Home Reading Skoog et al. Fundamentals of Analytical Chemistry. Sections 18A, 18C ( in chapter 18 ), Chapter 23

Potentiometric titration Potentiometric titration is a titration technique. It differs from classic titration only in a method of indicating the titration endpoint. Can be used for 1.Acid/base titration 2.Red/Ox titration 3.Complexometry Benefits: 1.More sensitive 2.Can be automated 3.Can be used for turbid or strongly coloured solutions 4.Can be used if there is no suitible indicator

There are two distinguishable situation in potentiometric acid/base titration: 1. Titration of a strong electrolyte by a strong electrolyte 2. Titration of a weak electrolyte by a strong electrolyte

Titration of a strong acid by a strong base V eq

Consider the following graph: Titration of a weak acid by a strong base pK a V eq /2 V eq

1/2 Determination of the endpoint

Voltammetry Electroanalytical methods that depend on the measurement of current as a function of applied potential are called voltammetric methods. Voltammetry is widely used by analytical, inorganic, physical, and biological chemists for fundamental studies of (1) oxidation and reduction processes in various media, (2) adsorption processes on surfaces, and (3) electron transfer mechanisms at solid/liquid interfaces. In analytical chemistry, voltammetric methods are used for the determination of trace metals in the environment, for the analysis of pharmaceutical compounds, and for the analysis of human tissue for medical purposes.

Red/Ox reactions In an reduction/oxidation reaction, electrons are transferred from one reactant to another. Example:Ce 4+ + Fe 2+ ↔ Ce 3+ + Fe 3+ In this reaction, an electron is transferred from Fe 2+ to Ce 4+ to form Ce 3+ and Fe 3+ ions. A substance that has a strong affinity for electrons, such as Ce 4+, is called an oxidant. A reductant, is a species, such as Fe 2+, that easily donates electrons to another species. oxidantreductant The reaction in the example occurs in a solution. Both the oxidant and reductant are solved species

Standard Red/Ox potential Ce 4+ + Fe 2+ ↔ Ce 3+ + Fe 3+ Ce 4+ + e - ↔ Ce 3+ Fe 2+ + e - ↔ Fe 3+ Half-reactions Both half-reactions are written towards reduced compounds. Why? Because it is conventional Half-reaction for reductant Half-reaction for oxidant Overall reaction = ―

Standard Red/Ox potential Ce 4+ + Fe 2+ ↔ Ce 3+ + Fe 3+ Ce 4+ + e - ↔ Ce 3+ Fe 2+ + e - ↔ Fe 3+ Half-reactions For each half-reaction we can write Nernst equation

Half-reaction for reductant Half-reaction for oxidant Overall reaction = ― Standard Red/Ox potential Assume that activities of all the reactants are equal to 1 Standard Red/Ox potential of reaction

Standard Red/Ox potential 1.70 V0.77 V E 0 = 1.70 – 0.77 = 0.93 V E 0 > 0 reaction is possible E 0 < 0 reaction is impossible If concentration of all the reactants is 1 M

(c) Skoog et al. Fundamental of Analytical Chemistry Standard Red/Ox potential Half-reactions with a more positive Red/Ox potential can be an oxidant for half-reactions with a less positive potential

Reactants can accept electrons from or donate electrons to an electrode, so the electrode can play a role of an oxidant or reductant e-e- e-e- e-e- e-e- Ce 4+ Ce Ce 4+ Ce Electrode is a reductant Electrode is an oxidant

Ce 4+ + e - ↔ Ce 3+ Cathode - ↔ Cathode + e - Ce 4+ + Cathode - ↔ Ce 3+ +Cathode ― = Cathode is a reductant Example 1 Reduction of Ce 4+ is possible while E cathode < 1.7. [Ce 4+ ] = [Ce 3+ ] = 1M

Example 1 [Ce 4+ ] = [Ce 3+ ] = 1M i, A E cathode, V The reduction of Ce 4+ will not happen till the potential of the electrode becomes less than 1.7 V. Current i is a characteristic of the rate of a reaction. When a reaction proceeds, there is transfer of charge between an electrode and the solution. There is the current. When the reaction stops, there is no charge transfer, no current.

Anode + + e - ↔ Anode Fe 2+ ↔ Fe 3+ + e - Anode + + Fe 2+ ↔ Anode + Fe 3+ ― = Anode is an oxidant Example 2 Oxidation of Fe 2+ is possible while E anode > 0.77 V. [Fe 2+ ] = [Fe 3+ ] = 1M

Example 2 [Fe 3+ ] = [Fe 2+ ] = 1M i, A E anode, V The oxidation of Fe 2+ will not happen till the potential of the electrode becomes larger than 0.77 V.

Voltammetric instruments Scheme is from Skoog et al. “Fundamental of Analytical Chemistry” The three electrode scheme Working electrode: where the electrochemical reaction takes place Counter (auxiliary) electrode serves to make a close circuit with the working electrode in order to measure the reaction current. Reference electrode serves to independently control the potential on the working electrode

Depending on the aggregate state, there distinguish two types of the working electrodes, solid-state and liquid mercury electrodes. The first voltammetric method was developed by Jaroslav Heyrovsky on the basis of drop mercury electrode in The method was called polarography. Mercury is dropping from the capillary. During the formation of the drop, current passes between the drop and the reference electrode. When the drop is detached and falls, the current discontinues but reestablishes again as a new drop begins to form. Polarographic cell with a drop mercury electrode

Modern polarographic equipment

The current in a polarographic cell is stable only when the surface of the drop is stable and constant. These periods are short. Because of a fluctuating nature of the DME the signal is noisy.

Especial sophisticated electric circuits and architectures of the DME are employed to suppress fluctuations of the signal.

E 1/2 = half-wave potential Polarogram of a mixture of Ag +, Tl +, Cd 2+, Ni 2+, Zn 2+ = standard potential of the electrode reaction = potential of the reference electrode

The E 1/2 potential is a characteristic of an ion and can be used or quantitative analysis. Qualitative analysis with polarography You must remember that the electrode reaction on DME involves the solvation of metal ions in mercury, in other words, the formation of amalgams

Table is from Skoog et al. “Fundamental of Analytical Chemistry” Qualitative analysis with polarography The standard potential of the electrode reaction can depend on the composition of an electrolyte solution

(c) Skoog et al. “Fundamental of Analytical Chemistry Quantitative analysis with polarography ilil limiting current i l = k a ∙C a

Quantitative analysis with polarography i Tl = k Tl C Tl i Cd = k Cd C Cd i Ni = k Ni C Ni i Zn = k Zn C Zn i Ag = k Ag C Ag