GB/T 20975.16-2020Methods for chemical analysis of aluminium and aluminium alloys—Part 16: Determination of magnesium content (English PDF)

4.1 Method summary

The sample was dissolved in hydrochloric acid, and the magnesium in the residue was recovered by filtration. Precipitating magnesium with sodium hydroxide in the presence of hydrogen peroxide, potassium cyanide and a small amount of iron And separated from a large amount of aluminum, zinc, copper, nickel and chromium. Dissolve the precipitate with hydrochloric acid, in the presence of potassium permanganate, separate a small amount of iron, manganese, Aluminum and titanium. The sample solution uses methyl thymol blue as the indicator, and titrates with the CDTA standard titration solution until the solution changes from blue to light gray Color is the end point to determine the magnesium 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 Zinc oxide.

4.2.2 Bromine water (saturated solution).

4.2.3 Hydrofluoric acid (rho=1.14g/mL).

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

4.2.5 Ammonia (rho=0.90g/mL).

4.2.6 Hydrochloric acid (about 0.05mol/L).

4.2.7 Hydrochloric acid (11).

4.2.8 Ammonia (11).

4.2.9 Nitric acid (2 3).

4.2.10 Sulfuric acid (1 3).

4.2.11 Ethanol (1 3).

4.2.12 Hydrochloric acid (1 5).

4.2.13 Ammonia (1 13).

4.2.14 Potassium cyanide solution (250g/L). Equipped when used. Very toxic, pay attention when using.

4.2.15 Sodium hydroxide solution (240g/L). Store in plastic bottles.

4.2.16 Sodium hydroxide solution (20g/L). Store in plastic bottles.

4.2.17 Hydroxylamine hydrochloride solution (18g/L).

4.2.18 Potassium permanganate solution (10g/L).

4.2.19 Ferric chloride solution (1mg/mL). Weigh 0.48g ferric chloride (FeCl3 6H2O) and dissolve it in 16mL hydrochloric acid (4.2.12), Transfer to a 100mL volumetric flask, dilute to the mark with water, and mix. This solution 1mL contains 1mg iron.

4.2.20 Ethylene glycol-bis(2-aminoethyl ether)tetraacetic acid (EGTA) solution (0.05mol/L). Weigh 1.9g EGTA and dissolve it in 25mL hydrogen Sodium oxide solution (4.2.16), transfer to a 100mL volumetric flask, dilute to the mark with water, and mix.

4.2.21 Magnesium standard solution A (rho2=1mg/mL). Use a certified standard solution to prepare. Or weigh 1.0000g magnesium (wMg>= 99.95%) placed in a 500mL beaker, add.200mL water, add 30mL hydrochloric acid (4.2.7) in total in batches, and after it is completely dissolved, transfer Put it into a 1000mL volumetric flask, dilute to the mark with water, and mix well. 1mL of this solution contains 1mg of magnesium.

4.2.22 Magnesium standard solution B (rho4=0.2mg/mL). Pipette 100.0mL magnesium standard solution A (4.2.21) into a 500mL volumetric flask, Add 6mL hydrochloric acid (4.2.7), dilute to the mark with water, and mix. 1mL of this solution contains 0.2mg of magnesium.

4.2.23 1,2-cyclohexanediaminetetraacetic acid (CDTA) standard titration solution A (c1~0.035mol/L). ---Preparation. Weigh 12.75g CDTA into a 1000mL beaker, add about 500mL water, add 10mL sodium hydroxide to dissolve Liquid (4.2.15), stir for a few minutes. Add 10mL~15mL sodium hydroxide solution (4.2.15) to completely dissolve. Use slow Quantitative filter paper is filtered in a 1000mL volumetric flask, washed with water and diluted to the mark, and mixed. Store in a polyethylene bottle. ---Calibration. Pipette 3 volumes (V2) of 25.00mL magnesium standard solution A (4.2.21) into a 500mL conical beaker, and add 5g ammonium chloride, diluted with water to about 100mL, add 100mL ammonia water (4.2.5), cool, add 0.05g~0.1g methyl musk Vanillol blue indicator (4.2.25), titrated with CDTA standard titration solution A (4.2.23) until the solution changes from blue to light gray, The color of the excess 2 drops does not change to the end point. The volume of CDTA standard titration solution (4.2.23) consumed for parallel calibration should not More than 0.15mL, take the average value (V1). ---Calculation. Calculate the concentration of CDTA standard titration solution according to formula (1).

