GB/T 20975.2-2018Methods for chemical analysis of aluminium and aluminium alloys—Part2: Determination of arsenic content (English PDF)

2.1 Method principle

The test portion is dissolved with a mixed acid of sulfuric acid, hydrochloric acid, and nitric acid. The iodide of trivalent arsenic is extracted from the hydrochloric acid solution with carbon tetrachloride, then transferred to the aqueous phase.

The trivalent arsenic is oxidized to pentavalent arsenic with iodine, to form a colored complex with ammonium molybdate. At a wavelength of 850 nm of a spectrophotometer, the absorbance of the complex is measured, to calculate the mass fraction of arsenic.

2.2 Reagents and materials

Unless otherwise stated, in the analysis, only identified guaranteed reagents and Grade 1 water are used.

2.2.1 Hydrochloric acid (rho=1.19 g/mL).

2.2.2 Hydrochloric acid (1+1).

2.2.3 Sulfuric acid (1+1).

2.5.4.4

Part of the test solution (2.5.4.3) and the blank test solution (2.5.3) are transferred into a 5 cm absorption cell. With water as a reference, at a wavelength of 850 nm of the spectrophotometer, the absorbance is measured.

The absorbance of the blank test solution (2.5.3) is subtracted from the absorbance of the test solution (2.5.4.3), and the corresponding amount of arsenic is found from the working curve.

2.5.5 Drawing of working curve

2.5.5.1 PIPETTE 0 mL, 1.00 mL, 2.00 mL, 3.00 mL, 4.00 mL, 5.00 mL, 10.00 mL of arsenic standard solution (2.2.13) into 50 mL volumetric flasks; and operate as specified in 2.5.4.3.

2.5.5.2 MOVE some of the series of standard solutions (2.5.5.1) into the 5 cm absorption cell. With water as a reference, at a wavelength of 850 nm of the spectrophotometer, the absorbance is measured. The arsenic content is taken as the abscissa; the absorbance, obtained by subtracting the absorbance of the "zero" concentration solution in the series of standard solutions from the absorbance of the standard solution, is taken as the ordinate; and the working curve is drawn.

2.6 Calculation of analytical results

The mass fraction w(As) of arsenic is calculated according to formula (1).

Where. m1 - The arsenic content found from the working curve, in micrograms (µg); m0 - The mass of the test portion, in grams (g). The calculation result retains 2 significant figures.

2.7 Precision

2.7.1 Repeatability

The absolute difference between the two independent determination results obtained under repeated conditions, within the average range given below, does not exceed the repeatability limit (r). The situations where the repeatability limit (r) is exceeded do not exceed 5%. […]

3.2.5 Mixed acid

Hydrochloric acid (3.2.3) and nitric acid (3.2.4) are mixed in an equal volume ratio.

3.2.6 Hydrochloric acid (5+95)

3.2.7 Potassium borohydride solution (10 g/L)

WEIGH 2.5 g of potassium borohydride in a beaker; USE sodium hydroxide solution (5 g/L) to dilute to 250 mL and SHAKE well. Prepared when used.

3.2.8 Thiourea-ascorbic acid solution (100 g/L)

WEIGH 25 g of thiourea and 25 g of ascorbic acid into a 300 mL beaker; USE water to dissolve and dilute to 250 mL, SHAKE well; and TRANSFER to a brown bottle. Prepared when used.

3.2.9 Standard stock solution of arsenic (1.0 mg/mL)

WEIGH 1.320 g of arsenic trioxide dissolved in 5 mL of sodium hydroxide solution (200 g/L); USE hydrochloric acid (3.2.3) to acidify until the Congo red paper is blue; TRANSFER the solution to a 1000 mL volumetric flask; USE water to dilute to volume, and MIX well. 1 mL of this solution contains 1.0 mg of arsenic.

3.2.10 Arsenic standard solution A (10 µg/mL)

PIPETTE 1.00 mL of standard stock solution of arsenic (3.2.9) into a 100 mL volumetric flask; USE hydrochloric acid (3.2.6) to dilute to volume, and MIX well. 1 mL of this solution contains 10 µg of arsenic. Prepared when used.

3.2.11 Arsenic standard solution B (0.1 µg/mL)

PIPETTE 1.00 mL of arsenic standard solution A (3.2.10) into a 100 mL volumetric flask; USE hydrochloric acid (3.2.6) to dilute to volume, and MIX well. 1 mL of this solution contains 0.1 µg of arsenic. Prepared when used.

3.3 Instrument

Atomic fluorescence spectrometer, with arsenic hollow cathode lamp.

3.4 Sample

The sample is processed into chips having a thickness of no more than 1 mm.

3.5 Analytical procedures

3.5.1 Test portion

Sample (3.4) is weighed according to Table 4, accurate to 0.0001 g. […]