Ascorbic Acid Determination in Natural Orange Juice

s a Teaching Tool of Coulometry and Polarography

Mauro Bertotti, Jorge Moreira Vaz, and Rogerio Telles

lnstituto de Quimica, USP,CP 26077,

CEP

05599-970, SBo Paulo, SP, Brazil

Electroanalytical techniques have-been defined as t he

application of electrical measurements for analytical pur-

poses I ) ,and they have undergone a large development in

the l ast two decades due to the improvements and simpli-

fications of instrumentation 2. ). In t his wav, th e use of

operational amplifiws led to more versatile elertrorhemi-

cal i n s t ~ m e n t s . he interest in thc detcrminatim of met-

als and organlr compound< t lower conccntratmns, espe-

riallv in envwonmenrnl nnaI\r.+es,ha hroughr about the

application of some electroanalytical methods in both aca-

demic and indus trial contexts. In spite of these important

aspects, the teaching of electroanalytical methods is a dif-

ficult task mainly due to the apparent aversion of students

relating to electrochemistry 4). n this sense, we have pro-

posed an attractive experiment involving coulometry and

polarography in order to determine ascorbic acid concen-

trations in natural orange juice. This experiment has been

performed by undergraduate students of a pharmacy

course. Our expectation is to present to these students a

practical experiment involving a common substance to al-

low understanding of the principles of operation of the cou-

lometer and polaromaph, besides pointing out the connec-

tion between th e measured phykcal p;operty and the

concentration of a substance. Impor tant concepts as Fara-

day s law. diffusive transport, and the ronnrction hrtwwn

the electrical quant ~t ie s f interest in clcctrorhcmistry

(i.e., potential a i d current) are emphasized strongly.

usual method for coulometric determination of ascor-

bic acid consists in the reaction of this substance with elec-

trogenerated bromine, using constant current (coulometric

titration) 5,6).The endpoint in these tit rations has been

determined by methods such as amperometry or poten-

tiometrv. With the aim to present to the students another

differek ele~tro~eneratedubstance able to oxidize the

ascorbic acid. the experiments also are performed using

12.

The two oxidant spkcies are generatea separately a t t h e

anode from the respective reduced forms by the applica-

tion of constant current , and t he detection of endpoint in

these different coulometric titrations is made by use of vis-

ual indicators. The reaction between ascorbic acid and io-

dine or bromine is shown below

ascorbic acid

dehydroascorbic acid

The determination of ascorbic acid in orange juice by po-

larography is connected with the measurement of the an-

odic limiting current originated from the oxidation of this

substance to dehydroascorbic acid a t the dropping mercury

electrode (7). The polarograms of solutions at di fferent

ascorbic acid concentrations ar e recorded and the calibra-

tion curve is plotted in order to verify the linearity between

limiting current and ascorbic acid concentration (Ilkovic

equation). The standard addition method is used in the de-

termination of the substance in orange juice. This analyti-

cal procedure minimizes possible interferences of the ma-

trix 8).

Experimental Section

Reagents

The supporting electrolyte for the coulometric titrations

was 1.0

M

HpSO4. The iodide is oxidized easily by air at

sufiicientlv acidic solutions: therefore. for the iodometric

titration an acetatelacetic acid (0.1 k0 1 ) buffer was

em~ loved s electrolvtic solvent. The salts used a s s tartine

mateAals were ~ 1 a n dBr, both analytical grade re-

agents. The indicators were 2% starch solution and 1%

methyl orange solution; thi s latt er for the bromine titra-

tion. An ascorbic acid solution was prepared at approxi-

mately 0.1 M concentration from the dissolution of n ade-

quat e quantity of the solid substance in a volumetric flask.

%s solution was stored in a piston buret to limit the con-

tact with air and to deliver more precise volumes to the

coulometric and polarographic cells. The solution was

standardized by a usual method of titration with ascorbic

acid of the chemicallv eenera ted iodine af ter oxidation of

iodide with a known hcantity of iodate 9). he solution of

ascorbic acid holds this concentration for one dav in the

mentioned storage conditions 10).

Coulometric Titrations

AMetrohm Herisau coulometer E211 was used in the

coulometric titrations and the constant current was ad-

justed a t 20

mA

This current value was calibrated by use

of a usual method, i.e., titration of a defined volume of

standardized AsOi solution with electrogenerated iodine

(starch as indicator) in Na?HPOa medium 8).The olati-

num generator electrodes were a gauze cylinder and a wire

spiral. These electrodes were resnectivelv the anode and

the cathode when iodine and broniine were generated. The

wire spiral was placed into a glass isolation tube contain-

ing a Na2S04 aline bridge in order to avoid contamination

of the solution due to electrogenerated species. The solu-

tion in the cell was continuouily stirred ;sing a magnetic

bar.

