----------------------------- --CHAPTER - III

FLOTATION AND SPECTROPHOTOMETRIC

DETERMINATION OF RHENIUM(IV) WUH

THIOCYANATE AND .AMIDINES

-----------------·----------

SliMJW\Y

Rbeoium(VII) 1a reduced to l'bllliw.(IV) by aeua

of stannoua chloride 1n 5 aolar aulpburic acid, the

l'heniua(IV) ia tben reacted with thiocyanate 110lution and

the reaul tant yellow thiocyanate coaplex 1a the extracted.

in benzene solutions of aaidinea, 'Which were ayntlleaized

earlier. The extracted complex in benzllle coagulates and

1a then dissolved by addi t1on of one al of 1-pentanol in

a total volume of 10 ml. The molar abaorpt1 vi ty of the

Re(IV)-SCN complex wi til these uidines Ue in tbe range

of (1.3-2.2)x1o4 L 110l-1cm-1 at 'A aax 435 nm. Out of these

the simplest compound, N,N'-diphenylbenzamidine (DPBA) gave

the most sensitive colour reaction and hence chosen for

detailed studies. The optimum acidity range was found

to be between 2.5 - 8.5 M sulphuric acid. Various analy­

tical parameters, viz., effect of solvents, dilution,

diverse ions, temperature, etc. were also studied. The

system obeyed Beer's law in the range of 5-70 .All Re/10 1111.

ot final volume. Out of various diverse ions, only

molybdenum was found to interfere seriously. This detect

was overcome by its prior separation with 8-hydroxy­

quinoline. The method was applied tor the recovery of

the metal to synthetic matrices and an ore saJDple. The

relative standard deviation ot. the method was found to be

..t 1.6 "·

MVIEW OF qm.T£R WORK

Several spectrophotoaetrio aetbOda tor the

determination of rhenium bave been clesoribed. Of these

the thiocyanate and stannous chloricle aethod1-3 h .. been

widely used !or the determination ot rbeniUII. The method

aut! era !rom interference !rom several aetal ions, viz.,

Cu, Se, Te, Au, w, M0 , Pt and v. Several alternative

procedures of thiocyanate method tor sepaxe tion ot

rhenium !rom tungsten and molybdenum bave been proposed 4-11 by various workers.

Reactions of perrhenate with pyrazoline d1 thio-12 carbamates were studied by Busev et al. Although the

reactions were highly sensitive but were accompanied by

a number of drawbacks which made this method less useful

e.g. the formation of Re-PDTC complex was slow and·

chloroform and iaoaJIIyl !Jlcohol extract only colourless

rhenium compound just after the addition ot reagent.

The colour appeared after 35-40 mins. Mo{VI) was chief

interferent and if present needed conversion into peroxide

by addition of hydrogen peroxide. The amount of peroxide

was also critical as Re(IV) may also be oxidized. Se,

Te, Sn, I-, and oxalic, nitric and nitrous acids ialso

interfered.

The thiocarbazide derivatives used by Borisova

et a113 were fairly sensitive but selectivity was low.

Apart fro• theae •etboda aeTeral other reacenta

baTe alao been •ployed for the apeotropbotoaetrio cleter­

llination of ~tDiwa. Tbey are o(-furild.1ox1ae, 14-16

toluene-3,4-dithiol, 17• 18 tetrapbenyl araaaiwa oblor1de19

thiourea, 20-24 th1oox1ne25, 1,5-dipbenyloarbObydra­

zide26- 28, niox1me, 29 tbioglycollio ao1d, 30, 2,2•,2•­

tripyridyl-methylene blue31 , rhoduine 6031 , 11etbyl

violet, 31 • 32 etc. In addition to these reacenta several

other reagents baye also been eaployed by different

workera. 33-41 All these reagents were sensitive but not

entirely adequate as they suffer from various experimental

difficulties such aa, critical pH and standing tille, poor

selectivity, waiting time, repeated extractions, etc.

Reviews have also been compiled by various authors on

the spectrophotometric determination of r.benium.22 •42 •43

Recently, D!Odifications to the thiocyanate method

have been suggested in which a second ligand is added to

enhance the colour intensity of the Re-Sdi COD!Plex.

Various pprazolone derivativea44 have been proposed for

this purpose but it include an essential two step extract­

ion in which Re(VII} is extracted first with pyrazolone

into chloroform and then the organic phase is treated

with thiocyanate, hydrochloric acid and stannous chloride

to give coloured complex. Mo and W interfere.

In another method thiourea and thiocyanate20•45

were used in presence of stannous chloride in which Mo,

V, w, Fe and Ni interfered.

The hexaaethylphospboraaide-Sci aetb04 suuested

by M1 tra and Mitre 46 a lao utilized a second llsand for

the .tonats.on of 11ized coaplu with rbeniua and thiocya­

nate in which hydraziDe sulphate w .. eaployed .. a

reducing agent. The aetbod is aenai tive but needa a

prolonged heating and also suffers .troa inter.tereoe of

many ions.

Similarly hydroxamic ac1ds47 and aetaclopramide

hydrochloride48 have also been uaed to increase tbe

sensitivity of the Re-sd: method. Hydrozaaic acids forud

mixed ligand complexes with Re-SCN while metaclopramide

hydrochloride formed ion-association co•plez. Both these

reagents are sensitive but suffer from the inter.t erence

of so11e common elements.

PRESENT WORK:

The present work 1a based on the extraction of

mixed ligand complex of rhenium(IV) witb N,N'-diphenyl­

benzamidine (D~BA) in presence o.t thiocyanate. Tin(II)

chloride is employed as a reducing agent for the reduction

of rhenium(VII) to rhenium{IV) state. The reagent, DPBA,

gives water insoluble yellow complex with rhenium(IV) in

presence of thiocyanate at 2.5 - 8.0 M sulphuric acid

media, which is then extractea aa a coagulated complex in

benzene and is dissolved by the addition of 1 ml 1-pentanol.

