GOI'ALA RAO and V~RESWARA RAO: Ascorbic Acid as a Reducing Agent. I I

29

b e s c h r i e b e n , z w e c k m / i B i g e r w e i s e a u f 2 5 0 - - 3 0 0 m l v e r d f i n n t , die Z u g a b e yon Schwefels/iure entf/illt oder wird niedrig gehalten. Titrationen mit 0,01 n C e r ( I V ) - s u l f a t l S s u n g e r g a b e n k e i n e r e p r o d u z i e r b a r e n W e r t e , d a g e g e n w a r e n die R e s u l t a t e bei V e r w e n d u n g d e r 0,1 n L 6 s u n g r e c h t b r a u c h b a r (Tab. 6).

Literatur 1 Siehe z. B. BILTZ-FISCIIER: Ausfiihrung quantitativer Analysen. 6. Aufl., S.4O0. Stuttgart: S. Hirzel 1953. - - e FISOHF~R,K., u. F. TIHELE: Z. anorg, allg. Chem. 67, 302 (1910); insbes. S. 312. - - 3 FRESENIUS-JANDER: Handbuch der analytischen Chemic, Band III, S. 764. Berlin, GSttingen, Heidelberg: Springer 1942. - - 4 Ausftihrliche Literatur bei N. H. FURMAN: ,,Cer(IV)-LSsungen als maBanalytische Oxydationsmittel" in ,,Neuere maBanalytische Methoden", 3: Aufi. 1951. Band X X X I I I der Sammlung ,,Die chemische Analyse", herausgegeben yon W. B5ttger. Stuttgart: F. Enke 1951. - - 5 Fu~A~r N. H., and J. If. WALLACE: J. Amer. chem. Soc. 53, 1283 (1931). - - 6 GLEU, K.: diese Z. 95, 305 (1933). --- 7 HARRIS, F. R.: Analyst (London) 75, 496 (1950); vgl. diese Z. 184, 156 (1951). - - s KLEMlg, W., u. P. IfENKEL: Z. anorg, allg. Chem. 220, 180 (1934). - - 9 LEWIS, 1).: Ind. Engng. Chem., anal: Edit. 8, 199 (1936); vgl. diese Z. 109, 121 (1937). INGEBORG LANGE, Freiberg/Sachsen, Agricolastr. 17

Department of Chemistry, Andhra University, Waltair (India)

hseorbie Acid as a Reducing Agent in Quantitative Analysis P a r t II*

Estimation of Mercuric Chloride through Reduction to Mercurous Chloride By G. GOPALA RA0 and U. VEERESWARA. RA0

(Eingegangen am 7. Dezember 1955) Ascorbic acid is coming into prominence in recent years as a valuable reductometric reagent in quantitative analysis. SZmCTGY6RoY120 was the first to observe that ascorbic acid reduces silver nitrate. This reaction was later studied in some detail by SVIlCBELY19 who found that the speed of the reaction depends to a large extent on the acidity or alkalinity of the reaction mixture. In slightly ammoniaeal medium, the reduction proceeds rapidly, yielding a black precipitate of finely divided metallic silver. I n a neutral medium, the reaction speed is much less and the precipitate obtained consists of large particles of a silvery sheen. When the medium becomes even slightly acid (about p~ 3) the reaction velocity is negligibly slow. This reaction has been utilised by STATlZlS16 for the gravimetric estimation of silver. A boiling neutral solution of silver nitrate is treated with an excess of ascorbic acid solution, and the precipitate is washed with hot water, ignited and weighed. ER~mr and * Part I : Z. analyt. Chem. 147, 338 (1955).

