96

G. P ~ ] ~

i ~ o and A. R. V~SUDEV~MVRT~:f

Bd. 187

sonst u n v o l l s t ~ n d i g wird. Bei K a t a l y s a t o r e n hingegen b e s t e h t keine obere K o n z e n t r a t i o n s g r e n z e ; allerdings muf3 m a n bei hoher K o n z e n t r a t i o n der P r o b e i m Spfilgas die T e m p e r a t u r des L a n g b r e n n e r s u m etwa 200~ hSher einstellen als bei niederer K o n z e n t r a t i o n . Abschliel~end werden die F u n k t i o n der Preglsehen Vergasungstechnik, insbesondere die AuslSsung der Druckanzeige, sowio die a n a l y t i s c h e n F o l g e r u n g e n erSrtert.

Literatur 1CLA~K,R. 0., U. G.I-I. STILLSO~: Analyt. Chemistry 19, 423 (1947). -2HA~]~TT, L. T.: Ind. Engng. Chem., anal. Edit. 10, 111 (1938); vgl. diese Z. 122, 382 (194t). -- 8 KAI~z, G.: diese Z. 166, 427 (1959). -- a K~-~z, G., u. H. HO~WATITSCH:diese Z. 184, 363 (1961). -- ~ P~EOL, F. : Quantitative organisehe Mikroana]yse, 7. Anti., S. 66; neubearbeitet yon I-I. RoTK. Wien: Springer 1958. -6 R]~m'LE~,H. : MJkrochemie 23, 285 (1936). -- 7 R]~I]tL~, H., u. E. W]~I~]3~ENN ~ : Chem. Fabrik 7, 63 (1934). -- s R o u G. L., A. 1~. NO~TON u. O. E. SV~D~RG: Ind. Engng. Chem., anal. Edit. 12, 688 (t940). -- 9STE'ZERYLal~K,A.: Analyt. Chemistry 17, 523 (1945). -- lo Vv.~Ef~A,~ . : Acta chim. Acad. Sci. hung. 26, 511 (1961). Univ.-Doz. Dr. GERALDK~rsz, Analytisches Instit~ut der Universit~t, Wien IX, W~hringerstral]e 38

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-12, India

Cerimetry in l~onaqueous Media Oxidation of Aseorbic Acid By

G. :PRABHAKAIt RA0 and A. R. VASUDEVAMURTHY With 1 Figure in the Text

(Received October 28, t961) I t is k n o w n t h a t the r e d u c i n g c a p a c i t y of ascorbie acid is v e r y m u c h i n h i b i t e d i n organic solvents 1,2 when compared to its a b i l i t y to reduce in aqueous media. F o r i n s t a n c e iodine i n glacial acetic acid or alcohol is n o t c o n s u m e d b y ascorbic acid i n the same solvent. I t has been shown t h a t a m m o n i u m h e x a n i t r a t o cerate i n acetonitrfle is a versatile oxidizing a g e n t i n n o n a q u e o u s m e d i a 4. This r e a g e n t was f o u n d to react w i t h ascorbic acid q u a n t i t a t i v e l y a n d the results o b t a i n e d are reported i n this c o m m u n i c a t i o n .

Experimental Reagents. Ammonium Hexanitrato Cerate. A standard solution of the reagent (0.05 N) was prepared in acetonitrile and its strength wa s checked up as described earlier ~. Ascorbic Acid. Aseerbic acid is not freely soluble at room temperature (25~ C) neither in ac~tonitrile nor in glacial acatic acid. I-Iowever on warming, glacial acetic

1962

Cerimetry in nonaqueous media

97

acid was found to dissolve considerable a m o u n t of the reagent, corresponding to a n a m o u n t to give a b o u t 0.05 N solution. The exact strength of t h e ascorbic acid solution was determined b y t i t r a t i n g against a n aqueous solution of iodine (liberated from a n acidified mixture of potassium iodate a n d iodide) as suggested b y E~DEY ~. This solution was suitably diluted with glacial acetic acid whenever it was found necessary. As has already been suggested glacial acetic acid was ~ used as a diluent to economise the use of acetonitrile 4. Potentiometric titrations were carried out using glass or a n t i m o n y electrodes as reference electrodes a n d p l a t i n u m as indicating electrode. The experimental set up used was the same /./g~ as described earlier ~. Procedure. Preliminary experiments indicated t h a t it was more advantageous to t i t r a t e a solution of a m m o n i u m n i t r a t e carafe against 0.905 ascorbie acid t h a n vice versa. 2 - - 5 ml of a m m o n i u m n i t r a t e carafe reagent are diluted with few ml of glacial acetic acid so as to h a v e a t i t r a n t volume of a b o u t 10 ml in the potena700 tiometric t i t r a t i o n cell. The solution I was magnetieally stirred a n d was tit r a t e d against 0,015 N ascorbic acid solution. The potential decreased with the progressive addition of reductant 0.50t .1 I a n d a steep jump was observed at the end point. A potential fall of about T~I of Ascorb/c ac/d F i g . i . Typical p o t e n t i o m e t r i c t i t r a t i o n curves o f 80--100 millivolts was observed with a m m o n i u m h e x a n i t r a t o cerate w i t h ascorbie acid a n t i m o n y p l a t i n u m system whereas in in non-aqueous m e d i u m , u s i n g glass (I) a n d t h e case of glass-platinum electrodes, a n t i m o n y ( H ) as reference electrodes the jump was of the order of 100 to 120 millivolts for a n addition of 0.03--0.04 ml of the reagent. F u r t h e r additions of the reagent produced very small changes in the potential. Titre values were found to be highly reproducible. The results of a few representative experiments are presented in the following table a n d the t i t r a t i o n curves ar e given in the Fig. 1. Table No.