Where.

rho2 --- The mass concentration of magnesium standard solution A (4.2.21), in milligrams per milliliter (mg/mL);

V2 --- The volume of the magnesium standard solution A (4.2.21) to be pipetted, in milliliter (mL);

24.305---The molar mass of magnesium, in grams per mole (g/mol);

V1 --- The volume of CDTA standard titration solution A (4.2.23) consumed for calibration, in milliliter (mL).

The calculation result retains four significant figures. The rounding of values shall be implemented in 3.2 and 3.3 of GB/T 8170-2008.

4.2.24 1,2-Cyclohexanediaminetetraacetic acid (CDTA) standard titration solution B (c3~0.01mol/L). ---Preparation. Weigh 3.64g CDTA into a 1000mL beaker, add about 500mL water, and add 5mL sodium hydroxide solution (4.2.15), stir for a few minutes. Add 10mL~15mL sodium hydroxide solution (4.2.15) to completely dissolve. Use slow speed Filter the amount of filter paper in a 1000mL volumetric flask, wash with water and dilute to the mark, and mix. Store in a polyethylene bottle. ---Calibration. Pipette 3 parts (V4) of 30.00mL magnesium standard solution B (4.2.22) into a 500mL conical beaker, and add 5g ammonium chloride, diluted with water to about 100mL, add 100mL ammonia water (4.2.5), cool, add 0.05g~0.1g methyl musk Vanillol blue indicator (4.2.25), titrate with CDTA standard titration solution B (4.2.24) until the solution changes from blue to light gray, The color of the excess 2 drops does not change to the end point. The volume of CDTA standard titration solution (4.2.24) consumed for parallel calibration should not More than 0.10mL, take the average value (V3). ---Calculation. Calculate the concentration of the CDTA standard titration solution according to formula (2).

Where.

rho4 --- The mass concentration of magnesium standard solution B (4.2.22), in milligrams per milliliter (mg/mL);

V4 --- The volume of the magnesium standard solution B (4.2.22) to be pipetted, in milliliter (mL);

24.305---The molar mass of magnesium, in grams per mole (g/mol);

V3 --- The volume of CDTA standard titration solution B (4.2.24) consumed during calibration, in milliliters (mL).

The calculation result retains four significant figures. The rounding of values shall be implemented in 3.2 and 3.3 of GB/T 8170-2008.

4.2.25 Methyl thymol blue indicator. Weigh 0.1g of methyl thymol blue and 10g of sodium chloride, grind and mix.

4.3 Apparatus

PH meter.

4.4 Sample

Process the sample into chips with a thickness of not more than 1mm.

4.5 Analysis steps

4.5.1 Sample

According to Table 1, weigh the sample (4.4) of the corresponding mass (m1) 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 reagent blank test with the sample (4.5.1). Record the volume of CDTA standard solution (V5) consumed during the titration of the blank solution.

4.5.4 Determination

4.5.4.1 Put the sample (4.5.1) in a 400mL beaker, cover with a watch glass, add water and hydrochloric acid (4.2.7) according to Table 1, and wait until the violent reaction stops After that, add 10mL nitric acid (4.2.9), slowly heat until the sample is completely dissolved, and boil for 10min. Wash watch dishes and cup walls with hot water and dilute the body It is accumulated to 150mL~200mL, filter with a slow quantitative filter paper, wash the filter paper and residue with hot hydrochloric acid (4.2.6) 8 to 10 times, and collect the filtrate And washing liquid in a 400mL beaker. If there is a large amount of residue, put the filter paper together with the residue in a platinum crucible, dry and ash at 550°C. Whole (don't burn the filter paper), let cool. Add 2mL sulfuric acid (4.2.10), 5mL hydrofluoric acid (4.2.3), add nitric acid (4.2.9) dropwise to the solution Clear. Heat and evaporate until the sulfuric acid fumes are removed, and burn for 10min (<=700°C). cool down. Add 5mL water and 2mL hydrochloric acid (4.2.7), add Heat to completely dissolve the precipitate (if turbidity needs to be filtered), combine this solution with the main sample solution. Concentrate the volume to about 100 mL.