30

G. GoP~A RAo and U. VEE~ESW~a~ARAO:

Buzs 5 carried out the titrimetrie estimation of silver nitrate with ~scorbic acid detecting the end point potentiometrically or with a suitable redox indicator like variamine blue B, 4-amino-4'-methoxy-diphenyl amine. At the end point, the solution should be made neutral or only slightly acid by adding sodium acetate. Solutions as dilute as 0.001 N. are reported to give satisfactory end points. STAT~IS and GATOSX7have employed ascorbic acid also for the quantitative determination of gold chloride, making use of the reaction: 2 AuC13 + 3 C6HsO6 --> 2 An -[- 3 CsH~O6 ~- 6 HC1. The gold chloride solution is acidified with dilute hydrochloric acid, heated to 80~ to 90~ C and then treated with a freshly prepared ascorbic acid solution. The heating is continued for five minutes. After cooling to room temperature, the precipitated metallic gold is filtered, washed with dilute hydrochloric acid, ignited and weighed. It is claimed that copper salts do not inter fere in this gravimetric procedure. STAT~IS and GATOS1T also studied the volumetric determination of gold chloride with ascorbic acid. The slightly acidified solution of gold chloride is treated in the cold with a known excess of freshly prepared ascorbic acid solution for five minutes with constant stirring. Then the unreacted ascorbic acid is estimated by titration with a decinormal iodine solution, using starch as the indicator. PvrrsYN and KozLov n employed ascorbic acid for the volumetric estimation of ferric iron with a potentiometric end point. FLASCm~ and ZAVAGYL6 titrated ferric iron with ascorbic acid in hydrochloric acid solution using potassium thiocyan te us the indicator. The disappearance of the pink color of the ferric thiocyanate is taken to be the end point. ERDEu and BoDo~ a made a careful study of this volumetric procedure. They recommended that the titration be carried out at 60~ C because the reaction is somewhat slow at the room temperature, specially towards the approach of the end point. These investigators give the caution that temperatures above 60~ C should be avoided to prevent the interference of the reaction between ferric iron and dehydroascorbic acid. EM~ERIEa stated that ascorbie acid reduces seleneous acid to give an orange-red colour..Working with a 5 per cent solution of seleneous acid he found the limit of sensitivity to be 0.01 mg. of ascorbic acid per ml. RUn~A~5employed the same reaction for the turbido-colorimetric determination of selenium in vegetable extracts, soils and water samples. I n the p r e s e n t investigation, we have s t u d i e d t h e use of ascorbic acid for the e s t i m a t i o n of m e r c u r y i n mercuric chloride. The reaction b e t w e e n mercuric chloride a n d ascorbic acid has already been e m p l o y e d for the d e t e r m i n a t i o n of ascorbic acid b y ROS~T~ALE~ In, as also b y SV~u RAYA:~A~AO, V]~ERESWARA~:~_A_Oa n d GOPALA I:~AOls . The r e a c t i o n b e t w e e n mercuric chloride a n d ascorbic acid m a y be represented b y the stoichiometric e q u a t i o n 2 HgCI~ -~ C~HsO s - ) Hg~Cl~ ~- C~H608 ~- 2 HC1. I t is well k n o w n t h a t a solution of mercuric chloride contains H g ~+ ions a t a negligible c o n c e n t r a t i o n . The composition of a s a t u r a t e d aqueous solution of the salt is given b y S H E ~ I L as, (HgCI~) : 2.6 • 10-~; (HgC1+) : 2 . 7 • 10-~; (HgC14~-) = 1 . 6 • 10-s; a n d (Hg ~+) = 3 . 5 • 10 -s. The c o n c e n t r a t i o n of t h e complex halide ions increases in the presence of a soluble halide like sodium or p o t a s s i u m chloride.

Ascorbic Acid as a Reducing Agent. II

31

The potentials* of the 2 Hg2+ ~<~-Hg~2+ couple have been measured by CART~I~ and Ro~I~SO~ 2 in perchloric acid medium against a calomel electrode and the value E 0 given as 0.913 volts. Similar measurements by POPOFF, RIDDICK, WIRTH and OvGI~l~ using a hydrogen reference electrode have yielded the value. 0.905 volts for the E 0. The E 0 potential corresponding to the reduction of mercuric chloride to mercurous chloride is, however, much less than this. The value is estimated by LATIME~s tO be 0.53 volt. C1- + 1/2 Hg2C12~- HgC12 (sat. solution) + e-. I t will be interesting to compare this value with the E o potential corresponding to the redaction of mercuric chloride directly to metallic mercury. An approximate idea of this potential may be got from the following data supplied by LATI~S,I~s 4 C1- + Hg = HgC142- d- 2 e-, E o = -4- 0.48 volt. T h u s the mercuric chloride-mercurous chloride couple with a potential of 0.53 volt appears to be a slightly stronger oxidizing agent than the mercuric chloride-mercury couple with a potential of + 0.48 volt. Hence it is reasonable to suppose that any reducing agent will first reduce the mercuric chloride to the mercurous chloride and then only reduce the mercurous chloride to mercury provided its redox potential is suitable for the purpose. I~A~TDALLand u 12 have determined the value of the mercurous chl0ride-mercury couple to be as follows: 2 C1- + 2 Hg = Hg2C12 d- 2 e-, E o = 0.2676. Hence, mercurous chloride will be reduced only by reductants having redox potentials lying well below 0.26. The theoretical considerations outlined above lead us to believe that it should be possible to achieve the reduction of mercuric chloride to any desired stage by the choice of a reductant with the appropriate redox potential. The redox potential of aseorbie acid changes appreciably with the change in the p~ of the medium, so that we have in ascorbic acid a single reductant whose reducing power can be altered at will by varying the p~ of the medium. This will be evident from the data in Table 1. Table 1. Variation o/the Redox Potential o/Ascorbic Acid with changing p s p~

Eoin volts

p~

1.05 2.16 3.04 4.00

0.326 0.260 0.209 0.154

5.19 6.32 7.24 8.67

Eoin volts I

0.115 0.078 0.051 --0.012

* The European notation, but not the American, is adopted in assigning the sign to the values of redox potentials.

