Commission Regulation (EEC) No 000/90 (repealed)Show full title

14. VOLATILE ACIDITY U.K.

1. DEFINITION U.K.

The volatile acidity is formed from the acids of the acetic series present in wine in the free state and combined as a salt.

2. PRINCIPLE OF THE METHOD U.K.

Titration of the volatile acids separated from the wine by steam distillation and titration of the distillate.

Carbon dioxide is first removed from the wine.

The acidity of free and combined sulphur dioxide distilled under these conditions should be deducted from the acidity of the distillate.

The acidity of any sorbic acid which may have been added to the wine must also be deducted.

Note: Part of the salicylic acid used in some countries to stabilize the wines before analysis is present in the distillate. This must be determined and deducted from the acidity. The method of determination is given in section 7 of this chapter. U.K.

3. REAGENTS U.K.

3.1. Crystalline tartaric acid (C₄H₆O₆). U.K.

3.2. 0,1 M sodium hydroxide solution (NaOH). U.K.

3.3. 1 % phenolphthalein solution in 96 % vol neutral alcohol. U.K.

3.4. Hydrochloric acid (ρ ₂₀ = 1,18 to 1,19 g/ml) diluted 1/4 (v/v). U.K.

3.5. 0,005 M iodine (I₂) solution. U.K.

3.6. Crystalline potassium iodide (KI). U.K.

3.7. 5 g/l starch solution. U.K.

Mix 5 g of starch with about 500 ml of water. Bring to the boil, stirring continuously and boil for 10 minutes. Add 200 g sodium chloride. When cool, make up to one litre.

3.8. Saturated solution of sodium borate (Na₂B₄O₇ · 10H₂O), i.e. about 55 g/l at 20 °C. U.K.

4. APPARATUS U.K.

4.1. Steam distillation apparatus consisting of: U.K.

1.

a steam generator; the steam must be free of carbon dioxide;

2.

a flask with steam pipe;

3.

a distillation column;

4.

a condenser.

This equipment must pass the following three tests:

(a)

Place 20 ml of boiled water in the flask. Collect 250 ml of the distillate and add to it 0,1 ml of 0,1 M sodium hydroxide solution (3.2) and two drops of the phenolphthalein solution (3.3). The pink colouration must be stable for at least 10 seconds (i.e. steam to be free of carbon dioxide).

(b)

Place 20 ml of a 0,1 M acetic acid solution in the flask. Collect 250 ml of the distillate. Titrate with the 0,1 M sodium hydroxide solution (3.2): the volume oft his used must be at least 19,9 ml (i.e. at least 99,5 % of the acetic acid entrained with the steam).

(c)

Place 20 ml of 1 M lactic acid solution in the flask. Collect 250 ml of the distillate and titrate the acid with the 0,1 M sodium hydroxide solution (3.2).

The volume of sodium hydroxide solution added must be less than or equal to 1,0 ml (i.e. not more than 0,5 % of lactic acid is distilled).

Any apparatus or procedure which passes these tests satisfactorily fulfils the requirements of official international apparatus or procedures.

4.2. Water pump. U.K.

4.3. Vacuum flask. U.K.

5. PROCEDURE U.K.

5.1. Preparation of sample: elimination of carbon dioxide. Place about 50 ml of wine in a vacuum flask; apply vacuum to the flask with the water pump for one to two minutes, shaking continuously. U.K.

5.2. Steam distillation U.K.

Place 20 ml of wine, freed from carbon dioxide as in 5.1, in the flask. Add about 0,5 g of tartaric acid (3.1). Collect at least 250 ml of the distillate.

5.3. Titration U.K.

Titrate with the 0,1 M sodium hydroxide solution (3.2) using two drops of phenolphthalein (3.3) as indicator. Let n ml be the volume of sodium hydroxide used.

Add four drops of 1/4 dilute hydrochloric acid (3.4), 2 ml starch solution (3.3) and a few crystals of potassium iodide (3.6). Titrate the free sulphur dioxide with the 0,005 M iodine solution (3.5). Let n″ ml be the volume used.

