338

G. GOPALAI~AOund V. N A I ~ c ~

RAO:

Literatur. 1 BYGD~, A.: Silieium als Vertreter des Kohlenstoffs in organischen Verbindungen, S. 71--76. Almquvist u. Wiksells Boktrykeri A.-B. Uppsala 1916. - 2 D~II~Ao~, E.: Dissertation Marburg/Lahn 1951. - - 3 DENNSTEDT, M., U. W. UTEm~Am~: Anleitung zur vereinfaehten Elementaranalyse nach der Makro- und Mikromethode, 5. Aufl. Otto MeiBners Verlag, Hamburg 1947. - - t DIELS, O., U. B. WOLF: Ber. dtseh, chem. Ges. 39, 694 (1906). - - 5 HEL~m~Iel~, ]3., u. W. R~IMAN~: Ber. dtsch, chem. Ges. 80, 164 (1947). -- GRoe~ow, E. G., and W. F. GImM: J. Amer. chem. See. 63, 798 (1941) ; Roc~ow, Einffihrung in die Chemic der Silicone, S. 166, Verlag Chemic 1952. Prof. Dr. H. KAUTSKY,Marburg/Lahn, Gutenbergstr. 18.

Chemical Laboratories, Andhra University, Waltair (South India).

Ascorbic Acid as a Reducing Agent in Quantitative Analysis. Part I. by G. GOPALA RA0 und V. NARAYANA RA0. With 2 figures in the text.

(Eingegangen am 12. April 1955.) A l t h o u g h ascorbic acid has been originally isolated in traces from p l a n t a n d a n i m a l sources, it is n o w available a t a fairly cheap price on a c c o u n t of its large scale commercial synthesis. The price of the acid has come d o w n steeply from a b o u t ~ 203 per ounce in 1934 to a b o u t $ 0.75 per ounce currently. The ascorbic acid m a r k e t e d b y several p h a r m a c e u t i c a l a n d chemical firms is ~ white, odourless, stable, crystalline solid, which is easily 99 to 99.5% pure. On a c c o u n t of its c u r r e n t large-scale availabil i t y a n d the p u r i t y of a r e a g e n t chemical, ascorbic acid deserves the special a t t e n t i o n of a n a l y t i c a l chemists. I n this a n d other papers to follow, we propose to s t u d y the a n a l y t i c a l applications of ascorbic acid, specially as a reducing agent.

Chemical properties o/ascorbic acid. According to Hirst and collaborators19, aseorbie acid has the structure I. In aqueous solutions, it enolises into a compound with structure II, which easily undergoes reversible oxidation into dehydroascorbie acid having the structure III. The dehydroaseorbic acid changes over into 2,3 diketo gulonie acid (IV), with the opening of the originM laetone ring. On oxidation with alkMine hypoiodite, this compound IV yields quantitatively a mixture of 1-threonic acid and oxalic acid.

Ascorbic Acid as a Reducing Agent. I.

339

According to BO~SOOK, DAVE~Og% JE~F~EYS and W~a~Eg t, the oxidation and decomposition of 2,3 diketo gulonic acid may also be brought about by atmospheric oxygen in alkaline solution. COOH

I

!

I

C=O

I

C=0

I

tt--C--Ott

I

C=O

I

H--C--

I 0

~-

I

f i

I

,

C--OH

0

C=0

I ~

C~O

tl C--0H

I C=0

F

I

H--C -----

H--C--

I

ItO--C--H

i CH2OH I

H0--C--II

1

H0--CH

i

[

]

]

O

C=O -~

I

C=O

]