32

G. GOPALARAO and U. VEEaESW~a~At~AO:

A c o m p a r i s o n of t h i s d a t a w i t h t h e d a t a concerning t h e r e d o x p o t e n t i a l s of t h e HgCI~--Hg2C12 a n d Hg2C12--Hg s y s t e m s a l r e a d y discussed shows t h a t i t should be possible to reduce m e r c u r i c chloride either t o t h e m e r c u r o u s chloride or t o t h e metallic m e r c u r y s t a g e w i t h ascorbic a c i d b y controlling t h e pH of t h e m e d i u m a n d o t h e r conditions. Our e x p e r i m e n t s show t h a t these e x p e c t a t i o n s of t h e reducing b e h a v i o u r of ascorbic acid can be realised in full. I t will be f o u n d t h a t a t p ~ 2.16 a n d below, t h e p o t e n t i a l of ascorbic acid is either equal t o or m o r e t h a n t h e p o t e n t i a l of t h e mercurous c h l o r i d e - m e r c u r y couple. I n a c c o r d a n c e w i t h this, we h a v e o b s e r v e d t h a t a t p ~ 2.2 a n d below, ascorbic a c i d does n o t r e d u c e m e r c u r o u s chloride to t h e m e t a l l i c m e r c u r y s t a g e even w h e n t h e ascorbie a c i d is t a k e n in large excess. A t pH 3.04 t h e m e r c u r o u s c h l o r i d e - m e r c u r y p o t e n t i a l is o n l y s l i g h t l y higher (by a b o u t 0.0586 volts) t h a n t h a t of t h e ascorbie acidd e h y d r o a s c o r b i c a c i d couple. A c c o r d i n g l y our e x p e r i m e n t s show t h a t ascorbic acid shows o n l y a slight r e d u c i n g p o w e r on t h e m e r c u r o u s chloride a t this p~. Some m e t a l l i c m e r c u r y results when t h e ascorbic acid is t a k e n in large excess e. g. w h e n t h e mole r a t i o exceeds 10 : 1. W h e n t h e p ~ of t h e m e d i u m is r a i s e d t o 4, we o b s e r v e d t h a t a p p r e c i a b l e r e d u c t i o n to metallic m e r c u r y o c c u r s even w i t h a mole r a t i o of H g : A H 2 ~ 1 : 2. As t h e p~ is r a i s e d further, we f o u n d t h a t t h e t e n d e n c y t o m e r c u r y f o r m a t i o n increases. This m a y be e x p l a i n e d b y t h e f a c t t h a t t h e p o t e n t i a l of t h e ascorbic a c i d s y s t e m a t t a i n s values i n c r e a s i n g l y below t h a t of t h e mercurous c h l o r i d e - - m e r c u r y couple as t h e p ~ is increased. H e n c e we m a y conclude t h a t t h e p ~ of t h e m e d i u m m u s t be k e p t below 3, if i t is desired t o achieve t h e q u a n t i t a t i v e r e d u c t i o n of m e r c u r i c chloride to m e r c u r o u s chloride, a v o i d i n g t h e f o r m a t i o n of m e t a l l i c m e r c u r y .

Experimental .Materials The ascorbie acid used in this investigation is the crystalline vitamin C manufactured by Hoffmann La Roche Company Ltd. of Basle, Switzerland. A solution of the requisite strength is prepared by dissolving a weighed quantity of the substance in triple distilled water. This precaution is neo~essary in view of the fact that the autoxidation of ascorbie acid in aqueous solutions is markedly eatalysed by copper ions. The solution was standardized with a standard solution of potassium permanganate ~ccording to the method of MU~Tn~Z and VlSWA~ADH~9 or with standard solution of potassinm iodate according to the method ofBALLE~TINE1. All other chemicals are of reagent quality. To control the p~ of the reaction mixture, buffer solutions composed with citric acid and sodium dihydrogen phosphate as recommended by MCILVAI~E have been used. WALPOLE buffers consisting of sodium acetate hydrochloric acid are unsuitable because of the marked decelerating action of chloride ions on the reduction of mercuric chloride.

