INDIAN J. CHEM., VOL. 21A, JUNE J982

CALCIUM 0OJY~EN 0

PHOSPHORUS oFig. 1 - A half-unit cell of octacalcium phosphate joined to

a full-unit cell of hydroxyapatite at the plane A-A (ref. 9).

hydrolysis of OCP under normal conditions", theexperiments were carried out at the boiling tempe-rature around IOO°C and at a high pH of 12. Thehighest uptake of 5 % of magnesium by the hydro-lytic solid corresponded to 2.08 g atom of mag-nesium per mol of apatite.

Brown et af.9 proposed a mechanism for a threedimensional growth of apatite about (100) plane(A-A plane in Fig. 1) which is common for bothOCP and HAP structures through an ion by ionrearrangement considered to occur via Eq. (I).

CasH2(PO .•)•.5H20+2Ca2+ =Ca1o(PO.MOH)2 + 4H+ + 3H20 ... (1)

According to Eq. (1), one mol of apatite is formedby the incorporation of two Ca 2+ ions released inthe medium by initial hydrolysis of OCP, into itsstructure. Since magnesium inhibits the hydrolysisinitially", the two Ca2+ ions required to form apa-tite will not be available and we propose that Mg2+ions present in the medium may follow the abovesequence of mechanism and the incorporation oftwo Mg2+ ions into HAP may be considered to occurvia Eq. (2).

Ca.H2(PO.),.5H20 + 2Mg2+ =CasMg2(PO.MOH)a + 4H+ + 3H20 ..(2)

If this mechanism were true, calcium and phosphateions should not be released into the medium and inour experiments practically no calcium and phosphateions were released during hydrolysis of OCP: Sincetriangular calcium sites are smaller than the columncalcium positions in HAP structure"; these placesmay be the possible loci for magnesium in magne-sium containing apatite. .

Authors (N.S.C.) and (p.P.M.) thank the UG~,New Delhi for financial assistance and R.C.L. grate-fully acknowledges the award of junior researchfellowship by the CSIR; New Delhi.

References

1. PATEL, P. R. '" BROWN, W.E.,J. Dent. Res., 54 (1975),72'3-736.

602

2. WEATHERELL,J. A. & ROBINSON.C., Biological Minerali-sation (Zipkin, I), ( John Wiley & Sons, New York)1973,4347. '

3. SIMPSON, D. R .• Clin. Or/hop .. 86 (1972), 260-286.4. BROWN, W. E., Na/ure,l96 (1967.), 1048.S. BROWN, W. E., LEHR, J. R., SMITH, J. P. & FRAZIER,

A. W., J. Am. chem, Soc., 79 (1957), 53J8.6. SINGH, R. P., CHICKERUR,N. S. & NARASARAJU,T. S. B.,

Z. anal. Chem., 237 (1968), 117.7. PIuBIL, R .• Analytical applicationof EDTA and related

compounds, Vol. 52 (Pergamon Press, New York) 1972,348.

8. TUNG, M. S., CHlCKERUR.N. S. & BROWN, W. E., J.oe« Res., 58 (1979), 369 (abst. 112).

9. BROWN.W. E., Cliu. Orthop., 44 (1966), 205. .JO. EANES,E. D., J. Dem. Res., spscial Issue B( 1979), 829.

Rectification through Anodic Aluminium Oxide LayersUnder Different Metallic Electrodes

R. C. SAINI·, R. K. NrGAMt & KULOIP S. DHINDSACentral Quality Testing Laboratory, Department of Chemistry

. & Biochemistry, Haryana Agricultural University,Hissar 125004

Received 9 June 1931; revised and accepted 18 January 1982

Rectification studies through the anodic aluminium oxidefilms on superpurity aluminium have been carried out usingvacuum-deposited metals, viz. Cu, Ni, Cd, Pb, Sn and AI ascounter electrodes. Both symmetrical and unsymmetrical cur-rent-voltage characteristics are shown by the films of differentthicknesses under AI and of same thickness under different metals.as counter electrodes. Suriace property and Huw density of thefilm are responsible Cor rectification phenomenon through anodicoxide layers.

MUCH work in recent years has been carried outon electrolytic faradaic rectificationv- but

only a little effort has been made to investigate recti-fication phenomenon using M/MxOy/M' sandwich,especially in the case of anodic oxide films on alumi-nium. The continuity of these films has been re-cently established", The aim of the present investi-gation is to study the mechanism of rectificationthrough these films using different vacuum-evapora-ted metallic counter electrodes and to examine thecorrelation between critical voltage and work func-tion of the counter electrodes, if any.

Aluminium specimens were cleaned, polished andencapsulated as described earlier'. The encapsulatedspecimen was anodized under constant currentconditions in non-aqueous glycol-borate electrolyteat 25° and current density = 1.0 mA/cm2• First setconsisted of 90V films with evaporated Cd, Cu, Ni,Pb, Sn, and AI. whereas the second set of 30, 50,80, J10 and J50V films with Al metal as counterelectrode. The metallic contact with oxide filmswas made by evaporating the metals under highvacuum (ZJO-I torr.), D.C microvoltmeter (Philipsmodel PP-9004) and a regulated power supply,o to JOV, which could read up to ±O.lV, were em-

tChemistry Department, M. D. University, Rohtak.

