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Current Efficiency andElectrogravimetricDetermination of the Numberof Electrons Involved inGalvanostatic Coulometry ofMercury MetalRoberto Tokoro a & Marina F. Maggi Tavares aa Instituto de Química da USP , C. Postal, 20780, SãoPaulo, SP, BrazilPublished online: 03 Jan 2007.

To cite this article: Roberto Tokoro & Marina F. Maggi Tavares (1986) CurrentEfficiency and Electrogravimetric Determination of the Number of ElectronsInvolved in Galvanostatic Coulometry of Mercury Metal, Analytical Letters, 19:21-22,2079-2094, DOI: 10.1080/00032718608080867

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ANALYTICAL LETTERS, 1 9 ( 2 1 & 2 2 ) , 2079-2094 (1986)

CURRENT EFFICIENCY AND ELECTROGRAVIMETRIC

DETERMINATION OF THE NUMBER OF ELECTRONS

INVOLVED IN GALVANOSTATIC COULOMETRY OF

ME RC U RY METAL

Roberto Tokoro* and Marina F. Maggi Tavares

*Institute de Qul’mica da USP - C . Postal 20780 Sao Paulo, SP - Brazil

KEY WORDS: Mercury oxidation, galvanostatic generation, electrogravimetry, current efficiency .

The efficiency of current during the oxidation

of mercury metal by galvanostatic coulometry was

total in the presence of thiosulphate. The

electrogravimetric determination of the number of

electrons involved in the oxidation of metallic

mercury in the presence of substances which have a

great affinity f o r mercury ions, showed that n=2 in

Copyright 0 1986 by Marcel Dekker, Inc.

2079

0003-27 191861 192 1-2079$3.50/0

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2080 TOKORO AND TAVARES

t h e e l e c t r o c h e m i c a l s t e p . On t h e o t h e r h a n d , t h e

mass v a r i a t i o n was n o t a good method t o d e t e r m i n e n

i n t h e absence o f complexing a g e n t due t o a c o u p l e d

chemica l p r o c e s s .

INTRODUCTION

The p r e s e n t s t u d y i s a b r a n c h o f t h e main work

conce rned w i t h t h e complex f o r m a t i o n between

m e r c u r y ( I 1 ) c a t i o n s and t h i o s u l p h a t e a n i o n s i n

aqueous media. Data f o r t h i s sys t em a r e r a r e i n t h e

l i t e r a t u r e . The c i t e d p a p e r s a r e ma in ly conce rned

w i t h t h e p o l n r o q r a p h i c d e t e r m i n a t i o n o f complex

format i o n

method. The p o l a r o g r a p h i c method is p r e f e r e d due t o

t h e d i f f i c u l t p r e p a r a t i o n o f m e r c u r y ( I 1 ) - t h i o -

s u l p h a t e s o l u t i o n s . I t i s known t h a t a l k a l i n e

m e r c u r y ( I 1 ) s o l u t i o n s h y d r o l y s e and t h a t t h i o s u l p h a t e

h y d r o l y s e s i n a c i d i c med ia , p r e c i p i t a t i n g s ix lp l iur .

S t u d i e s i n b u f f e r e d s o l u t i o n s were c a r r i e d o u t and

showed a ve ry narrow pH window where t h e two i o n s

c o u l d e x i s t w i t h o u t h y d r o l y s i n g . However, even i n

t h i s optimum pH window t h e mercury(II)/thiosulphate

s o l u t i o n i s n o t s t a b l e . A f t e r some t i m e ( a few

minu tes ) t h e s o l u t i o n t u r n s y e l l o w and a b l a c k i s h

and n o t w i t h a p o t e n t i o m e t r i c 1-5

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CURRENT EFFICIENCY AND ELECTROGRAVIMETRIC DETERMINATION 2081

p r e c i p i t a t e f o r m s , presumably mercury s u l p h i d e . I n

t h i s manner t h e time a v a i l a b l e from t h e m i x t u r e o f t h e

r e a c t a n t s u n t i l t h e beginning o f t h e d e c o m p o s i t i o n o f

t h e complex ( n o t i c e a b l e by the a o p e s r a n c e o f ye l lowish

c o l o r ) was t h e l i m i t i n g f a c t o r f o r t h e u s e o f

p o t e n t i o m e t r i c methods. O p t i m i z a t i o n o f t h i s

c o n d i t i o n was a c h i e v e d by c o u l o m e t r i c g e n e r a t i o n o f