Procedure [or the volumetric analysis An aliquot volume of the mercuric chloride solution is taken in a conical flask, and treated with 20 ml. of the !VIeILVAY~Ebuffer of the desired p~ and an excess

Ascorbic Acid as a Reducing Agent. I I

33

Table 2. Influence of p~ and Concentration o/Ascorbic Acid on the Reduction o/

Mercuric Chloride Pg

Mole ratio of AK2 :HgCI~

Time of reaction in minutes

4.0 4.0 4.0 4.0 4.0 4.0 4.0

1.2:1 1.2:1 1.2:1 1.2:1 1.2:1 1.2:1 1.2:1

5 5 10 l0 10 15 3O

0.1724 0.1120 0.1493 0.1113 0.0862 0.1357 0.1357

0.1724 0.1116 0.1487 0.1109 0.0862 0.1352 0.1353

4.0 4.0 4.0 4.0 4.0

10:1 10:l 5:1 5:1 2:1

15 3O 15 30 30

0.01783 0.01783 0.01783 0.01783 0.01783

0.03466 0.03646 0.03646 0.03648 0.03646

3.0 3.0

1.2:l 1.2:1

5 15

0.1357 0.1357

0.08414 0.1024

3.0 3.0 3.0 3.0

1.2:l 1.2:1 2.5:1 5:1

3O 60 15 15

0.1357 0.1357 0.01783 0.01783

0.1355 0.1357 0.01783 0.01783

3.0 3.0 3.0

10:1 10:1 10:1

3O 15 3O

0.01783 0.01783 0.01783

0.02540 0.02127 0.0254

grey color, reduction carried too far

2.2 2.2 2.2 2.2 2.2 2.2 2.2

1.2:1 1.2:1 1.2:1 1.2:1 2:1 2:1 5:1

5 10 3O 6O 15 30 15

0.1357 0.1357 0.1357 0.1357 0.01783 0.01783 0.01783

0.03522 0.04536 0.06088 0.07163 0.00960 0.01303 0.01646

reduction incomplete

2.2 2.2 2.2

5:1 10:1 10:1

30 15 3O

0,01783 0.01783 0.01783

0.01783 0.01783 0.01783

reaction complete

i

i

Amount of ]tgCl~ taken g.

Amount of HgC12reduced g.

l

Remarks

eaction complete

!grey color, reduction i carried too far

reduction incomplete reaction complete

of a standard solution of ascorbie acid. After keeping the mixture for the required time, it is treated with 20% potassium iodide, 5 ml. of 1:1 sulphuric acid and 1.0 ml. of 0.5% starch solution. The mixture is then diluted to 250 ml. and titrated with standard potassium iodate solution. The end point is indicated by the appearance of a deep blue colour which is stable for about a minute in these titrations. As the end point is approached, it is desirable to carry out the titration by adding the iodate solutien dropwise. For the succes of the titration, it is necessary to maintain the concentration of sulphuric acid, potassium iodide and the dilution at about the values mentioned here. The quantity of potassium iodate consumed in the titration corresponds to the ascorbic acid that has been left over. This is subtracted from the quantity of ascorbic acid originally added and the difference arrived at corresponds to the amount of mercuric chloride taken. Z. anal. Chem., Bd. 150

3

34

G. GOPALARAo and U. VEERESWA~ARAO:

All the results in Table 2 (p. 33) are expressed on the basis that the reduction of mercuric chloride stops at the mercurous chloride stage. That the reduction has proceeded partially to the metallic mercury stage is shown by the grey colour of the precipitate. Further, if any mercuric chloride has escaped reduction it can be detected by the appearance of a transient scarlet red color of the mercuric iodide during the subsequent addition of potassium iodide, specially if the addition is done dropwise, before titration with the potassium iodate. The results are presented in Table 2. From the results in Table 2, it is evident that at p~ 4, the reduction of mercuric chloride proceeds quantitatively to the stage of mercurous chloride when the ascorbic acid is only in slight excess; but when the aseorbic acid is present in large excess the reduction proceeds further to the mercury stage. Somewhat similar is the case with the reduction a~ p~ 3. At p~ 2.2 the reduction does not proceed to the mercury stage whatever may be the concentration of acsorbic acid and whatever may be the time of contact of the reactants. I t will Mso b e noted that the speed of the reduction at this pH increases with increase in the concentration of ascorbic acid. I f this is taken in a concentration 5 to 10 times that of mercuric chloride the latter is quantitatively reduced to mercurous chloride. I t will also be noted that the speed of reduction increases in general, with an increase in the p~ value of the medium.

