Electrochemical Methods Dr M.Afroz Bakht

Potentiometry Potentiometry is a method of analysis used in the determination of concentration of ions or substances by measuring the voltage [potential difference] developed between two electrodes. Electrons transfer from reductant to oxidant, in the same solution [ i.e. electron transfer takes place in one phase] will produce electromotive force [emf] which is known as oxidation potential. In some other cases, transfer of electrons takes place between element and its ions [ i.e. electrons transfer takes place between two phases], in this case emf produced is known as electrode potential.

Potentiometers Potentiometer is the instrument used in Potentiometer determinations. It consists of : 1)Galvanic Cell : Which consists of two half cells connected with salt bridge, one of the two half cells is known as reference electrode and the other which contain the sample to be analyzed is known as indicator electrode. 2) Voltage divider: Which supply galvanic cell with external potential opposing that of the galvanic cell till we obtain zero current i.e. potential difference obtained by voltage divider equal that of the galvanic cell.

Conductometry This method of analysis is based on measuring electrolytic conductance. Conductance is the ability of electrolyte solution to carry the electric current. Electric current pass through metallic conductors, e.g. iron wire or copper wire in the form of electron flow. But conduction of electricity through an electrolytic solution involve migration of cations towards cathode and anions toward anode, i.e. current is carried by all ions present in solution. C = 1/R Conductance ( C ) of a solution is a reciprocal of its resistance C = conductance & R = resistance ) Unit of resistance = Ohm Unit of conductance = Ohm -1 or Siemens.

Factors affecting conductance 1)Nature of ions The velocity by which ions move towards the electrode carrying the electric current, varies according to their nature. Velocity of ions α charge Velocity of ions α 1/size 2) Temperature Conductance is increased by increase of temperature, as viscosity is decreased. An increase of temperature by 1˚C is accompanied by 2 % increase in conductance.

3) concentration of ions Conductance is directly proportional to the ions concentration. 4) size of the electrodes The conductance of electrolyte (C) is directly proportional to surface of electrode ( A ) and inversely proportional to the distance between the two electrodes ( L ). i.e C α A & C α 1/L C α A/L C = K. A/L Where K is a proportionality constant known as specific conductance or conductivity.

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

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

Example of conductometric titrations 1)Acid-base titration a)strong acid against strong base e.g HCl # NaOH HCl + NaOH → NaCl + H 2 O H + + Cl - + Na + + OH - → Na + + Cl - + H 2 O First, H + present in the acid sample causes high forming H 2 O, which doesn’t conduct electricity, therefore conductance will be decreased during the progress of the reaction although H + is replaced by Na + ( as H + has higher mobility (349.8) if compared with that of Na + (50.1). After complete reaction excess NaOH added will give free OH - as well as Na +, therefore conductance is increased. N.B. Chloride ion (Cl - ) has constant effect on conductance as its concentration does not change during titration.

Example of conductometric titrations b) Weak acid against strong base (e.g.CH 3 COOH # NaOH) CH 3 COOH + NaOH → CH 3 COONa + H 2 O Before titration low initial conductance is observed due to low [H + ] obtained during dissociation of the weak acid CH 3 COOH. During titration we can observe slight decrease of potential due to consumption of initial H + During progress of titration we can observe slight increase in conductance due to the presence Of CH 3 COO - and Na + and nearly constant [H + ] due to the buffer action of the produced CH 3 COONa and the remaining CH 3 COOH. After complete reaction excess NaOH will lead to increase in conductance due to increase of Na + and OH - after the end point.

Example of conductometric titrations Titration of NaCl with AgNO 3. NaCl + AgNO 3 → AgCl + NaNO 3 NaCl + Ag + + NO 3 - → AgCl + Na + + NO 3 - During titration of NaCl against AgNO 3, Ag + precipitate Cl - as AgCl and NO 3 - replace Cl - in the medium. As mobility of NO 3 - (71.5) is nearly equals that of Cl - (76.3) we observe nearly no change in conductance during titration. After complete precipitation of Cl -, excess Ag + (61.9) and NO 3 - (71.5) will increase conductance of solution. It is important to choose titrant which give sharp change in conductance after the e.p., titrant should have mobility that varies greatly from the sample because the products are weakly ionizable leading to decrease in conductance during titration till the end point.