Hg2+ c a t i o n s i n t h e p r e s e n c e o f t h i o s u l p h a t e . T h i s

a l lowed t h e p o t e n t i o m e t r i c measurement due t o t h e

s t a b i l i z a t i o n o f t h e complex o v e r a l a r g e r p e r i o d o f

t i m e . The d e c o m p o s i t i o n o f t h e complex c o u l d be

d e t e c t e d by t h e c o l o r change a s w e l l a s by p o t e n t i a l

i n s t a b i l i t y a f t e r few m i n u t e s .

PURPOSE

The o b j e c t i v e o f t h e p r e s e n t s t u d y i s t h e

d e t e r m i n a t i o n o f t h e c u r r e n t e f f i c i e n c y f o r t h e

o x i d a t i o n o f mercury m e t a l i n d i f f e r e n t s u p p o r t i n g

e l e c t r o l y t e s . The number o f e l e c t r o n s i n v o l v e d i n t h e

e l e c t r o c h e m i c a l s t e p o f t h e p r o c e s s i s a l s o d e t e r m i n e d .

HYPOTHESIS

Based on p r e l i m i n a r y s t u d i e s and c h e m i c a l

b e h a v i o r t h r e e main hypoteses were e s t a b l i s h e d :

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1) E l e c t r o c h e m i c a l o x i d a t i o n s t r a i g h t t o Hg2+ (n=2)

Hgo - Hg2+ + 2e electrochemical s tep

Hg + Hgo II? Hg2 coupled chemical s tep 2+ 2+

2) S t cpwise e l e c t r o c h e m i c a l o x i d a t i o n t o Hg2+ (n=2)

~ g ' --- ~ g + + l e first electrochemical s t ep

~ g + --- ~ g ' + + l e second electrochemical s tep

coupled chemical s tep 2+ H Z + + Hgo --3 HgZ

3) E l e c t r o c h e m i c a l o x i d a t i o n t o Hg+ (n=1)

2x(Hg0 -- Hg+ + l e ) electrochemical s tep

Hg+ + Hg+ + Hg2 f i r s t chemical s tep (dimeri zat ion)

second chemical s tep (disproportionationj

2+

- H:+ + Hgo

E X P E R I M E N T A L D A T A

Mercurous c a t i o n s were g e n e r a t e d i n two w a y s :

c h e m i c a l l y and c o u l o m e t r i c a l l y . The chemica l

g e n e r a t i o n o f mercurous c a t i o n s was done i n non

complexing media, t a k i n g 5 m l o f m e t a l mercury i n a

2 0 m l b e a k e r and add ing a s t a n d a r d s o l u t i o n o f

m e r c u r i c ( I 1 ) p e r c h l o r a t e ( a c i d i c s o l u t i o n ) . T h e

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s o l u t i o n was l e f t d u r i n g 2 h o u r s u n d e r a b l a n k e t o f

n i t r o g e n and m i l d l y s t i r r e d , k e e p i n g t h e t e m p e r a t u r e

a t 2 5 O C . The c h e m i c a l r e a c t i o n t h a t o c c u r r e d w a s :

t h e c o r r e s p o n d i n g e q u i l i b r i u m c o n s t a n t b e i n g :

The c o u l o m e t r i c g e n e r a t i o n o f m e r c u r o u s c a t i o n s

a t c o n s t a n t c u r r e n t i n non c o m p l e x i n g media was

c a r r i e d o u t p l a c i n g 5 m l o f m e t a l mercu ry i n t o 20 m l

c e l l , and a d d i n g a known volume o f s t a n d a r d s o l u t i o n

o f sodium p e r c h l o r a t e . A c o n s t a n t c u r r e n t o f 2 0 mA

was imposed d u r i n g 30 m i n u t e s .