Influence o/Chloride, Bromide and Iodide Ion8 I t is well known that mercuric ions complex with chloride, bromide and iodide ions. Even mercurous chloride appears to form a complex with chloride ions as evidenced by its increased solubility in hydrochloric acid or sodium chloride solution. The solubility of mercurous chloride in water is 0.28 rag. at 24.6~ C according to KOHLRAUSCI~7 per 100 ml. of solution; while it is 0.41 rag. at 25 ~ C in water containing 0.585 rag. of sodium chloride, according to RICItARDS and ARCttIBALDla. The solubility is 3.4 rag. of mercurous chloride in water containing 3.169 g. of hydrochloric acid at 25 ~ C. In view of such complex formation, it will be interesting to study the effect of chloride, bromide and iodide on the speed of reduction of mercuric chloride by ascorbic acid: The results of such studies are given in table 3. A scrutiny of the results shows that chloride, bromide and iodide ions markedly inhibit the speed of reduction and that the deceleration is in the order I - > B r - > C1-. Even a small concentration of C1- of the order of 10 -1 gram ion per litre inhibits the reaction to the extent of about 80 percent at p~ 2.2. I t is because of this high decelerating action of the C1- ion that the quantitative reduction of mercuric chloride now proposed fails with acetate-hydrochloric acid buffers, while it succeeds with the citric acid phosphate buffers.

Ascorbic Acid as a Reducing Agent. II

35

Table 3. Influence o/ Chloride, Bromide, Iodide, Nitrate and Perchlorate lone ou the

Reduction o/Mercuric Chloride by Ascorbic Acid Amount of HgCl~ taken = 0.01783 g. Time of reaction is 15 rots. Pg

1 of ttgCl~ Overall concentration [ 3Iole ra~io of Amount reduced of addendum i Att~:ttgCl~ g.

2.2 2.2 2.2 3.0 3.0 2.2 2.2 2.2 3.0 3.0 3.0 2.2 2.2 2.2 3.0 3.0 3.0 2.2 2.2 2.2

1.00 0.20 0.10 1.00 0.20 1.00 0.20 0.10 1.00 0.20 0.10 1.00 0.20 0.10 1.00 0.20 0.10 1.00 0.50 0.20

5{ KC1 M KC1 M KCI M KC1 M KC1 M KBr M KBr M KBr M KBr M KBr M KBr 3/[ K I M KI M KI M KI M KI M KI M NAN03 NI NAN03 M NaNQ

10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 lO:l 10:1 10:1 10:1 10:1 10:1 10:1

0.002053 0.002051 0.003429 0.003429 0.003929 0.001783 0.001780 0.001783 0.002743 0.003429 O.003929 0.000686 0.000686 0.000686 0.001783 0.001783 0.002743 O.O10960 0.011660 0.011660

3.0 3.0 3.0 2.2 2.2 2.2 3.0 3.0 3.0

1.00 0.50 0.20 1.00 0.20 0.10 1.00 0.20 0.10

M NaNOs M NaNO3 M NaNO~ M NaCIO~ M NaCl04 M NaCl04 M NaClO~ M NaC104 iV[ NaCIO 4

10:1 10:1 10:i 10:1 10:1 10:1 10:1 10:1 10:1

0.017800 0.017800 0.017780 0.017800 0.017760 0.017830 0.017810 0.017880 0.017830

Remarks

Jreduction not carried I to the mercury stage

Influence o / N i t r a t e Ions T a b l e 3 also i n c l u d e s r e s u l t s o f s t u d i e s o n t h e effect o f n i t r a t e ions on the speed of the reaction. These results show that nitrate ion exerts a s m a l l r e t a r d i n g effect a t pE 2.2 w i t h t h e r e s u l t t h e r e a c t i o n t i m e is t o be v e r y m u c h i n c r e a s e d if q u a n t i t a t i v e r e d u c t i o n is d e s i r e d a t pH 2.2. I t is i n t e r e s t i n g t o n o t e t h a t e v e n a l o w o v e r a l l c o n c e n t r a t i o n of n i t r a t e (0.2 M.) e x e r t s a b e n e f i c i e n t effect o n t h e course o f t h e r e a c t i o n . Q u a n t i t a t i v e r e d u c t i o n o f t h e m e r c u r i c c h l o r i d e t o m e r c u r o u s c h l o r i d e is o b t a i n e d in 15 m i n u t e s e v e n a t p ~ 3 in t h e p r e s e n c e o f n i t r a t e , a l t h o u g h t h e c o n c e n t r a t i o n o f a s c o r b i e a c i d is k e p t v e r y h i g h ( t h a t is, A H 2 : ItgCl~ 10:1) ; whereas under these conditions, the reduction proceeds further t o t h e s t a g e o f m e t a l l i c m e r c u r y , i f n i t r a t e is n o t a d d e d . 3*

G. GO~ALA I~AO a n d U. VEEgESWn~X 1~O :

36

Table 4. Influence o/Nitrate and Perchlorate on the Reduction o[ Mercuric Chloride by Ascorbic Acid. A m o u n t of HgC12 t a k e n = 0.01783 g Concentration o[ addendum

