Ionic Mass Transfer and Concentration Polarization at Rotating Electrode#

Ra te s of ionic mass t rans fe r a t nickel electrodes ro t a t i ng abou t the i r axes in the center of s t a t i o n a r y electrodes were s tudied using the fer r i fer rocyanide couple in a lkal ine solut ions . A general mass t r ans fe r corre la t ion was found to apply equal ly well to dissolut ion ra tes of ro t a t i ng solids and to rates of ionic mass t rans fe r at r o t a t i ng electrodes. This corre la t ion takes into account physical proper t ies of the sys tem as well as geometr ic and hydrodynamic factors. The corre la t ion allows pred ic t ion of l imi t ing cur ren t s and concen t ra t ion po la r iza t ion a t r o t a t i ng electrodes under a wide range of condi t ions . The na ture of po lar iza t ion involved in reduct ion of Fe(CN)6 -a and oxidat ion of Fe(CN)~ -4 was also inves t iga ted . Po la r iza t ion was found to depend s t rongly on the presence of electrode poisons. Wi th freshly p repared solut ions, under exclusion of l ight , and wi th ca thodica l ly t r ea t ed nickel electrodes, re la t ive ly small chemical polar izat ions were de termined. For ro ta t iona l speeds not exceeding Reynolds n u m b e r 11,000, chemical po lar iza t ion was found to be negligible in compar ison wi th concen t r a t ion polar izat ion. Under such condi t ions , the fer ro-ferr icyanidc couple can be conven ien t ly used to ob ta in mass t rans fe r ra tes for var ious hyd rodynamic condi t ions, or conversely, to ver i fy the va l id i ty of mass t r ans fe r equat ions by a compar ison of exper imenta l and ca lcula ted values of l imi t ing cur ren ts and concen t ra t ion polar izat ion. The ro ta t ing electrode model was found to be most su i tab le for s tudy ing the na tu re of electrolyt ic polar iza t ion phenomena beeause of un i fo rmi ty of the cur ren t d i s t r ibu t ion and the hydrodynamic diffusion layer at the electrode surface.