The maximum absorbance of the coloured complex in benzene

+ 1-pentanol (9: 11 v/v) is at 435 nm. 'lhe method is

abple, rapid, .. lective and tr" trOll ripd control of

experimental variable• auoh aa wluae of aqueoua pbaae,

a110uat of reagent, ataacU.nc ti•e, te•perature, etc.

EXPERIMJIITAL

Apparatus:

{j f;

A Carl Zeias Jena "spekoi' with •etched 1-cm quartz

cuvettea was employed tor tbe absorbance •eaaureaenta.

For all pH measurements syatronic pH meter Type-322 waa

used. Precipitates were collected 1n aintered glass

crucible G-3 porosity. Pear shaped aeparatory !unnela

(125-ml) with short stem were used for aolvant extraction.

Graduated apparatus of standard calibrations were used

for measurements.

Benzeae:

Purified benzene was employed for preparation of

reagent solutions and all experimental work.

Reagents:

All reagents and chemicals used were ot analytical

grade (B.D.H./Merck).

Standard rhenium solution:

A stock solution of rheaium was prepared by

dissolving accurately weighed A.R. grade potassium perrhe­

nate 0.5 g in double distilled water and diluted to ane

litre in a volumetric flask with distilled water and

standard1aed49•

f)7

Solution of ~!dines:

o.'"' w/v (0.01 M) solutiona of different

aa1d.1nes in benzene were prepared. tor extraction purpoaes

except in case of solvent effect studies.

T1n(II) chlorid.e and potasa1ua thiocyanate aolutiona

5", w/v t1n(II) chloride solution in 5 M aulpbur1c

acid was prepared. 20", w/v aqueous solution ot thio­

cyanate was prepared. Both the solutions were prepared.

daily.

All the aqueous solutions eaployed for extraction

purposes were preaaturated with benz~e.

Colour reaction:

All the eight amid1nea react with Re(IV) in

presence of thiocyanate in the acidity range of 2.5-8.0 M

sulphuric acid to give a water insoluble yellow coaplex

extractable into benzene. This coaplex 1n oenzene gets

coagulated after extraction and is dissolved by the

addition of 1 ml, 1-pentanol after rejection of the

aqueous phase. The complex absorbs maximum at 435 nm

against a reagent blank.

Recommended procedure for the spectrophotometric determination of rhenium:

An aliquot of the solution containing 30 .ug of

rhenium was placed 1n a 125-ml separatory funnel. To

this 1.G 111 tin(II) chloride solution was added followed

by addition of 3.6 ml aulpburic acid solution ( 10 M) •

This was reacted with 1.~ •l potassium thiocyanate solu­

tion. ·.lbe volume of the aqueoua pbase wu acl;)uated to

10 •l with distilled water. The Mtal was then extracted

with 9.0 ml benzene solution of reasent llld equJ.libratecl

for t...o minutes. The phaaas were allowed to separate.

After rejectinl the aqueous phase, the coasulated

complex in benzene was dissolvecl by addition of 1.0 ml

of 1-pentanol. The organic extract was dried over

anhydrous sodium sulphate 2.0 g. The absorbance of the

complex was meas~ed at 435 nm against a reagent blank as

reference.

Spectral data of RheniUIII-SCI-DPBA complex in Benzene:

Eigbt N,N'-diarylbenzsubstitutedbenza.idines were

tested towards the extraction of the metal in presence

of thiocyanate ions. These showed the same )\ 118x and

the molar absorptivity of the complexes with these ami dines

lie in the range of (2.2- 1.3)x104 L 111ol-1 e~~-1 at )\•ax

435 nm. Table I shows the spectral data of various

amidines. Of these, the simplest compound DPBA gave most

sensitive colour reaction and therefore was ch~sen for

detailed studies.

Table - 1

SPECTRAL DATA OF RHBUUM(IV)-ScN-DPBA COMPLEX IN BEN~E

[ Re"r+ ] • 30 ~g/10 ml, 1.61x1o-5 M in aq. phase; [ Sn2+] • 0.03 M;

[ H2so4 ] • 4.0 M; ( Sc'R ] = 0.3 M; [ Amidines] • 0.01 M.

X @)-C=N-@)

I y

Z-----L_~- N- H

N,N'-diphenylbenzamidine (DPBA)

---------··- ------------------- ------·--------··---------·------s.No. Substitution 1n X,Y,Z rings of DPBA

X y z

A max (nm)

E.. L mol-1cm-1 Sandell's

sensitivity pg Re ca-2

·-------------------------------........................................ ----------------------------1 • H H H 435 22,300 0.0083 2. Cl-2-C6H4 H H 435 20,100 0.0092 3. H Cl-2-C6H4 Cl-4-C6H4 435 18,000 0.0103 4. H Cl-2-c6H4 H 435 13,700 0.0135 5. Cl-2-c6H4 Cl-4-C6H4 H

' 435 18,300 0.0102

6. Cl-2-c6H4 H Cl-2-C6H4 435 17,000 0.0109

7. Cl-2-c6.tt4 H Cl-4-C6H4 435 16,700 0.0111

a. H (CH3

) 2-2,6-c6H3 Cl-4-C6H4 435 15,600 0.0119

------------------· -------'---~ -·

ltESULTS AND DlSCUSSIOII

Absorption apectraa

The absorption spectra ot the reacent blank (A)

and Re(IV) -sci-DPBA in 1-pentenol + benzene (E),

Re(IV)-Sei-DPBA in 1-pentanol (C), Re(IV)-scl in

1-pentanol (B) and Re(IV)-S~ in 1-pentanol + benzme

(D) are shown in Figure 1. All these complexes show

nearly the same value !or adsorption maximum at 435 n11.