Add the saturated sodium borate solution (3.8) until the pink coloration reappears. Titrate the combined sulphur dioxide with the 0,005 M iodine solution (3.5). Let n″ ml be the volume used.

6. EXPRESSION OF RESULTS U.K.

6.1. Method of calculation U.K.

The volatile acidity, expressed in milliequivalents per litre to one decimal place, is given by:

A = 5 (n − 0,1 n′ − 0,05 n″).

The volatile acidity, expressed in grams of acetic acid per litre to two decimal places, is given by:

0,300 (n − 0,1 n′ − 0,05 n″).

6.2. Repeatability (r) U.K.

r

=

0,7 meq/litre

r

=

0,04 g acetic acid/litre.

6.3. Reproducibility (R) U.K.

R

=

1,3 meq/litre

R

=

0,08 g acetic acid/litre.

6.4. Wine with sorbic acid present U.K.

Since 96 % of sorbic acid is steam distilled with a distillate volume of 250 ml, its acidity must be deducted from the volatile acidity, knowing that 100 mg of sorbic acid corresponds to an acidity of 0,89 milliequivalents or 0,053 g of acetic acid and knowing the concentration of sorbic acid in mg/l as determined by other methods.

7. DETERMINATION OF SALICYLIC ACID ENTRAINED IN THE DISTILLATE FROM THE VOLATILE ACIDITY U.K.

7.1. Principle U.K.

After the determination of the volatile acidity and the correction for sulphur dioxide, the presence of salicylic acid is indicated, after acidification, by the violet colouration that appears when an iron (III) salt is added.

The determination of the salicylic acid entrained in the distillate with the volatile acidity is carried out on a second distillate having the same volume as that on which the determination of volatile acidity was carried out. In this distillate, the salicylic acid is determined by a comparative colorimetric method. It is deducted from the acidity of the volatile acidity distillate.

7.2. Reagents U.K.

7.2.1. Hydrochloric acid (HCl) (ρ ₂₀ = 1,18 to 1,19 g/l). U.K.

7.2.2. Sodium thiosulphate, (Na₂S₂O₃ · 5H₂O) in a 0,1 M solution. U.K.

7.2.3. 10 % (m/v) solution of iron (III) ammonium sulphate (Fe₂(SO₄)₃ · (NH₄)₂SO₄ · 24H₂O). U.K.

7.2.4. 0,01 M solution of sodium salicylate. U.K.

Solution containing 1,60 g/l of sodium salicylate (NaC₇H₅O₃).

7.3. Procedure U.K.

7.3.1. Identification of salicylic acid in the volatile acidity distillate. U.K.

Immediately after the determination of the volatile acidity and the correction for free and combined sulphur dioxide, introduce into a conical flask 0,5 ml hydrochloric acid (7.2.1), 3 ml of the 0,1 M sodium thiosulphate solution (7.2.2) and 1 ml of the iron (III) ammonium sulphate solution (7.2.3).

If salicylic acid is present, a violet coloration appears.

7.3.2. Determination of salicylic acid U.K.

On the above conical flask, indicate the volume of the distillate by a reference mark. Empty and rinse the flask.

Subject a new test sample of 20 ml wine to steam distillation and collect the distillate in the conical flask up to the reference mark. Add 0,3 ml pure hydrochloric acid (7.2.1), and 1 ml of the iron (III) ammonium sulphate solution (7.2.3). The contents of the conical flask turn violet.

Into a conical flask identical to that carrying the reference mark, introduce distilled water up to the same level as that of the distillate. Add 0,3 ml pure hydrochloric acid (7.2.1) and 1 ml of the iron (III) ammonium sulphate solution (7.2.3). From the burette run in the 0,01 M sodium salicylate solution (7.2.4) until the violet coloration obtained has the same intensity as that of the conical flask containing the wine distillate.

Let n″ ml be the volume of solution added from the burette.

7.3.3. Correction to the volatile acidity U.K.

Subtract the volume 0,1 × nXXX ml from the volume n ml of 0,1 M sodium hydroxide solution used to titrate the acidity of the distillate during the determination of volatile acidity.