H~C--OH

I

HO--CH

r

CH20H

I

CH20H

CH2OH

II

llI

IV

According to Fox and L~vY 1~, the active charcoal "Norite" converts ascorbie acid in aqueous solution into dehydroascorbie acid, almost completely and reversibly. With small amounts of l~orite the reaction is rapid and proportional to the amount used. The dehydroaseorbic acid thus prepared is reported to be stable in aqueous solution for several days, if saturated with carbondioxide and stored at 0 ~C. According to ENGELKARDT and Bu~:IN ~, dehydroascorbie acid is completely destroyed at p~ 7.0, when the temperatare is raised to 60 ~ C; at the room temperature, about 80--90 per cent is destroyed in 10--20 minutes. W~sT and RI~EHA~T51 claim that copper ions catalyse the decomposition of dehydroascorbic acid; the disappearance of ascorbic acid and dehydroascorbic acid was found to go parallel With the formation of oxalic acid and 1-threonie acid. The significance of ascorbic acid for quantitative analysis is its great reducing capacity. I t has been shown that it reduced gold salts to metallic gold, silver salts to metallic silver, mercuric chloride to metallic mercury or mercurous chloride, according to the conditions; selenions acid to free selenium, cupric salts to cuprous salts, ferric salts to ferrous salts, trivalent cobalt salts to the divalent cobalt salts and ceric salts to cerous salts. Ascorbic acid also reduces iodine, iodate, bromine, bromate, chlorate, permanganate, dichromate, vanadate, phosphomolybdate, tungstate, phosphotungstate, ferricyanide, etc. Ascorbic acid is also known to reduce some dyes like 2,6 dich]orophenol-indophenol, methylene blue, thionine, and eaeotheline. HIRST and HER]3~RT2~ found that when solutions of ascorbic acid are titrated with potassium permanganate, the reagent is decelorised almost instantaneously until the equivalent of about 1.3 atoms of oxygen has been added. A slower reaction then follows, which ends somewhat indefinitely when the permanganate equivalent of 2 atoms of oxygen has been used uD. Thereafter, the reaction proceeds still more slowly without a definite end point. S U ~ A N t2 carried out experiments adding excess o f ceric sulphate to an ascorbie acid solution, keeping for fifteen minutes, and titrating the unreacted ceric sulphate solution with a standard solution of Moa~'s salt. He found that one mole of ascorbic acid required an amount of eeric sulphate corresponding to three atoms of oxygen. 1-threonic acid is reported to be formed in the reaction according to the equation. C~HsO 8 + 6 Ce+~ + 3 H20 --> C4Hs05 + 2 C02 + 6 Ce+a + 6 H +. 22 ~

340

G. Go~x~x l~xo u n d V. N ~ x r ~ N ~

R~o:

P T r r s r ~ a n d Koznov 35 showed t h a t the reaction between ferric salt a n d ascorbic acid~