The c o m p a r i s o n o f t h e s e two d i f f e r e n t methods o f

g e n e r a t i n g mercu rous i o n s showed t h a t i n t h e

s u p e r n a t a n t s o l u t i o n Hg2 and Hg2+ a r e i n e q u i l i b r i u m ,

t h e amount o f Hg b e i n g s m a l l .

2 +

2 +

C u r r e n t E f f i c i e n c y and E l e c t r o g r a v i m e t r y

I n t h e c o u l o m e t r i c g e n e r a t i o n o f m e r c u r o u s

s p e c i e s a t c o n s t a n t c u r r e n t i t i s i m p o r t a n t t o

d e t e r m i n e t h e c u r r e n t e f f i c i e n c y t o make s u r e n o s i d e

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r e a c t i o n s a r e o c c u r r i n g s i m u l t a n e o u s l y . The number o f

e l e c t r o n s i n v o l v e d i n t h e e l e c t r o c h e m i c a l s t e p i s

s t a t e d i n t h e l i t e r a t u r e

r e a c t i o n :

g i v i n g t h e o v e r a l l 6-8

2 + 2 Hgo - 2e - Hg2

t h e f o l l o w i n g chemica l r e a c t i o n i s w e l l known:

Hg2+ + Hgo - Hg; +

I n t h i s way t h e r e i s a c o n t r o v e r s y abou t t h e number

o f e l e c t r o n s (one o r two) i n v o l v e d i n t h e o x i d a t i o n

o f mercury m e t a l t o c a t i o n s i n t h e e l e c t r o c h e m i c a l

s t e p .

To e l u c i d a t e t h i s f a c t , e l e c t r o g r a v i m e t r i c

measurements were done u s i n g t h e f o l l o w i n g p rocedure :

A known mass o f mercury was s u b m i t t e d t o e lectrolysis .

A f t e r t h e e l e c t r o l y s i s t h e r ema in ing mercury was

washed t h o r o u g h l y w i t h w a t e r , d r y e d c a r e f u l l y w i t h

h y g r o s c o p i c p a p e r , washed w i t h a l c o h o l and d r y e d

c a r e f u l l y w i t h h y g r o s c o p i c p a p e r , and f i n a l l y d r y e d

w i t h h o t a i r u s i n g a h a i r d r y e r d u r i n g 30-60 seconds.

A f t e r c o o l i n g t h e mercury was weighed. The d i f f e r e n c e

i n mass c o r r e s p o n d s t o t h e amount e l e c t r o l y s e d . A

Metrohm c e l l EA876-20 was used . A mercury poo l was the

anode and ;i p l a t i n u m e l e c t r o l e , t h e c a t h o d e , b o t h

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TABLE 1

Supporting Generated Theoretical Number of Efficiency

electrolyte mass (g) mass (8) ELECTRONS* (%I

NaC104 1.93M 0.0745C 0 .0021 0.0746 1 99.9

Na2S203 0.08M 0 .0370~ 0.0016 0.0373 2 99.2

HC104 0.(),2M

NaC104 1.76M

EDTA 0.04M 0 . 0 4 5 2 t 0.0020 0.0373 2 "121"

NaC104 1.84M

* See t e s t .

s e p a r a t e d p r o p e r l y w i t h a s i n t e r e d g l a s s a g a r - a g a r

p l u g . The volume o f s o l u t i o n was 1 0 . 0 0 m l , t h e

c o n s t a n t c u r r e n t 2 0 . 0 0 mA and t ime 3 0 m i n u t e s . The

sys t em w a s m a i n t a i n e d unde r a b l a n k e t o f n i t r o g e n

and t h e t e m p e r a t u r e k e p t a t ( 2 5 i 0.2)°C. The

s u p p o r t i n g e l e c t r o l y t e was sodium p e r c h l o r a t e i n non

complexing med ia , and t h i o s u l p h a t e and EDTA as

complexing a g e n t s o f m e r c u r i c ( I 1 ) i o n s i n aqueous

media. The same amount o f e l e c t r i c i t y was p a s s e d

th rough a l l s o l u t i o n s . The e x p e r i m e n t a l d a t a a r e

c o n s i g n e d i n TABLE 1.

