Indian JournaJ of ChemistryVol. 34A, July 1995, pp. 540-545

Effect of phenols on the polarographic and spectral behaviour of hexavalentmolybdenum in sulphuric acid solutions

Amany M A Ibrahim" & Mohamed I IsmailDepartment of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt

Received 5 July 1993; revised 6 July 1994; accepted 17 January 1995

The polarographic and spectral behaviour of hexavalent molybdenum in sulphuric acid solutions havebeen studied in the absenceand presenceof phenols. The processesof reduction pass ultimately to trivalentmolybdenum via two to four reduction wavesdepending on the acid and the phenol concentrations. Thepresence of resorcinol or pyrogallol causes the reduction waves to split or intermingle due to the surfaceactivityof the phenols. In the presenceof highconcentrationsof pyrogallol,the sulphatecomplexestransforminto the pyrogallol complex. Also, these phenols cause the polymerization of the monomer molybdenumspecies. The nature and the kinetic parameters of the waves in different media have been investigated.

It is well known that molybdate does not produce awave from netural or alkaline solution but welldefined polarogram is obtained in acidic media. In allacids, two to four waves were observed, depending onthe acidity, which were ascribed 1 - 5 to reduction toMo(V), to the red trivalent molybdenum, to the greentrivalent molybdenum and to Mo(OHh. The polaro-graphic behaviour of hexavalent molybdenum inH2S04 of different concentrations shows that itundergoes successive reduction to quinque andtrivalent states",

The polymerization of H2Mo04 at differentconcentrations occurs at pH 4-6 to form complexes ofthe type (HMo04)n, (Mo04)n+1 and (H2Mo04kdepending on the concentration and the type(HMo04)n, and (H2Moo)n +1 and (H2Mo04)n,depending on the concentration and the pH of thesolution. At pH> 6 there is no polymerization whilein strong acid solutions monomeric forms"-? of themolybdenyl ions Mooi+ or MoO(OH)i+predominate.

Nothing much is known about the effect of phenolson the polarographic or spectral behaviour ofMo(VI) in acidic solutions. These' phenols maybehave as surface active substances (SAS) and at thesame time as complexing agents. Thus, the effect oftwo different phenols, resorcinol and pyrogallol, onthe electro-reduction of hexavalent molybdenum inH2S04 will be investigated to prove the dependence ofthe electrode behaviour upon the type and concen-tration of the phenol present.

Materials and MethodsA stock solution of 0.05 mol dm - 3 ammonium

molybdate was prepared by dissolving the properamount of (NH4)6M07024 . 4H20 (AR, BOH) indeionised water. The dilute solution wasstandardized by a recommended procedures.Resorcinol and pyrogallol (BOH) were used as such.Freshly prepared solutions or the required weighedamount of the solid were used before theexperiment.

The polarograms were recorded by RadiometerP04 type recording polarograph using a droppingmercury electrode with rate of flow mercury, m = 2.15mgs ' and drop time, t= 3.~5 s at mercury height,h = 52 em. A saturated calomel electrode (SeE) wasused as reference electrode. Oepolarizer concen-trations studied were 5 x 10- 4 mol dm - 3 in H2S04

solutions. The electrolysis at controlled potential wasverified using a potentiostat Tacusel of type ASA4/60. The electrolytic cell used was the same as thatdescribed by Peltier et al", The absorption spectrawere studied with the aid of a Unicam Sp 1800automatic, recording spectrophotometer using I emmatched cells.

Results and Discussion

Polarographic and spectral behaviour of hexavalentmolybdenum in sulphuric acid solutions

The polarogram of 5 x 10-4 mol dm - 3 Mo(Vl) inthe presence of different concentrations of H2SO4-ranging from 0.1 to 5.0 mof dm - 3 exhibits two or threewaves depending on the acid concentration used,(Fig.l). In these media, Mo(VI) undergoes successivereduction to quinque-and trivalent states where bothMo(VI) and Mo(V) can exist in two forms showing

IBRAHIM et al.: EFFECT OF PHENOLS ON POLAROGRAPHIC & SPECTRAL BEHAVIOUR OF Mo(VI) 541

11)

8

«=>.

