Commission Regulation (EEC) No 000/90 (repealed)Dangos y teitl llawn

24. pH U.K.

1. PRINCIPLE U.K.

The difference in potential between two electrodes immersed in the liquid under test is measured. One of these two electrodes has a potential which is a function of the pH of the liquid, while the other has a fixed and known potential and constitutes the reference electrode.

2. APPARATUS U.K.

2.1. pH meter with a scale calibrated in pH units and enabling measurements to be made to at least ±0,05 pH unit. U.K.

2.2. Electrodes: U.K.

2.2.1. Glass electrode, kept in distilled water. U.K.

2.2.2. Calomel-saturated potassium chloride reference electrode, kept in a saturated solution of potassium chloride. U.K.

2.2.3. Or a combined electrode, kept in distilled water. U.K.

3. REAGENTS U.K.

3.1. Buffer solutions U.K.

3.1.1. Saturated solution of potassium hydrogen tartrate, containing at least 5,7 g of potassium hydrogen tartrate per litre (C₄H₅KO₆) at 20 °C. (This solution may be kept for up to two months by adding 0,1 g of thymol per 200 ml.) U.K.

pH	3,57 at 20 °C
	3,56 at 25 °C
	3,55 at 30 °C

3.1.2. Solution of potassium hydrogen phthalate, 0,05 M, containing 10,211 g of potassium hydrogen phthalate (C₈H₅KO₄) per litre at 20 °C. (Maximum keeping period, two months.) U.K.

pH	3,999 at 15 °C
	4,003 at 20 °C
	4,008 at 25 °C
	4,015 at 30 °C

3.1.3. Solution containing: U.K.

monopotassium phosphate, KH2PO4 …	3,402	g
dipotassium phosphate, K2HPO4 …	4,354	g
water to …	1 000	ml

(maximum keeping period, two months)

pH	6,9 at 15 °C
	6,88 at 20°C
	6,86 at 25 °C
	6,85 at 30 °C

Note: Commerical reference buffer solutions may also be used. U.K.

4. PROCEDURE U.K.

4.1. Preparation of the sample for analysis U.K.

4.1.1. For must and wine: use the must or wine directly. U.K.

4.1.2. For rectified concentrated must: dilute the rectified concentrated must with water to produce a concentration of 25 ± 0,5 % (m/m) of total sugars (25 ° Brix). U.K.

If P is the percentage concentration (m/m) of total sugars in the rectified concentrated must, weigh a mass of

and make up to 100 g with water. The water used must have a conductivity below 2 microsiemens per cm.

4.2. Zeroing of the apparatus U.K.

Zeroing is carried out before any measurement is made, according to the instructions provided with the apparatus used.

4.3. Calibration of the pH meter U.K.

Calibrate the pH meter at 20 °C using buffer solutions of pH 6,88 and 3,57 at 20 °C.

Use the buffer solution of pH 4,00 at 20 °C to check the calibration of the scale.

4.4. Determination U.K.

Dip the electrode into the sample to be analysed, the temperature of which should be between 20 and 25 °C and as close as possible to 20 °C. Read the pH value directly off the scale.

Carry out at least two determinations on the same sample.

The final result is taken to be the arithmetic mean of two determinations.

5. EXPRESSION OF RESULTS U.K.

The pH of the must, the wine or the 25 % (m/m) (25 ° Brix) solution of rectified concentrated must is quoted to two decimal places.