24

M. u FA~AH and S. Z. 5flKUAm:

16st sich Barinmsulfat in einem zwei- bis dreifachen UberschuB an A D T A rasch u n d quantitativ nach kurzem Aufkochen. Wird start des Trigthano]amins N a t r i u m h y d r o x y d zugegeben und die LSsung d a m i t a u f p~ 12 gebracht, so geht Bariumsulfat noch schnel]er in L6sung bzw. der erforder]iche UberschuB ~n ~ D T A k a n n geringer sein. Die anschlieBende Titration mit MagnesinmchloridlSsung ist d a n n abet wegen des fehlenden Puffers nieht durchfiihrbar. A n Stelle des Trigthanolamins eignen sich auch Monogth~nolamin u n d andere Puffer, die in diesem pH-Bereich wirksam stud. Literatur 1 BELCHER,R., D. GIBBOI~SU. T. S. WEST: Chem. and Ind. 1954, 127; vgl. diese Z. 143, 210 (1955). -- 2 In H~.r.LE~, A. : Verfahren zur Untersuchung der AnBenluft und deren Bedeutung fiir die Lufthygiene, Schriftenreihe des Vereins ffir Wasser-, Boden- und Lufthygiene Nr. 10: ]~eitrgge zur AuBenlufthygiene, Stuttgart: Fischer 1955. -- 3 T~T~WErLE~, I(., u. W. PILz: Naturwissenschaften 41, 332 (1954); vgL diese Z. 144, 380 (1955). Dr. F. l~U)ILEl% l~. I-IEt~BOLSttEII~:EI~und G. WOLF, Staatliches Institut fiir Hygiene und Infektionskrankheiten, Saarbrficken

Atomic Energy Establishment, Cairo

Trivalent Molybdenum as Volumetric Reducing Agent By M. Y. FARAH and S. Z. )IIKHML With 8 Figures in the Text

(Eingegangen am 24. September 1958) I n studying the applicability of lower-valency m o l y b d e n u m 1~, tungsten 14 and u r a n i u m 1 as volumetric reducing agents, it was found t h a t t h e y could be used for numerous titrations involving higher valency ions. Although some of these new titrants were of limited applicability, y e t t h e y h a d the advantage over more powerful ones, in being more stable and in t h a t no special air-tight devices were needed during the titration experiments involving their use. I n a previous investigation, one of us 13 had shown t h a t qtfinquevalent m o l y b d e n u m in 2 N hydrochloric acid, was successfully applied for t h e determination of ferric, dichromate, vanadate, eerie, iodate and b r o m a t e either alone or in mixtures of two and three together. F u ~ I A ~ and MuaaAY 5 had investigated the stability of such solution and found t h a t it was possible to work with quinquevalent molybdenum, without precautions to exclude air if the solution was used within short time. Studies

Molybdenum(III) as Volumetric Reducing Agent

25

of the oxidation rates of Mov had shown also 3, that neither light nor heating for four hours at 80 ~ C had any appreciable effect in favouring oxidation; only copper and platinised platinum catalysed it. An increase in acidity of the medium to about 8N tIC1 made such solution suffer oxidation in one week's duration only to the extent of 0.90/0 as compared to 6.3 and 14.7~ in 4 N and 2 N acid respectively. Formal potentials for the penta-/hexavalent molybdenum system were measured by F o ~ s T n ~ and F~ICX~ a and found to be 0.532, 0.551 and 0.697 V in 2, 4 and 8N hydrochloric acid respectively. T o v R ~ u and En- S H a Y ~2 reinvestigated the redox potentials of that system and observed discontinuity with acid concentration. In studying the effect of zinc and lead amalgams on molybdic-sulphurie acid solutions, 1~I~'s and I~A~zo ~ isolated trivalent and quadrivalent molybdenum forms, which they determined afterwards through fen~ic solutions. Similar oxidation titrations of trivalent molybdenum were undertaken by M ~ m ~ T T ~ and YVT~MA9 in phosphoric acid media using mixed eerie-permanganate oxidiser; however serious discrepancies were lacking interpretation. G A r ~ o ~ used MolII to reduce nitro and nitroso groups 1~o amino compounds and extended r method to the determination of picric acid and cupferron. The objec~ of the present investigation is to extend the use of trivalent molybdenum in hydrochloric and sulfuric acid media to various other detel~q~inations. Although the slope of formal potentials follows a parallel decrease in lower acid normalities, as in the case of the pentavalent form, falling from 0.3 to 0.1 V between 9N and 1N hydrochloric acid respectively 4, yet the solution in higher acid concentration was preferably thought of as an appropriate titrant with the hope of using it in ordinary opened burettes. From theoretical considerations, the equilibrium constunts of some reactions and their corresponding degrees of completion, i.e. the fractions failing to react at the equivalence point, confirmed the applicability of the suggested reagent for the reduction of ferric, dichromate, vanadate, cerie, iodate, b r o m a t e . . , etc.