6 -J'

~-

Fig. I-Polarographic behaviour of5 x 10-4 mol dm - 3 Mo(VI)in H2S04 solutions; 1-0.1, 2-0.5, 3-1.0 and 4-5.0 mol dm - 3

H2S04

two separate waves-on i-E curves. Accordingly, thereduction of Mo(VI) at DME in the presence ofH2S04,as supporting electrolyte is in harmony withthose of other authors+!" and may take placeaccording to one of the following 'Schemes:The splitting of the waves (A) and (B) can beattributed to the presence of two different species ofeach of the Mo-sulphate complexes which arepresent in equilibrium and get reduced separately+!".This is further supported by studying the spectralbehaviour ofMo(VI) and Mo(V) in sulphuric acid ofdifferent concentrations, (Fig.2). The UV spectra ofammonium molybdate in water solutions exhibit twobands at 210 and 230 nm corresponding to theabsorption of the monomer H2Mo04 and topolymers of the type (HMo04)n and (Mo04)"+ I (ref.7)respectively. The intensity of the first band isapproximately twice that of the second one indicatingthat, the monomer species is predominant in thewater solution. It is apparent that the positionsof the

Z& 101011111wovalll,)

"'0 (VI) & • Mo(V)-~----,5;n91& wova(A) Two wovu(S,ond~)

Z& 1010(111)wova(1I2)

Schama 1

:0-5 ->5moldm-3)

101o(VII • Mo(V) 2a • "'0(111)5in91a wova (A) 5in91a wova (I)

Schem& Z

a,..-----<MOO(V)

WQYC(~) .....!!.....,IoIO(III)

« Si"\lI«_ •• (II)"-'---:-:-'i"IoIo(V)

wava(AZ

MO(VI)------ __ -<Twu waves (A,CIIICI"J)

Scheme 3

1.~

( b)(a)1.2

•• 1.0uc.•~..o.•.0«

~90 210 230 25C 270 2S0 l1C lSC i30 210. 310 350 3S0 L3o.

Fig. 2-Effect of H~S04 concentration on the spectrum of (a)1.0 x 10-4 mol drn ":' Mo(VI), (b) 3.0 x 10-4 mol dm - 3 Mo(V).1-0.0, 2-0.1, 3-0.5, 4-\.0, 5-5.0 and 6- 10.0 mol dm -.l H~S04

bands and their intensities change on varying [H2S04]

from 0.1 to 10 mol dm -.3. In strong acid solutionsmonomeric forms of the 'molybdenyl ions pre-dominate, the formula of these ions being Mooi + or

MoO(OH)i +. It is likely that the ion MoO~ + is formed inconcentrated solutions of sulphuric acid, and occursas M002S04 (ref.6, II) which can be obtained bydissolving Mo03 in concentrated H2S04. Thus, thepresence of sulphuric acid brings about progressivedepolymerization of the polymeric forms (HM.o04).n,(Mo04)" + I by incorporating the molybdenyl ions III

sulphate complexes of the type M002S04 or[Mo02(S04hF - .

In water solution, the spectrum of Mo(V) show twobands at 234 and 320 nm indicating the presences oftwo different species of Mo(V). On increasing theH2S04 concentration, the bands are blue shiftedwhile the absorbance decreases indicating thepolymerization of Mo(V) species. When theconcentration of H2S04 is further increased to 5.0 moldm - 3, the intensity of the two bands suffers markedincrease due to the interaction of the acid moleculeswith the Mo(V) species to yield the sulphatecomplexes.