C~Hs06 d- 2 Fe +a --> CeHeO 6 -}- 2 Fe +z d- 2 I-I+

can be used for t h e estimation of ferric iron, carrying t h e t i t r a t i o n to a potentiometric end point. FLASC~KA a n d ZAVAGrZL"z4 t i t r a t c d ferric iron in hydrochloric acid solution with ascorbic acid using potassium thiocyanate as indicator. The disappearance of the pink color of ferric thiocyanate is t a k e n to be the end point. E~DEY a n d BODO~ 11 carried out a detailed investigation on t h e estimation of ferric iron with ascorbic acid. The reaction is r a t h e r slow a t t h e room temperature, specially towards the end point. Hence the authors propose carrying out t h e t i t r a t i o n a t 60 ~ C. Temperatures above 60 ~ C should be avoided, as otherwise t h e reaction between dehydroascorbic acid a n d ferric iron becomes appreciable. I t was found t h a t in acid medium (0.1 N a n d above) the a m o u n t of ferric iron reduced b y dehydroascorbic acid during a five m i n u t e t i t r a t i o n is negligible. SC]tUnEK a n d FLODE~ER Ss have carried out the colorimetric determination of ferric iron after reducing t h e same with ascorbie acid a n d eomplexing t h e ferrous salt formed with ~-~'-dipyridyl. The reduction with aseorbie acid a n d t h e consequent formation of the coloured complex, ferrous-dipyridyl, is completed in t e n minutes instead of sixty minutes required b y sulphurous acid as reducing agent. Moss and ~ELLON al also recommended this method as being very accurate. I n a review of the investigations on the reducing properties of aseorbie acid, E M ~ E ~ I E 6 stated that solutions containing i mg of ascorbic acid pe r ml. gave an orange red colour with selenious acid (50/0 solution in water) a n d a brownish red with uranyl acetate. The limit of b o t h reactions is a t a b o u t 0.01 mg. of aseorbic acid per ml. The same a u t h o r stated t h a t dilute aseorbic acid solutions reduce gold chloride in aqueous solution. ST~TmS a n d GA~os ~1 found t h a t gold chloride is quantitatively reduced b y aseorbic acid according to the equation: 2 AuCla ~- 3 C~HsO 6 -+ 2 Au ~- 3 CeHeO6 -j- 6 HCI. According to SV~BELu ~4, a 5 % solution of silver n i t r a t e is reduced in a slightly ammoniacal medium b y as little as 0.003 rag. of ascorbic acid. Less reduction occured in neutral or acid solution. ST~TmS a~ employed t h e reaction between ascorbic acid a n d silver nitrate for the estimation of t h e latter. ERDEY a n d BODORs %itrated ~ chlorate solution in dilute hydrochloric acid a t 60 ~ C with ascorbie acid in the presence of selenious acid a n d manganous sulphate. ERDEY, :BODOI~ a n d B c z i s TM also determined potassium iodate b y t i t r a t i o n with ascorbic acid with the addition of selenious acid as catalyst a n d a solution of mercuric chloride as complexing agent to b i n d the iodide formed in the reaction. These authors 13 estimated bromate volumetrically b y t i t r a t i o n with a solution of ascorbie acid in dilute hydrochloric acid medium in t h e presence of sclenious acid, mercuric chloride a n d manganous sulphate. Go~ R~o a n d SUBBA RAO ~s found t h a t the reaction between ascorbic acid a n d silver bromide induces the reaction between silver bromide a n d sodium sulphite. LEWNE 27 states t h a t ascorbic acid differs from all other organic compounds so far tested in t h a t it possesses the unique a n d specific property of reducing in t h e cold a n acidified selenite solution with t h e formation of a brick red colour characteristic of free selenium. The reagent is made u p b y mixing 100 ml. of 20/0 sodium selenite a n d 20 ml. of concentrated hydrochloric acid. PIT~mELLI a n d PITTA~ELn~ ~ found t h a t ascorbic acid precipitates mercurous chloride from a solution of mercuric chloride a n d t h a t the hydrochloric acid formed in the reaction gives a mes of the progress of the reaction. I~DOW~). a n d M~FRO~ ~ determined the kinetics of the reaction making use of the conductivity

Ascorbic Acid as a Reducing Agent. I.

34l

method; They found t h a t the reaction is complete in a b o u t t h i r t y six hours. R O S ~ T~ALER3~ determined aseorbie acid b y heating the same in 0.1 N hydrochloric acid solution with excess of mercuric chloride a t 100 ~ C for four hours. The solution is cooled a n d treated with a decinormal solution of sodium hydroxide a n d 10 per cent sodium nitrate solution. The mercurous chloride precipitate is separated b y filtration t h r o u g h asbestos and washed with 10% sodium nitrate solution. I t is dried a t 105 ~ C a n d weighed. SC~rA~C~AYA~A RAO, V ~ S W ~ A Rao a n d GOPALA I~AO~a made a detailed investigation of the reaction between ascorbie acid and mercuric chloride. They have found t h a t , ff the ascorbie acid is t a k e n in excess over the mercuric chloride a n d the pR of the solution is kept above 4, the reduction proceeds even to the stage of metallic mercury. Below this pH, however, the reduction proceeds only to the stage of mercurous chloride. W h e n the mercuric chloride is t a k e n in excess, t h e reaction proceeds only to the stage of mercurous chloride u n d e r all pH-values from 1 t o 8. They have shown t h a t a t pR lying between 2 to 8, (controlled b y the use of sodium acetate-acetic acid or citric acid-disodium phosphate buffers) when aseorbie acid is treated with excess of mercuric chloride, the reaction proceeds rapidly even in the cold with t h e quantitative formation of mercurous chloride. B y estimating the mercurous chloride formed iodometrieally the ascorbic acid t a k e n can be determined.