Experimental a) Preparation and Standardization of ~eagent Trivalent moly;odenum solutions were obtained by the electrolytic reduction of pm'e molybdic acid on a platinised cathode in approximately 8~~hydrochloric acid. The process was performed in a cell having a capacity of 300 ml with a platinum spiral anode and a platinised wire-gauze cathode separated from each other by an AG1 sintered-glass diaphragm. There is a slight polarisation and a quantitative current yield. The acidity was always kept above 8~~ to avoid formation of pentavalen~ molybdenum. Alternately, recourse was made to ~MA~- method s based on reduction through metallic mercury in 91Xhydrochloric acid. The tervalent solution in strong acid is salmonpink in colour, while that in weak acid (2~N)is emerald green and less stable; however if filtered from mercury, washed with 1/1 hydrochloric acid

26

M. Y. :FAmt~ and S. Z. MI~:K~L:

and titrated immediately with ceric sulphate while adding trivalent molybdenum from the burette, it was found to give excellent results so long as no trace of copper or iron is present. A typical standardisation experiment is reported in Fig. 1. It was conducted in 2N sulphurie acid, in the hot (80 ~ C), the equilibria were reached instantaneously and a peak inflection, amounting to 475 millivolts per 0.1 ml reduetant added, was recorded. Stock solutions in 8 N hydrochloric acid, were found to be sufficiently stable within reasonable lengths of time; thus, a 0.1201 N solution decreased in titer to 0.0648 after two months.

b) The Titration Device The titration vessel consisted of a well-steamed pyrex glass beaker fitted with a ground joint stopper containing the necessary inlets for the indicator platinum rod electrode, the salt bridge, the tip of the burette, the mercury-seal stirrer, inlet and outlet for inert gas atmosphere. The tervalent solution was delivered from an ordinary opened 10 ml NPL burette, graduated to 0.02 ml and the titration performed in a current of carbon dioxide.

c) Preparation of Miscellaneous Solutions Approximately deeinormal solutions of ferric chloride were prepared by dissolving the appropriate amount of Carlo Erba (pro analysi) product in pure hydrochloric acid of the required strength; the iron content of such solution was determined gravimetrically as ferric oxide according to the well-known procedure. Potassium dichromate, iodate and bromate of the analar grade were dried up to 120 ~ C before bringing to solution. In preparing the dichromate as standard, the procedure recommended by Fv~x~ss Gwas adopted. The titrations with bromate and iodate were always conducted potentiometrieally, alternately L ~ c ' s volumetric procedure s, involving the use of chloroform as a solvent for the liberated iodine, was applied; few drops of concentrated sulphuric acid were also added as catalyst. In the case of volumetric titration using bromate, methyl orange was applied as indicator. The eerie and vanadate solutions were prepared by heating the calculated quantity of pure ceric sulphate, vanadie acid or ammonium vanadate respectively in enough sulphuric acid until dissolution, leaving to cool and diluting to volmne. The ceric sulphate was standardised against analar sodium oxalate, while the vanadium content was determined volumetrically by reduction through an excess of analar decinormal ferrous ammonium sulphate which reduces the vanadate to the green vanadyl form and after oxidising excess ferrous through persulphate, the reduced vanadyt w~s titrated against standard permanganate.

Results and Discussion Some r e p r e s e n t a t i v e results which were f r e q u e n t l y t e s t e d for t h e i r r e p r o d u c i b i l i t y are s u m m a r i s e d in t h e following Tables 1 a n d 2 an d r e p r e s e n t e d g r a p h i c a l l y in Figs. 1 to 8 (see p. 28/29 ).

a) Trivalent Molybdenum as Reductant /or Ceric, Dichromate, Ferric and Vanadate F r o m t h e results shown in T a b l e 1 it is e v i d e n t t h a t t h e i n d e p e n d e n t r e d u c t i o n of d i c h r o m a t e , ccric, ferric an d vanada~e could be a c h i e v e d u n d e r a wide r a n g e of conditions. F o r d i c h r o m a t e in h y d r o c h l o r i c acid media, r e d u c t i o n was i n s t a n t a n e o u s a n d t h e t i t r a t i o n s were carried o u t in t h e cold. I n s u l p h u r i c acid media, however, t h e a t t a i n m e n t of equi-

