Velp Scientifica
– UDK139 Operating Manual
10004762/B5
108
3) Titration:
titrate using the solution 0.1M of sodium hydroxide in presence of 2 drops of
phenolpthalein solution, being
n
the volume in ml used.
Add 4 drops of diluted hydrochloric acid 1/4 , 2ml of starch-water, and some crystals of
potassium iodide (in order to make more visible the colour change). Titrate the free
sulphur dioxide using the solution 0.005M of iodine. Being
n’
the volume in ml used.
Add the saturated solution of sodium tetra-borate until the pink colour appears again.
Titrate the combined sulphur dioxide using the solution 0.005M of iodine . Being
n’’
the
volume in ml used.
4) Results:
- the volatile acidity expressed as milli-equivalents per liter, with 1 decimal, is:
A = 5 (
n
0.1
n
’ 0.05
n’’
)
- the volatile acidity expressed as grams of acetic acid per liter with 2 decimals , is:
0.300 (
n
0.1
n’
0.05
n’’
)
5) Wines containing sorbic acid:
inside the first 250ml of distillate is included also 96% of the sorbic acid present; for this
reason it is necessary to subtract its acidity from the volatile acidity considering that
100mg of sorbic acid correspond to an acidity of 0.89 milli-equivalents or to 0.053g of
acetic acid, knowing the amount of sorbic acid (mg/l) determined separately.
6) Determination of the salycilic acid present in the distillate of the volatile acidity:
After the determination of the volatile acidity and after the correction for the sulphur
dioxide, the presence of salycilic acid is revealed, after the addition of acid, by the
presence of a violet colour forming after addition of an iron III salt.The determination of the
salycilic acid ,present in the distillate of the volatile acidity, is to be performed in a second
distillate having the same volume of the distillate used for the dosage of the volatile
acidity.
In such a distillate the salycilic acid is determined by a comparative colorimetric method. It
is then subtracted from the distillate acidity.
a) Reagents:
- hydrochloric acid (HCl) (
20
= 1.18-1.19 g/ml)
- sodium thio-sulphate (Na
2
S
2
O
3
-5 H2O) in solution 0.1M
- solution of ammonic ferric sulphate
Fe
2
(SO
4
)
3
x (NH
4
)
2
SO
4
x 24 H
2
O
10% (w/v).
- solution of sodium salycilate 0.01M: solution containing 1.60 g/l of sodium salycilate (Na
C
7
H
5
O
3
).
b) Procedure:
-characterisation of the salycilic acid in the distillate of the volatile acidity:
immediately after having determined the volatile acidity and the correction for the free and
combined sulphur dioxide, add in the flask 0.5ml of hydrochloric acid, 3ml of solution 0.1M
295