Pure AscorbicAcid

z a s a n O x i da n t The electrolytic solution contained

100 mL of acetatel acetic acid buffer and 1mL of starch

solution. The solution was 0.1 M in iodide. 0.125-mL

s a m ~ l ef standard ascorbic acid solution was added to the

cell (a 200-mL beaker) and the constant current was ap-

plied until the appearance of blue color. Previous experi-

ments were made with electrolyte in the absence of a

reducing agent to verify the time required for the visuali-

Volume 72 Number 5 May 1995 445

 

Table 1. Results of Coulometric Titrations of Pure Ascorbic Acid

Natura l Orange Juice

Using Bromine and Iodine as Electrogenerated Oxidants I = 19.90

The juice was obtained by

Eleclroanalytical Ascorbic Time (s) Ascorbic Recovery Trials

technique acid added acid found ( )

mM)

mM)

Coulometry 12 Method 102.5 126.0f .2 104.0f 0.1 101.5 3

Br2

Method 102.5 125.8 f 0.8 104.7 0.6 102.1 3

Polarography Standard addition 0.91 0.92 0.01 101.1 3

Table 2 Comparison of the Results of Ascorbic Acid Determinations

in Natural Orange Juice by Coulometric with iodine and bromine)

and Polarographic standard addition procedure) ~ e t h o d s ~

Eleclroanalytical

technique

Ascorbic acid found (mM) Trials

Coulometry 12 Method 2.46M.02 3

Rrg Method 6R7+004 3

Polarography Standard addition 2.43M.07

T h e percent re overy

for

polarographywas 9 G .

zation of the blue color of the iodine-starch complex (blank

correction).

Brz as an Oxidant

The detection of endpoint in these

coulometric titrations was based on the reaction of the

electrogenerated bromine with methyl orange 11 ) . Be-

cause ascorbic acid is first oxidized by bromine, the end-

point can be found by the reaction of the excess of the

oxidant with the indicator, the red color of the solution

(acidic medium) is changed to colorless. Initially, three

drops of methyl orange were added to the solution con-

tained in the cell (0.1

M

H2S04 and 0.1 M B r 100 mL))

and the current was applied in order to determine the time

required for the indicator oxidation (bl ank correction).

Then, three new drops of methyl orange and a 0.125-mL

sample of standard ascorbic acid were added to the cell for

the titration.

squeezing fresh oranges and fil-

tering to remove solids. The re-

sulting solution was stocked in a

closed flask under nitrogen at-

mosphere. Five milliliters of this

juice were added to the coulomet-

ric cell for the analysis with io-

dine. In spite of the presence of

th e juice, the excess of iodine

(blue color) was easily observed.

In the coulometric t i t rat ions

with bromine the added volume

of orange juice was 2 mL in order

to diminish the interference of

th e juice color in the detection of

the endpoint.

Polarography. The polaro-

m h i c determinations were car-

riea out with a Sar gent Welch

3

Polarograph Model XVI using a

conventional H-cell and a dropping

mercury electrode (SCE). All the

potentials were measured against

a saturated calomel electrode. The

solutions were bubbled adequately with nitrogen. Chloride

from the SCE gives an anodic wave in the region of ascor-

bic acid electroxidation (anticipat ion of the mercury oxida-

tion); therefore, t he working electrode compartment was

separated from the reference electrode by a s intered glass

disk and 4 agar-saturated potassium nitr ate saline

bridge. This procedure minimizes the migrat ion of chloride

ions from the reference electrode to the t est solution.

Calibration curves were constructed from the polaro-

grams recorded after the addition of known volumes of

standard ascorbic acid solution (0.100 to 0.400 mL) to 10.0

mL of 0.5M KN03 contained i n t he polarographic cell. The

polarograms of the blank and the solutions were recorded

from -0.1 V going in the positive direction, unti l the ap-

pearance of the anodic wave corresponding to the mercury

oxidation. The polarographic determination of ascorbic

acid in the juice was performed by use of the standard ad-

dition method. A0.5

M

KNOB o-

lution was used as supporting

electrolyte (8.0 mL). Then, a 2.0-

mL sample of the filtered natura l

juice was added to the po-

larographic cell and the polaro-

gram of the resulting solution

as recorded. Afterwards, four

aliquots (0.050 mL each) of the

standard ascorbic acid contained

in the piston buret were added to

the cell and t he respective po-

~1.

larograms were obtained.

