Chapter 17 Electroanalytical techniques Topics Elelectrogravimetric analysis Coulometry Voltammetry Electrolysis – provide external voltage to force non-spontaneous redox reaction to occur Elelectrogravimetric analysis Goal: Plate soluble species onto the surface of an electrode, quantitatively. Cu2+ + 2e- → Cu(s) E° = 0.399 V + 2H2O → O2 + 4H + 4e- E° = 1.229 V 2+ + 0.010 M Cu solution, [H ] = 1M and pO2 = 1 atm Ecell = 0.339 – 0.05916/2 log (1/0.01) – 1.229 = -0.949 V Need to supply a voltage of at least -0.949 V Overpotential Ohmic potential Concentration polarization After experiment is complete dry the electrode and re-weigh to determine the mass of copper that has been plated Problem 17-8 With a two electrode set-up, E(cathode) increases through the course of the experiment, which is problematic from a selectivity standpoint. 3-electrode cell – the potentiostat Holds E(cathode constant) More expensive $10-30 K Longer analysis time Coulometry Redox oxidant is generated by electrolysis 2Br- → Br2 + 2e- forced to occur through applied voltage (at constant current) large current Br2 + cyclohexene → dibromocyclohexane Detector cell Apply a voltage of 0.25 V (do not want analyte to be oxidized) A small current is passes through the circuit in the presence of Br-, 20 µA. In the absence of Br-, the current is essentially zero. - Anode 2Br- → Br2 + 2e - Cathode Br2 + 2e- → 2Br Detection of endpoint Problem 17-15 4 g of KI were added to 50.00 ml of a H2S solution. I- was generated through electrolysis of I-. The titration required 812 s at 52.6 mA. Calculate the concentration of H2S. -3 4 [H2S] = (812 s)*(52.6*10 C/s)*(mol e-/9.649*10 C)* 6 (1 mol H2S/2 mol e-)*(34.08 g/mol H2S)*(10 µg/g)*(1/50.00 ml) = 151 µg/mL Voltammetry Potentiometer Measure current as a function of the applied voltage Quantitative and qualitative Polarography - voltammetry with a mercury dropping electrode Simplest voltammetric experiment Stirring Analyte is reduced at the working electrode Linearly scan applied voltage 10-5 M Square wave Similar, but step the applied voltage Charging current vs. Faradaic current Reduction of charging current 10-7-10-8 M Charge Stripping Plate the electrode with sample then reverse the voltage and measure the current. Current is proportional to the amount of analyte deposited on the electrode. 10-10-10-12 M Cyclic voltammetry (no stirring) Reversible, quasi-reversible, irreversible Problem 17-27 Standard addition analysis Tap water 0.89 µA Tap water +100 ppb 1.18 µA Tap water +200 ppb 1.35 µA Tap water +300 ppb 1.53 µA Tap water +400 ppb 1.71 µA Tap water +500 ppb 1.83 µA st. add plot for prob 17-27 2 1.5 1 current 0.5 0 -600 -100 400 900 ppb Cu2+ slope 0.001849 y-inter 0.952857 x-inter -515.456 unk con 515 ppb .