Time in minutes

4.0

1.00 1~ NaNOa 1.00 1V[N~NOa 1.00 Yf NaNO 3 1.00 IV[ NaNO 3 2.00 M NaNOa 2.00 M NaNO 3 2.00 M NaN03 2.00 M N~NO 3 1.00 ~ N~CIO~ 1 . 0 0 ~ N~CIO a 1.00 M NaClO a 2.00 1~I N~CIO 4 2.00 M NaClOa 2.00 M N~CIO 4 1.00 M N~CIO 4 1.00 ~ N~CIO 4 1.00 M NaCtO~ 2.00 5I NaCIO 4 2.00 IYi N~CIO 4 2.00 lV[ NaC104

5 10 10 10 5 10 15 20 5 10 15 5 10 15 5 10 15 5 10 15

10:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 10:1 10:1 10:1 10:1 10:1 10:1 5:1 5:1 5:1 5:1 5:1 5:1

0.01779 0.01780 0.01783 0.02578 * 0.01780 0.01783 0.01805" 0.02984" 0.01780 0.02578 * 0.02712 * 0.01783 0.02156* 0.02305 * 0.01780 0.01783 0.01966* 0.01780 0.01779 0.01783

5,0

1.00 1.00 L00 1.00 2.00 2.00 2.00 1.00 1 00 1.00 1.00 2.00 2,00 2,00 2,00

5 5 10 15 5 ]0 15 5 5 i0 15 5 10 15 3O

10:1 5:1 5:1 5:1 5:1 5:1 5:1 10:1 5:1 5:1 0:1 5:1 5:1 5:1 5:1

0.01966* 0.01783 0.02441 * 0.02712* 0.01772 0.01783 0.02238 * 0.01822" 0.01780 0.01966* 0.02238" 0.01783 0.01783 0.01783 0.02174"

PH

M N~NO s M NaNO s M NaNO~ ~ NaNO 3 R NaNO~ M N~NO 3 M NaNO 3 M NaC10, M N~CIO a ~ NaCIO~ M N,~CIO~ M N~C104 M NaClO 4 M N~CIO~ M NaCIO~

~Iole ratio of ascorbie acid and HgCl~

Amount of Hg01~ reduced g.

* The asterisk indicates t h a t the reduction is carried to t h e mercury stage. I n all other cases, t,he reduction to the mercurous chloride stage is quantitative.

Influence o/Perchlorate Ions The results obtained with sodium perchlorate are still more interesting. Unlike sodium nitrate, sodium perchlorate does not retard the reduction a t PH 2.2, w h e n t h e c o n c e n t r a t i o n o f a s c o r b i c is k e p t t e n t i m e s t h a t o f mercuric chloride. Sodium perchlorate, however, exerts the same b e n e f i c i a l a c t i o n as s o d i u m n i t r a t e o n t h e c o u r s e o f t h e r e d u c t i o n a t

Ascorbie Acid as a Reducing Agent. I I

37

pH 3. E v e n a t a l o w c o n c e n t r a t i o n o f (0.1 M.) s o d i u m p e r c h l o r a t e p r e v e n t s t h e f u r t h e r r e d u c t i o n o f m e r c u r o u s c h l o r i d e t o m e t a l l i c m e r c u r y a t p ~ 3. In view of the inhibitary action of nitrate and perehlorate on the f o r m a t i o n of m e t a l l i c m e r c u r y a t p ~ 3, f u r t h e r e x p e r i m e n t s w e r e c o n d u c t e d a t PH 4 a n d pH 5. T h e r e s u l t s a r e g i v e n in T a b l e 4.

Recommended Procedure/or the Determination o/Mercuric Chloride with Asvorbic Acid An aliquot of the neutral or nearly neutral solution of mercuric chloride is taken in an Erlenmeyer flask, and treated with 30 ml. of Mc ILVAI~E buffer (p~ 2.2) and an excess of a known volume of a standard solution of ascorbic acid for 40 minutes; the quantity of ascorbic acid added should be enough to give a ratio ascorbic acid: HgC12 of at least 5:1. Then 5 ml. of 20~o potassium iodide, 5 ml. of 1 : 1 sulphurie acid and 1.0 ml. of 0.5% percent starch solution are added to the reaction mixture which Table 5 is diluted to 250 ml and titrated with a standAmount of mercuric chloride ard potassium iodate solution. The end point S. 1~o. is indicated by a deep blue colour of the starch taken in g. ~ u n d in g. iodide complex stable for at least one minute. 0.00892 0.00890 1. For a sueces ful titration, the concentration of 2. 0.03566 0.03560 the sulphuric acid is not to exceed 0.5 N. The 3. 0.04458 0.04450 initial reaction between mercuric chloride and 4. 0.0668 0.0664 ascorbic acid can also be conducted at PE 3, 0.0904 0.0900 5. provided the ratio of ascorbic acid: HgC12 lies 0.2714 0.2710 6. between the limits 2:1, and 5:1, in which case the time of reaction will vary from about 30 to 15 minutes respectively. The volume of potassium iodate run down corresponds to the amount of unreacted ascorbic acid. Knowing the amount of ascorbic acid originally added, we can determine the amount of mercuric chloride taken on the basis that the mercuric chloride is reduced to mercurous chloride. The results of some typical experiments are given in Table 5.

