function, into, reference electrodes and indicator electrodes. 1) Reference electrodes: In this type electrode should has a known and constant potential, therefore used as reference (or standard), to measure the potential of indicator electrode, through galvanic cell. e.g Normal hydrogen electrode (N.H.E.), Calomel electrode, silver electrode. a) Normal hydrogen electrode (N.H.E)

19. 2) Indicator electrodes • An indicator electrode is that which is sensitive to the concentration of one of the participants or product of reaction. Its potential changes rapidly with the change of concentration of a particular ion. It must give rapid response. Its response must be reproducible. • These electrodes are classified into two classes: • 1-Electrodes where redox reaction (electron transfer) takes place at electrode surface, e.g metallic electrodes. • 2-Electrodes where charge (ions) exchange takes place at specific membrane surface, e.g ion selective electrodes (or specific ion electrodes).

20. Applications 1- Direct potentiometry: Calibration curve method • in which we plot the potential of galvanic cell versus a series of standard solution prepared from extra pure grade of the substance to be analysed. • Then the potential developed when indicator electrode is immersed in solution of the substance to be analysed is measured. • From calibration curve we can obtain the concentration of the sample.

21. 2) Indirect potentiometry: (Potentiometric titration or potentiometric determination of end point)