The reagent blank also absorbs at this wavelength and

hence it was used as a reference !or all meaaurements.

Effect o! solvent:

Effect of various solvents on the extraction o!

Re(IV)-SCN with amidine bas been studied by dissolving

DPBA in different solvents (Table 2). It was found that

solvents like 1-pentanol, chloroform, benzene, methyl­

isobutylKetone, toluene, o-dichlorobenzene, ethyl acetate

and butyl acetate extracted the complex ~ile carbon

tetrachloride did not extract the complex. The complex

coagulated in benzene, toluene and o-dichlorobenzene but

dissolved on addition of 1-pentanol, methyl isobutyl

.Ketone or ethylacetate. The colour of the complex was

stable in benzene or toluene only when 1-pentanol was added.

The molar absorptivity of the complex in these solvents

lie in the range of (0~5 - 2.5) x 104 L mol-1cm-1 at

435 nm as shown in the table 2. Benzene was chosen !or

0.4

w 0.3 u z <(

m 0::

~ 0.2 m <(

0.1

71

425 450 475 500

WAVELENGTH,nm

FIG.1.ABSORBANCE SPECTRA OF RHENIUM (JV1 WITH THIOCYAN.A TE AND WITH OR WLTHOUT DPBA.

A.ABSORPTION SPECTRA OF REAGENT BLANK OF DPBA(0.010M) IN BENZENE+ 1-PENTANOUS:t,V/V)

B.ABSORPTION SPECTRA OF Re(IV)-SCN IN"1-PENTANOL.

C. ABSORPTION SPECTRA OF Re (IV)-SCN-DP~A IN 1-PENTANOL*

D.ABSORPTION SPECTRA OF ReliV)-SCN IN BENZENE + 1-PENTANOL

1 (9: 1. V/V)

E .ABSORPTION SPECTRA OF Re(J.V) -SCN-DPBA IN BENZENE•1-PENTANOL ( 9:1, V/V),

Table - 2

J:lo'FECT O.F SOL~TS ON 'DiE EXTRACTION OF Re(IV)..Sc!l-DPB.A

OOMPLEX

( Re'J+

[ Sn2+

[ H2so4

[ sc:R

[ DPBA

]

J ]

]

]

• 30 ..uc/10 81 1.(1x10-5 M

in aqueoua Pb••• • 0.03 M

• 4.0 M

• 0.3 M

• 0.01 M

----------·------·------·---------S.No. Solvent Molar absorpti­

vity at 435 na (L mol-1CII-1)

-------------------------·---------------------------1 •

2

3

4

5

6

7

8

9

10

11

12.

----

1-Pentanol

Chloroform

Ethyl acetate

Methyl isobutyl .ketone

n-Butylacetate

Carbon tetrachloride

Benzene, toluene, a-dichlorobenzene

Benzene + ethyl acetate/ methylisobutyl ~etone (9:1)

Toluene + 1-pentanol (9:1)

o-Dichlorobenzene + 1-pentanol (9: 1)

Benzene + 1-pentanol (9:1)

Benzene + 1-pentanol (8:2)

---

5,300

18,600

25,800

16,700

13,600

No extraction

Coagulated

Complex unstable

18,200

Complex unstable

22,300

17,000

l-1 {)

I •-

,_. ,~ t't)

detailed work because of ita sel.ectin extraction of

rheniu., wide optimum acidity ~· and hip aolubili ty ot

reagent in it.

Effect of aciditys

The acidity of the aquaoua phase waa ma1Dta,Ple4

with 10 M sulphuric acid. The optiaua working reDge

lies between 2.5 - a.o M sulphuric acid as abown in

Table 3 and Fig. 2. Hydrochloric acid waa also tried but

the molar absorptivity was low aa compared to sulphuric

acid (1.2x104 L mol-1cm-1). Moreover, the ra*• of extract­

ion of metal was more rapid and selective in sulphuric

acid media hence it was chosen for detailed studies.

All work was performed at 4.0 M sulphuric acid.

Effect of reagents:

Effect of DPBA:

Effect of DPBA on the extraction of complex is

shown in Table 4 and Fig. 3. A 0.001 M DPBA in benzene

was found to be sufficient, in presence of constant

excess of thiocyanate at 4.0 M sulphuric acid. Addition

of further excess of the reagent upto 0.02 M caused no

adverse effect. In practice a 0.01 M reagent was used.

Effect of thiocyanate:

The effect of amount of thiocyanate on the extract­

ion of Re(IV) was studied at constant excess of DPBA

Table - '

EFFECT Oi' ACIDITY ON THE EXTRACTION or Re(IV)-sc:Jr-DPBA

COMPLEX IN B~ZDIE

[ Re!':t+ ]

[ sn2+]

[ sai }' [ DPBA )

30 )JC/10 al 1 • $Gox1o-5 M in aqueoua phaae.

