GB/T 20975.4-2020Methods for chemical analysis of aluminium and aluminium alloys—Part 4:Determination of iron content (English PDF)

4.1 Method summary

The sample is dissolved in hydrochloric acid and hydrogen peroxide, and the iron is reduced with hydroxylamine hydrochloride. The pH of the test solution is controlled to be 3.5-4.5, so that the divalent iron ions and the adjacent Phenanthroline forms a stable orange-red complex, and its absorbance is measured at a wavelength of 510.0nm with a spectrophotometer to determine the iron content.

4.2 Reagents

Unless otherwise stated, only reagents and laboratory secondary water confirmed to be analytically pure are used in the analysis.

4.2.1 Hydrogen peroxide (rho=1.10g/mL).

4.2.2 Hydrochloric acid (5 1).

4.2.3 Hydrochloric acid (1 1).

4.2.4 Sodium hydroxide solution (200g/L), stored in plastic bottles.

4.2.5 Hydroxylamine hydrochloride solution (10g/L).

4.2.6 Phenanthroline solution (2.5g/L). Weigh 2.5g of phenanthroline (C12H8N2·H2O) or 3g of phenanthroline hydrochloride (C12H8N2·HCl·H2O) dissolved in warm water and cooled. Dilute to 1000mL with water and mix well.

4.2.7 Nickel chloride (NiCl2·6H2O) solution (1g/L).

4.2.8 Buffer solution. Weigh 272g of sodium acetate (CH3COONa·3H2O) and dissolve it with 500mL of water. After filtering, add 240mL of ice. Acetic acid (rho=1.05g/mL), dilute to 1000mL with water, and mix well.

4.2.9 Mixed solution. Combine hydroxylamine hydrochloride solution (4.2.5), o-phenanthroline solution (4.2.6) and buffer solution (4.2.8) to (1 1 3) The product is mixed with each other and stored in a brown bottle. The storage period does not exceed four weeks.

4.2.10 Iron standard storage solution. Use a certified standard solution for preparation first. Or weigh 0.2860g of Ozone which has been burnt at 600°C in advance Ferric fluoride [w(Fe2O3)>=99.99%], put in a.200mL beaker, add 30mL hydrochloric acid (4.2.3), heat to completely dissolve, cool, Transfer to a 1000mL volumetric flask, dilute to the mark with water, and mix. 1mL of this solution contains 0.2mg iron.

4.2.11 Iron standard solution A. Pipette 25.00mL iron standard stock solution (4.2.10) into a 100mL volumetric flask and dilute to the mark with water. Mix well (prepared when used). 1mL of this solution contains 0.05mg iron.

4.2.12 Iron standard solution B. Pipette 25.00mL iron standard stock solution (4.2.10) into a 500mL volumetric flask and dilute to the mark with water. Mix well (prepared when used). This solution 1mL contains 0.01mg iron.

4.3 Apparatus

Spectrophotometer.

4.4 Sample

Process the sample into chips not larger than 1mm.

4.5 Analysis steps

4.5.1 Sample

According to Table 1, weigh the sample (4.4) of the corresponding mass (m0) to the nearest 0.0001g.

4.5.2 Parallel experiment

Do two tests in parallel and take the average value.

4.5.3 Blank test

Do a blank test with the sample (4.5.1).

4.5.4 Determination

4.5.4.1 Place the test material (4.5.1) in a 250mL beaker, cover with a watch glass, add 25mL of hydrochloric acid (4.2.3) in batches, and wait vigorously. After the reaction, add 7 to 8 drops of hydrogen peroxide (4.2.1), if it is a pure aluminum sample, add a nickel chloride solution (4.2.7), and slowly heat until the sample is complete Dissolve completely, boil for 5 minutes, and cool. Aluminum alloys that are not easily dissolved by hydrochloric acid or have high silicon content can be dissolved in alkali. Place the sample (4.5.1) in In a 100mL silver beaker (or polytetrafluoroethylene beaker), carefully add 20mL of sodium hydroxide solution (4.2.4), and cover with a silver watch (or polytetrafluoroethylene cover), heat until the sample is completely dissolved, if the mass fraction of silicon in the sample is >=4.00%, keep the test solution at a temperature slightly lower than the boiling point 20min, carefully add the amount of water lost due to evaporation, boil for 2min~3min, wash the cup wall and watch dish with water, and dilute the test solution to about 25mL, add 20mL hydrochloric acid (4.2.2), boil slightly until the solution is clear, and cool.