Estimation o/ ascorbic acid solutions. Iodine a n d 2,6-dichlorophenot-indophenol are the two reagents t h a t are extensively employed for t h e estimation of ascorbic acid. T I L ~ S et al. 4~were the first to introduce t h e dye for the estimation of ascorbic acid in p l a n t a n d animal extracts. Their estimations were carried out in a neutral medium, b u t under these conditions, eysteine, g h t a t h i o n e and ferrous salts interfere. SV~BET,r a n d v o ~ SZE~TGu aS, BIRCH, HARRIS and RAYa, a n d WOLF~, v i ~ ECK~LE~ and EM~E~I~ 5~ proposed carrying out t h e t i t r a t i o n in a n acid medium to avoid t h e interference of glutathione a n d ferrous salts. Cysteine and ergothionine, however, reduce the dye even in acid medium. According to BAs~ a n d I~ATH2 ferrous salts also reduce t h e dye in the presence of such organic acids as oxalic, malonie, malic, tartaric a n d citric acids. According to GAw~o~ a n d BErG ~7, ferrous salts reduce t h e dye in the presence of phosphoric acid. Bl~ow~ a n d An~_~5 also find t h a t ferrous salts are quantatively oxidlsed b y the dye in t h e presence of phosphoric acid. The dye method of estimation of ascorbic acid has other disadvantages. The dye solution has to be standardised against a solution of known aseorbic acid concentration. Moreover, we have often found t h a t old samples of the dye are insoluble in water even when preserved in sealed tubes. The iodine method suffers from the drawback t h a t s t a n d a r d solutions of iodine (specially of the order of 0.001 N) change in strength rapidly a n d require frequent standardlsation at hourly or even shorter intervals in the case of the dilute solutions. Potassium iodate suggested b y B A ~ I ~ r 1 appears to be satisfactory for t h e direct t i t r a t i o n a n d standardisation of ascorbie acid in aqueous solution. While it has the same sensitivity as the iodine method, i t is n o t subject to m a n y of its disadvantages. The permanganate method of ~uR~n-r a n d V x s w A ~ n ~ ~ is also quite satisfactory, although permanganate solutions m a y not be expected to be as stable as those of potassium iodate. Their method consists in t i t r a t i n g a solution of ascorbie acid with a dilute solution of potassium permanganate, using starchpotassium iodide as indicator. The permanganate consumed corresponds to t h e oxidation of ascorbic acid to t h e stage of dehydroascorbic acid. SCHVL~K, KovXcs a n d R6zsA ~0 proposed potassium bromate for the estimation of ascorbie acid. A 10 ml. sample containing 1--2,000 rag. of the substance is placed

342

G. G o ~ x

g x o u n d V. N ~ x Y ~ x

R~o:

i n a 100 ml. flask, a n d treated with 0.5 gm. of potassium bromide a n d 5 to 10 ml. of 10% hydrochloric acid. I t is t h e n t i t r a t e d with a 0.1 or 0.01 N potassium bromate solution using one drop of a 0,2% solution of p-ethoxy-chrysoidine as indicator. One mole of ascorbic acid requires two atoms of bromine. The procedure is recommended %o be suitable for the estimation of uscorbic acid in medicinal preparations. PITTA~LLI a n d PITT~nLLI 3~ appear to have been t h e first to use ferric chloride for the estimation of ascorbic acid. The t i t r a t i o n was carried out in acid medium using a 4 % solution of a m m o n i u m thlocyanate as indicator. The ferric chloride was standardised against a solution of ascorbic acid of k n o w n strength. S~BAn~VA 26~ estimated ascorbic acid b y t i t r a t i o n with a s t a n d a r d ferric chloride solution a t 90 ~ C a n d p g 4 to 4.5, using a m m o n i u m thlocyanate as indicator. N~scr~mcTO 88 carried out a similar t i t r a t i o n in dilute hydrochloric acid medium, while K_HO~UTOV~ ~a carried out t h e titration in sulphuric acid medium with ferric chloride, using stareh-potassinm iodide as indicator. I n carrying out this method it m u s t be borne in m i n d t h a t ascorbic acid decomposes in aqueous solution on boiling. According to H o u ~, V i t a m i n C suffers only slight destruction when heated rapidly to boiling, b u t undergoes 2 0 % destruction after boiling for 15 minutes. Other methods of estimation of ascorbic acid depending on the reduction of phosphomolybdic acid or germanomolybdic acid 9, silicomolybdic acid (FgJITA, IWATAXE a n d 1VLrY~T.t16), iungstie acid (FuJITA, IW• and M~xls), phosphotungstic acid or arsenotungstic acid (Vo~EscE a n d RE~EZZ~O~S), potassium ferricyanlde (T~cBEats), methylene blue ( l Y I ~ i ~ a n d BO~Sm~ORE~), thionine (WA~K~ a n d H ~ a o v T ~) and their numerous modifications have so far been studied for eolorimetric determination only. I t remains to be investigated as to how far those reactions can be used as the basis of volumetric procedures.

