1001
S.E. S.EL WAKKAD, H. A. R r m A N D I. G . EFL~ID
by other strong absorptions, such as those resulting from C-H deformation of the phenyl ring. It is interesting to note that the observed metal-ligand bands are spread over a wide frequency region, while the metal specific bands of bisacetylacetone metal chelates are observed in the range of 700-GOO cm.-l.lo If metal-specific absorptions of both types of compounds are due to the similar mode of metalligand vibrations, we could expect the absorptions of bisacetylacetone-ethylenediimine metal chelates to occur in a higher frequency region than those of bisacetylacetone metal chelates, since the covalent metal-ligand bonds of the former compounds have the higher bond energies. At present, however, it is difficult to compare the absorption bands of these compounds, because there are many modes of vibration, such as metal-oxygen and -nitrogen stretching, in-plane and out-of-plane deformation vibrations, all of which result in change of dipole moment, and hence are responsible for infrared absorptions. In general, the order of decreasing frequency of bands of each set of metal-specific absorptions in the
T'ol. 69
metal chelates of a given ligand is: Pd > Ni and Co > Cu. On the other hand, the stability of complexes of these bivalent metal ioiis usually follows the order Pd > Cu > Ni > The relationship between the metal-ligand stretching force coilstant and the metal-ligand bond strength, which is a measure of the stability of complexes, might be expected to lead one to predict Pd > Cu > Ni > Co as the order of decreasing frequency.I7 However the greater mass of Pd might also be espected t,o shift its absorption frequencies t o lower values. The irregularity of the Cu-ligand frequencies reported indicates that the absorptions may arise from a complicated, rather than a simple mode of vibration. For all the metals investigated, it is possible that other factors, such as resonance effects involving the d-orbitals of the metal ion, may also influence the metal-ligand vibrations. (16) D. P. Mellor a n d L. Rlaley, Nature, 159, 370 (1947); 161, 4313 (1948). (17) Bellainy and Branch recently rei>orted t h e lineal relatioiisliiii hetween t h e stability of salicylaldehyde metal ellelates a n d t h e shift of t h e ohrlute onrbonyl freqiieiicy in 11300 eiii.-1 region. However, n o a t t e m p t has been made i n lower freqiiency region, J . Clrern. S o c . , 4491 (1954).
THE ELECTROCHEMICAL BEHAVIOR OF THE TUYGSTEN ELECTRODE AND THE NATURE OF THE DIFFERENT OXIDES OF THE METAL RY S.E. S.EL WAKKAD, H. A. RIZICAND I. G. EBAID Department of Chemislry, Faculty of Science, Cairo University, Cairo, Egypt, and John Harrison Laboratory of Chemisti,y, University of Pennsylvania, Philadelphia, Pa. Received February 7 , 1855
The limited results previously reported on the behavior of the tungsten electrode in solutions of different pH values are conflicting. This is clarified here by calculating the potentials of the different oxides of tungsten and comparing them with the experimental results. It is found that the behavior of the tungsten electrode depends upon whether i t is massive or i n the powdered form. From this study it has been found possible to define clearly the pH range over which the tungsten electrode can function properly as an indicator electrode for hydrogen ion activity. The anodic oxidation of tungsten a t very low current density is studied and the nature of the different oxides of tungsten which are a p t to be formed on t h e electrode surface has been revealed. From all these studies it is shown that the tungsten electrode is far better than t h e antimony electrode as an indicator electrode for the hydrogen ion activity since u. calibration curve can stand for much longer period without any of the appreciable drift characteristic of the antimony electrode.
Quite recently the electrochemical behavior of the potentials of the different osides of tungsten are calantimony electrode mas studied by El Wakkncli~2 culated and compared with the esperimental results. and the factors which govern the electrode behavior It is shojvn that the behavior of t!ie tungsteil elerwere defined. The present investigation deals with tarodedepends upon whether it is i n the massixre or the tungsten electrode which has been the subject in the powdered form. From this study it has been found possible to define clearly the pH range over of a limited amount of experimental ~ v o r k . ~The -~ results obtained by different workers are conflicting which the tungsten electrode can function properly and the pH range over which the electrode can give as a n indicator electrode for the hydrogen ion sct8i\-correct measurements for the hydrogen ion activity ity. The anodic osida.tion of tungsten a t very low as well as the time during which a calibration curve current density is studied and the nature of the difcan give satisfactory values are obscure. The pre- ferent oxides of tungsten which can be formed on cise determination of these factors is complicated the surface of the electrode has been revealed. not only for the different types of osides given by From these studies it is concluded that this electungsten but also by the great variety of com- trode in it,s behavior as an indicator electrode for pounds with bases given by these osides, especially the hydrogen ion activity is far better than the anthe trioxide. I n this investigation, howeirer, the timony electrode. A calibration ci1r.i.e in case of the tungsten electrode can stand for a much longer (1) S. E. 9. El Wakkad, J . Cham. Soc., 2894 (1950). period without the c1inracterist)icdrift of the anti(2) S. E. S. El Wakkad and A. Hickling, THISJOURNAL, 57, 203 (1953). mony electrode. (3) J. E. Baylis, Ind. E n g . Chem., 16, 852 (1023). Experimental (4) H. C. Parker, $bid., 17, 737 (1925). I. The Tungsten Electrodes.-Tlic tiingsteti Plrc(ro(lcs (6) A . L. Holven, ibzd., 21, 905 (1429). (6) H. T. 9. Britton and E. N. Dodd, J. C k e m . Soc., 82r) (lr)'31).
