GB 5009.74-2014National Food Safety Standard -- Method for Limit Test of Heavy Metals in Food Additives (English PDF)

3.1 Reagents

3.1.1 Nitric acid (HNO3).

3.1.6 Phenolphthalein (C2OH14O4).

3.1.7 Hydrogen peroxide (H2O2).

3.2 Preparation of reagents

3.3 Standard substance

High-purity lead nitrate [Pb(NO3)2], a standard substance whose purity is 99.99% or

which passes the national certification and is granted of the certificate of standard

substance.

3.4 Preparation of standard solutions

3.4.1 Lead standard stock solution (1 mg/mL). weigh 0.159 8 g of high-purity lead

nitrate [Pb(NO3)2] to dissolve in 10 mL of nitric acid solution (1%),

3.4.2 Lead standard working solution (10 µg/mL). absorb 1.0 mL of lead standard

stock solution (1 mg/mL) to pour into a 100 mL volumetric flask, add water to the scale.

4.5 Electronic balances. sensitivities 0.1 mg and 1 mg.

4.6 Water baths.

4.7 Pressure digestion tanks.

5.1 Preparation of inorganic specimens

The "specimen treatment" of inorganic specimens may be the pretreatment as required

by the specifications of all products including dissolution or digestion.

5.2 Preparation of organic specimens

The "specimen treatment" of organic specimens may generally be conducted in

accordance with the following methods, in addition to the specification requirements of

all products.

a) wet digestion. weigh 5.00 g of specimen, place in a 25 mL conical flask, add 10

mL ~ 15 mL of nitric acid to moisten the specimen, heat on an electric hot plate

after placing aside for a moment (or overnight), take down for cooling after the

reaction becomes gentle, add 5 mL of sulphuric acid along the flask wall,

continue to heat until the solution in the flask becomes brown, add nitric acid

continuously (add some perchloric acid if necessary) until the organic materials

are fully decomposed, continue to heat until lots of sulfur dioxide white smoke is

generated and finally the solution shall be colourless or yellowish.

b) dry digestion. weigh 5.00 g of specimen, place in a hard glass evaporating dish

or quartz crucible, add an appropriate quantity of sulphuric acid to moisten the

specimen, use a soft fire on an electric furnace for carbonization before adding

6.1 Tube A (standard tube). absorb the lead standard working solution (containing

not less than 10 µg), whose lead content is equivalent to the specified heavy metal

limit, pour into a 50 mL Nessler tube (if the specimen is treated, the same quantity of

reagent blank solution as the specimen solution shall be absorbed meanwhile), add

water to 25 mL, mix up, add 1 drop of phenolphthalein indicator solution, use diluted

hydrochloric acid (6 mol/L) or diluted ammonia hydroxide (1 mol/L) to adjust the pH to

neutral (when the phenolphthalein red just fades), add 5 mL of acetate buffer solution

of pH 3.5, and mix up as standby.

6.2 Tube B (specimen tube). take one Nessler tube matched with tube A, add 10 mL

~ 20 mL (or an appropriate quantity) of specimen solution, add water to 25 mL, mix up,

add 1 drop of 1% phenolphthalein indicator solution, use diluted hydrochloric acid (6

mol/L) or diluted ammonia hydroxide (1 mol/L) to adjust the pH to neutral (when the

phenolphthalein red just fades), add 5 mL of acetate buffer solution of pH 3.5, and mix

up as standby.

6.4 Add 10 mL of freshly-prepared hydrogen sulphide saturated solution to all tubes,

add water to the 50 mL scale, mix up, observe on the white background after placing

in a dark place for 5 min.

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