Stability el ascorbic acid solutions. Solid ascorbic acid under dry conditions is perfectly stable towards air, light a n d heat. I t is only in aqueous solution t h a t it is oxidised a n d decomposed. Oxidation b y air in aqueous solution is catalysed b y copper ions a n d to a lesser degree b y ferric ion a n d also b y enzymes of the oxidase t y p e such as ascorbic acid oxidase, phenoloxidases a n d peroxidases. E v e n traces of cupric ions are catalytically active. However, NIAeso~ 2s reported t h a t no oxidation took place even a t 100 ~ C when the copper ion concentration was reduced to one p a r t in 650,000,000 parts of water. Z ~ v A ~a found t h a t ultraviolett light could also catalyse the oxidation of ~seorbie acid to dehydroascorbic acid. L ~ n ~ 2G has reported t h a t the addition of 0.5 per cent stannous chloride or 1.0 per cent thiourea retards the loss of ascorbic acid in metaphosphoric acid (5 per cent) solution even in the presence of traces o f copper ions. M ~ x a~ observed t h a t a saturated solution of sodium chloride acts as a b e t t e r stubiliser t h a n metuphosphorie acid. I n a sample extructed with metaphosphoric acid t h e loss of ascorbic acid is 89 per cent in 30 days, whereas in a similar sample extracted with sodium chloride the loss was only 14 per cent. ERDEY a n d BoI)o~ 1~found t h a t ascorbic acid can be stubilised in aqueous solution a t 0 ~ C in a n atmosphere of carbon dioxide with the addition of Trflon B (EDTA) as stabiliser. I t can also be stabilised in a solution containing formic acid a n d Trilon B ; solutions thus stubilised show a deterioration of a b o u t 0.1 per cent per day a t the room temperature. In the present investigation, we have carried out numerous experiments to study the stability of ascorbie acid in water at concentrations usually e n c o u n t e r e d i n q u a n t i t a t i v e a n a l y s i s (0.1 t o 0.05 N ) . I n c i d e n t a l l y , w e h a v e a l s o s%udied t h e s t a b i l i t y o f s o l u t i o n s o f 0,01 N t o 0 . 0 0 6 N c o n c e n t r a t i o n .

343

Ascorbic Acid as a Reducing Agent. I.

The ascorbic acid solutions are prepared in water that was distilled twice in an all-pyrex distillation apparatus and the strength determined p e r i o d i c a l l y , w i t h p o t a s s i u m i o d a t e , w h e n p r e s e r v e d u n d e r v a r y i n g cond i t i o n s . I n t h e e x p e r i m e n t s p e r t a i n i n g t o r e s u l t s r e p o r t e d i n t a b l e 1, t h e ascorbic acid was kept in a 250 ml. measuring flask without any special p r e c a u t i o n s t o e x c l u d e air. T h e f l a s k w a s k e p t s t a n d i n g o n t h e w o r k t a b l e exposed to the diffuse light of the laboratory. Table 1. Stability o/ascorbic acid solution in air. Volume of potassium iodate solution (0.1 2r corresponding to 5.0 1111. of" aseorbic acid solution Time in hours

in water ml.

