Home Reading Skoog et al. Fundamental of Analytical Chemistry. Chapters 18, 19, 21

Overall error of measurement Overall error = Systematic Error Random Error + There are two schemes of summation of the errors: additive and quadratic Additive Quadratic Errors are additive Variances are additive

Electrochemical Analysis Electrochemical methods employ processes occurring in a system when electrical current passes through the system or the system is influenced by the electro-magnetic field to obtain information about its qualitative and quantitative composition. We will speak only about methods involving the passage of electrical current. All such methods imply a direct contact of two electrodes with a solution of an electrolyte. A vessel in which the electrodes contact with the solution is called an electrochemical cell. General scheme of an electrochemical cell Cathode is the negatively charged electrode. It attracts cations. Anode is the positively charged electrode. It attracts anions Cathode is the negatively charged electrode. It attracts cations. Anode is the positively charged electrode. It attracts anions

Conductometry Conductometry is a method of analysis based on the measurement of the conductivity of solutions. When difference of potentials is applied to the electrodes, the transfer of charged particles is initiated: cations go to the cathode, anions go to the anode. The conductance correlates with the concentration of charged species in the solution. ― + ― + l The measured property is the resistance of the solution R [ohm]. Reciprocal resistance is called conductance G = 1/R.

― + ― + l  – specific resistance l – distance between electrodes S – cross-sectional area  – specific conductance or conductivity [R] = Ohm =  [G] = S = 1/  [  ] = S/m The ratio l/S can be measured with scale for solid conductors but it is NOT so easy to do with liquid conductors. Then this ratio (cell constant) is to be found by calibration with a standard solution.

Conductance  depends on the total concentration of charged particles in a solution. Conductivity = Observed conductance × cell constant

Molar conductivity [c] = mol/L The total conductivity of an electrolyte solution is additive with respect to ions’ conductivities only for strongly diluted solutions, so called at infinite dilution. This is not true for concentrated solutions of electrolytes. The Kohlrausch law

Conductance and concentration We cannot measure separately the conductivity of ions. We can only measure the total conductivity. So, the conductometry is a method of the determination of total salinity. Nowadays, it is used for the determination of total salinity in pipelines, in water purification systems and so on. Overall concentration

Measurement of conductance Modified Wheatstone bridge circuit Conductivity cell

Conductometers Desktop conductometer Portable conductometer On-line conductometer

Water purification system with molded on-line conductometer

Potentiometry Potentiometry is a method of analysis based on the measurement of the potential of electrochemical cells, in which one electrode is selective to a certain sort of ions, called potential determining ions.

Theory of potentiometry Cu + 2Ag + = Cu Ag CuSO 4 solution Copper electrode Low resistance circuit e-e- Silver electrode AgNO 3 solution Salt bridge Cu = Cu e - Ag + + e - = Ag Copper is dissolving Silver is reducing AnodeCathode Transfer of charge from an electrode to the solution and from the solution to an electrode by dissolving copper ions from the anode into the solution and by transfer of electrons from silver cathode onto silver cations in the solution. ReductantOxidant Cu|Cu 2+ (0.02M)||Ag + (0.02M)|Ag

e-e- Cu Cu 2+ e-e- Ag Ag + Conductor Electrolyte solution + _ As a result of electrochemical reaction electrons leave copper electrode and it is charged positively. Electrons are accumulated on the silver electrode and it is charged negatively. Potentials are formed on each electrodes. Difference of these potentials determines the electromotive force of this electrochemical cell.

e-e- Cu Cu 2+ e-e- Ag Ag + Conductor + _ 0.02M CuSO M AgNO 3 || E = 0.698E = E cell = emf = E cat – E anod = – = Half-reaction

Nernst equation Potential of electrode relates to the concentrations of the reactants and the products of a half-reaction through the Nernst equation. Consider the following reversible half-reaction aA+bB = cC+dD E 0 = standard electrode potential R = gas constant, J/mol K T = temperature, K n = number of electrones F = Faraday constant, coulombs

Cu e - = Cu(s) Ag + + e - = Ag(s) AgCl(s) + e - = Ag(s) + Cl -

Standard electrode potential E 0 is a potential of a half-reaction when the concentrations of all participants are equal to 1. Values of the standard potential for a number of common Red/Ox reactions are tabulated. (c) Skoog et al. Fundamentals of analytical chemistry

emf of an electrochemical cell is a difference of the potentials of two electrodes Half-reaction 1 Half-reaction 2 In order to make the instrumental signal a function of the analyte concentration, we have to fix the potential of the second electrode. We need one electrode to be the reference electrode, an electrode with a constant potential.

Potentiometry Reference electrode|Salt bridge|Analyte solution|Indicator electrode A principal scheme of a potentiometric cell Salt Bridge Hydrogen reference electrode Ag Indicator electrode

Reference electrodes 1. Calomel Electrodes 2. Silver/Silver chloride electrodes Hg I Hg 2 CI 2 (sat'd), KCI( x M) II Ag I AgCI(sat'd), KCI( x M) II Half-reaction: *Reference electrodes can contain a KCl solution of different concentration. Most frequently x = 0.1; 1, saturated.

Silver/Silver chloride reference electrode Silver wire AgCl on Ag wire KCl solution Ceramic quartz or glass fiber junction

One-beaker potentiometric cell

Indicator electrodes Type of indicator electrode Determined ions MetallicMetal cations MembraneInorganic cations; Organic and inorganic anions lon-Sensitive Field Effect Transistors (ISFETS) Inorganic cations and anions; solvated gases

pH – electrodes. pH-metry Glass membrane electrodes pH-responsive glass membrane Internal electrolyte solution Internal reference electrode Lead wire Mercury connection Insulated connection cable Shielding Silicate glass structure (c) G. A. Perley, Anal. Chem., 1949, 21, 395.

Examples of pH-electrodes

Measurement of pH with glass electrodes Potentiometric cell for pH measurement E IRE E b = E 2 – E 1 E ERE EjEj E b = boundary potential E 1, E 2 = potentials at the surface of the glass membrane E j = junction potential Ag/AgCl

Measurement of pH with glass electrodes C(H + ) 2 = concentration of the internal solution; C(H + ) 2 = constant log C H = - pH (T = 25 0 C)

slope = Include E IRE, E ERE, E j pH E cell (25 0 C) Calibration plot