Commission Regulation (EEC) No 000/90 of 17 September 1990 determining Community methods for the analysis of wines (repealed) (c. 2676)

ANNEX U.K.

11. CHLORIDES U.K.

1. PRINCIPLE U.K.

Chlorides are determined directly in the wine by potentiometry using an Ag/AgCl electrode.

2. APPARATUS U.K.

2.1. pH/mV meter graduated at intervals of at least 2 mV. U.K.

2.2. Magnetic stirrer. U.K.

2.3. Ag/AcCl electrode with a saturated solution of nitrate potassium as electrolyte. U.K.

2.4. Microburette graduated in 1/100 ml. U.K.

2.5. Chronometer. U.K.

3. REAGENTS U.K.

3.1. Standard chloride solution: 2,1027 g of potassium chloride, KCl (max. 0,005 % Br), dried before use by leaving in a desiccator for several days, are diluted in distilled water and made up to one litre. 1 ml of this solution contains 1 mg Cl⁻. U.K.

3.2. Silver nitrate titrating solution: 4,7912 g of analytical grade silver nitrate, AgNO₃ are diluted in a 10 % (v/v) alcohol solution and made up to one litre. 1 ml of this solution corresponds to 1 mg Cl⁻. U.K.

3.3. Nitric acid, of at least 65 % purity ( ρ₂₀ = 1,40 g/ml). U.K.

4. PROCEDURE U.K.

4.1. 5,0 ml of standard chloride solution are measured into a 150 ml cylindrical vessel placed on a magnetic stirrer, diluted with distilled water to approximately 100 ml and acidified with 1,0 ml of nitric acid (at least 65 %). After immersing the electrode, titrate by adding the silver nitrate titrating solution with the microburette, with moderate stirring. Begin by adding 1,00 ml for the first 4 ml and read the corresponding millivolt values. Add the next 2 ml in fractions of 0,20 ml. Finally, continue the addition in fractions of 1 ml until a total of 10 ml has been added. After each addition, wait for approximately 30 seconds before reading the corresponding millivolts. Transfer the values thus obtained onto graph paper against the corresponding millilitres of titrating solution and determine the potential of the equivalence point on the basis of the singular point on the curve obtained. U.K.

4.2. 5 ml of the standard chloride solution are measured into a 150 ml cylindrical vessel with 95 ml of distilled water and 1 ml of nitric acid (at least 65 %). Immerse the electrode and titre, whilst stirring, until the potential of the equivalence point is obtained. This determination is repeated until a good degree of agreement in the results is obtained. This check must be carried out before each series of measurements of chlorides in the samples. U.K.

4.3. 50 ml of wine for analysis are measured into a 150 ml cylindrical vessel. Add 50 ml of distilled water and 1 ml of nitric acid (at least 65 %) and titrate using the procedure described in 4.2. U.K.

5. EXPRESSION OF RESULTS U.K.

5.1. Calculations U.K.

If n represents the number of millilitres of silver nitrate titrating solution, the chloride content in the tested liquid is:

20 × n	expressed as milligrams of Cl per litre,
0,5633 × n	expressed as milliequivalents per litre,
32,9 × n	expressed as milligrams of sodium chloride per litre.

5.2. Repeatability (r): U.K.

r

=

1,2 mg Cl per litre

r

=

0,03 meg per litre

r

=

2,0 mg NaCl per litre

5.3. Reproducibility (R): U.K.

R

=

4,1 mg Cl per litre

R

=

0,12 meg per litre

R

=

6,8 mg NaCl per litre

6.

Note: For very precise determination. U.K.

Refer to the complete titration curve obtained during determination of the test liquid with the silver nitrate solution. U.K.

(a)

Measure 50 ml of the wine to be analysed into a 150 ml cylindrical vessel. Add 50 ml of distilled water and 1 ml of nitric acid (at least 65 %). Titrate using the silver nitrate solution, adding 0,5 ml at a time and recording the corresponding potential in millivolts. Derive from this first titration the approximate volume of silver nitrate solution required.

(b)

Recommence determination in the same conditions. Begin by adding 0,5 ml of titrating solution at a time until the volume added is 1,5 to 2 ml less that the volume determined in (a). Hereafter add 0,2 ml at a time. Continue to add the solution beyond the approximately located equivalence point in a symmetrical manner, i.e. by adding 0,2 ml and then 0,5 ml at a time.

The end point of the measurement and the exact volume of silver nitrate consumed are obtained: U.K.

• either by drawing the curve and determining the equivalence point,

• or by the following calculation:

  

  Where:

  V

  =

  volume of titrating solution at equivalence point;

  V′

  =

  volume of titrating solution before the largest potential change;

  Δ V_i

  =

  constant volume of the increments of titrating solution, i.e. 0,2 ml;

  ΔΔ E₁

  =

  second difference in potential before the largest potential change;

  ΔΔ E₂

  =

  second difference in potential after the largest potential change.

Example: U.K.

Volume of AgNO3 titrating solution	E potential in mV	Difference Δ E	Second difference ΔΔ E
0	204
		4
0,2	208		0
		4
0,4	212		2
		6
0,6	218		0
		6
0,8	224		0
		6
1,0	230		2
		8
1,2	238		4
		12
1,4	250		10
		22
1,6	272		22
		44
1,8	316		10
		34
2,0	350		8
		26
2,2	376		6
		20
2,4	396

In this example, the end point of the titration is between 1,6 and 1,8 ml: the largest potential change (Δ E = 44 mV) occurs in this interval. The volume of silver nitrate titrating solution consumed to measure the chlorides in the test sample is: U.K.