0 1 2 24 48 72 96 120 144 168 216 288

3.400 3.380 3.350 3.25O 3.225 3.075 2.925 2.775 2.650 2.500 2.150 * 1.870

* Solution turns yellow.

J

in 0.04 N sulfuric acid solution ml.

in 2.0 lq sulfuric acid sohltion ml.

4.025 4.025 4.000 3.960 3.925 3.900 3.850 3.776

3.400 3.38O 3.350 3.250 3.150 3.060 2.925 2.810

3.700 3.675 ** 3.360

2.725 2.550 2.300 *

** Fungous growth started.

I n t a b l e 2, w e r e c o r d t h e r e s u l t s o n t h e s t a b i l i t y o f a s c o r b i c s o l u t i o n s preserved under carbondioxide. Table 2. Stability o/ascorbic acid solution in carbondioxide atmosphere.

Time in hours

Volmne of potassium iodate solution (0.1 N) required for 5.0 ml. of the ascorbic acid solution In water in 0.04 N sulfuric in 2.0 5T sulfuric acid solution acid solution ml.

0 3.400 1 3.400 2 3.400 24 3.350 48 3.330 72 3.330 96 3.325 120 3.325 144 3.320 168 3.300 216 3.300 264 3.230 Solution turns yellow.

ml.

ml.

4.025 4.025 4.020 4.020 4.000 4.000 4.000 3.960

3.400 3.400 3.400 3.310 3.300 3.250 3.225 3.210

3.925 3.925 3.925

3.210 3.160 * 2.990

344

G. GOPALAl~xo und V. N ~ v A ~ A Rxo:

The results are represented graphically in diagram 2 (Fig. 2). Experiments have also been made with dilute solutions of the order of 0.0066 N and the results obtained are represented graphically in diagram 1 (Fig. 1).

O

20

ztO

6"0

80

100

7Z0

lqO

150

180

ZOO HgUP3

Fig. 1. Stability of Ascorbie Acid in Dilute Aqueous Solution. (a) in 0.0& N It, S04 solution in air; (b) in 2 N ] ~ S O t solution in air; (e) in 0.04 ~ ~ 8 0 4 solution in C0~ atmosphere; (d) in 21~ tt~SO4 solution in C02 a~mosphere. g

q 0

r_

I0

,,~

d' -)

0

d

0

20

90

" 80

80

/00

720

7~/0

7B0

780

200 Hours

Fig. 2. Stability of Ascorbie Acid in Concen~rmted Aciueous Solution. (a) in 0.04 I~ H 2 S O 4 solution in ~ir; (b) in 2 BT ]12S04 solution in air. (c) in 0.0~ ~ I12S04 solution in C0~ atmosphere; (d) in 2 BT II~SO~

solution in C02 atmosphere.

I t is known from the work of KlCISH~AMURTI and Gn~I 2~ and of WATA~r 5~ that oxalic acid protects ascorbic solution against atmospheric oxidation. Hence the stability of ascorbic acid over long periods has been studied in the presence of oxalic acid. The results are presented in table 3. I n the table 4, we record the results on the stability of ascorbic acid solution preserved under carbondioxide. Our experiments show that ascorbic acid is more stable in concentrated solution than in dilute solution in the presence of air. The same pheno-

Ascorbic Acid as a Reducing Agent. I.

345

Table 3. Stability ot ascorbic acid solution in air in the presence ot oxalic acid. Oxalic acid concentration: 0.05 molar Volume of potassium iodate solution (0.1 I% required for 5.0 mi. of ascorbie acid solution T i m e in hours

0 1 2 3 24 48 72 96 120 144 168 216 240 264

in 0.04 sulfuric acid solution iul,

in 2.0 :N sulfuric acid solution

4.025 4.O25 4.020 4.020 4.020 4.010 3.950 3.950 3.870

3.400 3.400 3.400

3.800 3.750 3.710 3.625

mi.