According t o F a r a d a y ' s L a w t h e f o l l o w i n g

e x p r e s s i o n i s v a l i d : n = (W) i t

mF

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E x p e r i m e n t a l l y t h e same amount o f e l e c t r i c i t y was

t r a n s f e r r e d . T h e o r e t i c a l l y t h e mass o f mercu ry

e l e c t r o l y s e d depends on t h e number o f e l e c t r o n s

i n v o l v e d i n t h e p r o c e s s . T h e r e f o r e , i f n = 1 t h e m a s s

m = k / n l and i n t h e c a s e o f n = 2 , i n 2 = k / n 2 . T h u s ,

m l = 2 . m 2 . C o n s i d e r i n g t h e f i r s t h y p o t h e s i s

e n u n c i a t e d e a r l i e r , where t h e number o f e l e c t r o n s i s

t w o , t h e v a r i a t i o n o f t o t a l mass o f mercu ry m e t a l

w i l l b e due t o t h e v a r i a t i o n o f mass c o r r e s p o n d i n g t o

t h e e l e c t r o c h e m i c a l s t e p and t o t h e v a r i a t i o n o f m a s s

r e l a t e d t o t h e f o l l o w i n g c h e m i c a l s t e p . I n non

complex ing m e d i a , where Hg2+ i s n o t s t a b i l i z e d , t h e

c h e m i c a l s t e p a l w a y s o c c u r s . I n t h i s way, t h e

v a r i a t i o n o f t o t a l mass ( g r a v i m e t r y ) a s i n d i c a t o r o f

t h e number o f e l e c t r o n s i n v o l v e d i n t h e electrochemical

s t e p i s n o t c o r r e c t . However , i n c o m p l e x i n g m e d i a ,

where t h e l i g a n d s t a b i l i z e s H g 2 + , t h e p r o c e s s o c c u r s

w i t h c h e m i c a l s t e p b l o c k a g e . Now t h e v a r i a t i o n o f

mass i s a c t u a l i n d i c a t o r o f t h e number o f e l e c t r o n s

i n v o l v e d i n the electrochemical s t e p . I f the ,:roccss proceeds

i n c o m p l e x i n g media f o r Hg , a c c o r d i n g t o t h e f i r s t

and s e c o n d h y p o t h e s e s , t h e mass v a r i a t i o n w i l l b eha l f

t h e mass v a r i a t i o n i n non complex ing med ia . I f t h e

p r o c e s s f o l l o w s t h e t h i r d h y p o t h e s i s , t h e mass

v a r i a t i o n w i l l b e t h e same i n corr!ilcxint. a!ld i n non

1

2 +

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complexing media. The T A B L E 1 shows t h a t t h e mass

e l e c t r o l y s e d i n t h i o s u l p h a t e media i s h a l f t h e mass

e l e c t r o l y s e d i n absence o f complexing a g e n t . T h i s

l e a d s us t o t h e c o n c l u s i o n t h a t t h e p r o c e s s f o l l o w s

h y p o t h e s e s 1 and 2 , w i t h two e l e c t r o n s , n = 2 , and

r u l e s o u t h y p o t h e s i s 3 . By now we a r e n o t a b l e t o

d i s t i n g u i s h t h e f i r s t and t h e second h y p o t h e s e s

u s i n g g r a v i m e t r y . I n t h e c a s e o f EDTA i t seems t h a t

t h e complexa t ion i s n o t c o m p l e t e ; a l i m i t a t i o n maybe

p r e s e n t due t o l i b e r a t i o n o f p r o t o n .

r e a c t i n g c h e m i c a l l y w i t h H g , y i e l d i n g an i n c r e a s e i n

t h e mass o f mercury and making t h e e f f i c i e n c y a p p e a r

t o be o v e r one hundred p e r c e n t . The e f f i c i e n c y o f

c u r r e n t was o b t a i n e d by t h e r e l a t i o n o f g e n e r a t e d

mass and t h e o r e t i c a l mass.