The number of electrons participating in each wavewas defined coulometrically and the diffusioncoefficients of the reducable species at differente acidconcentrations were calculated (Table I). Onconsidering the wave (A) representing the uptake ofone electron, one may assume that this wave shouldbe a reversible one. But it is not the case as indicated bythe polarographic parameters (Table I). Theirreversibility characteristics of the wave may be dueto its ill-defined nature and to the presence of theMo(VI)-sulphate complex. However, the kineticparameters of the daughter waves (AI) and (A2)indicate the increase in the reversible behaviour withincrease in acidity. Analysis of the waves (Bj) and (B2)

Indicate that the rate determining step should involve

542 INDIAN J CHEM, SEe. A, JULY 1995

Table I-Polarographic results obtained for the electroreduction of 0.5 mol dm -.1 ammonium molybdate in presence of differentconcentrations of H~S04

[H~S041 D n Wave id -1::1 ~ S K<" LGtmol dm-3 em- S-I (IlA) (NHE) (ern S-I) (kcul mol- 1)

0.10 A 2.60 -0.18 7.50 7.IOxlO-2 41.8 J2 BI 2.00 0.10 11.20 2.20 x 10-4 50.41.15 x 10-5 2 B~ 4.20 0.41 15.40 1.50 x 10-9 80.1

0.50 A 2.60 -0.38 15.00 1507 16.91.04 x 10-' 2 B 5.60 0.08 9.10 5.60 x 10-4 48.2

-4.00 l.4xlO-3 45.91.0 A 2.50 -0.37 14.80 857 12.5

9.45 x 10-6 2 B 5.30 0.11 7.5 4.30 x 10-4 48.93.7 I. 10 x 10-3 46.4

5.0 AI 1.20 -0.39 15.0 1698 10.86.70 x 10 6 A~ 1.00 -0.26 19.8 316.2 15.3

2 B 4.40 -0.40 15.3 9.70 x 10-3 41.1

10.0 6.00 x 10-3 42.3D. diffusion coefficient; n, number of electrons; S, slopes of analysis

I/~ _-1._/

.: B.I 2 ,\

I 3/.~I /: ..I / ~..,/

I / "".>5I //62~ B'/· .../

II .~;:::;/

!./ 61I'A2/

I ,:0/

AI :'::~21.0 li/

/.ifo 1II-......L.---:.I--:l-:--:-1----:1':-~~ 0 L...£..!...J....--L_.l--L-.....JL-.....I....J

-0.1 -0.3 -O.!>-0.7 -0.9 -1.0 0.0 -0.2 - 0.4 -0.6 -0.11-1.0

EeV) vs SCE ElV) vs SCE

Fig. 3-Effect of resorcinol on the polarographic behaviour of5.0 x 1O-4mol dm-3 Mo(VI) in; (a)O.1 H~S04' (b) 5.0moldni-3H~S04' 1-0.0%, 2-.0.2%, 3-1.0%, 4-2.0% and 5-5.0%

resorcinol

(a)

10

8

the uptake of two electrons and the reduction processis irreversible and the irreversibility along the upperpart is higher than that along the lower one.

Effect ofresorcinolIn 0.1 mol dm - 3 H2S04 and 0.2% resorcinol, the

first wave (A) joins completely the wave (BI) forminga composite wave whereas wave (B2) shifts to morenegative potentials (Fig.3). As the concentration ofphenol is increased, the Eli2 and it! of the compositewave are not largely affected whereas, wave (B2)

suffers progressive shift to more negative potentialuntil it gets vitiated by the hydrogen wave. In 0.5,1.0and 5.0 mol dm-3 H2S04 and on using low

70(b)

6.C

5.0

«~ 4.0

'0

30

2.0

concentration of resorcinol, the waves (A) and (B)each gets divided into two waves (AI,A2) and(BI,B2respectively. On increasing the percentage ofresorcinol, the whole polarogram shifts to morenegative potential and the waves At. A2 and BI joineach other forming a composite wave. However, inthe case of 5.0 mol dm - 3 H2S04 the waves (AI) and(A2) do not join the wave (BI) on increasing theconcentration of the phenol up to the maximumpercentage used; only the wave (AI) shifts to morenegative potentials to join the wave (A2) (Fig.3).

