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MANUAL DE INSTRUCCIONES CODIGO 80191 REV D
J
.P. SELECTA s.a.u.
ENE/2020
13 Determination of sorbic acid (SO
(Method of determination by spectrophotometry of ultraviolet absorption)
Principle:
Sorbic acid separated by steam dragged distilling is determined in the distillate by spectrophotom-
etry of ultra violet absorption. The substances which interfere in the measure of the absorption
in the ultra violet are eliminated by evaporation to dryness of the distillate of the sample, slightly
alkalized by a solution of calcium hydroxide.
Necessary reagents:
-
Crystallized tartaric acid.
-
Solution of hydroxide of calcium Ca(OH)
-
Reference solution of sorbic acid C
in approximately 2 ml of solution 0.1M of sodium hydroxide. Pour into a calibrated flask of 1000
ml, level with water. 26.8 mg of potassium sorbate C
with water up to 1000 ml.
Necessary material:
-
DE-1626 distilling unit by steam dragging.
-
Water bath at 100ºC.
-
Zirconium or porcelain capsule of 55 mm in diameter.
-
Spectrophotometer permitting the measuring a wavelength of 256 nanometers with 1
cm quartz tanks.
Procedure:
DISTILLATION
1- Place 10 ml of wine into the small bubble tube, add 1 or 2 g of tartaric acid. Put it in the DE-
1626 distilling unit and collect 250 ml of distillate in the gauged flask.
PATTERN CURVE
2- By dilutions, prepare with water - from the reference solution - four pattern solutions which
contain 0.5, 1, 2.5 and 5 mg of sorbic acid per litre respectively.
3- With a spectrophotometer, measure the respective absorbance at 256 nm of pattern solutions
with regards to the distilled water.
4- Trace the absorbance curve regarding the concentration of solutions. The relation is lineal.
DETERMINATION
5- Introduce 5 ml of distillate in a capsule of 55 mm diameter, add 1 ml of calcium hydroxide
solution and a drop of copper sulphate solution. Evaporate to dryness in a bath of boiling water.
6- Collect the residuum with some mililitres of distilled water, drag quantitative in a 20 ml gauged
flask and drag with the rinsing water, measure the absorbance at 256 nm in comparison to witness
solution obtained with 1 ml of solution of hydroxide of calcium and diluted at 20 ml with water.
7- Take the value of the measured absorbance and subtract the C concentration of the solution
in sorbic acid.
Note: In current practice, this evaporation or dryness may not be necessary. Measure the absorb-
ance directly in the diluted distillated at 1/4 against the distilled water.
EXPRESSION OF THE RESULTS
The concentration in sorbic acid of the wine expressed in milligrams per litre equals: 100 x C
C = concentration in sorbic acid of the analysed solution by spectrophotometry, expressed in
milligrams per litre.
Ctra. NII Km 585.1 Abrera 08630 (Barcelona) España
e-mail: selecta@jpselecta.es - website: http://www.jpselecta.es
(Sujetas a modificaciones sin previo aviso)
, approximately 0.02M.
2
H
O
of 20 mg per litre: Dissolve 20 mg of sorbic acid
6
8
2
H
KO
can be dissolved in water and filled
6
7
2
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