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