Gravimetric Estimaiton o/Mercuric Chloride From the foregoing sections, it is evident that mercuric chloride can be reduced to mercurous chloride quantitatively at p~ 2.2 in 30 minutes if the ascorbic acid is taken five times in excess or at 3.0 p~ in l0 to 15 minutes time. In the volumetric method proposed an aliquot of the mercuric chloride is treated with a known excess of ascorbie acid, and the excess of aseorbie acid is determined by titration with standard potassium iodate solution. Alternatively, the precipitated mercurous chloride can be estimated gravimetrically. In our experiments the m~rcurous chloride was filtered through a tared Jena sintered glass crucible (No. 4), washed with cold water moderately, dried at 100 to ll0~ The results are given in Table 6 (p. 38). T h e r e s u l t s in T a b l e 6 s h o w t h a t m e r c u r i c c h l o r i d e c o u l d be e s t i m a t e d g r a v i m e t r i c a l l y also w i t h t h e a i d o f a s c o r b i c a c i d as r e d v c i n g a g e n t . T h e g r a v i m e t r i c p r o c e d u r e g i v e s r e s u l t s s o m e w h a t l o w b e c a u s e o f t h e loss o f a s m a l l q u a n t i t y o f m e r c u r o u s c h l o r i d e in t h e w a s h wat,-c. T h e s t a n d a r d g r a v i m e t r i e p r o c e d u r e p r e s c r i b e d b y HILLE~nANI) a n d L r ~ DELLS r e q u i r e s * Applied Inorganic Analysis, p. 218, J o h n Wiley & Sons, 1;)53.

G. GOPALARAO and U. V~EnESWAgAKAO:

38

Table 6

PH

Time in minutes

Mole ratio of AK~:ttgCl~

3.0

10 15 60 60 60 60 60 60 10 15 30 15 15 15 15 15 15

1.20:1.00 1.20:1.00 1.20:I.00 1.20:1.00 1.20:1.00 1.20:1.00 1.20:1.00 1.20:1.00 1.20:1.00 1.20:1.00 1.20:1.00 1.20:1.00 1.20:i.00 1.20:I.00 1.20:1.00 1.20:1.00 1.20:1.00

4.0

Amountof ]~gCl~ taken in g.

found in g.

0.1180 0.1180 0.1180 0.1770 0.2360 0.2950 0.0786 0.0590 0.1180 0.1180 0.1180 0.0590 0.0786 0.1180 0.1770 0.2360 0.2950

0.1036 0.1064 0.1174 0.1764 0.2354 0.2900 0.0785 0.0585 0.1174 0.1174 0.1176 0.0587 0.0784 0.1174 0.1763 0.2355 0.2940

t r e a t m e n t with phosphorous acid for 12 hours. The present m e t h o d requires 10 to 15 m i n u t e s only a t pH 4.0 a n d 60 m i n u t e s at pj: 3.0 taking aseorbic acid only in slight excess. I f the aseorbic acid is t a k e n i n greater excess the t i m e at pH 3 can also be reduced to 15 to 30 m i n u t e s .

Summary 1. Conditions have been established for the q u a n t i t a t i v e r e d u c t i o n of mercuric chloride to mercurous chloride b y aseorbic used. The speed of r e d u c t i o n increases with increasing p~ a n d increasing c o n c e n t r a t i o n of ascorbic acid. A t p~ 2.2, the r e d u c t i o n takes place q u a n t i t a t i v e l y to mercurous chloride w i t h o u t a n y f o r m a t i o n of metallic m e r c u r y , w h a t e v e r m a y be relative c o n c e n t r a t i o n s of the reactants. A t this Pm the r e d u c t i o n is complete i n i 5 to 30 m i n u t e s for aseorbic acid c o n c e n t r a t i o n s t a k e n 10 times i n excess or 5 times i n excess respectively of the mercuric chloride. A t p~ 3 a n d 4, ascorbie acid should be t a k e n at concentrations lower t h a n this a n d the times of reaction also carefully controlled if the f o r m a t i o n of m e r c u r y is to he avoided. A t p~ 5 a n d above, it is difficult to limit the r e d u c t i o n to the mercurous chloride stage w i t h o u t proceeding f u r t h e r to the metallic m e r c u r y stage. 2. Chloride, b r o m i d e a n d iodide have been f o u n d to i n h i b i t the r e d u c t i o n v e r y markedly. The control of p~ should be made with buffer solutions free from chloride ions. I t is for this reason t h a t acetatehydrochloric acid buffers have been f o u n d u n s u i t a b l e , while phosphatecitric acid buffers are f o u n d to be quite suitable.