Ammonium nitrato aerate t a k e n in I moles x 10 -8

I

163.5 163.5 136.2 131.9 131.9 175.9 Z. analyt. Chem., Bd. 187

Ascorbic acid consumed in moles • 10 -6

Mole ratio ascorbic acid: cerate

40.60 41.01 33.83 32.78 33.16 44.25

1:4.03 1:3.99 1:4.03 1:4.02 1:3.98 1:3.98

l~ean value:

,1:4.005

Electrode p a i r used

Sb~Pt Sb-Pt Sb-Pt Glass-Ft Glass-Pt Glass-Pt 7

98

G. PI~AB~AI~A_~RAO and A. R. VASeDEVAI~UI~TI~Y

Bd. 187

I t is seen from the results of the table t h a t one mole of ascorbic acid consumes four equivalents of the oxidant at the end point of the reaction. I t is interesting to note t h a t common oxidizing agents like iodine, oxidize ascorbic acid in aqueous medium to dehydro ascorbic acid stage, involving only two equivalents of the oxidant. The reaction involved in such a system is known to be the oxidation of ascorbic acid to dehydro ascorbic acid. CO HO--C II HO--C

CO-I o

OC o ]C

K--~-

--K~

o

1 H--C.--

i

[

HO--C--H

HO--C--H

t

I

CH~OH

CH~OH

I n the case of oxidation with ceric sulphate is has been reported a t h a t ascorbic acid consumes a little more t h a n two equivalents of the oxidant in aqueous medium. However it is known t h a t under certain experimental conditions 2, with alkaline hypoiodite and acid p e r m a n g a n a t e the oxidation of ascorbic acid m a y go to a further stage (beyond dehydroascorbic acid) giving rise to threonic acid and oxalic acid. CO--HO--C

COOK

I

H--C--0H

o KO--C

I

H--C--

T

HO--C--H

-~

I KO--C--K ] CH20H

COOK

+

/ v00H Oxalic acid

Threonic acid

I

CH2OH Ascorbic acid This process requires four equivalents of the oxidizing agent per mole of ascorbic acid. I n the present investigation it is possible to infer from titre values t h a t this type of reaction takes place, for, every mole of ascorbic acid consumes four equivalents of the reagent. Thus the oxidation of ascorbie acid in non-aqueous media seems to be more energetic t h a n in the case of aqueous medium. As has already been mentioned, it is essential t h a t while carrying oUt the titrations, ascorb]c acid has to be added to the oxidizing agent, otherwise the reaction was found to be sluggish particularly towards the

1962

Cerimetry in nonaqueous media

99

end. The discharge of the colour of the a m m o n i u m hexa nitrato cerate solution consequent on reduction was v e r y slow and one has to wait for considerably long time before registering a stable potential corresponding to a n y particular titre value. The addition of ascorbic acid to the oxidizing agent is free from these disadvantages. Other ions such as, Au3+; I-Ig~+ and Br etc. ~ have been reported recently, to react with ascorbie acid in acetic acid solution. I t was observed t h a t two equivalents of the oxidant were consumed b y ascorbic acid in such systems. However, stoichiometric relationship, could not be established in several of these cases from the titre values. F r o m the results reported in this communication, it is possible to evolve an analytical procedure for the estimation of ascorbic acid present in non-aqueous media. The a c c u r a c y of such a m e t h o d is enhanced as the n u m b e r of aqttivalents of the oxidant consumed is v e r y m u c h higher t h a n with the conventional methods.

Summary The reaction between ascorbic acid and a m m o n i u m h e x a nitrato cerate was studied potentiometrically in the mixed solvent glacial acetic acid acetonitrile medium. I t was f o u n d t h a t one mole of aseorbie acid consumes four equivalents of cerate in non-aqueous medium. This reaction can be made use of to estimate potentiometrically ascorbie acid with a m m o n i u m nitrato cerate in non-aqueous media, using either glass or a n t i m o n y as reference electrode and platinum as indicator electrode. Zusammenfassung Die i~eaktion zwischen Ascorbins/~nre u n d A m m o n i u m h e x a n i t r a t o eerat wird in einem Medium y o n Eisessig + Acetonitril untersueht. I m niehtw/~13rigen Medium v c r b r a u c h t 1 Mol Ascorbins/iure 4 ~ q u i v a l e n t e Cerat. Die I~eaktion k a n n zur potcntiometrischen B e s t i m m u n g yon Ascorbins~urc b c n u t z t wcrden; dabei wird als Indieatorelektrode Platin und als Vergleichselektrode die Glas- oder Antimonelektrode verwendet. Acknowledg'ement. Thanks are due to ProfessorM.~.A. RAo for his interest in the work.

References 1E~nmr, L., and G. I~ADY: Acta chim. Acad. Sci. hung. 15, 81 (1958); cf. Z. analyt. Chem. 166, 127 (1959). -- 2 H ~ r ~ T , 1~. W., E. L. I-II~sT, E. G. V. Pw.~eIvAL,R. g. W. I~mc~or~DsandF. S~T~: J. chem. Soc. (London) 1988, 1270. -8 L~o~m~D% It., and W. NoEs~.~: Z. analy~. Chem. 122, 3 (1941). -- ~ l~Ao, G. PRAB]tAKA~, and A. I~. VikSIFDI~VA~[IJI~TIt~.* Z. analyt. Chem. 182, 358 (1961). Dr. A. 1~. VASm)V,VA ~URTmr, D. Sc., Assist. Professor, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 12 (S. India) 7*