Reference electrode Luggin Tube

Variable speed motor

High impedance voltameter

Electrolyte level

Plexiglas cylindrical container

Stainless steel cylindrical anode (+ve)

Multi impeller cathode (-ve)

A Multi range ammeter

voltmeter

12 volt  d.c. power supply

Fig. 1 –  Cell and electrical circuit.

Fig. 2 –  Three dimensional diagram of the proposed impeller.

used (Fig. 3); the apparatus consisted of 20 L plastic storage tank, 0.33 horse power plastic centrifugal pump and the cell. A by-pass was used to regulate the solution velocity in the cell; all valves were made of plastic.

The electrical circuit consisted of 12 V d.c. power supply with a built-in voltage regulator connected in series with a multi-range ammeter and the cell. Before each run the cell was filled with the solution (2650 cm3) and the rotational speed of the impeller was adjusted at the required value. The  limiting

Fig. 3 –  Apparatus used in case of continuous operation.

current of the cathodic reduction of K3Fe(CN)6 at the impeller cathode was determined by increasing the current stepwise and measuring the corresponding cathode potential   against a reference electrode by means of a digital high impedance voltmeter until the limiting current plateau was obtained. The reference electrode consisted of a nickel wire placed in the cup of a glass luggin tube filled with the cell solution; the tip of the luggin tube was placed at a distance 0.5–1 mm from the rotat- ing impeller surface (Fig. 1). For the continuous operation of the cell at a certain impeller speed, solution was introduced to the cell from the storage tank at the required velocity by means of the centrifugal pump; solution velocity was mea- sured volumetrically by means of a graduated cylinder and a stopwatch. Table 1 summarizes the range of variables studied in the present work. In order to change Sc, the physical proper- ties (p, µ and D) of the solution were changed by changing the concentration of the NaOH supporting electrolyte (Berger and Hau, 1977). All NaOH concentrations used in the present work are sufficient to eliminate mass transfer of the ferricyanide

Table  2 – physical properties of the ferri/ferrocyanide system at    25 ◦ C.

Solution composition p  (g/cm3 ) µ × 102 (Po) D × 106 (cm2 /s) Sc

0.025 M K3 Fe(CN)6 + 0.1 M K4 Fe(CN) + 1 N NaOH 1.054 1.2236 6.693 1735

0.025 M K3 Fe(CN)6 + 0.1 M K4 Fe(CN) + 2 N NaOH 1.084 1.489 5.508 2494

0.025 M K3 Fe(CN)6 + 0.1 M K4 Fe(CN) + 3 N NaOH 1.1236 1.964 4.335 4032

Table 3 – physical properties of CuSO4–H2SO4  system at 25 ◦ C.

Solution composition p  (g/cm3 ) µ × 102 (Po) D × 106