39

Ascorbic Acid as a Reducing Ager~t. I I

3. T h e r e a c t i o n has b e e n u t i l i s e d for t h e v o l u m e t r i c or g r a v i m e t r i c d e t e r m i n a t i o n o f m e r c u r i c chloride. F o r t h e v o l u m e t r i c d e t e r m i n a t i o n a n a l i q u o t o f t h e n e u t r a l s o l u t i o n o f m e r c u r i c c h l o r i d e is t r e a t e d w i t h a k n o w n e x c e s s o f s t a n d a r d s o l u t i o n o f a s c o r b i c a c i d u n d e r t h e specified conditions and the unreacted ascorbic acid estimated by titration with a s t a n d a r d s o l u t i o n of p o t a s s i u m i o d a t e a f t e r a d d i n g d i l u t e s u l p h u r i c a c i d ( o v e r a l l c o n c e n t r a t i o n , 0.5 N), d i l u t e s o l u t i o n of p o t a s s i u m i o d i d e a n d s t a r c h . B l a n k s r u n a t t h e s a m e t i m e h a v e s h o w n t h a t a s e o r b i c a c i d does not undergo any autoxidation under the conditions of the experiment and at the concentrations employed. 4. T h e g r a v i m e t r i e d e t e r m i n a t i o n is c a r r i e d o u t m e r c u r o u s chloride, d r y i n g a n d w e i g h i n g t h e same.

by

washing

the

References 1 BALLE~TI~E, R.: Ind. Engng. Chem., anal. Edit. 18, 89 (1941). - - 2 CA~T~,R, S. R., and R. ROBINSOn-: J. chem. Soc. (London) 1927, 267. - - ~ E ~ [ E ~ E , A. : Acta brevia neerl. Physiol. Pharm~col. Microbiol. 4, 141 (1934). - - 4 E~DEY, L., and E. BoDom Analyt. Chemistry 24, 418 (1952); cf. Z. analyt. Chem. 137, 293 (1952/53). - 5 ERDEY, L., and H. BuzAs: Acta chim. Acad. Sci. hung. 4, 195 (1954); cf. Z. analyt. Chem. 147, 70 (1955). 6 FLASC~K~, tI., and It. ZAVAOYL: Z. analyt. Chem. 132, 170 (1951). - - 7 KOI-ILI~AUSCtt:, F. : Z. physik.Chem. 64, 150 (1908). - - s L~TL'~E~, W. M. : The Oxidation States of the Elements and their Potentials in Aqueous Solutions. p. 175. Prentice ttal[ Inc. 1952. - - 9 MURTHu K. S., and C. R. VISW~t~AD~A~: Proc. nat. Inst. Sci. Indi~ 10, 217 (1944). - - 10 PO~OFF, S., J. A. R m m c K , V. WInT~ and D. Ouo~: J. Amer. chem. Soc. 53, 1195 (1931). - - 11 P T I T S ' ~ , B. V. and t~OZLOV, V. A. : Z. anal. Chim., 4, 35 (1949); cfi Z. analyt. Chem. 132, 171 (1951). - 12 RAS;DALL,~VI.,and L. E. YOUNG: J. Amer. chem. Soc. 50, 989 (1928). - - la RmHARDS, T. W., and E. H. A~C~gBALD: Z. physik. Chem. 40, 385 (1902). - - 14 ROSENTHALER, L. : Z. Vitaminforsch. 9,342 (1939). - - 15RUD~A, M. N. : Current Sci. 21,229 (1952) - 16 STATt{IS, E. E. C. **Analyt. Chemistry 20, 271 (1948). --17 STATItIS, E. E. C., and H. C. GATOS: Ind. Engng. Chem., anal. Edit. 18, 801 (1946). - - ~s S V ~ u RAO, S. V., U. VEEI~ESWARARAO, and G. GO~Ar,~ R~o: Z. analyt. Chem. 145, 88 (1955). - - zo SWa~ELY, J. L. : Biochemic. J. 29, 1547 (1935). - - ~0 Sz~N~r Gu A.: Biochemic. J. 29, 1545 (1935). Prof. G. GO~LA R.(O, Department of Chemistry, Andhra University, Waltaic (India)