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1622114 Ab 098 4 En 1 Essay

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Application Bulletin 98/4 e

Determination of ascorbic acid (vitamin C) and its compounds Branch
General
analytical
chemistry;
organic
pharmaceutical industry; food, stimulants,
flavours; biochemistry, biology;

chemistry;
beverages,

Bi-voltammetric determination
with iodine
Instruments

Titrator with MET mode

Keywords

10 mL burette

Ascorbic acid; titration; bi-voltametric titration; photometric titration; polarography; vitamin C; ISO 6557/2; branch 1;
branch 3; branch 4; branch 7; branch 8; 6.0309.100;
6.1115.000

Stirrer

Electrode
Double Pt-sheet electrode

6.0309.100

Summary
In addition to the natural occurrence, ascorbic acid (vitamin C) is added to food and beverages as an antioxidant. It can
also be found in various pharmaceutical products.
Ascorbic acid, as well as its salts and esters, can be
determined by titration or by means of polarography. The
determination is based on the oxidation of ascorbic acid to
dehydroascorbic acid.
HO

HO
O

HO
HO

O
OH

- 2 H+ - 2 e

O

HO
O

Reagents

Volumetric iodine solution, c(I2) = 0.01 mol/L

Glyoxal, w(C2H2O2) = 40%

Sodium hydroxide solution, c(NaOH) = 1 mol/L

Sulfuric acid, w(H2SO4) = 25%

.
Disodium thiosulfate pentahydrate, Na2S2O3 5 H2O

Acetic acid, c(CH3COOH) = 2 mol/L

O
O

Solutions

For the titrimetric determination bi-voltametric or photometric equivalence point indication can be used. The bi-voltametric indication is independent of inherent coloration of the
sample.

Titrant

c(I2) = 0.01 mol/L

Glyoxal solution

w(C2H2O2) = 40%
200 mL w(C2H2O2) = 40% is
adjusted with c(NaOH) = 1 mol/L
to pH = 7.0.
The solution has to be stored in a
dark bottle in a refrigerator.

The polarographic method is the most selective of the
described methods, since other reducing or oxidizing
substances do not interfere.
Standard
Disodium thiosulfate
pentahydrate

Disodium thiosulfate pentahydrate
is dried at 120 °C for 2 h and
cooled down in a desiccator for at
least 1 h.

Sample preparation

Drinks, fruits and vegetable juices can be analyzed
directly

Page 1 of 9

Application Bulletin 98/4 e
Determination of ascorbic acid (vitamin C) and its compounds

For tablets and other vitamin preparations, a diluted
solution in degased dist. water is prepared. Thereof an
aliquot is used for the titrimetric determination.
Foods, stimulants and animal feeds are extracted using
the appropriate procedures.

Calculation
Titer
f=

ms
VEP1 × 2 × cI2 × MA

f:
ms:
VEP1:

Analysis
Titer
Approximately 50 mg Na2S2O3 · 5 H2O is weighed into a
titration beaker with an accuracy of 0.1 mg, 5 mL
c(CH3COOH) = 2 mol/L is added and the solution is diluted
to approximately 80 mL with deionized water. The solution is then titrated with c(I2) = 0.01 mol/L until after the first
equivalence point.
Sample
50 mL sample (containing 0.05…0.5 mg ascorbic acid) is
pipetted into a titration beaker, 2 mL glyoxal solution is
added and the mixture is stirred briefly and allowed to stand for 5 min. After the addition of 5 mL sulfuric acid w(H2SO4) = 25% it is titrated with c(I2) = 0.01 mol/L until after the
equivalence point.

2:
cI2 :
MA :

βAA:
VEP1:

MET Ipol

Vol. increment

0.1 mL

Signal drift

50 mV/min

Max. waiting time

26 s

I(pol)

1 μA

EP criterion

30 mV

EP recognition

greatest

Mass concentration of ascorbic acid (AA) in
mg/L
Titrant consumption until the first equivalence
point in mL

cI2 :

Concentration of titrant in mol/L

f:
MAA:

Correction factor («titer») without unit...

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Keywords

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