The data indicate that the concentration ofresorcinol used for splitting of the two waves (A andB) decreases as the concentration of the acid is

mRAHIM et al.: EFFECT OF PHENOLS ON POLAROGRAPHIC & SPECTRAL BEHAVIOUR OF Mo(VI) 543

Table 2~Polarographic results obtained for the e1ectroreduction of 0.5 mol dm - 3 ammonium molybdate in the presence of differentconcentrations of H2S04 and resorcinol

[H2SO4] Resorcinol Wave id -£1/2 S K"o .6G~

mol dm-3 °/. (VA) (NHE) (em S-l) (kcal mol")

"0.1 1.0 A+BI 4.2 -0.08 4.0 5.20 x 10-3 48.4

B2 3.7 0.56 5.1 4.40 x 10-6 60.1

0.5 2.0 AI +A2+nl 3.4 -0.12 5.0 1.20 x 10-2 40.6

B2 3.1 0.50 5.4 5.90 x 10-6 59.4

1.0 5.0 A+BI 3.2 -0.09 4.5 7.30 x 10-3 41.8

B2 3.3 0.50 5.0 9.IGx 10-6 58.3

5.0 5.0 A2 1.4 -0.29 ~.7 0.79 29.9

BI 1.4 -0.03 13.4 9.50 x 10- 3 42.31.5 0.31 10.0 1.90 x 10-6 62.3

Table 3-Polarographic results obtained for the electroreduction of 0.5 mol dm - 3 ammonium molybdate in the presence of differentconcentrations of H2S04 and pyrogallol

[H2SO4] Pyrogallol Wave id -£1/2 S /(,,0 .6G'mol dm"? % (VA) (NHE) (em S-l) (kcal mol- 1)

0.1 5.0 A 2.3 -0.12 13.2 0.12 34.7

BI+B2 4.4 0.35 10.0 1.0 x 10-7 63.5

0.5 0.2 A 2.6 -0.26 12.5 5.5 25.1

BI 2.7 0.06 13.5 4.70 x 10-4 48.7

B2 2.6 0.45 8.8 3.30xlO-7 67.01.0 5.0 A 2.0 -0.24 13.4 6.20 24.8

BI 3.4 0.14 13.3 4.1 x'lO-s 54.7

5.0 A 2.2 -0.24 12.5 3.82 26.1

BI 4.3 0.14 15.0 2.20 x IO-s 56.3

5.0 8.0 A 0.8 -0.32 10.0 3.80 27.7

BI I.7 0.07 10.9 4.10 x 10-4 49.0

increased. The formation of the composite wave (AI,A2 and BI) on the other hand, requires a higherconcentration of resorcinol as the concentration ofthe acid is increased.

These phenomena can be interpreted on the basisof the surface activity of resorcinol which causes shiftof the waves to more negative potentials without thedisappearance of any of these waves. Thus, the waves(Ab A2, BI and B2) are due to the reduction of twotypes of Mo(VI)-and Mo(V)-sulphate complexes.On the other hand, the presence of resorcinol favourspolymerization of the monomer molybdenumspecies whereas depolymerization of the polymericones is favoured by H2S04 as was also suggested bythe UV spectra, where the intensity of the band at 210nm decreases as the concentration of the phenol isincreased.

The effect of resorcinol on the kinetic parametersand the nature of the waves can be gathered from thedata in Table 2. The reduction processes along thedifferent waves become more diffusion controlledand the irreversibility increases in the presence of

resorcinol than in its absence. None of the wavesobserved is reversible.

Effect oj pyrogallolIn order to study the effect of pyrogallol on the

polarographic behaviour of Mo(VI), the spectralbehaviour ofMo(VI) in H2S04 and in the presence ofpyrogallol should be investigated to throw light onthe nature of probable species. The spectrum of3 x 10-4 mol dm ":' Mo(VI) and 3 x 10-4 mol dm-3pyrogallol in water solution exhibits two bandslocated at 230 and 363 nm (Fig.4). The disappearanceof the band at 21 0 nm and the existence of the band at230 nm indicate that pyrogallol decreases theconcentration of the monomer H2Mo04 andenhances the dominance of the polymeric species(HMo04)n, MoOS + ~The new band appearing ats 363nm is due to a complex formed between Mo(VI) ionsand pyrogallol as supported by the increase in theintensity of the band as the concentration ofpyrogallol is increased. The complex formation isfurther supported by the change in colour of the

544 INDIAN J CHEM, SEe. A, JULY 1995

solution to yellow and finally to orange by increasingthc concentration of pyrogallol.