Results

Table presents the electroly-

sis times for the coulometric ti-

trati ons of the s tandard ascorbic

acid samples using the two pro-

2

posed methods, and the amount

of ascorbic acid determined from

C

ASCORBIC

ACID mM

each method. The resul ts found,

using both iodine and bromine as

oxidants for pure ascorbic acid,

are reproducible and a difference

Figure1.Polarograms o 0.5M KN03 solution 0)nd after addition o pure ascorbic acid aliquots 1-4).

than 2 was obtained in

The resulting calibration curve is also depicted.

comparison with iodometric

446 Journal of Chemical Education

 

standardization 9).A partial ex-

planation for the higher results

of the coulometric analysis may

be related to th e presence of im-

purities, so tha t the current effi-

ciency was not 100 .The data

obtained i n th e coulometric titra-

t ions of the natur al juice ar e

shown in Table

2,

where the dis-

crepancies between the titration

with bromine and iodine are evi-

dent. The explanation for these

results is related to the greater

oxidizing power of the bromine,

com ~on nds f the na tural iuice

besides ascorbic acid. This ex-

oeriment shows th e higher selec-

tivity of ascorbic acid-oxidation

bv iodine. The ~o la ro ma ms nd

calibration curve obiained for

th e analys is of pure ascorbic acid

are shown in Figure 1 The linear

plot was perfectly adiusted to the

I

2

ASCORBIC ACID ADDED mM

Figure 2. Polarograms o natural orange juice (0) and after the addition of pure ascorbic acid aliquots

14 ) (0.5 K N 0 3 . The extrapolation of the straight line to x-axis represents the ascorbic acid concen-

tration

n

the juice contained

n

the polarographic cell.

experimental points, also passing through the origin. So,

the linear dependence of the limiting curre nt with respect

to ascorbic acid concentration was proved. Figure 2 pre-

sents th e polarogram of natu ral juice i n KN08 as well as

the re sulting polarograms after th e addition of known

amounts of standard ascorbic acid. The anomalous de-

crease of the cur rent before th e mercury oxidation may be

associated with unknown compounds present in the or-

ange juice, because this unexpected effect was not oh-

served in the polarograms of Figure 1 pur e ascorbic acid).

However, thi s anomaly does not interfere in th e analytical

application of polarography for this study. The results

found from the standard addition method (Fig. 2 are in

excellent agreement with the ones obtained using electro-

generated iodine in the constant current coulometric ex-

periment, a s shown in Table 2.

Acknowledgment

We thank Lniz Roberto de Moraes Pitombo for helpful

suggestions and Paulo Celso Isolani for correcting the Eng-

lish.

Literature Clted

1. Kolthaff,

I

M.J Eledroehrm. Soc 1971,118, 5C4C.

2. Bond, A . M .

M od em Polomgmgmgphie Me tho ds in Analytical ChamLslry;

Marcel

Dek-

ter New Ynrt 19RO

~ . .. ~ ..

3. Bard, A. J.; F a u l k n e ~ . R. E l ~ f r n c h ~ m i m lethods: Wiiey: New York, 1980.

4. Chambers,J Q. J. C h e m E d u c 1983.60.259-262,

5. Marsh,

D

J.;Jacobs. DL :

eening

H J Chem. Educ. 1973.50,626628.

6.

G~en span D ;

Burch6eld,

D

E.:Veening, H.

J.

Chem. Educ

1985.62.688690.

7. Milner,

G. W C. The Princ~ples nd Applications ofPoia mpm phy; Lonprmans: Lon-

don, 1966.p 602.

8.

Willard,H :Mertitt,

L. L., Jr; ean

A In sb u m n to l Metha lr ~ f A n a i ~ ~ i ~ : V ~

Nostrand: New

York,

1965.

9. KolthofC

I.

M.

VolumelricAnolysis;

Interscience:New

York,

1957.

p

626.

10 Erdey, L.; Bodor, E. A n d C ha m. 1952,24,418420.

11. skoog,. west, M w ~ L ~ ~ ~ ~ P ~ ~ ~ I ~f ~ n a i y t i m ~hemistry; saunaers:i l a -

delphia, 1982, p379 .

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447