GB/T 1819.12-2017Methods for chemical analysis of tin concentrates - Part 12:Determination of silicon dioxide content - The silicomolybdenum blue spectrophotometry and sodium hydroxide titration (English PDF)

2.1 Principle

The test portion is decomposed by fusion with sodium peroxide, leached with boiling water and acidified with hydrochloric acid. In a dilute hydrochloric acid medium, silicic acid reacts with ammonium molybdate to form silicomolybdic yellow. The hydrochloric acid concentration is increased, oxalic acid and tartaric acid are added to eliminate interference from phosphorus, arsenic and other elements, and the silicomolybdic yellow is reduced to silicomolybdenum blue with ascorbic acid. The absorbance is measured with a spectrophotometer at a wavelength of 810 nm.

2.2 Reagents

Unless otherwise stated, only reagents of recognised analytical grade and grade 2 water shall be used in the analysis.

2.2.1 Sodium peroxide.

2.2.2 Anhydrous ethanol.

2.2.3 Hydrochloric acid (1+1).

2.2.4 Hydrochloric acid (1+3).

2.2.5 Potassium permanganate solution (30 g/L).

2.2.6 Ammonium molybdate solution [(NH4)6Mo7O24·4H2O] (100 g/L): filter before use; store in a plastic bottle.

2.2.7 Oxalic acid-tartaric acid mixed solution: dissolve 100 g of oxalic acid (H2C2O4·2H2O) and 40 g of L(+)-tartaric acid (C4H6O6) in water, dilute to 1 000 mL with water; store in a plastic bottle.

2.2.8 Ascorbic acid solution (20 g/L): prepare fresh before use.

2.2.9 Silicon dioxide standard stock solution: weigh 1.000 0 g of silicon dioxide (wSiO2 99.99 %, previously ignited at 800 °C to constant mass and cooled to room temperature in a desiccator) into a platinum crucible, add 5 g of anhydrous sodium carbonate, mix, cover with a further 1 g of anhydrous sodium carbonate, and fuse in a muffle furnace at 1 000 °C for 1 h. Remove, allow to cool slightly, rinse the outside of the crucible clean with water, and place it in a polytetrafluoroethylene beaker containing 100 mL of hot water to leach the melt (heat gently if necessary). When leaching is complete, wash and remove the crucible and cool. Transfer the solution to a 1 000 mL volumetric flask, dilute to the mark with water and mix. 1 mL of this solution contains 1 mg of silicon dioxide. Store in a plastic bottle.

2.2.10 Silicon dioxide standard solution A: pipette 25.00 mL of the silicon dioxide standard stock solution (2.2.9) into a 500 mL volumetric flask, dilute to the mark with water and mix. 1 mL of this solution contains 50 µg of silicon dioxide. Store in a plastic bottle.

2.2.11 Silicon dioxide standard solution B: pipette 50.00 mL of silicon dioxide standard solution A (2.2.10) into a 250 mL volumetric flask, dilute to the mark with water and mix. 1 mL of this solution contains 10 µg of silicon dioxide. Store in a plastic bottle.

2.3 Apparatus

Spectrophotometer.

2.4 Sample

The particle size of the sample shall not exceed 0.074 mm.

The sample shall be dried in an oven at 105 °C ± 5 °C for 1 h and cooled to room temperature in a desiccator before use.

2.5 Analytical procedure

2.5.1 Test portion

Weigh the test portion according to Table 1, to the nearest 0.000 1 g.

Table 1 - Mass of test portion, dilution volume, aliquot volume (wSiO2, %: test portion, g; dilution volume, mL; aliquot volume, mL; cell, cm):

0.10 to 0.20: 0.20; 200; 10.00; 3.

> 0.20 to 1.00: 0.10; 200; 10.00; 3.

> 1.00 to 4.00: 0.10; 200; 10.00; 1.

> 4.00 to 10.00: 0.10; 250; 5.00; 1.

2.5.2 Number of determinations

Carry out two independent determinations and take the mean value.

2.5.3 Blank test

Carry out a blank test together with the test portion.