4.5.4.2 Add 5 mL of ferric chloride solution (4.2.19) to the sample solution (4.5.4.1), and transfer the sample solution into the pre-filled 70 mL hydrogen Sodium oxide solution (4.2.15) in a 400mL beaker, mix well. Add 2mL hydrogen peroxide (4.2.4), boil for 5min~10min. take under. Add 5mL potassium cyanide solution (4.2.14), boil for 5min. Dilute the volume with hot water to approximately 250 mL. Cover the watch glass to keep warm (but Do not boil) not less than 20min. Filter with slow quantitative filter paper, wash the precipitate and filter paper 5 times with hot sodium hydroxide solution (4.2.16). Keep Beaker. Dispose of the filtrate after harmless treatment.

4.5.4.3 Dissolve the precipitate in the original beaker with a mixture of 50 mL of hot hydrochloric acid (4.2.12) and 2 mL of hydroxylamine hydrochloride solution (4.2.17). Wash the filter paper thoroughly with water, keep the volume less than 70 mL, and heat until the sample solution is clear. Drop bromine water (4.2.2) until the color of bromine remains unchanged And excess 3mL, boil to remove excess bromine, dilute with water to a volume of about 80mL, and cool.

4.5.4.4 Adjust the sample solution to pH 4 with ammonia water (4.2.8), then adjust the sample solution to pH (4.4 ± 0.2) with ammonia water (4.2.13), and heat to boiling, Add 0.3g zinc oxide (4.2.1) (first make a paste with water and then move it into the sample solution), shake well. Add potassium permanganate solution (4.2.18) dropwise to stable Set the khaki, heat to boiling. Remove, add 2mL ethanol (4.2.11), shake well. Place it on a boiling water bath and heat for 5 min, add 2 mL of ethyl acetate Alcohol (4.2.11), mix well. Place on a boiling water bath and continue heating for 10 minutes. Filter the sample solution on a 500mL cone with slow quantitative filter paper In the cup, wash the sediment and filter paper 8 times with hot water and cool.

4.5.4.5 Adjust to slightly alkaline with ammonia (4.2.5) (check with test paper). If the sample solution is colorless, add 8mL potassium cyanide solution (4.2.14), If the sample solution is blue, add potassium cyanide solution (4.2.14) dropwise while stirring until the sample solution becomes colorless and an excess of 8 mL. Add 1mL EGTA solution (4.2.20), 100mL ammonia (4.2.5), 0.05g~0.1g methyl thymol blue indicator (4.2.25), shake well.

4.5.4.6 When the mass fraction of magnesium is <=0.50%, use CDTA standard titration solution B (4.2.24); when the mass fraction of magnesium is >0.50%, use CDTA Standard titration solution A (4.2.23); titrate until the sample solution changes from blue to light gray, and the color of the excess 2 drops does not change as the end point. Record consumption CDTA standard titration solution (4.2.24 or 4.2.23) volume (V6).

4.6 Processing of test data

The magnesium content is calculated as the magnesium mass fraction wMg, calculated according to formula (3).

Where.

c ---The concentration of CDTA standard titration solution (4.2.24 or 4.2.23), in moles per liter (mol/L);

V6 --- The volume of the CDTA standard titration solution (4.2.24 or 4.2.23) consumed during titration, in milliliters (mL);

V5 --- The volume of the CDTA standard titration solution (4.2.24 or 4.2.23) consumed in the blank test, in milliliter (mL);

24.305---The molar mass of magnesium, in grams per mole (g/moL);

m1 --- The mass of the sample, in grams (g).

The calculation result is expressed to two decimal places. 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 Tolerance

The difference between the analysis results between laboratories should not be greater than the allowable difference listed in Table 3.

5.1 Method summary

The sample is dissolved in hydrochloric acid and hydrogen peroxide, and placed at the wavelength of 285.2nm or 279.6nm in the atomic absorption spectrometer to be in the presence of strontium chloride An air-acetylene lean flame is used to determine the absorption value of magnesium to determine the magnesium content.

5.2 Reagents

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

5.2.1 Pure aluminum (wAl>=99.996%, wMg<=0.0002%).

5.2.2 Nitric acid (rho=1.42g/mL).

5.2.3 Hydrofluoric acid (rho=1.14g/mL).

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

5.2.5 Hydrochloric acid (1 1).

5.2.6 Sulfuric acid (1 1).

5.2.7 Strontium chloride solution (50mg/mL). Weigh 76g of strontium chloride in a 500mL beaker, add 400mL of water to dissolve, and transfer In a 500mL volumetric flask, dilute to the mark with water, mix well, and store in a plastic bottle.