0.03 M

0.3 M

0.01 M ----------- -·· _____________ , _____ _ s.No. Concentration of sulphuric

acid 1n aqueous phase M

Absorbance, 435 Dll

----------------- ---· --- ---1 0.5 0.220

2 1.0 0.250

3 1.5 0.290

4 2.0 o.320

5 2.5 0.360

6 3.0 0.:560

7 4.0 0.:560

8 5.0 0.360

9 6.0 0.:560

10 7.0 0.360

11 a.o o.360

12 9.0 0.260

--- -··-- -- --- ---

74

0.~

E c

Ltl M O.J ...., ~

w u z ~ 0.2 a: 0 Ul m <( 0.1

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

SULPHURIC ACID, MOLARITY AQUEOUSPHASE

FIG.2. EFFECT OF ACIDITY ON THE EXTRACTION OF RQ(IV)

-SCN-DPBA COMPLEX IN BENZENE • 1-PENTANOL{SJ,V/V)

[Re <vu!J = 1.61 x 105M, [sn (It U = o. 03 M. [scNJ = o. 30M

[_bP 8 A] =0. 010 M

Table - 4

EFI.<'ECT OF AMOUNT OF DPBA ON 'l'HE EXTRACTIOM OF

Re(IV)-s~-DPBA OOMPLEX IN'l'O BDZDE

[ Re~+ ]

[ sn2+ ]

[ H2so4

)

( sdl ]

----

30 AA&/10 al 1. &k1o-5 M 1n aq. phaae

0.03 M

4.0 M

0.3 M

------ -----·---·--·----------S.No. Concentration of

DPBA in benzene M X 10-3

Absorbance, 435 Dll

------- - -··---·------·-------1 o.oa 0.110

2 0.1 0.150

3 0.3 0.245

4 0.5 0.295

5 1.0 0.360

6 5.0 0.360

7 10.0 0.360

8 15.0 0.360

9 20.0 0.360

10 3Q.O 0.310

11 40.0 0.250

--- - -- -------

7 {j

. 0.4 E c Li'l ,.., -.#

w-o.J. u z 4: m 0::: ~ 0.2 m 4:

0.1

"i7

o,L-______ L_ ____ ~------~------~~

0.01 0.02

MOLARITY, DPBA

0.03 0.04

FIG.3.EFFECT OF AMOUNT OF DPBA ON THE. EXTRACTION OF Re (IV)-SCN-DPBA COMPLEX

IN BENZENE+1-PENTANOL{9:1 V/V) AT l..OM

H2S04 •

[Re(VIl U = 1.6Jx105M , [sn(ll ~= 0.03 M,

[scNJ =a. 30M

'i' ()

(0.010 M) keeping all other .ariablea cooataDt. A 0.18 M

sdN waa found nec .. aary for a conatant aad aaxiau. develOP­

ment of colour and a further additioa of sdl upto 0.35 M

cauaed no adverae effect upoa extraction. Reaulta are

tabulated in Table 5, and ahown 1n FiiUl'e 4. In practice

0.30 M thiocyanate waa uaecl for extraction purpoaea.

Effect of reducing agent1

Tin(II) chloride wu uaed for the reduction of

the metal into Re(IV) in 4.0 M sulphuric acid. Tbe

effect of reduction of Re(VII) into Re(IV) was studied

over a temperature of 20°- 35°C. ibe reaction was found

to be instantaneous. It was noticed that at least 0.01 M

tin(II) chloride was required for maximum developaent of

colour and remained constant upto o.o4 M tin(II) chloride.

Dilution was made after reduction of the metal. Results

are listed in Table 6 and shown in Figure 5. In practice

0.03 M tin(II) chloride was employed for extraction

purposes.

Effect of volume, temperature and standing t1ae1

The variation of volume of aqueous phase between

5 - 30 ml did not cause any adverse effect on the extract­

ion of the metal. In practice, the volume of the aqueous

phase was always adjusted to 10 ml before extraction.

Results are shown in Table 7 and Figure 6.

Table - 5

EFFECT Oi' AMOUNT OF THIOCYANATE ON '!HE EXTRACTION OF

ri.e( IV) -sef;-DPBA OOMPLEX INto BEJUU.E

[ Ref~+ ] • 30 Ioiii 10 al &:i -5 in aq. pbaae 1 •. x10 M

[ Sn2+ ] • 0.03 M

[H2so4 ] • 4.0 M

[ DPBA J • 0.01 M in benzene

------------------------------S.No. Concentration of Absorbance

Thiocyanate 435 OBI M

----- ----- -- --1 o.oa 0.070

2 0.10 0.130

3 0.120 0.190'

4 0.150 0.230

5 0.18 o.36o

6 0.20 0.360

7 0.30 0.360

8 0.35 0.360

9 0.40 0.320

10 0.45 0.280

11 0.50 0.230

-------- -- ___ ..,..._ ____

... , <J ( .

0.~

E c: ~ 0.3 ~ -w u z <( 0 2 . m . 0:: 0 (f)

m <( 0.1

0~--~----~--~-----L----L---~

0.10 0.20 0.30 0.~0 0.50

MOLARITY ,THIOCYANATE

FIG.t..EFFECT OF AMOUNT OF THIOCYANATE ON

THE EXTRACTION OF Re(IV)-SCN-DPBA

COMPLEX AT t..OM H2SOt, IN BENZENE•

1-PEN TANOL (9: 1, V/V)

~e(VII] l.G1 x 105M , [?n (II U = 0.03 M.

@PBA] = 0 .OlOM

(j()

Tible - 6

EFFECT OF .AMOm.T Olo' TIN (II) CHLORIDE OJI THE EX'l'RACTIOII

OF Re(IV)-sdf-DPBA OOMPLEX 1111'0 BEIIZDUt

• 30 .uc/10 Ill [ Re7• 1 1.Gax1o-5 M in aq. phaae

( H2S04 ]

[ sdf ]

( DPBA]

• 4.0 M

• 0.3 M

• 0.01 M in benzene

------ -----------s.No. Concentration o~ atannoua

chloride in aqueous phase M X 10-3

-------------- -1 5.0

2 10.0

3 20.0

4 30.0

5 40.0

6 50.0

7 60.0

8 ao.o

----- ---- -

-·----Absorbance,

435 llll

--0.325

0.360

0.360

0.360

0.360

0.320

0.290

0.230

---

81

0.4

E c:

L() 0.3 M

-.s ~

..... u z ~ 0.2 m a: 0 (fl

m ~ 0.1

0.02 0.04 O.D6 0..08 MOLARITY, TIN(II) CHLORIDE

FIG. 5. EFFECT OF AMOUNT OF TIN{II) CHLORIDE

ON THE EXTRACTION OF ReOV)-5 CN-0P8A

COMPLEX IN BENZENE+ 1-PENTANOL (9."1,V/V)

AT L..OM,H2SOt,

[Re (VIID:l.61x105M ~ [seN]= o. 30M,

[pPBA}0.010fv1

Table - 7

EFI'ECT OF AMOUMT OF AQUEDUS HIASB 011 'l'HI BX'l'RACTION Of

Re(IV)-ScJl-DPBA COMPLEX INTO BDlzmiB

[ Sn2+]

(H2so4 ]

[ scS ]

[ DPBA]

• 30 .uV 10 Ill

1. S4K1o-5 M 1n aq. ph••• • 0.03 M

• 4.0 M

• 0.3 M

w. 0.01 M 1n benzene

-------------------Ratio of volume of

s .No. aqueous phase to organic phase

-----------------------· 1 1 • 2 •

2 1 • 1 •

3 2 • 1 •

4 3 • 1 •

5 4 • 1 •

6 5 • 1 •

-----------·------

0.4

E <::

In 0.3 (")

~

w u z <{ 0.2 ID 0: 0 U1 ID

<{ 0 0 1

o~--~----~--~----~----~--~ 10 20 30 40 50

VOLUME OF AQUEOUS PHASE ,m1

FIG.G. EFFECT OF AMOUNT OF AQUEOUS PHASE

ON THE EXTRACTION OF ReUV)-SCN-DPBA

COMPLEX AT 4-0M H2SOt, IN BENZENE

• 1-PENTANOL { 9:1 ,V/V)

[Re{VIIl) 1.61 x165M, [sn01~=0.03M, [scN]=0.30M, [DPBA]=0.010M.

,. r Od

'l'be nriation in teaperature of a.queoua pbase

from 20°C to 35°C bad no attect on colour llltealtJ and

absorbance of the •ixed OOIIPlexes. The extracted coaplex

was stable tor at least two boura at roo• t•perature.

Effect of Electrolytes:

Variation of ionic stren1th ot the at~ueoua pbaaa

between 1.0 and 1.5 M with electrolytes such as potassiua

chloride and umonium aulpbate d1d nllt affect the spectral

characteristics or the rate of reaction.

Beer' a lawJ

The system obeyed Beer's law 1n the range of

0.5 - 7 ppm. This was tested by plotting the absorbance

of the complex vs concentration of the metal {Qg/10 ml)

Table 8, Fig. 7. A aeries of extractions were pertoraed

from the solutions containing .fixed amount o.f reagent in

benzene (o.01 M) and varying amounts of rhenium. Sandell

recommends50 an optimum range o.f 0.2 - 0.7 units for

absorbance measurements and therefore the practical ran1e

.for the determination o! rhenium by present method is

1.5- 60 ~g Re/10 ml.

Molar absorptivity and sensitivity:

The molar absorptivity o! the complex (calculated

on the basis of rhenium constant) is 22,300 L mol-1cm-1

and Sandell's sensitivity of the colour reaction is

o.ooa ..ug of Re per cm2 at >.. max 435 na.

Table - 8

CALIBRATION CURVE DATA FOR 'l'HE DI:.TERMINATIOM OF RH~IUM

Wl'lli DPBA AND 'l'HIOCYANATE

[ Sn2+ ) -[ s~ ] • 0.:5 M

(H2so4 ] • 4.0 M

( DPBA] • 0.01 M in benzene

-----------------·----·-----·-----S.No. RheniWII(VI)

.ug/10 ml Absorbance

435 llll

---------------------·----·--1

2

3

4

5

6

7

8

9

5

10

15

20

30

40

50

60

70

o.o6o 0.120

0.180

0.240

0.360

0.490

0.600

0.720

0.840

--------- - ----·------------

E; f;

0.8

E " ~ 0.6

w u z ~ 0.4 0:: 0 (I)

m <{ 0.2

20 40 60 so RHE Nl UM (IV),,gg I 10mt

FIG. 7.CALIBRATION CURVE FOR DETERMINATION

OF RHENIUM AS Re(IV)-SCN-DPBA COMPLEX

IN BENZENE •1-PENTANOL (9:1,VIV ).

bl

Preciaioru

To determine preciaiCD ot the aetbod, atatisti•al

inveatiptions were undertake by -.king 10 replicate

measur .. mts each containing 30--""' Re/10 al. Tbe standard

deviatiOD and relative standard deYiatiOD 1a given in

Table 9.

Effect ot diverse ions:

Tbe etfect of diverse ions, was studied by making

a separation of Re(IV) (30 ..u&) in presence ot individual

iODs aa reported in the procedure. Their tolerance limite

are reported in the Table 10. Among various lana studied

only molybdenum was found to interfere aerioual.y. Ita

inter! erence was removed by the prior aepara tion of the

metal by precipitating molybdenum with 8-hydroxyquino-

11ne51. Small amounts of copper are tolerated in the

determination but wnen present in amounts greater than

150 ppm, it inhibits colour devel.opaent, hence it also

requires prior separation. This can also be achieved by

&-hydroxyquinoline procedure.51

Application of the method:

The application of the atethod bas been tested by

analysing synthetic matrices and an ore sample. Results

are tabulated in Table 11.

