GB/T 12456-2008 English PDF (GBT12456-2008)

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GB/T 12456-2008: Determination of total acid in foods
GB/T 12456-2008
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 67.050
X 04
Replacing GB/T 12456-1990
Determination of total acid in foods
(ISO 750:1981, Fruit and vegetable products -
Determination of titratable acidity, NEQ)
ISSUED ON: JUNE 25, 2008
IMPLEMENTED ON: JANUARY 01, 2009
Issued by: General Administration of Quality Supervision Inspection and
Quarantine of PRC;
Standardization Administration of PRC.
Table of Contents
Foreword ... 3 
1 Scope ... 4 
2 Normative references ... 4 
3 Acid-base titration ... 4 
4 pH potential method ... 7 
Determination of total acid in foods
1 Scope
This standard specifies the analytical procedures for the determination of total
acid in food by acid-base titration and pH potential method.
This standard applies to the determination of total acid in foods such as fruit
and vegetable products, beverages, dairy products, alcoholic beverage, bee
products, starch products, cereal products, condiments. The acid-base titration
method of this standard is not applicable to colored or turbid opaque test
solutions.
2 Normative references
The provisions in following documents become the provisions of this Standard
through reference in this Standard. For the dated references, the subsequent
amendments (excluding corrections) or revisions do not apply to this Standard;
however, parties who reach an agreement based on this Standard are
encouraged to study if the latest versions of these documents are applicable.
For undated references, the latest edition of the referenced document applies.
GB/T 601 Chemical reagent - Preparations of standard volumetric solutions
GB/T 604 Chemical reagent - General method for the determination of pH
ranges corresponding the color transition of acid-base indicators
GB/T 6682 Water for analytical laboratory use - Specification and test
methods (GB/T 6682-1992, neq ISO 3696:1987)
3 Acid-base titration
3.1 Method summary
According to the principle of acid-base neutralization, lye is used to titrate the
acid in the test solution and phenolphthalein is used as an indicator, to
determine the titration end point. Calculate the total acid content in the food
according to the consumption of lye.
3.2 Reagents and solutions
Take a representative sample of at least 200 g; place it in a mortar or tissue
masher; add the same amount of boiled water as the sample; use a mortar to
mash it; or use a tissue masher to mash it; mix it well; place it in a closed glass
container.
3.4.3 Solid and liquid samples
Take at least 200 g of the sample according to the solid-liquid ratio; use a mortar
to grind it or use a tissue masher to mash it. Mix it well. Place it in a closed glass
container.
3.5 Preparation of test solution
3.5.1 Specimen with total acid content less than or equal to 4 g/kg
Use fast filter paper to filter the sample (3.4). Collect the filtrate for determination.
3.5.2 specimen with total acid content greater than 4 g/kg
Weigh 10 g ~ 50 g of specimen (3.4), accurate to 0.001 g. Place it in a 100 mL
beaker. Use water boiled at about 80 °C to transfer the contents of the beaker
into a 250 mL volumetric flask (total volume is about 150 mL). Place it in a
boiling water bath and boil for 30 minutes (shaking 2 ~ 3 times to dissolve all
the organic acids in the sample in the solution). Take it out. Cool to room
temperature (about 20 °C). Use boiled water to make its volume reach to 250
mL. Use fast filter paper to filter it. Collect the filtrate for determination.
3.6 Analytical procedures
3.6.1 Weigh 25.000 g ~ 50.000 g 1) of test solution (3.5); make it contain 0.035
g ~ 0.070 g acid. Place it in a 250 mL conical flask. Add 40 mL ~ 60 mL of water
and 0.2 mL of 1% phenolphthalein indicator. Use 0.1 mol/L sodium hydroxide
standard titration solution (if the sample’s acidity is low, it may use 0.01 mol/L
or 0.05 mol/L sodium hydroxide standard titration solution) to titrate it to reddish,
which does not fade for 30 seconds. Record the consumed volume of the 0.1
mol/L sodium hydroxide standard titration solution (V1).
The same sample to be tested shall be determined twice.
3.6.2 Blank test: Use water to replace the test solution. Follow the steps 3.6.1.
Record the consumed volume of the 0.1 mol/L sodium hydroxide standard
titration solution (V2).
3.7 Result calculation
1 It may also measure 25.00 mL ~ 50.00 mL test solution; covert it to the mass value based on the
density of test solution.
4.6 Analytical procedures
4.6.1 Test solutions for fruit and vegetable products, beverages, dairy
products, alcoholic beverage, starch products, cereal products and
condiments
Weigh 20.000 g ~ 50.000 g2) of test solution (4.5); make it contain 0.035 g ~
0.070 g acid. Place it in a 150 mL beaker. Add 40 mL ~ 60 mL water. Turn on
the power of the pH meter. After the pointer is stable, use a pH 8.0 buffer
solution (4.1.2) to calibrate the pH meter. Place the beaker containing the test
solution on the electromagnetic stirrer and immerse the glass electrode and
calomel electrode. Press the pH reading switch; turn on the stirrer; quickly use
0.1 mol/L sodium hydroxide standard titration solution (if the sample has low
acidity, use 0.01 mol/L or 0.05 mol/L sodium hydroxide standard titration
solution) for titration. Observe the change of pH value of the solution at any time.
When approaching the end of titration, slow down the titration speed. Add half
a drop at a time (at most one drop) until the pH of the solution reaches the end
point. Record the volume of the standard titration solution consumed by sodium
hydroxide (V3).
The same sample to be tested shall be determined twice.
4.6.2 Bee products
Weigh about 10 g of a well-mixed specimen, accurate to 0.001 g; place it in a
150 mL beaker; add 80 mL of water; proceed the following according to the
steps 3.6.1. Use a 0.05 mol/L sodium hydroxide standard titration solution for
titration at a rate of 5.0 mL/min. Stop dropping when the pH reaches 8.5.
Continue to add 10 mL of 0.05 mol/L sodium hydroxide standard titration
solution. Record the total volume value (V3) of the 0.05 mol/L sodium hydroxide
standard titration solution consumed. Immediately use the 0.05 mol/L
hydrochloric acid standard titration solution to back titrate to pH 8.2. Record the
volume value (V5) of the 0.05 mol/L hydrochloric acid standard titration solution
consumed.
The same test sample shall be determined twice.
4.6.3 Blank test
For the operations of 4.6.1 and 4.6.2, use water instead of the test solution to
do a blank test. Record the volume (V4) of the sodium hydroxide standard
titration solution consumed.
The pH at the end point of various acid titrations: phosphoric acid, 8.7 ~ 8.8;
2 It is also possible to measure 25.0 mL ~ 50.00 mL test solution; convert it to mass value according to
the density of the test solution.