On the other hand, die spectrum of 3 x 10-4 moldm - 3 Mo(VI) and 3 x 10-4 mol dm - 3 pyrogallol in0.1 mol dm - 3 H2S04 consists of only ope band at 230nm (Fig.4); the presence of H2S04 retards theformation of the pyrogallol-molybdate complex.This is evident from the disappearance of the bandnear 363 nm and also the disappearance of the yellowcolour formed by 'the addition of pyrogallol to themolyhdate in water solution.

The spectrum of Mo(V) in water consists of twobands at 234 and 316 nm (Fig.S). Addition of3 x 10-4

mol dm - 3 pyrogallol to this solution changes thebrown colour to orange while the first band at 234 nmis unaffected but suffers slight red shift and thesecond band at 316 nm suffers increase in intensity

2.0

1 B (a)

1.6

~ i.,u0.. 1.2"c0 i.c.•"..

( b)

O.L

02.l···~·~:':':'·:~:~::":'::·······1;'0 2S0 290' no »o LlO LSC 210 2S0 290 )]C j70 <.ro<,;o

wa v e t e n q t h , n m

Fig. 4--- Effect of pyrogallol on the spectrum of 3.0 x 10- 4 moldm'" Mo(YI)in; (a) water. (b)O.1 mol dm -, H~S04_ 1-3 X 10-4•

2-6x 10-" 3-l.SX 10-'. 4-3x 10-3 and 5-6x 10-3 mol dm "?

pyrogallol

without change in position. Further increase inpyrogallol concentration brings about the same effectin addition to the appearance of a new broad bandnear 430 nm with increase in intensity.

In the presence of 6 x 10- 2 mol dm - 3 pyrogallol,the position of the first band is not affected whereas itsintensity starts to decrease until it disappears in thepresence of I x lO- 2 mol dm - 3 pyrogallol (Fig.S).These results indicate that pyrogallol forms acomplex with the monomeric Mo(V) species in watersolution and at the same time brings aboutpolymerization of the monomer species.

In the case of 0.1 mol dm - 3 H2S04, the broad bandlocated near 430 nm does not appear and the colour ofthe solution does not change to orange indicating thatpyrogallol does not form a complex with Mo(V)species in the presence of O.I mol dm - 3 H2S04(Fig.S). '

The effect of pyrogallol on the electro-reduction ofhexavalent molybdenum in sulphuric acid is differentfrom that of resorcinol (Fig.6). In 0.1 mol dm - 3

H2S04 where the polarogram consists of three waves{A, B, and B2), a small amount of pyrogallol (0.2%)causes wave (B,) to join completely wave (B2)' Underthese' conditions a pyrogallol complex is obtained inthe presence of pyrogallol instead of two sulphatecomplexes in its absence. On increasing theconcentration of pyrogallol, the EI/'2 of the first waveis not affected whereas the second wave suffersshifting to more negative potentials as a result of thesurface activity of the phenol.

In O.S and 1.0mol dm - 3 H2S04 and in the presenceof 0.1 % pyrogallol, the wave (B) divides into twoapproximately equal portions B. andBjindicatingthat Mo(V) occurs in two sulphate complex formswith different stabilities (Fig.6). When theconcentration of pyrogallol is increased, the height of

1.L, ,

1.2

"-

u1.0c

'"DL 0.80

'".CJ0.6«

O.L

(a) ( b)

