Electroanalytical and Thermal Methods
Electroanalytical and Thermal Methods
This domain tests potential, current, charge and heat as analytical signals: potentiometry, pH, ion-selective electrodes, conductometry, coulometry, voltammetry, TGA, DSC and DTA.
Core rules
- Nernst equation: E = E° − (2.303RT/nF) log Q. At 25 °C the slope is −59.16/n mV per decade. pH electrodes are calibrated with buffers bracketing the sample; slope should be 95–105% of theoretical.
- Ion-selective electrode selectivity follows the Nikolsky–Eisenman equation. Matrix, ionic strength and junction potential must be controlled.
- Coulometry uses Faraday's law: n = Q/(zF), with Q = ∫I dt. Voltammetry measures current versus potential; diffusion control gives the Randles–Ševčík relation.
- TGA measures mass loss versus temperature; DSC measures heat flow; DTA measures temperature difference. Heating rate shifts transitions; calibrate with indium and zinc.
- Thermal analysis is often qualitative; quantitative purity by DSC requires the van't Hoff method and careful baseline treatment.
Worked example
A pH electrode reads 177 mV in pH 4.01 buffer and −1 mV in pH 7.00 buffer. Slope = (−1 − 177)/(7.00 − 4.01) = −178/2.99 = −59.5 mV/pH. Percentage of theoretical = 59.5/59.16 × 100 = 100.6%, acceptable at 95–105%. A slope of 52 mV/pH would require rejection or reconditioning.
Traps
- Calibrating pH with only one buffer.
- Ignoring temperature compensation and ionic strength.
- Using TGA mass loss without buoyancy correction.
- Interpreting a DSC melting peak without checking heating rate and calibration.
- Treating a non-Nernstian ISE as ideal.