27

Molybdenum(III) as Volumetric Reducing Agent Table 1 Titratio:a 1 CelV in 2N H:,SO4 2 K2Cr~O7 in 0.SN HC1 3 K~Cr20~ in 2N HC1 4 K2Cr207 in 4N tIC1 5 K2Cr20 ~ in 0.] N H2S Q 6 K~Cr20v in 0.5N H2SO4 7 K2Cr20 v in 2N H~S04 8 K~Cr20~ in 4N H2S04 9 1%C1a in 0.1N HC1 10 FeC13 in 1 N HC1 11 FeCIa in 2N I:[C1 12 FeCla in 4N HC1 13 Vanadate in 2N HC1 14 Vanadate in 4N HC1 15 Vanadate in 6N HC1

~Iax. inflection per 0.1 mll~Iorg sol. (mY)

MoIri sol. calculated Milliliter

l~folH sol. used ~[illiliter

l~rror Percent

475 272 388 436 299 291 286 294 39 12 8

10.55 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00 6.31 6.31 6.40

10.55 12.00 12.05 12.05 11.93 11.88 12.00 11.90 11.93 11.90 12.00 12.00 6.36 6.33 6.50

nil nil + 0.4 + 0.4 -0.6

Max. inflection per 0.1 ml oxidant (mV)

Oxidant calculated ~s

Oxidant used Milliliter

Error Percent

13

12.00 12.00 12.00 12.00 6.48 5.00 6.48 13.50

11.50 11.60 11.90 11.30 5.50 4.50 6.50 13.35

1

100 192 442

-

1.o

nil -0.8 -0.6 -0.8 nil nil +0.8 +0.3 + 1.6

Table 2 Titration 16 17 18 19 20 21 22 23

Iodate in 2N I-IC1 Iodate in 4N HC1 Iodate in 6N tiC1 Iodate in 8N I-IC1 Bromate in 2N HC1 Bromate in 4N HC1 Bromate in 6N HC1 Ceric in 4N It:C1

161

168 47 20 33 47 362

--

4.2

--

3.3

--

0.8

--

5.0

--

15.0

--

10.0

+

0.3

-

1.1

l i b r i u m was slower. This is to be a t t r i b u t e d to a m o r e p r o n o u n c e d pass i v a t i o n of th e electrode in c o n t a c t w i t h t h e d i c h r o m a t e solution. B y h e a t i n g to a b o u t 80 ~ C, t h e speed of a t t a i n m e n t of e q u i l i b r i u m was m u c h e n h a n c e d . T h e o p t i m a l error n e v e r surpassed 1~ in t h e worst cases. M o r e o v e r in 0.1 N acid (Exp. 5), th e end of t i t r a t i o n was characterised b y t h e a p p e a r a n c e of a pale blue eolouration due to m o l y b d e n u m blue. I t is t h u s possible to p e r f o r m th e t i t r a t i o n v i s u a l l y af t er some eiectrom e t r i c trials. T h e a c c u r a c y of t h e t i t r a t i o n , p a r t i c u l a r l y w h e n followed p o t e n t i o m e t r i e a l l y affords an a d d i t i o n a l m e t h o d for s t a n d a r d i s i n g triv a l e n t m o l y b d e n u m in presence of sulphuric as well as h y d r o c h l o r i c acid n p to concentrations of 4 N . F i g s . 2 an d 3 illustrate t h e r e s p e c t i v e p o t e n t i o m e t r i e t i t r a t i o n curves for d i c h r o m a t e in h y d r o c h l o r i c an d su!phuric acids, w i t h i n t h e r a n g e of 0.1 to 4 N . I n t h e case of r e d u c t i o n o f ferric, t h e m a x i m a l inflection is m u c h less p r o n o u n c e d t h a n in t h e case of d i c h r o m a t e ; it fluctuates b e t w e e n 1 an d

28

FA~A~

M. Y.

a n d S. Z. M : ~ _ a ~ :

0.000

O.800

L200

0.700

)'-'

I

0.500 ~

O.qO0

O . q O 0 ~

,

O.200 0

~I

0.300

8

K2Cr207/}70.5N BCt

~

,

, 2.ON ,,

,--o

n

" ~.ON

1

0

If

) ~

i

"

]

I 8

I 72

IF

m ~ Momsotuh'on

m{ Mo'~oluiion

Fig. 1. Titration of CeIV in 2 ~ sulfuric acid by Morn solution

Fig. 2, Titration of dichromate in hydrochloric acid medium by MerrI solution

I

""

i ^

x

*

0.700

0. 700 O.gO0 0.6"00 -.~ 0.500

0,500

%

O"q OO ~c.---o K g Cr207 /n 04 N _

,

~

, 0.SN

I~...