Table - 9

DETERMIIIATION Oi' Re(IY)-sciLDPBA COJIPLII D11'0 sgZUII

[ Rett.t. 1 • 3() Mg/10 Ill 1 • .-x1o-5 M 1n aq.pbaae

[ srl• 1 • 0.03 M

[H2so4 ] • 4.0 M

[ sci] • 0.3 M

[ DPBA] - 0.01 M

----- - --No. of Absorbance deterai-nations

Mean Standard Relative absorbance deviation standard

deviation

-----------------·----· -- --0.355

0.360

0.360

0.360

10 0.360 0.359 .:!: 0.0035 .:!: 1.6 "

o.36o

0.360

0.360

0.360

0.360

------·----------------·---

(' ( o.J

DO

Table - 10

EFFECT OF FOREIGN IONS IN THE DHBlltlNATION OP' RHI!JIIUM(IV)

[ Ret!+ 1 • 30 ~10 •1 1.&f't.x10_, M

in aquaoua phaae

[ sn2• ] • 0.03 M

[ H2S04 ] • 4.0 M

[ sdfi I • 0.3 M

[ DPBA ] • 0.01 M

---------------------------------Ion Added aa

--------------------I II

'l'oleranceb Absorbance limi t(mg) at 435 nm

------III IV --------------------- ---- 0.360

Ni(II) NiS04.6H20 5 0.360

Mn(II) MnC12 .4H20 1 0.360

Co(II) coso4 .7H2o 5 0.360

V{V) NH4 vo3 0.5 0.360 ' Zn(II) .znso

4.7H2o 10 0.360

Cd(II) 3CdS04. 9H20 2.5 0.360

Zr(Il) Zr(S04) 2 .4H20 3.0 0.360

Ca(Il} caco3 5.0 0.360

Mg(II) M@O.:L/~;H2o 4.0 0.360

Al(III) Al~;SH2o 0.1 0.355

U(VI) UO 2 (NO 3

) 2 .6H20 0.5 0.360

----------------------------- -

81 Table - 10 (contd)

--------I II III IV --- --- -La(III) La(No 3)'5. 61-12.0 0.05 0.360

Hg(II) HgCl2 0.2 0.360

.Fe(III) io'e(No3

) 3.9H20 20 0.360

Tl(I) TlCl 0.5 0.360

Mo(IV) (NH4)6M~024"4H20 1oo• 0.360

Cr(III) K2so4cr2 (so4) 3.~o 15 0.360

Bi(III) Bi(No3)3 0.1 0.360

Pd(II) Pdtlo';)2. 0.1 0.360

Be(II) Beso4 2.5 0.360

Sr(II) Sr(No 3) 2 !.0 0.360

Sb(III) K(~bO) c4H2o.Y2H2.0 0.08 0.360

W(VI) Na2wo4.2~0 0.03 0.360

cu(II) euso4 .5H2o 0.15 0.360

Nb(V) Nb205 O.l 0.360

Se(IV) Na2sea3 0.03 0.360

EDTA Na2EDTA 1.0 0.360

Citrate Sodium citrate 1.0 0.360

Tai'trate NaKC4H4o6 .4H20 10 0.360

Ti(IV) K2TiO(C2b4)~2H20 o.a 0.360

Phosphate N~4 0.5 0.360

Arsenate Na2HAs:,~o4 • 7H2o o.a 0.360

----- ------ ---a • precipitation with 8-hydroxyquinoline at pH 3. b ~± 2'% ev-ror

Table - 11

DETERMINATION OF RHEIUUM IN sYM'lHETIC MATRICES AND ORE

--------------------Sample Coapoai tion/

Certified value*

Recovery Rb.U\111

--------------------------------·----------------------

1. Mo-Re Synthetic 1000 ag + Matrice 30 ~ Re

II. Mo-Cu Synthetic 60 mg CU + Matrice 30 ~ Re

III. Ore sample8 o.o~ • cu, 2.0~ • Mo, 40 PPJIIII Re*

29.6 ,ug

29.8 ,ug

________________ , ______________ _ a - Obtained from National Bureau c1' Mines, Nagpur.

b - Four measurements.

(' ':.> U.~

Synthetic Matrices:

(\ '. oJ 'J

In the synthetic •ixture rbeiua waa detenairled

by adjusting the Pi of the solution to 3.0 by aodium

acetate ~md precipitating molybdenum or copper by

8-hydroxyquinoline51 • The •trt•l content o! the solution

was then determined as described in the procedure.

Ore saJDple:

The ore was dissolved in aqua regia (10.0 ml) in

a 250-ml beaker and heated to remove fumes of oxides of

nitrogen. After cooling, 3 ml of sulphuric acid waa added

to residue and evaporated almost to dr,yness (2 times).

The dried mass was dissolved in distilled water and diluted

to a desired volume. An aliquot of this solution was taken

in a beaker for the prior separation of rhenium from

molybdenum and copper by 8-hydrOIJ.Yquinoline method51 •

After precipitating and separating molybdenum and copper

from rhenium, it was determined spectrophotometrically as

described in the recommended procedure.

Composition:

The stoichiometry of rbenium(IV) mixed complex with

DP.BA ·and thiocyanate has been determined by curve-fitting

method. 52 The results obtained indicate the formation of

1 : 1 : 3 (Metal : Reagent : Thiocyanate) mixed ligand

complex 1n benzene. The mechanism inwlved in reduction

of Re04 with tin(II) chloride 1D preaence of tbiocyanate

haa been atudled by varioua workera53-55. The probable

extraction equlllbriua •Y be apreaaecl u followaa

R.O~+ + I S<!f + HA0

+ H-~ [ Re O(SCII) ,l H .HA0

where subscript HA and o denote the DPBA end orsanic

phase respectively.

Ratio of rhenium to DPBA:

Curve-! ittlng aethod:

The ratio of rhenium to DPBA was deterllined by

curve-fitting method (by plott1nglogPcf<1Psotl::;n-:c va log

molarity of DPBA) • The results obtained indicate tbe ratio

of Re : reagent to be 1 : 1 in Re(IV)-sdl-DPBA complex '

(Table 12).