5.2.8 Aluminum solution A (20mg/mL). Weigh 20.00g pure aluminum (5.2.1), place it in a 1000mL beaker, cover with a watch glass, and add to the total The amount is 800 mL of hydrochloric acid (5.2.5). After the violent reaction stops, slowly heat until it is completely dissolved, add a few drops of hydrogen peroxide (5.2.4), and boil Boil for a few minutes to decompose excess hydrogen peroxide and cool. Transfer the solution into a 1000mL volumetric flask, dilute to the mark with water, and mix.

5.2.9 Aluminum solution B (1mg/mL). Pipette 50.00mL aluminum solution A (5.2.8) into a 1000mL volumetric flask and dilute to the mark with water. Mix well.

5.2.10 Magnesium standard storage solution. weigh 1.000g of magnesium (wMg>=99.95%) into a 1000mL conical beaker, add.200mL water And 30mL hydrochloric acid (5.2.5), after the sample is completely dissolved, transfer it to a 1000mL volumetric flask, dilute to the mark with water, and mix. This solution 1mL contains 1.0mg magnesium.

5.2.11 Magnesium standard solution C. Pipette 25.00mL magnesium standard stock solution (5.2.10) into a 500mL volumetric flask and dilute to the mark with water. Mix well. 1mL of this solution contains 0.05mg of magnesium.

5.2.12 Magnesium standard solution D. Pipette 25.00mL magnesium standard solution C (5.2.11) into a 250mL volumetric flask, dilute to the mark with water, and mix uniform. 1mL of this solution contains 0.005mg of magnesium.

5.3 Apparatus

Atomic absorption spectrometer with magnesium hollow cathode lamp. The instrument should meet the following conditions.

---Characteristic concentration. In a solution consistent with the matrix of the measurement sample solution, the characteristic concentration of magnesium should not be greater than 0.008µg/mL.

---Precision. Use the highest concentration of standard solution to measure the absorbance 10 times, and the standard deviation should not exceed 1.0% of the average absorbance; Use the lowest concentration standard solution (not the "zero" concentration solution) to measure the absorbance 10 times, and the standard deviation should not exceed the highest concentration The average absorbance of the standard solution is 0.5%.

---Linear working curve. Divide the working curve into five equal sections according to the concentration, the difference between the absorbance difference of the highest section and the absorbance difference of the lowest section The ratio should not be less than 0.7.

5.4 Sample

Process the sample into chips with a thickness of not more than 1mm.

5.5 Analysis steps

5.5.1 Sample

Weigh the mass (m2) as 0.50g sample (5.4), accurate to 0.0001g.

5.5.2 Parallel test

Do two tests in parallel and take the average value.

5.5.3 Blank test

Weigh 0.50g pure aluminum (5.2.1), accurate to 0.0001g, instead of the sample (5.5.1), and do a blank test with the sample.

5.5.4 Determination

5.5.4.1 Place the sample (5.5.1) in a 250mL beaker, cover with a watch glass, add 10mL of water, and add 20mL of hydrochloric acid in batches (5.2.5), after the violent reaction stops, add an appropriate amount of hydrogen peroxide (5.2.4) dropwise, slowly heat until the sample is completely dissolved, boil for 10 min, cool but. If there is insoluble matter, filter and wash the precipitate. Place the residue and filter paper in a platinum crucible, ash (do not burn the filter paper), and burn at about 550°C Burn and cool. Add 2mL sulfuric acid (5.2.6), 5mL hydrofluoric acid (5.2.3), add nitric acid (5.2.2) dropwise until the solution is clear. Heat to evaporate to dryness, Burn at 700°C for a few minutes and cool. Dissolve the residue with as little hydrochloric acid (5.2.5) as possible (filter if necessary), and combine this sample solution with the original In the filtrate.

5.5.4.2 When the mass fraction of magnesium is 0.0020%~0.050%, transfer the sample solution (5.5.4.1) into a 250mL volumetric flask (V9) and add Add 20mL strontium chloride solution (5.2.7), dilute to the mark with water, and mix. […]