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Indian Journal of ChemistryVol. 20A, March 1981, pp. 286-288

Studies on a Stationary Disc Electrode In a UniformlyRotated Fluid

R. SRINIVASANDepartment of Chemistry, Indian Institute of Technology, Powai, Bombay 400 076

Received 25 July 1980; accepted 14 August 1980

A rotor with a new geometry, to be employed in convective diffusion studies, is proposed. Experimentalverification on a stationary disc electrode shows that a diffusion layer, reproducible in nature. is obtained with thisrotor. This diffusion layer is also shown to be comparable to that of conventional rotating disc electrode andstationary disc electrode in a uniformly rotating fluid.

THE rotating disc electrode (rde) is an efficienttool for the study of kinetics of electrodereactions. However, the rde arrangementposes a few but important problems pertinent to theelectrical contacts and sticking of gas bubbles at theactive surface. To overcome such difficulties Matsudaand coworkersv" and Bucur et af.3 successfully de-monstrated the use of stationary disc electrode in auniformly rotating fluid (SDERF), following thetheory developed by Matsuda-, This set-up hadeither a disc" or a cone shaped rotor" placed coaxiallyover a stationary disc electrode]. This, when rota-ted, produced a flow at the electrode surface, whichwas similar in pattern, but opposite in direction tothat ofrde.

In the present study a modified rotor has been deve-loped and used along with a stationary disc electrode.The diffusion layer produced by it is reproducible andhas identical thickness as that of rde5,6 and SDERP.

cylinder was placed below it (as in Fig. 2), the fluidhad to move along its radial direction. The massloss due to such a motion of the fluid was compensa-ted by the suction of the same through the holes atthe hollow-solid junction.

The flow thus created at the electrode surfacewas similar to that of rde and SDERF, but its direc-tion opposite to the latter.

Materials and MethodsDescription of the rotor - The elevation and the

plan ofthe rotor are shown in Fig. 1. It was a perspexcylinder hollowed along its axis from one end (facingthe disc electrode) to a height of 4 em. Above this therod had a solid inside with a conical projection intothe hollow portion. At the junction of the hollow andsolid portions, three circular holes displaced symmet-rically along the walls of the cylinder were provided.The axis of each of those holes were at 45° inclinationto the wall of the cylinder, and parallel to the sidesof the conical projection. The hollow portion ofthe rotor was internally threaded in the form of ahelix. Those threads started right from its open endand ran up to the holes at the hollow-solid junction.

When this cylinder, immersed in a fluid from itsopen end to a height well above the holes, was rotatedin the direction of the threading, the fluid was pusheddown along its axis due to the motion of the thread(and also aided by gravity), and thrown out of theopening at the bottom. When a disc coaxial to the

3 Holes, equally spacedat 45

0

Fig. 1 - Section and plan of the rotor showing the internaltSathyanarayana and Rathnakumar' have successfully threading, the solid portion and the holes at the hollow-solid

used a conical rotor with threading along its wall. junction. Dimensions are in millimeters.

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Acme thread depth2 m m ,12 thread I inch -i"'--..--:J---

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SRINIVASAN; A ROTOR WITH A NEW GEOMETRY

Fig. 2 - Electrochemical cell. [SDE, stationary disc electrode;E, perspex holder for SDE; A, platinum counter electrodes;B, reference electrode; and D, water trap. Arrow heads indicatethe direction of flow of the electrolyte through the rotor].

Measurements - Figure 2 shows the experimentalset-up. A polished and cathodically degreased plati-num disc electrode of 0.75 em radius, embedded in aperspex holder, was introduced from the bottom of aspherical cell of one litre capacity. The electrodeholder was a cylindrical shaft whose height was madeadjustable by as crew-nut arrangement. The rotorwas placed from the top of the cellcoaxial to the discelectrode. Its inner and outer diameter at its hollowportion were 0.9 em and 1.6 em respectivelyj , whereit had a internal threading of acme type with 29°thread angle. The pitch and the depth of this threadwere 1/12 inch and 0.2 em respectively. The threeholes located at the hollow-solid portions of thecylinder, had a total cross sectional area of 1.13em2.Experiments were carried out at various distancesbetween the disc electrode and the rotor in the rangeof I to 20 mm.

Another rotor with a similar description as above,but with a thread pitch of 1/6"was also made and theexperiments carried out.

Two platinum counter electrodes were placed wellaway from the disc electrode. A dip-type calomelelectrode with the test electrolyte was used as re-ference electrode. Its luggin capillary ran along thewall of the cell and its tip ended near the discelectrode.

