Frequency modulated rotating disc electrodes

Rosamilia and Miller [35] presented instrumentation developments allowing rapid potential scans to be made at the HMRDE. They demonstrated a linear concentration calibration in the range 2-10 x 

10 6 mol dm-3 for dopamine on a carbon RDE with scan rates up to 80 mV s 1 and 15 Hz modulation of rotation rate. 

A detailed examination of the mass transport effects of the HMRDE has been made. At low rotation speeds and for small amplitude modulations (as defined in Section 10.3.6.2) the response of the current is found to agree exactly with that predicted by the steady-state Levich theory (equations (10.15)-(10.17)) [27, 36, 37]. Theoretical and experimental application of the HMRDE, under these conditions, to cases where the electrode reaction rate constant was comparable to the mass-transfer coefficient has also been made [36]. At higher rotation speeds and/or larger amplitude modulations, the observed current response deviated from the expected Levich behaviour. 

One special case of HMRDE is stopped-rotation voltammetry (SRV) introduced by Wang [44]. As the name suggests, the rotation is switched 

A variant of SRV is pulsed rotation voltammetry (PRV) [48-50]. Here, the rotation rate of a RDE is switched between two well separated speeds 

---

Fig. 9. Polarization curve of an Fe-disc Pt-split-ring electrode with hydrodynamic square wave modulation. In 1 M NaOH with anodic and cathodic scan including capacity of the Fe disc (dashed curve), modulation frequency of rotation co = 0.05 Hz (insert), simultaneous detection of Fe(II) and Fe(III) ions at Pt half rings [12].

This is the same asymptotic modulation frequency dependence as for a small element of a channel electrode, and arises because the concentration boundary layer is the same thickness over the whole of the rotating disc. 

Another example for the HMRRD electrode is given in Fig. 9 for Fe in alkaline solutions [12, 27]. The square wave modulation of the rotation frequency co causes the simultaneous oscillation of the analytical ring currents. They are caused by species of the bulk solution. Additional spikes refer to corrosion products dissolved at the Fe disc. This is a consequence of the change of the Nemst diffusion layer due to the changes of co. This pumping effect leads to transient analytical ring currents. Besides qualitative information, also quantitative information on soluble corrosion products may be obtained. The size of the spikes is proportional to the dissolution rate at the disc, as has been shown by a close relation of experimental results and calculations [28-30]. As seen in Fig. 7, soluble Fe(II) species are formed in the po-... 

---