NOTES

ployed for this investigation. Throughout this studya ten megohm resistance was kept in series with thespecimen. Thes ample was kept in an automatic aircir-culating oven at 40°. The current-voltage behaviour-of aluminium oxide films under different electrodeswith respect to zero bias, was both symmetrical andunsymmetrical. The symmetry of the films dependedmarkedly upon the nature of counter electrode, withlesser contribution by oxide film thickness. Ahundred-fold increase in the current was observed onthe application of d.c bias in the range 0 to 6Vaeross the oxide film. The current-voltage charac-teristics of first set at lower voltage, i.e. 0 to IV,exhibited the phenomenon of breakdown similarto the typical back-diode static characteristics. Thecurrent through Cd and Sn specimen increased byan order of ten upto 5.1 and 35V, respectively.Cadmium specimen showed further ten-fold rise inmagnitude of current at 5.2V beyond which a conti-nuous sparking was observed. For Sn specimen,at the stage of 35V, sparking was observed at thesurface of the film but the specimen still retainedits assymmetric conduction characteristics even aftera sparking. For all the specimens, except Ni,forward critical voltage was nearly independent ofthe nature of counter electrode, whereas the reversewas not true. This could be substantiated by thefact that the thermionic work function for thesemetals varied from 4.0 to 4.5eV. The work functionpotential depends upon the density of states in theconduction band of the metal in contact with theinsulating aluminium oxide film. An increase inthe critical voltage, the voltage after which thecurrent showed an abrupt rise in current, in therange 2 to 6V, with increasing film thickness wasobserved in the present study. This showed a directrelationship between flaw densitys-? and oxide thick-ness. A slight variation in the magnitude of criticalvoltage for both forward and backward directionsfor a given thickness could be attributed to the sur-face state of bulk Al (experimental sample) and thinAl (vacuum-evaporated) metal.

It appears that both factors i. e. the thermionicwork function of metal electrode and the presenceOf electrons and weak-spots 'flaws' in particular,contribute towards rectification through the oxidefilms under the influence of applied field. Mechanisti-.'cally, the applied voltage across an anodic oxidefilm on aluminium specimen activates the free currentcarriers-electrons and flaws or voids-inside thefilms. Some of these current carriers attain asufficiently high energy under the field applied andthen on arriving at the surface of the film overcomethe work function barrier of the counter electrode;and get emitted from the soilid suddenly, Thesecurrent carriers are responsible for the rectificationcharacteristic through the anodic oxide films.

One of the authors (R. C. S) acknowledges thefinancial assistance as Post-doctoral fellowship bythe CSIR, New Delhi.

Ref'erences1. RANOARAIAN, S. K., J. electroanal, Chem.

Electrochem .• 56 (1974). 27.interfacial

2. DYER, C. K., Electrocomponent Sci, Techno/ .• 1(2) (1974),122.

3. NIGAM, R. K., SAINI, R. C. & KAPooR, R., Indian J. Chem ••17A (1979), 17. .

4. NIGAM, R. K., SAINI, R. C., KAPOOR, R. & KATYAL,S. c., Indian J. Chem .• 20A (1981), 541.s. GIAEVER, 1., Phys. Rev .. Letts .• 5 (1960), 147.

6. ALWITI, R. S. & HILLS, R. G., J. electrochem .• Soc.,112 (1965), 974.

7. SHIMIZU, K. & TAJIMA,S., J. electrochim. Acta, 24 (1979),309.

Determination of Standard Potentials of Copper-Copper Pyrophosphate & Nickel-Nickel Pyrophos-

phate Electrodes

M. P. SINGH*t & WAHID U. MALIK:Department of Chemistry, University of Roorkee, Roorkee

Received 15 October 1981; revised and accepted 4 February1982

Copper-copper and nickel-nickel electrodes reversible topyrophosphate ioos have been prepared and their standard poteq-tials determined. The values of the standard electrode potentials(EO) found are : -0.358 V and -0.388 V respectively. Thee.m.f. values of the metal-metal pyrophosphate cells, set upwith and without transport, are also reported.

EARLIER, we have reported cobalt-cobalt elect-rode! which is reversible with respect to pyro-

phosphate ions. In continuation of this work _wereport now the preparation of metal-metal pyro-phosphate electrodes [where metal is copper(II) ornickel(II)], the e.m.f, vaues of the metal-metal pyro-phosphate cells with and without transport as wellas the potentials of the two electrode systems. Theexperimental arrangements were similar to thosereported by Poarce and Ough 2 with some minormodifications.

All the reagents employed were of AR grade (BDH)except pyrophosphoric acid and sodium pyrophos-phate which were E. Merck products. All thesolutions Were made -in doubly distilled water.Mercury, used for preparing the amalgam andmercurous chloride, was triply distilled.

Preparation of the complex - Solutions of cupricchloride (nickel nitrate in the case of nickel complex).and pyrophosphoric acid (cone. 1.0 M each) weremixed in I : 2 molar ratio. These mixtures wereconcentrated by heating on a water-bath. On cool-ing, the precipitates settled down; these were filtered,washed several times with water and then dried at90"C. The sodium amalgam electrode was preparedas described in the literature".

Metal electrode - A thin film of metal (copperor nickel) was deposited on a platinum foil (areaI sq. em) by the electrolysis of a solution containingcopper/nickel sulphate (56 %), boric acid (3 %) and

tPresent address: Department of Chemistry, AtarraP. G. College, Atarra, Distt. Banda (U.P.).

. :Present address; Vice Chancellor, Kashmir University,Srmagar.

603~