The electrolyte contained 1.5 X 10-4 M hexacya-noferrate(III), 1.45 X 10-4 M hexacyanoferrate(II)and 0.1 M potassium chloride, all of AR grade anddissolved in doubly distilled water. Fresh electrolytewas prepared for each experiment from a stock solu-tion and deaerated for 5 hr using pure nitrogen.Experiments were then repeated at different hexacya-noferrate(II)/hexacyanoferrate(III) concentrations.

Steady-state galvanostatic cathodic polarisationmeasurements were made at various rotor speeds

tThe internal diameter was measured between the twohills of the thread (see Fig. 1).

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ranging from 6.2 rps to 59.2 rps. All experimentswere carried out at 25°C.

Results and DiscussionFigure 3 shows the dependence of the limiting

current (h) on the distance between the stationarydisc electrode and the rotor, at a constant rotor speedof 31.7 rps. The Is. which was as high as 210 p.Aupto a distance of 2 mm, fell down to 143p.Aat 3 mmand at a distance > 3 mm it remained constant.Hence all experiments were carried out with theelectrode at a 5 mm distance from the rotor, unlessstated otherwise.

A plot of Is. against cube root of angular velocity(wI/3) of the rotor is found to be linear passingthrough the origin (curves 1 and 2, Fig. 4). Thisgives the relation

Ii. = K. WI/3 ••• (1)where K. is the slope which depends upon the threadpitch. Such a dependence shows that the downward

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INDIAN J. CHEM., VOL. 20A, MARCH 1981

push of the fluid by the rotor, as described above(§ 3), is true. A decrease in KI for an increase in thethread pitch is in the right direction. This is be~ause,with increase in the pitch value (or decrease III thenumbr of threads per unit distance), the energytransferred from the rotor to the fluid should dec-rease. This in turn should decrease the velocity of thefluid moving down along its axis. The exact relationbetween Is. and the thread pitch is yet to be establi-shed. .

For a rotor with a given thread pitch, the K, valueswere found to be identical at all rotor-electrode dis-tances from 3 mm to 20 rnm. However, the errorlimits in Is. values increased with the increase in thisdistance, as can be seen in Fig. 3. [The K. valuesincreased significantly when the rotor-electrodedistance was below 3 mm (curve 3; Fig. 4)].

The diffusion layer thickness ( a ) was calculated forthe rotor of pitch 1/12 inch at 48.2 rps using the rela-tion, (3), where n is the number of equivalents, F isthe Faraday, A is the area of the electrode, D is thediffusion coefficient and Co the bulk concentration.D for cation was taken as 7.6 X 10-6 em". sec-1(ref. 7). The S-value came out to be 1.17 X 10-3 em.This is quite close to the values obtained by rde5,s(0.9 X 10-3 em) and SDERP (1.2 X 10-3 em),

h = nFACo (3)o

The limiting current was found to increase linearlywith the concentration of the electrochemically activespecies.

The present rotor produces a diffusion layer com-parable in its thickness to that of rde and SDERF.

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Hence the sensitivity of measurements of concentra-tions and other electrochemical parameters shouldbe as good as that of rde. In addition the problemsregarding electrical contracts and the sticking of airbubbles are absent in the present set-up.

The consistency in h versus wl/3 relation for allvalues of the pitch of the rotor and electrode-rotor

-distances indicates that the boundary layer pro-duced is stable and reproducible and no turbulancesare produced within the rps range employed.

A visual observation of the flow pattern using sawdust particles showed it to be reproducible andfollows a path as described earlier. Experimentaldetermination of the velocity profiles is being attemp-ted.

AcknowledgementThe author is indebted to Prof. Hira Lal and Prof.

S. Sathyanarayna (Indian Institute of Science,Bangalore) for useful discussions.

References1. MATSUDA, H., J. electroanal. Chem., 35 (1972), 77; 38

(1972), 159.2. HAMADA,S., IToH, M., MATSUDA,H. & YAMADA,J., J.

electroanal. Chem., 91 (1978), 107.3. BRUCUR, R. V., BARTES,A. & MECEA, V., Electrochim.

Acta, 22 (1977), 499; 23 (1978), 641; 24 (1979),173.4. SATHYANARAYANA,S. & RATHNAKUMAR,V., Indian Institute

of Science, Bangalore, (Private communication).5. GREGORY,D. P. & RIDDIFORD,A. c., J. chem. Soc., (1956),

3756.6. HOGGE& KRAICHMAN,J. Am. chem. Soc., 76 (1954), 1431.7. KORYTA, J., DVORAK, J. & BOHACKOVA,V., Electroche-

mistry (Methuen, London), 1970, 116.

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