Wavf'\~ngt h I nm

Fig. S-Effect of pyrogallol on the spectrum of 3 x 10-4 mol dm-' Mo(Y) in: (a) water,(b) 0.1 mol drn " H~S04' 1-0.0, 2-3x 10-4, 3-9x 10-', 4-3x 10-'. 5-6x 10-' and 6-1 x 10-3

mol dm -, pyrogallol

IBRAHIM et al.: EFFECT OF PHENOLS ON POLAROGRAPHIC & SPECTRAL BEHAVIOUR OF Mo(VI) 545

(b)10 (a)

,,,o /-Ql -0.3 -o.~ -0.7 -0.9 -1.1

E(V) vs !:ICE

o~~ __ ~ __ ~~ __ ~~~0.0 -0.2 -0.' -0.6 -O.B -1.0

E{V) v s sc E

Fig. 6--Effect of pyrogallol on the polarog~aphic behaviour of 5 x 10-4 mol drn " c1 Mo(VI) in;(a) 0.1 H2S04 [I -0.0%, 2-0.2%, 3-1.0%, 4-5.0% and 5-8.0'10 pyrogallol], (b) in 1.0 mol dm - 3

H2S04 [1-0.1%', 2-\.0%, 3-3.0oio. 4-5.0% and 5-8.i)% ,pyrogallol]

wave (81) increases at the expense of wave (82)together with the appearance of a minimumfollowing the rising portion of wave (BI)' Theappearance of the minimum along (81) can be .attributed to the formation of an insol uble species ofMo(III) and/or due to the adsorption of this phenolon the mercury surface. The increase in the limitingcurrent of wave (8,) at the expense of wave (82)

indicates that, Mo(V)-sulphate complex transformsinto Mo(V)-pyrogallol complex. In the presence ofmore concentrated pyrogallol ( 5.0%), both 'theminimum and the wave (82) disappear completelyprobably as a result of the elimination of the inhibitivefilm by the adsorbed phenol.

In the presence of 5.0 mol dm':' 3 H2S04 and 0.2%pyrogallol, the two waves (AI and A2)join each otherand at the same time wave (8) splits into two portions(81 and B2)' As the concentration of pyrogallol isincreased wave (82) shifts to more negative potentialsuntil it disappears in the presence of higher pyrogallolconcentration (8%); contrary to the behaviour in thepresence of 0.5 and 1.0 mol dm - 3 H2S04, wave (82)

does not increase in height at the expense of wave (B2)

indicating that the transformation of the sulphatecomplex into the pyrogallol complex is retarded byincreasing the acid concentration of the supportingelectrolyte.

Thus, in more dilute solution, 0.1 mol dm - 3

H2S04, the two Mo(V)-sulphate complexes can

transform into a Mo(V)-pyrogallol complex by theaddition of 0.2% phenol. As the concentration ofH2S04 is increased, this transformation becomesdifficult. The amount of pyrogallol required forcomplete transformation of sulphate complex intopyrogallol complex is found to increase as the acidstrength is increased.

The kinetic parameters of the reduction processes'(Table 3), indicate that wave (A) proceeds lessirreversibly than wave (al) and the presence ofpyrogallol renders the reduction processes to becomemore diffusion controlled.

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Electrochem, 32(3) (1971) 463 .. 3 'Millal R K & Mohan L, Z Naturforsch B. 24(8)( 1969) 1053.4 Kolthoff I M & Hodara L, J electroanal Chem, 4 (1962) 369.5 Johnson M G & Robinson R. J anal Chem, 24 (1952) 366.6 Ramana Rao D V & Jain S. J sci ind Res. 15 (1956) 667.7 Babko A K & Nabivanets 8 I, Zh Neog Khim, 2 (1957) 2085.8 Vogel A I, TeXI hook ofquantitative inorganic analysis. theory

and practice (Longman's Green, London), 1951. p.469.9 Peltier D. Legauyader M & Tacussel J, B//ll Sac Chini Fr. II

(1963) 2609.10 Shnaiderman S Ya & Khrustalev G I, Zhur Obskches Khim, 29

(\959) 20.II Perrin D, Organic complexing reagents (Inter Science, New

York) (1964), p.293.