Expe rilll ental:

30 .ug of Re(IV) solution was taken in a separatory

funnel. To thia was added 1.,5 ml tin(U) chloride solu­

tion followed by 3 .e ml 10 M sulphuric acid and 1.~ ml

20% (w/v) potassium thiocyanate solution. The volume of

the aqueous phase was diluted to 10.0 ml. The content of

the funnel was then equilibrated with 9.0 ml benzene

solution of reagent for ten minutes with varying amounts

of reagent in benzene. The aqueous phase was rejected

and the coagulated complex was dissolved in 1 ml 1-pentanol.

The coloured extract was transferred into a 50-ml beaker

Table - 12

CUHVE-FITTINQ METHOD fOR DIE DETERMDIATION C'6 RHDUUM

TO DPBA IN Re(IV) -sdl-DPBA SYSTEM

[ Re'1+ 1

[ Sn2+]

[H2so4 )

[ s~ J

•

•

•

•

30 »a/10 Ill 1.61x10-5 M in aqueoua pbaae

0.03 M

4.0 M

0.3 M

-------------·----·--------Cone entration of DPBA M X 10-3

Log M Absorbance 435 na

--------------·-----· o.oa - 4.096 0.11

0.10 - 4.000 0.15

0.30 0.245

0.50 - 3.301 0.295

Log])

------ 0.356

- 0.146

0.328

0.656

---- T ·----------------- -------

containing anhyclroua aodi\11 sulpbate. Absorbance waa

meaaured at 435 na acainat a reqent blulk. 'l'he Yalues

(' {' ~{}

of log not the obserYed absorbance, 1! 1 are plotted apinst

log aolarity ot DPBA, 'X' in each oaae (Table 11). The

absorbance ot Re(IV)-Scg-DPBA aixed oo~lex increased with

concentration ot reagent. The slope ot the strai&ht line

was found to be 1.2 which is close to integer 1, which

indica tea the formation ot 1:1 (Re : DPBA) mixed coaplex

in benzene. Results are shown in Table 12 and Figure e.

Ratio of rhenium to thiocyanate:

Curve-fitting method:

The ratio ot metal to thiocyanate in the Re(IV)­

SCN-DPBA complex is found to be 1 : 3 by curve-t! tting

method (Table 13).

Experimental:

An aliquot containing 30 .J1C of rhenium solution

was placed in a 125-ml separatory funnel, then 1.0 ml

tin(II) chloride solution, followed by 3.8 al sulphuric

acid solution (10M) was added. Varying quantities of

thiocyanate solution was introduced. Dilution ot the

aqueous phase was made upto 10.0 ml. It was equilibrated

.for 10 mins. with 9.0 ml of benzene solution of the

reagent (o.01 M). TJte extract after rejecting the aqueous

phase and dissolving the complex in 1.0 al 1-pentanol was

dried over anhydrous sodium sulphate in a 50-1111 beaker.

Table - 1'

CURVE-FITTING ME'niOD io'OR 'ntE DETERMINATION OF RHOIIUM 'ro

THIOCYANATE IN Re(IV)-sdl-DfBA SYSTEM.

[ Re"f:+ ) 30 ~10 Ill, -5 • 1. 61x10 M in aqueoua pbaae

[ Sn2+ 1 • 0.03 M

( H2S04 ] - 4.0 M

[ DPBA ] • 0.01 M

---------------- ----·-·--Concentration of Log M Absorbance thiocyanate M 435 Dll

------ ------ -----------o.os - 1.096 0.07 - 0.617

0.10 - 1.000 0.130 - 0.247

0.12 - 0.921 0.190 o.~

0.15 - 0.824 0.230 0.247

---------------------------------

(J' -, < f

A 0\ 0

....J

0 A SlOPE:1.2 Sl0PE:2.9

-1~-----------L----------~~----------~-----------J----------~ -4 ..g -2 -1 0

log [oP BA J 1 log [seN"]

FIG.8. CURVE-FITTING METHOD FOR DETERMINING THE RATIO OF DPBA/SCN TO

ReOV) IN Re(lV) SCN DPBA COMPLEX

·A. . lOG .D VS LOG [pPBA] IN THE PRESENCE OF CONSTANT EXCESS OF THIOCYANATE(0.30M)

B. . LOG 1) VS LOG [SCN] IN THE PRESENCE OF CONSTANT

EXCESS OF DPBA (0.010M)

·~ '-:::>":'

('C) 'J.

The abaorbance waa aeaaured at 4" na apiDat r•1•t

blank. The alope of the curve, obtained by plott1n1 lu:SD of

"b~orbanct. 1Y1 va 101 aolarlty of tblooyaaate 'X' wu

2.9, whicb 1a cloae to lnteser '• llldlcatiDI the ratio of

rhenlua to thiocyanate aa 1 1 3. 'l'be reaulta are sinn iD

Table 13 and ~i~Ure 8.

Detection liai t:

'l'be detection l1m1 t of the aethod waa .found to be

0.001 .ug Re/al in the mixed. aolv8lt.

Comparison with other methods:

Several colorimetric methoda for determination of

rhenium have been described. Reasents 1Jl routine uae are

thiocyanate 1- 11, d.. -furildioxiae 14- 16, thiourea20- 24,

1-5,diphenyl carbohydrazide26- 28, thioox1ae25, methyl

violet31,32, tetraphenyl arsonium chloride, 19 4 aetbyl­

nioxiae29, toluene 3,4 d1thiol17• 18 etc. In addition to

these many other organic reagents have also been described

for the determinB tion of rhenium33-41 • All these methods

lack selectivity and are subject to interference to one ~ or more elements such as Mo, Cu, Cd, Bi, Nb, Se, V, W,

Ni, etc. which are commonly associated with rhenium in

many alloys and complex materials. Some of the established

methods are dis cussed as under:

The method ia basecl OD 1be recluct1oa of aoic11o

perrhenate solution by atannoua chloride 1n preaenoe of

potassium thiocyanate ions to .tcra a yellow to reel

coloured coaplex which is extractable with oraanio

solvents. The broad vilible abaorpt1on oeatering at

about 430-440 •JJ is usually .. ployed for aeasurementa.

