SOP: Determination of pH Value by Glass Electrode pH Meter

GIFSQ — Global Institute for Food Safety & Quality

Principle

pH is the logarithm of the reciprocal of the hydrogen-ion concentration, expressed in grams per litre. It measures active acidity, which influences the flavour and palatability of a product and its processing requirements. The glass-electrode pH meter is the quickest and most reliable method available for measuring pH; its usual range for common fruits is pH 2 to 5 (most commonly 3 to 4).

Apparatus required

• Glass-electrode pH meter (line-operated models are preferred for routine use where a stable alternating-current source is available)
• Beakers

Reagents

a. M/20 potassium acid phthalate buffer (pH taken as 4 at 20 °C): weigh accurately 10.207 g of potassium acid phthalate (AR grade), dissolve in freshly boiled and cooled distilled water, and make up to one litre with distilled water. Add one drop of toluene and shake well. Used for standardizing the instrument. (The book notes the British Standards Institution gives the pH of M/20 potassium hydrogen phthalate as 4.)

b. Saturated potassium chloride solution for the calomel electrode: prepare from pure crystals of KCl (AR grade) and distilled water; always keep a 3/4-inch layer of crystals in the bottle.

c. M/20 sodium borate (borax) buffer (pH 9.18 at 20 °C): dissolve 19.071 g of sodium borate (AR grade) in distilled water in a standard 1-litre graduated flask and make up to volume with carbon-dioxide-free distilled water. Add a drop of toluene and shake well. The pH of this buffer changes markedly with temperature (see note below).

d. Citric acid–phosphate buffer (for intermediate pH values): prepare the two stock solutions — Solution 1: M/5 Na₂HPO₄·12H₂O = 71.648 g per litre; Solution 2: M/10 citric acid (C₆H₈O₇·H₂O) = 21.01 g per litre — and mix to 200 ml in the proportions given in the book’s Table 28-9 for the required pH. Add a drop of toluene after mixing.

Procedure

Standardize the pH meter with the pH 4 phthalate buffer (a buffer of pH 4 suffices for determining the pH of fruits and vegetables and their products)

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Rinse the electrodes, immerse them in the sample, and read the pH

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Record the temperature of the measurement along with the pH value

Precautions

• Buffer solutions are not stable for longer than about one month — prepare them fresh after that interval; a drop of pure toluene per litre inhibits mould growth.
• Many buffers, particularly above pH 7, have a large pH/temperature coefficient: use the buffer pH value at the same temperature as the sample, and always state the temperature with the result.
• The book’s listed pH values of the borax buffer at temperatures of 0, 10, 20, 30, 40, 50, 60 and 70 °C are carried as printed (9.25, 9.18, 9.10, 9.03, 8.95, 8.88 and 8.81); the series is incomplete in the available copy, so check the printed book when reporting temperature-corrected values.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.