nwd wwp of the folloiviirg typcs:
Oct., 1955
1005
T H S ELECTROCIHEJIIC.4L BEHAVIOR O F THE T U N G S T E N ELECTRODE
( A ) Massive Tungsten Rod or Filament.-The rod electrode \viis of pure tungsten (B.D.H.) 4 . 2 e m . long and 0.4 cni. iii disinekr, while the filament was one cni. long and 0.40 nim. in diameter. The electrodes were sealed directly to Pyrex glass tubes. Each electrode before use was refreshed by immersing it several times in concentrated solution of sodium hydroxide, rubbing it with filter paper, washing thoroughly with 3 stream of redistilled water, and finally washing with the solution in which it would be esamiiied. (B) Aged Tungsten Rod or Filament.-This was t,he same as ( A ) but i n this case the same electrode w:~sused throughout without being refreshed. ( C ) Powdered Tungsten.-This was a B.D.H. saniple which had been shaken with the solution for solnetmimeand then used directly to cover a platinum cont,act for electrical ineasurement,s. 11. The Tungsten-Tungsten Trioxide Electrodes .-As most previous authors have att'riliuted the l)ehnvior of the tuiigsteii elect,roda ns :m indicntor electrotle for liytlrogrii tlic beion activity to the forinn tion of tungsten t~,ioxide,~ hnvior of tlie followitig electrode also I V ~ Se s m ~ i n e d . (D ) Powdered Tungsten-Tungsten Trioxide. --I ti this case the powdered metal was mixed thoroughly with the trioxide prepared as described by Arcliibald.8 The niisture was shaken with the solution for some tinie, and then allowed to set,tle on a platinum contact for electrical measuremen t)s. 111. The Solutions.-The solutions i n n.liic,li the e k e trodes were cxamiiietl were buffers, except i n t,he extrerne alkaline range of pH where sodium hydroxide solutions were used. From pH 0.65 to 5.00 we used the sodiurn acetat.e-hydrochloric acid buffer mistures. -4buffer solution of pH 5.79 was prepared from succinic acid and boi~ix. From pH 7.05 to 9.25, boric acid-boixx buffer mixtures were used, while for pH values 10.15, 10.85 and 11.03, sodium carboiiatehydrocliloric acid mistures were prepared.0 For the extreme alltalitie range, sodium hydroxide solutions of conceiitratious 0.01, 0.05, 0.10, 0.50, 1.00 and 2.50 M were used. All the solutions were prepared from highly purified niatei,ials nntl their pH values were c:rrefully checked with the hydrogen dectrode and d i e t i possible with thc quinhydrone electrode. Electrical Measuremants.-The electrical mmwrements were performd i n t,lie usual manlier i n an air thermostat fixed :it 25 =t0.02". These mcasutwnents irere carried out i n duplicn,t,ewiih diffcrentlp prepared stock solutions using :t saturated c:tlomel electrode as the referenre half cell. Thr e.1n.f. measurements were carried out usiiig a calibrated meter biidge on which nccurate rendiiigs could bo talieii to 0.02 cin. A cadmium cell cnlihr:tted by the National Physic:tl Inhorntory and an Onwootl Mirror galvanonieter having a sensit)ivity of 190 nim./micro-:~iiil)ere \yere used.
Results and Discussion
I. The Behavior of the Tungsten Electrodes in Solutions of Different pH Values.-Consideration of the variation of potential with time as well as with the pH of the solution for the different types of tungsten electrodes revealed the following. poten(1) Electrodes of Type A.-Equilibrium t8ialswere recorded in all so!utions after 1-2 hours from the time of immersion in unstirred solutions. The equilibrium values remained constant for 48 hours which \vas tlie time limit for our experiments. Bot'li the tungsten rods and the filnnients n'ei'c found to give the same equilibrium potentJials in the same solut,ions. steady-stage (2) Electrodes of Type B.-The potential ] d u e s were renched in d l solutiolis directly af tela immersion and reninind qtii te con( 7 ) 0 . U:ltts and 12. ( ' . R. :'iiuoner, " 1 ' l i r F J l ~ o t r u i l tPi,teriti;il ~ Bc.I~:tvior of Coi,roilitig Aletali i t ) . A ~ i i i e o uSulubiuns," 0sfut.d Pi,rv.;, Nc.n. ) ' O I I;, N. Y.. 1933, 11. 372. (8) E . FI. .ArcIiibald, "Tile Pregnrntioti of Pure I n o s b n r e s , " .John \Vile). atid S o n s , In[,., Neiv YorI