3.310 3.240 3.120 2.975 2.775 2.650 2.400 2.000 * 1.600

* Solution turned yellow. Table 4. Stability o/ascorbic acid solution under carbondioxide in the presence of oxalic acid. Oxalic acid concentration: 0.05 molar Volume of potassium iodate solution (0.1 N) corresponding to 5.0 ml. of ascorbie acid solution

Time in hours

0 1 2 3 24 48 72 96 120 144 168 192 216 240 264

in 0.04 suffm'icacid solution ml.

in 2.o ~ suffuric acid solution ml.

4.025 4.025 4.020 4.020 4.020 4.020 4.020 4.020 4.010

3.400 3.400 3.400

3.980 3.980 3.980 3.980 3.970

3.310 3.290 3.225 3.160 3.025 2.950 2.850 2.450 * 2.000

* The solution turned light yellow. m e n o n w a s o b s e r v e d b y p r e v i o u s w o r k e r s 26 w h o c a m e t o t h e s a m e c o n clusion that the higher the concentration of ascorbic acid the more s t a b l e i t is. O u r r e s u l t s s h o w ~ h a t Chere is:

346

G. GOPALARAO und V. NA~AYA~ARAO:

1. A d e t e r i o r a t i o n of a b o u t 14% in 4 d a y s ' t i m e in a p p r o x i m a t e l y 0.066 N solutions a n d a b o u t 6 2 % in a b o u t 0.0066 N solutions. 2. D e t e r i o r a t i o n is v e r y m u c h d i m i n i s h e d w h e n t h e solution is r e n d e r e d slightly acidic (0.04 N snlphurie acid). I n this case, t h e r e is 4.5% deter i o r a t i o n in a p p r o x i m a t e l y 0.066 N solutions a n d a b o u t 42~/o deterior a t i o n in a b o u t 0.0066 N solutions in a b o u t 4 d a y s ' t i m e . 3. W h e n t h e a c i d i t y is i n c r e a s e d t o overall 2.0 N sulphuric acid t h e degree of p r o t e c t i o n afforded is less. 4. R e m o v a l of air b y s a t u r a t i n g t h e solution w i t h c a r b o n d i o x i d e a n d p r e s e r v i n g t h e solution in a c a r b o n d i o x i d e a t m o s p h e r e coupled w i t h slight acidification gives a p r o t e c t i o n - - 0 . 6 % d e t e r i o r a t i o n in 0.066 57 solutions a n d a b o u t 2 5 % d e t e r i o r a t i o n in 0,0066 57 solutions in a b o u t 4 d a y s ' t i m e . I t will be seen t h a t solutions of t h e o r d e r of 0,05 N are f a i r l y s t a b l e for over a week when p r e s e r v e d u n d e r c a r b o n d i o x i d e even when t h e y are n o t acidified ( a b o u t 1.4~ d e t e r i o r a t i o n ) . 5. T h e a d d i t i o n of oxalic a c i d i m p r o v e s t h e s t a b i l i t y s l i g h t l y b o t h in air a n d e a r b o n d i o x i d e . Our results show t h a t ascorbic a c i d shows a fair degree of s t a b i l i t y a t c o n c e n t r a t i o n s u s u a l l y used b y t h e a n a l y t i c a l c h e m i s t s ; even a g r e a t e r s t a b i l i t y t h a n some inorganic r e d u c i n g a g e n t s like s t a n n o u s chloride, chromous chloride, t i t a n o u s chloride a n d s o d i u m sulphite. I t is therefore v e r y i n t e r e s t i n g to p u r s u e t h e a p p l i c a t i o n of ascorbic a c i d as a reduct o m e t r i c r e a g e n t in a n a l y t i c a l c h e m i s t r y . T h e r e d o x p o t e n t i a l values of ascorbic acid also are p a r t i c u l a r l y e n c o u r a g i n g as t h e y v a r y from a b o u t - - 0 . 0 1 2 volts to 0.326 volts w i t h t h e v a r i a t i o n of t h e p ~ from 8.67 t o 1.05. W o r k on t h e use of ascorbic a c i d as a r e d u c t o m e t r i c r e a g e n t is in progress in these l a b o r a t o r i e s .

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