GB/T 5009.61-2003Method for analysis of hygienic standard of products of melamine for food packaging (English PDF)

8.1 Principle

Under acidic conditions formaldehyde reacts with phenylhydrazine hydrochloride and, on oxidation, forms a red compound, which is quantified by comparison with a standard series. The minimum detection limit is 5 mg/L.

8.2 Reagents

8.2.1 Phenylhydrazine hydrochloride solution (10 g/L)

Weigh 1.0 g of phenylhydrazine hydrochloride, dissolve it in 80 mL of water, add 2 mL of hydrochloric acid (10+2), dilute with water to 100 mL, filter and store in a brown bottle.

8.2.2 Potassium ferricyanide solution (20 g/L)

Potassium ferricyanide solution, 20 g/L.

8.2.3 Hydrochloric acid (10+2)

Measure 100 mL of hydrochloric acid and dilute with water to 120 mL.

8.2.4 Formaldehyde standard solution

Pipette 2.5 mL of 36% to 38% formaldehyde solution into a 250 mL volumetric flask, dilute with water to the mark, standardise by the iodometric method, and finally dilute so that each millilitre is equivalent to 100 µg of formaldehyde.

8.2.5 Formaldehyde standard working solution

Pipette 10.0 mL of the formaldehyde standard solution into a 100 mL volumetric flask and dilute with water to the mark. Each millilitre of this solution is equivalent to 10.0 µg of formaldehyde.

8.3 Analytical procedure

Pipette 10.0 mL of the acetic acid (4%) soaking solution into a 100 mL volumetric flask, make up to the mark with water and mix. Then pipette 2 mL of this diluted solution into a 25 mL colorimetric tube.

Pipette 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mL of the formaldehyde standard working solution (equivalent to 0, 2, 4, 6, 8 and 10 µg of formaldehyde) into separate 25 mL colorimetric tubes and add water to 2 mL.

To the sample tube and each standard tube add 1 mL of phenylhydrazine hydrochloride solution, shake to mix and allow to stand for 20 min. Add 0.5 mL of potassium ferricyanide solution to each tube and allow to stand for 4 min; add 2.5 mL of hydrochloric acid (10+2) to each tube, then make up to 10 mL with water and mix.

Within 10 min to 40 min, using a 1 cm cell and setting the zero with the zero tube, measure the absorbance at a wavelength of 520 nm, plot the standard curve and compare.

8.4 Calculation of results

X = (m × 1000) / (10 × V/100 × 1000)

where:

X - content of formaldehyde in the soaking solution, in milligrams per litre (mg/L);

m - mass of formaldehyde in the diluted solution taken for the determination, in micrograms (µg);

V - volume of the diluted soaking solution taken for the determination, in millilitres (mL).

The calculated result is reported to three significant figures.

8.5 Precision

The absolute difference between two independent determination results obtained under repeatability conditions shall not exceed 10% of their arithmetic mean.