4.5.4.2 When the iron mass fraction is 0.0010%~0.050%, transfer the sample solution (4.5.4.1) into a 100mL volumetric flask, if not The soluble matter is filtered with medium-speed quantitative filter paper, and the precipitate is washed with hot water. When the iron mass fraction >0.050%~3.50%, the sample solution (4.5.4.1) Transfer to the corresponding volumetric flask (V0), if there is insoluble matter, filter with medium-speed quantitative filter paper, wash the precipitate with hot water, and dilute with water to Mark, mix, and then pipette the corresponding volume (V1) of the solution in a 100mL volumetric flask.

4.5.4.3 In a 100mL volumetric flask, add 25mL mixed solution (4.2.9), dilute to the mark with water, and mix. Leave it for 30 minutes. If When the mass fraction of copper in the sample is >=5.0% or the mass fraction of zinc is >=4.0% or the mass fraction of nickel is >=2.0%, or the combined mass fraction of copper, zinc, and nickel >=5%, add 25mL mixed solution (4.2.9), then add 20mL o-phenanthroline solution (4.2.6) (when drawing the working curve, also Correspondingly add 20mL o-phenanthroline solution), dilute to the mark with water, and mix. Leave it for 30 minutes.

4.5.4.4 Move part of the sample solution (4.5.4.3) into the absorption tank, and take the blank test (4.5.3) as a reference. At spectrophotometer wavelength The absorbance was measured at 510.0nm. Find the corresponding iron mass (m1) from the working curve.

4.5.5 Drawing of working curve

4.5.5.1 According to the mass fraction of iron in the sample, the preparation of the series of standard solutions is divided into the following two types.

a) When the iron mass fraction is 0.0010%~0.010%. pipette 0mL, 1.00mL, 2.00mL, 5.00mL, 8.00mL, 10.00mL iron standard solution B (4.2.12), put them in a set of 100mL volumetric flasks, dilute to about 50mL, add 25mL mixed solution (4.2.9), dilute to the mark with water, and mix. Leave for 30min;

b) When the iron mass fraction is >0.010%~3.50%. pipette 0mL, 1.00mL, 2.00mL, 4.00mL, 6.00mL, 8.00mL iron standard solution A (4.2.11), put them in a set of 100mL volumetric flasks, dilute to about 50mL, add 25mL mixed solution (4.2.9), dilute to the mark with water, and mix. Leave it for 30 minutes.

4.5.5.2 Move part of the series of standard solutions (4.5.5.1) into the absorption tank, and use the reagent blank solution (without iron standard solution) as the reference, and The absorbance was measured at a wavelength of 510.0nm with a spectrophotometer. Use iron mass as the abscissa and absorbance as the ordinate to draw the working curve.

4.6 Processing of test data

The iron content is calculated as the mass fraction of iron wFe, calculated according to formula (1).

When the iron mass fraction is less than 1.00%, the calculation result retains two significant digits; when the iron mass fraction is >=1.00%, the calculation result is expressed as small Count the last two digits. Numerical rounding is implemented in 3.2 and 3.3 of GB/T 8170-2008.

4.7 Precision

4.7.1 Repeatability

The measured value of two independent test results obtained under repeatability conditions, within the average range given below, the two test results The absolute difference does not exceed the repeatability limit r, and the case of exceeding the repeatability limit r does not exceed 5%. Repeatability limit r uses linear interpolation according to the data in Table 2 Method or extension method.