~ OXIO0

H2SO~I ,

~

"

t

0.300

•215 o~o

O.3OO I

0

o---oFeCL3inOjN H CI,

I

g

I

8 TZ m~. M o ~ solu//on

~

.

O.206

J

T6"

Fig. 3. Titr~tio~ of d~ehrom~te in sulfuric acid m e d i u m b y ~/~oT:~ soIution

u

u 2.oN

"

" ~.ON "

I

I

q

J

u

,

',

~ 12 m ~ hlo ~solulion

~ig. 4. Titration of Fe ]zz ~n hydroohlorIG ~cid m e d i u m b y M e IT~ solution

T~"

Molybdenum(llI)

29

as Volumetric Reducing Agent

0 . 0 0 0 ~

1.000

0.900

0.<700

c - - - o V0~ i'n 2 90 H EL . " qN " ~- 0.6"06 " " CAI "

0.50~

i

,

\

[f

"~- 0.700

t

0.~00

\

I

0.500 - -

i 0300

q

\

8 ~ L M o m solulion

o---o /'r •215 "--*

. .

217 H C I . - .SN "SN

. .

0.400

12

i<

8

12

m{ f~ 103 s o / u / i o n

~ig. 5. Titration of vanadate in hydrochloric acid medium by 1~o~II solut,ion

Fig, 6. Oxidation titration of EOHT by iodate iu hydrochloric acid medium

0.300

1.200,

8 12 ~L K 8r 03 s o / u / i o n Fig. 7. OxidaDion titration of ~ o ~ by bromate in hydrochloric acid medium

IG

"

0

t

t~

# mL

12

OeS~-soluli'on

Fill. 8. Oxidation titration of ~ o TiT by CeIV in 41~ hydrochloric acid

I~

30

M. Y. F ~

and S. Z. M I ~ :

12 mV per 0.1 ml, between 4N and 0.11~ hydrochlorid acid respectively. The titration m a y be performed in the cold, but better in the hot. Again the end-point in 0.1 N media is characterised b y the appearance of a pale blue colouration which became more intense on progressive addition of reducing agent. I t is therefore necessary to accustom the eye to this visual end-point b y preferentially performing parallel electrometric titrations. Fig.4 shows representative titration curves of ferric iron in hydrochloric acid solutions of various concentrations, using trivalen~ molybdenum as reductor. As for the titration of vanadate, theoretical prediction prooved t h a t the E 0 values of the system VO2+/VO 2+ as determined b y I~ l~ and CAgP~T~g 2 are sufficiently far removed from the system under investigation, to make the process quantitative. Experiments 13, 14 and 15 (Fig.5) confirmed this view. The titration is recommended to be conducted in lower than in higher acidity; the m a x i m u m increase in e. m. f. per 0.1 ml amounted approximately to 200 mV in 4Nhydrochloric acid. The equilibria were attained within few seconds and the maximal error approximated 0.3o/0 . Several parallel experiments yielded concordant results.

b) Oxidation Titration o/Mo III through Iodate, Bromate and Cerie Solutions I t was also found, t h a t both oxidants (iodate and bromate) can quantitatively oxidise tervalent molybdenum; however, owing to their instability in acid media, they should be delivered from the burette. In this case, when trivalent molybdenum was diluted using hydrochloric acid solutions of concentrations ranging between 2 and 8 N, the titrations had to be conducted in an inert atmosphere and at room temperature in order to minimise the oxidisability of the reductant sample under assay. The most favourable conditions were arrived at for iodate with 6N acid, whereby the equilibria were reached instantaneously. However, three minutes were always allowed to elapse to insure reproducibility. With 2 and 8N media, 20 rain were needed and the results are low owing to oxidisability of Mo III meanwhile. The results obtained by the usual volumetric methods involving the use of chloroform as a solvent for the liberated iodine, were found to coincide closely with those obtained potentiometrically. Analogous results with respect to the effect of acidity on the attainment of equilibrium and the accuracy of both potentiometrie and volumetric procedures (using methyl orange as indicator) were also obtained with bromate solutions. The accuracy attained b y these methods in 6N hydrochloric acid, being equal to t h a t attained with ceric sulphate solutions (Exp. 1, see also Fig. 1) permits the use of iodate and bromate as additional standardising agents for Mom.