some H~~~ i s

Complimentary S t u d i e s w i t h Spo t T e s t s

The e a r l i e r s t u d i e s were n o t a b l e t o d i s t i n g u i s h

whe the r t h e o x i d a t i o n o f mercury o c c u r s s t e p w i s e o r

whe the r t h e two e l e c t r o n s a r e removed a t o n c e . I f t h e

p r o c e s s i n v o l v e s two s t e p s , t h e f o r m a t i o n o f

m e r c u r y ( I 1 ) i s n o t p o s s i b l e i n a c o u l o m e t r y experiment

i n t h e p r e s e n c e o f c h l o r i d e , which s t a b i l i z e s H g 2 . I f

t h e o x i d a t i o n i s s t r a i g h t t o H g ( I I ) , t h e a p p e a r a n c e

2 +

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o f Hg(1) i n .:he s o l u t i o n i s due t o a chemica l s t e p ,

b u t some Hg(I1) i o n s w i l l be p r e s e n t i n t h e s o l u t i o n .

I n t h i s way t h e d e t e c t i o n o f Hg(I1) i n t h e s o l u t i o n

i s a s t r o n g argument i n t h e e l u c i d a t i o n o f t h e

mechanism. The e l e c t r o g r a v i m e t r y e x p e r i m e n t i n t h e

p r e s e n c e o f c h l o r i d e f a i l e d b e c a u s e o f l o s s o f

ca lome l p r e c i p i t a t e d u r i n g t h e washing o p e r a t i o n . The

l i t e r a t u r r c i t e s d i p h e n y l c a r b a z i d e a s an o r g a n i c

r e a g e n t f o r H g 2 + , b u t c h l o r i d e can i n t e r f e r e w i t h t h e

t e s t depend ing on i t s c o n c e n t r a t i o n .

Mercurous c a t i o n s were g e n e r a t e d c h e m i c a l l y from

s t a n d a r d s o l u t i o n o f Hgz+ and Hg y i e l d w a s a p p r o x i m a t e l y

2 0 mM o f Hg2 . The s u p e r n a t a n t was t a k e n and

i d e n t i f i c a t i o n t e s t s f o r Hg2+ were pe r fo rmed w i t h

d i f f e r e n t amounts o f C 1 - p r e s e n t . When c h l o r i d e was

added , p r e c i p i t a t i o n o f ca lomel o c c u r e d . From t h e

t e s t we can o b s e r v e t h a t t h e p r e s e n c e o f c h l o r i d e up

t o a 1: 2 p r o p o r t i o n ( 1 Hg;' : 2 C1-) a l l o w e d t h e

i d e n t i f i c a t i o n o f Hg2+. Above t h e 1 : 2 p r o p o r t i o n t h e

t e s t was n e g a t i v e . C h l o r i d e seems t o be a s t r o n g e r

l i g a n d f o r mercury t h a n d i p h e n y l c a r b a z i d e , t h u s

masking t h e t e s t . (TABLE 2 ) .

2 +

Mercurous c a t i o n s were g e n e r a t e d by galvanostatic

cou lomet ry i n t h e a b s e n c e o f complexing a g e n t , t o

y i e l d an a p p r o x i m a t e l y 1 8 . 6 mM s o l u t i o n . The amount

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2 + TABLE 2 Spo t T e s t f o r t h e p r e s e n c e o f Hg

IC1-1 added o v e r T e s t f o r Hg2+ w i t h d i p h e n y l c a r b a z i d e s upe r n a t an t

mM

IC1-1 added o v e r T e s t f o r Hg2+ w i t h d i p h e n y l c a r b a z i d e s upe r n a t an t

mM

0

11

2 1

4 1 . 2

4 8 . 6

6 9 . 5 -

8 2 . 6 -

2 + T A B L E 3 Spo t T e s t f o r t h e p r e s e n c e o f Hg

I C 1 - 1 added o v e r T e s t f o r Hg2+ w i t h p12 c a r b a z i d e s u p e r n a t a n t s o l u t i o n

a f t e r e l e c t r o l y s i s

2 1

3 5 . 2

39

4 0 . 5 -

4 5 . 5

5 3

1 2 3

o f c h a r g e was c a l c u l a t e d w i t h t h e a s sumpt ion n = 2

and t h e occurence o f t h e chemica l r e a c t i o n between

Hg2+ and Hg. Tak ing t h e s u p e r n a t a n t s o l u t i o n , t e s t s f o r

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Hg2+ were done i n t h e p r e s e n c e o f d i f f e r e n t amounts o f

c h l o r i d e .