4.7.2 Reproducibility

The measured values of two independent test results obtained under reproducibility conditions, within the average range given below, the two test results The absolute difference does not exceed the reproducibility limit R, and the case of exceeding the reproducibility limit R does not exceed 5%. Reproducibility limit R According to the data in Table 3, the linear internal Obtained by interpolation or extension method.

5.1 Method summary

In an acidic solution, use potassium dichromate standard titration solution to directly titrate ferrous ions, and excess potassium dichromate will refer to sodium diphenylamine sulfonate. The indicator is oxidized to purple, which is the end point, and the iron content is determined by this.

5.2 Reagents

Unless otherwise stated, only reagents and laboratory tertiary water confirmed to be analytically pure are used in the analysis.

5.2.1 Ammonium fluoride solution (100g/L).

5.2.2 Sulfuric acid-phosphoric acid mixed acid. In 300mL sulfuric acid (13), slowly add 10mL phosphoric acid (rho=1.69g/mL), and mix well.

5.2.3 Potassium dichromate standard titration solution (c=0.015mol/L). Weigh 4.4130g and dry it in an electric oven at 120°C ± 2°C Constant amount of working standard reagent potassium dichromate (relative molecular weight 294.186), placed in a 300mL beaker, dissolved in water, and transferred to 1000mL In a volumetric flask, dilute to the mark with water and mix well.

5.2.4 Sodium diphenylamine sulfonate indicator solution (4g/L).

5.3 Specimen

Process the sample into chips not larger than 1mm.

5.4 Analysis steps

5.4.1 Sample

Weigh the mass (m) as 0.50g sample (5.3), accurate to 0.0001g.

5.4.2 Parallel test

Do two tests in parallel and take the average value.

5.4.3 Determination

5.4.3.1 Place the sample (5.4.1) in a 400mL beaker, add 15mL ammonium fluoride solution (5.2.1), 30mL sulfuric acid-phosphoric acid mixed acid (5.2.2), heat until the sample is completely dissolved, and cool.

5.4.3.2 When the iron mass fraction is 3.50% ~ 40.00%, filter the sample solution (5.4.3.1) with filter paper (or absorbent cotton) in 500 mL In the conical beaker, wash the precipitate 6 to 7 times, and dilute the sample solution to about 150 mL. When the mass fraction of iron is >40.00%~65.00% At the time, filter the sample solution (5.4.3.1) with filter paper (or absorbent cotton) in a 250mL volumetric flask (V2), wash the precipitate 6 to 7 times, dilute with water Release to the mark and mix well. The aliquot volume (V3) is 50.00mL sample solution in a 500mL conical beaker, add 15mL sulfuric acid-phosphoric acid mixture Synthetic acid (5.2.2), diluted to about 150mL.

5.4.3.3 Add 2 to 4 drops of sodium diphenylamine sulfonate indicator solution (5.2.4), and titrate to the solution with potassium dichromate standard titration solution (5.2.3) A steady purple color is the end point. Record the volume (V) of the potassium dichromate standard titration solution (5.2.3) consumed.

5.5 Processing of test data

The iron content is calculated as the mass fraction of iron wFe, calculated according to formula (2).

The calculation result is expressed to two decimal places. The rounding of values shall be implemented in 3.2 and 3.3 of GB/T 8170-2008.

5.6 Precision

5.6.1 Repeatability

The measured value of two independent test results obtained under repeatability conditions, within the average range given below, the two test results The absolute difference does not exceed the repeatability limit r, and the case of exceeding the repeatability limit r does not exceed 5%. Repeatability limit r uses linear interpolation according to the data in Table 4 Method or extension method.

5.6.2 Reproducibility

The measured values of two independent test results obtained under reproducibility conditions, within the average range given below, the two test results The absolute difference does not exceed the reproducibility limit R, and the case of exceeding the reproducibility limit R does not exceed 5%. Reproducibility limit R is linear within Obtained by interpolation or extension method.