Molybdenum(III) as Volumetric l%eduehlgAgent

31

A t t e m p t s to oxidise t e r v a l e n t m o l y b d e n m n b y eerie solutions even i n 4 N hydrochlorie acid media (Exp. 23) gave slightly higher results, owing to a slight oxidisability, although precautions to exclude ah" b y a carbon dioxide atmosphere were taken.

Summary The purpose of the present i n v e s t i g a t i o n is to i n t r o d u c e t r i v a l e n t m o l y b d e n u m as a new a n a l y t i c a l reducing agent, i n hydrochloric as well as sulphurie acid media. The solution is prepared i n 9 N acid b y electrolytic or m e r c u r y r e d u c t i o n of the h e x a v a l e n t form. I t is preferentially stocked a n d used i n high acid concentrations, w h e r e b y its s t a b i l i t y in air is optimal, failing to h a l f its t i t e r after n e a r l y two m o n t h s . I t has been tested w i t h success as r e d u e t a n t for eeric, dichromate, ferric a n d v a n a d a t e . I t has the a d v a n t a g e of being used i n o r d i n a r y opened burettes. T i t r a t i o n s were followed p o t e n t i o m e t r i e a l l y i n general, a n d a l t e r n a t e l y v i s u a l l y i n decinormal media, w h e r e b y m o l y b d e n u m blue appears at the end-point. The percentage error i n fifteen discussed experiments n e v e r surpassed ~ 1~ Some oxidation t i t r a t i o n s were also u n d e r t a k e n using iodate a n d bromate, with the same degree of accuracy, provided t h e y were conducted i n 6 N acid. I n weaker acid concentrations, equilibria were more slowly attMned, with a more or tess p r o n o u n c e d deviation, due to some oxidisability t h a t necessitates eon~rolled i n e r t atmosphere.

References 1 BELC]:][EI:~, I~., O. GIBBONSand T. S. WEST: Analyt. Chemistry 26, 1025 (1954);

cf. Z. analyt. Chem. 145, 293 (1955). -- ISSA,I. N., and I. M. EL-S~]~IF: Anal. chim. Acta (Amsterdam) 15, 4 (1956); cf. Z. analyt. Chem. 155, 372 (1957). -CAIarENTEI~,J. E. : J. Amer. chem. Soe. 56, 1947 (1934). -- 3 FAlcAg,M. u : M. Se. Thesis, CMro 1946. -- 4 Fon~sT~R, F., and E. F ~ I c ~ : Z. physik. Chem., Abg. A 146, 81, 177, 231 (11930); Z. angew. Chem. 36, 458 (1923). -- 5 Fu~NA~, N. H., and W. M. MvR~Au J. Amer. chem. Soe. 58, 1689 (1936); cf. Z. analyt. Chem. 111, 419 (1937/38). -- ~ FU~NESS,W.: Analyst 75, 2 (1950); ef. Z. analyt. Chem. 182, 126 (1951). -- ~ G~eSn~]~o, M. V. : Zavodskaja Laborat. 10, 245 (1941). -- s LA~c, 1~. : Z. analyt. Chem. 106, 12 (1936). -- 9 ]~RINETTE, 1~., and L. F. YUTEIVI~k:Trans. Illinois State Aca& Sei. 34, 2, 119 (194i). -- a0 PA~TINGTO~,J. g., and A. B. tIA~T: J. chem. Soe. (London) 36, 1532 (1940). -- 11 I~INS, A.~ and J. R. IR~NZO: An. Fisica Quire. 42, 645, 761 (1946). -- ~: Tou~:Y, A. R., and H. K. E ~ - S ~ u J. chem. Soc. (London): 45, 140 (1949). -- ~ Tou~KY, A. R., M. Y. F~A~t and H. K. EL-SHaluu Analyst 78, 258, 262, 266 (1948). -- ~ TouaKY, A. R., I. M. Iss~ and A. ~ . AM~N: Anal. chim. Acta (Amsterdam) 10, 168 (1954); cf. Z. analyL Chem. 1~7, 53 (1955). Dr. M. F ~ A ~ , Atomic Energy Establishment, Sh. Tahrir, Dokki Cairo (Egypt)