Again when ( C 1 - 1 i s be low t h e p r o p o r t i o n 1 : 2 t h e t e s t

is p o s i t i v e . Above t h i s i t i s n e g a t i v e . C h l o r i d e

i n d e e d i n t e r f e r e s w i t h t h e t e s t . (TABLE 3 ) .

G a l v a n o s t a t i c c o u l o m e t r y e x p e r i m e n t s f o r

o x i d a t i o n o f Hg i n t h e p r e s e n c e o f d i f f e r e n t

c o n c e n t r a t i o n s o f c h l o r i d e were p e r f o r m e d . The c h a r g e

t h a t went t h r o u g h t h e mercu ry p o o l e l e c t r o d e was 2 + s u f f i c i e n t t o g e n e r a t e a p p r o x i m a t e l y 1 8 . 6 mM o f HgZ ,

a s s u m i n g t h a t n = 2 . The a n a l y s i s o f e a c h s u p e r n a t a n t

s o l u t i o n i s g i v e n be low:

2 + T A B L E 4 S p o t T e s t f o r t h e p r e s e n c e o f Hg

I C 1 - 1 p r e s e n t i n t h e Tes t f o r Hg2+ w i t h

-

s o l u t i o n s s u b m i t t e d t o e l e c t r o l y s i s $4, c a r b a z i d e

mM

1 0 +

2 0 +

36 -

37.2 -

37.2 -

38 -

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To p r e c i p i t a t e a l l mercurous i o n s w i t h chlor ide1.37 mM

o f C 1 - a r e needed. However, t h e t e s t was n e g a t i v e from

36 mM o n , b e f o r e t h e s t o i c h i o m e t r y f o r ca lomel

f o r m a t i o n was r e a c h e d . T h i s means t h a t i n a coulometry

expe r imen t i n t h e p r e s e n c e o f c h l o r i d e no Hg2+ i s

formed unde r t h e p r e s e n t s p o t t e s t s e n s i t i v i t y .

C h l o r i d e s t a b i l i z e s Hg'. The d i m e r i z a t i o n can o c c u r

t h r o u g h t h e chemica l s t e p s Hg+ + C 1 - - HgCl and

2 HgCl - Hg2C12. T h i s e x p e r i m e n t shows t h a t i n t h e

e l e c t r o - o x i d a t i o n o f Hg i n t h e p r e s e n c e o f C 1 - t h e

c h a r g e i s t r a n s f e r e d i n o n e - e l e c t r o n s t e p and Hg(1)

i s s t a b i l i z e d by C 1 - . I n t h e a b s e n c e o f C 1 - t h e

o x i d a t i o n goes up t o Hg(I1) i n two r a p i d one-

- e l e c t r o n s t e p s . (TABLE 4 ) . 2 + Another t e s t was done i n which 1.37.2 mM o f Hg2

were g e n e r a t e d c o u l o m e t r i c a l l y i n t h e p r e s e n c e o f

c h l o r i d e . Using s u l f i d e as a r e a g e n t f o r Hg2 , t h e

t e s t was n e g a t i v e . A c o u l o m e t r i c b l a n k t e s t i n t h e

a b s e n c e o f c h l o r i d e proved t h e p r e s e n c e o f Hg by

means o f t h e a d d i t i o n o f s u l f i d e .

2 +

2 +

F i n a l l y , a g a l v a n o s t a t i c cou lomet ry e x p e r i m e n t

i n which Hg was o x i d i s e d t o y i e l d 1.18 mM i n t h e

p r e s e n c e o f d i f f e r e n t amounts o f t h i o s u l f a t e was

pe r fo rmed . Af t e r e l e c t r o l y s i s t h e p r e s e n c e o f Hg2

was i n v e s t i g a t e d w i t h C 1 - . The amount o f Hg

2 +

2 +

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2 +

T e s t f o r Hg2

TABLE 5 Spo t T e s t f o r t h e p r e s e n c e o f Hg2 2 + S20; p r e s e n t i n t h e

s o l u t i o n s u b m i t t e d

mM t o e l e c t r o l y s i s w i t h C 1 -

1 8 . 6

37.2

55.8

g e n e r a t e d was a p p r o x i m a t e l y 1 9 m M , c o n s i d e r i n g n = 2 .

