GB 5009.239-2016National food safety standard - Determination of Acidity in Food (English PDF)

3.1 Reagents

3.1.1 Sodium hydroxide solution (NaOH).

3.1.2 Cobalt sulfate heptahydrate (CoSO4·7H2O).

3.1.7 Trichloromethane (CHCl3).

3.2 Preparation of reagents

3.2.1 Sodium hydroxide solution standard solution (0.100 0 mol/L)

Weigh 0.75 g of working chemical potassium acid phthalate which has been dried to

constant weight in an electric drying oven at 105°C ~ 110°C; add 50 mL of water free

of carbon dioxide to dissolve; add 2 drops of phenolphthalein indicator (10 g/L); use

prepared sodium hydroxide solution to titrate until the solution appears pink; maintain

it for 30 s. Meanwhile, carry out blank test.

3.2.2 Reference solution

Dissolve 3 g of cobalt sulfate heptahydrate in water; add dropwise to 100 mL.

3.2.5 Distilled water free of carbon dioxide

Boil water for 15 min to expel carbon dioxide; cool and confine the water.

4.1 Analytical balance. sensitivity 0.001 g.

4.2 Basic burette. capacity 10 mL; minimum scale 0.05 mL.

4.7 Pulverizer. it is capable of pulverizing specimen so that more than 95% of

specimen passes sieve CQ16 [equivalent to pore diameter 0.425 mm (mesh number

40)]; it will not generate heat in the grinding chamber when pulverizing specimen.

4.8 Oscillator. reciprocating type; oscillation frequency 100 /min.

4.9 Intermediate speed qualitative filter paper.

5.1 Mild powder

5.1.1 Specimen preparation

Transfer all specimen to a clean, dry container (with a seal lid) which is about 2 times

of the volume of specimen; immediately put on the lid; rotate and oscillate repeatedly

to mix up specimen. During the operation, efforts shall be taken to avoid specimen

from exposure to the air.

5.1.2 Determination

Weigh 4 g of specimen (accurate to 0.01 g) to place into 250 mL conical flask. Use

measuring cylinder to take 96 mL of water of about 20°C (3.2.5) to dissolve specimen

once again; stir before allowing to stand for 20 min.

5.1.3 Blank titration

Use 96 mL of water (3.2.5) to conduct blank test; read the millimetres of sodium

hydroxide standard solution consumed (V0). The volume of sodium hydroxide

consumed by blank shall not be less than 0, or else, distilled water as required shall

be prepared and used once again.

5.2 Milk and milk products

5.2.1 Preparation of reference solution

Add 2.0 mL of reference solution to conical flask containing an equivalent volume of

corresponding solution; rotate it gently to mix up; then obtain standard reference colour.

If several similar products are to be determined, the reference solution can be used for

the whole determination process, but the time shall not be more than 2 h.

5.2.4 Condensed milk

Weigh 10 g (accurate to 0.001 g) of specimen mixed up to place in 250 mL conical

flask; add 60 mL of water newly boiled and cooled to room temperature; mix up; add

2.0 mL of phenolphthalein indicator; use sodium hydroxide standard solution for

titration after mixing up; rotate the flask while adding dropwise until the colour is similar

to that of reference solution and does not fade away within 5 s.

5.2.5 Casein

Weigh 5 g (accurate to 0.001 g) of specimen ground and mixed up to place into conical

flask; add 50 mL of water (3.2.5); store for 4 h ~ 5 h at room temperature (18°C ~ 20°C)

or heat to 45°C in water bath and maintain it at the temperature for 30 min; add another

5.2.6 Blank titration

Use an equivalent volume of water (3.2.5) to conduct blank test; read the millimetres

of sodium hydroxide standard solution consumed (V0) (applicable to 5.2.2, 5.2.4 and

5.2.5). Use 30 mL of neutral ethanol-diethyl ether mixture to carry out blank test; read

the millimetres of sodium hydroxide standard solution consumed (V0) (applicable to

5.2.3).

5.3 Starch and derived products

5.3.1 Specimen pretreatment

Specimen shall be mixed up.

5.4 Grain and grain products

5.4.1 Specimen preparation

Take 80 g ~ 100 g of specimen mixed up; use pulverizer to pulverize specimen so that

more than 95% of specimen passes sieve CQ16 [pore diameter 0.425 mm (mesh

number 40)]; mix up all specimen sieved after pulverization; place in ground flask; and

immediately determine specimen prepared.

5.4.3 Blank titration

Use 30 mL of water (3.2.5) to carry out blank test and record the millimetres of sodium

hydroxide standard solution used (V0).

9.1 Sodium hydroxide standard solution. as in 3.2.1.

10.1 Analytical balance. sensitivity 0.001 g.

10.2 Basic burette. scale division 0.1 mL, accurate to 0.05 mL; or automatic burette

which can meet the same requirements for use.

10.3 pH meter. equipped with glass electrodes and appropriate reference electrodes.

10.6 Thermostat water bath.

11.1 Specimen preparation

Transfer all specimen to a clean, dry container (with a seal lid) which is about 2 times

of the volume of specimen; immediately put on the lid; rotate and oscillate repeatedly

to mix up specimen. During the operation, efforts shall be taken to avoid specimen

from exposure to the air.

11.3 Blank titration

Use 100 mL of water (9.3) to carry out blank test and record the millimetres of sodium

hydroxide standard solution used (V0).

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