100

The method is sensitive but it sutfera froa several

experimental difficulties such as Yariat1on of colour

intensity of ocmplex with respect to analytical variables

and interference of many ions. Mo(IV) and ii(VI) inter­

fere seriously as they give siailar reaction with

thiocyanate procedures so that require prior aeparatioa4-11

by pre-. ex~traction or by gravimetric precipitation methods.

O#ler interfering elements incl.ude Ni, Co, Cr, V, Ce,

Pb, etc. Several reagents to enhance the sensitivity o!

the thiocyanate method have also been. describedby various

workers32• 44-48•

Thiourea21- 24:

Thiourea reacts with pentavalent rhenium to form

a soluble coloured compound. i'he absorption maximum of

the sol.ution is at 390 mJJ. The system obeys Beer's law

between 5 - 200 ..ug Re/ml. The optimum concentration of

acid for the compl.ex formation 1a 2.5 - 5.0 N hydro­

chloric acid and of reducing agent is 0.8 - 1.0% stannous

chloride. The col.ouration develops within 20 minutes and

1 (. 1 J -

is stable for •ny hours. fbe interferinl ele•ets

include M0 • w, Cd, Bi, a,, 8e ancl Te. AD altel'llative

procedure suuestecl by RJabcbikov and Boriaon21 involves

the fora tion of t ive COIIPlU:ea ot rbeniua wi tb 1:b1o\U"ea

wbicb have d1tferent absorption spectra. They turtber

said that these five coJII)lu:u were ot cU.t!erent colo\U"s

i.e., yellow, orange, pink, blue and green and abowed

their absorption maxilllum at 390 1 4201 520, 570 and 700 m.u

respectively. By their studies (Ryabcb1kov and Borisova)

it has been found that thiourea may reduce Re(V\\)to Re(v)

forming complex compound wi tb absorbance maxima at 520,

570 and 700 m,u depending an acidity medium, consequently

this method is found dependent not only upon tbe concen­

tration of reducing agent but also on the acidity ot the

solution.

A yellow green complex is formed in hydrochloric

acid solution. It has an absorption maxima at 436 nm.

It has been used for determination of rllenium in

molybdenite and molybdenite roaster flue dust. Inter­

fering elements include Mo, Fe, Cu, Se, Ni, Au and

silica.

~ -furild1oxime14-16 :

In presence of tin chloride this reagent reacts

with perrhenate to form a red coloured· compound, which

absorbs maximum at 532 m,u in acetone. The molar

abaorpti vi ty of tbe oo•plu i• aua~eated to be 41,300

L 1101-1011-1• The method thO\IIb unaitiYe suffers froa

interference of several ions •uch as Mot Cu, Pd, S~,

No;, .,.-, etc. and is also time cons"•'"' •• the absorb­

ance of the complex can be read only after 60 1111Dutea of

extraction. The interference of Mo 1a overcome by

Mo- ~tbylxanthai.e extraction.

Tbiooxine (8-mercaptoquinoline) 25 a

It reacts wi tb perrhenate ions in strongly acid

medium (8-11 N HCl) to form a cblorofora extractable

complex. The absorption maxiiiiWII of the extract is at

438 III)J.. 'lhe molar extinction coatticient 1a 8,470.

Beer• s law is obeyed between 1 - 40 .ug of Re/ml of

chloroform. Nitrate md arsenite must be abaillnt.

Tetraphenyl arsonium chloride19:

The method suggested by Andrew and Gentry involves

the extraction of the complex in chloroform with ~max

at 255 nm. The complex is developed at a Iii 8 - 9

adjusted by sodium hydroxide and citric acid solution.

The method suffers from low sensitivity (E • 3600 L

mol-1cm-1 at A max) and poor selectivity as a number

of metal ions interfere with the method.

N,N '-diphmylbmzaa!di.Dea

In the pre .. nt aethod. rbeniua(YII) 18 red.uoed. to

rhenium(IV) with t1n(II) cblorid.e ln sulpburic acid.

mediua (2.5- e.o M H2so4) and. ooaplexed. witb N,N'­

diphenylbenzuidine in presence of thiocyanate. The

yellow Re(IV)_s~-DPBA coaplex is extrected. 1n benzene

in which it is coagulated and is d.iasolYed. by tbe addition

of 1 ml 1-pentanol, havins a sharp maxi• at 435 nm witb

molar absorptivity 22,300 L aol-1cm-1• The metbod 1a

fairly selective and free from rigid control of nrious

analytical variables such as volume of aqueous phase,

acid! ty, temperature, etc. The aensi t1 vi ty of the present

method is also good.

OOMCI.USlON

N,N°-dipbenyl })czuicl1De haa been found to })e

highly aelective and aenaithe rea1ent tor extractive

spectrophotometric determination ot aicrQir .. aaounta

of rhenium. The method 1a a1aple0 rapid and relatively

free from ri1id control of experimllltal Yariablea auch

aa acidity, volume of aqueoua phaae0 t .. perature0 etc.

10<1

A distinct advantaKt of thia aethod ia that a nullber of

common eleaenta viz. Ni(II), Co(II), Cr(III), V(Y), W(VI),

Be(II), etd. do not interfere.

1.

2.

3.

4.

5.

6.

7.

a.

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