N o f o r m a t i o n o f Hgf'was o b s e r v e d . (TABLE 5 ) . T h i s

means t h a t t h e o x i d a t i o n t a k e s p l a c e s t r a i g h t t o Hg(I1) 2 + ( n = 2 ) i n one s t e p t h a t t h e i n t e r a c t i o n between Hg

and Hg does n o t o c c u r due t o t h e s t a b i l i z a t i o n o f Hg

w i t h S20;. T h i s complex decomposes o v e r some t i m e

y i e l d i n g b l a c k HgS.(Hg(S203)21 _ H 2 3 HgS + SO:-+ S 2 0 i + 2H'.

2+

CONCLUSION

The e l e c t r o o x i d a t i o n o f mercury i n non

complexing s u p p o r t e l e c t r o l y t e s goes d i r e c t l y t o

H g ( I I ) , i n a r a p i d sequence o f two o n e - e l e c t r o n s t e p s ,

f o l l o w e d by t h e c o u p l e d chemica l p r o c e s s :

Hg2+ + Hg - Hg; + ( n = 2)

I n a complexing medium t h a t s t a b i l i z e s t h e

i n t e r m e d i a t e H g ( I ) , f o r i n s t a n c e C1-, ' t h e o x i d a t i o n

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s t o p s a t H g ( I ) , p r e c i p i t a t i n g c a l o m e l a s t h e

f o l l o w i n g c o u p l e d c h e m i c a l p r o c e s s ( n = 1 ) . No

f o r m a t i o n o f Hg2+ o c c u r s i n t h i s way. 2 + I n a complex ing medium t h a t s t a b i l i z e s Hg , f o r

example S 2 0 J , t h e e l e c t r o o x i d a t i o n goes d i r e c t l y t o

Hg( I I ) a n d no f o l l o w i n g c o u p l e d c h e m i c a l r e a c t i o n

o c c u r s ( n = 2 ) . No f o r m a t i o n o f Hg2 o c c u r s i n t h i s

way. The e f f i c i e n c y o f c u r r e n t i s t o t a l .

2 +

A C K N O W L E D G E M E N T S

The work was s u p p o r t e d by f u n d s r e c e i v e d fromthe

B r a s i l i a n F o u n d a t i o n FAPESP (Fundaqao d e Amparo S P e s -

q u i s a do E s t a d o d e Sao P a u l o ) u n d e r g r a n t np 80/1487-5.

Dr. Pe te r Tiedemann r e a d i n g e n g l i s h m a n u s c r i p t .

REFERENCES

1. I.M. K o l t h o f f and C.S . M i l l e r , J . A m e r . Chem. S O C , 6 3 , 1 4 0 5 , ( 1 9 4 1 ) . _ _ -

2 . D . G . D a v i s , A n a l . Chem., __ 3 0 , 1 7 2 9 , ( 1 9 5 8 ) .

3 . C . J . Nyman and T . S a l a z a r , Ana l . Chem., - 3 3 , 1467, ( 1 9 6 1 ) .

4 . T . Murayama, T . Sawasak i a n d S . S a k u r a b a , B u l l . Chem. SOC. J p n . , - 4 3 , 2820 , ( 1 9 7 0 ) .

5 . L . K u l l b u r g , Acta Chem. S c a n d . , A28, 9 7 9 , (1974) .

6 . J . Ravenda , C o l l e c t . Czech. Chem. Commun., - 6 , 4 3 3 , ( 1 9 3 4 ) .

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2094 TOKORO A N D TAVARES

7. I.M. Kolthoff and J.J. Lingane, "Polarography", Interscience Publishers, New York, 1952, p.573.

8. T. Biegler, J.Electroana1. Chem., - 357, 374, (1963).

9. F. Feigl, "Spot Tests in Inorganic Analysis", Elsevier Publishing Company, New York, 1972, p.307.

Received August 25, 1986 Accepted September 5, 1986

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