Metal electrodes for continuous amperometric measurement of free hydrofluoric acid in acidic solutions containing...

Titanium-based electrodes have been demonstrated to be useful for measuring free HF concentration in the range 0.01-0.07M at temperatures between 19 and 61 degrees C in acidic, complexed HF solutions, by measurement of the current density resulting when the electrode potential is approximately +0.6 V with respect to an inert electrode. The current density, i, is linearly related to the HF concentration when the solution is adequately stirred to remove diffusion effects at the electrode, and is independent of HNO(3) concentration up to 1.75M. The dependence on the absolute temperature is ln i = - A/T + B + C ln T, and the activation energy at an absolute temperature T(r) is R(A + T(r)C), where A,B and C are constants and R is the gas constant. For a Ti-2% Pd electrode, the activation energy for anodic dissolution is found to be 43.7 +/- 0.2 kJ/mole at 312 K. The equations relating the current density to HF concentration at different temperatures can be combined to give a single equation for [HF] as a function of both i and T.