2.5.4 Determination

2.5.4.1 Place the test portion (2.5.1) in a 30 mL silver crucible, add 1.5 g of sodium peroxide (2.2.1), mix, cover with 1 g of sodium peroxide (2.2.1) and put on the lid. Place in a muffle furnace at 400 °C, raise the temperature to 650 °C and fuse for 10 min. Remove and allow to cool slightly.

2.5.4.2 Rinse the outside of the crucible clean with water, place it in a 300 mL polyethylene beaker, add 70 mL of boiling water, and when leaching is complete wash and remove the crucible.

2.5.4.3 Slowly add 25 mL of hydrochloric acid (2.2.3) while swirling, and cool. Transfer the solution according to Table 1 into a volumetric flask of the corresponding volume, dilute to the mark with water and mix; filter through a fast filter paper, discarding the first portion of the filtrate.

2.5.4.4 Pipette an aliquot of the filtrate according to Table 1 into a 100 mL volumetric flask, make up to 10 mL with the blank test solution (2.5.3), add 5 mL of hydrochloric acid (2.2.4), and add potassium permanganate solution (2.2.5) dropwise until the solution turns faintly red; add 20 mL of water and mix.

2.5.4.5 Add 8 mL of anhydrous ethanol (2.2.2) and mix. While swirling continuously, add 7.5 mL of ammonium molybdate solution (2.2.6) and mix. Allow to stand at 15 °C to 25 °C for 20 min.

2.5.4.6 Add 30 mL of hydrochloric acid (2.2.3) and 5 mL of oxalic acid-tartaric acid mixed solution (2.2.7), mix, immediately add 5 mL of ascorbic acid solution (2.2.8), dilute to the mark with water and mix. Allow to stand at 15 °C to 35 °C for 30 min.

2.5.4.7 Transfer part of the coloured solution into a cell as specified in Table 1 and measure the absorbance with a spectrophotometer at a wavelength of 810 nm, using the coloured solution of the blank test carried out with the test portion as reference. Read the corresponding mass of silicon dioxide from the calibration curve.

2.5.5 Preparation of the calibration curve

2.5.5.1 For SiO2 contents of 0.10 % to 1.00 %

2.5.5.1.1 Pipette 0 mL, 1.00 mL, 2.00 mL, 3.00 mL, 4.00 mL and 5.00 mL of silicon dioxide standard solution B (2.2.11) into a series of 100 mL volumetric flasks, add 5 mL of hydrochloric acid (2.2.4), dilute with water to 35 mL and mix. Proceed as in 2.5.4.5 to 2.5.4.6.

2.5.5.1.2 Transfer part of the coloured solution into a 3 cm cell and measure the absorbance with a spectrophotometer at a wavelength of 810 nm, using the reagent blank coloured solution as reference. Plot the calibration curve with the mass of silicon dioxide as abscissa and absorbance as ordinate.

2.5.5.2 For SiO2 contents of more than 1.00 % to 10.00 %

2.5.5.2.1 Pipette 0 mL, 1.00 mL, 2.00 mL, 3.00 mL, 4.00 mL, 5.00 mL and 6.00 mL of silicon dioxide standard solution A (2.2.10) into a series of 100 mL volumetric flasks, add 5 mL of hydrochloric acid (2.2.4), dilute with water to 35 mL and mix. Proceed as in 2.5.4.5 to 2.5.4.6.

2.5.5.2.2 Transfer part of the coloured solution into a 1 cm cell and measure the absorbance with a spectrophotometer at a wavelength of 810 nm, using the reagent blank coloured solution as reference. Plot the calibration curve with the mass of silicon dioxide as abscissa and absorbance as ordinate.

2.6 Calculation of results

The silicon dioxide content is expressed as mass fraction wSiO2, in %, and calculated using formula (1):

wSiO2 = (m1 · V0 × 10^-6) / (m0 · V1) × 100 (1)

where:

m1 - mass of silicon dioxide read from the calibration curve, in micrograms (µg);

V0 - total volume of the test solution, in millilitres (mL);

m0 - mass of the test portion, in grams (g);

V1 - volume of the aliquot of the test solution, in millilitres (mL).

The result is expressed to two decimal places.

2.7 Precision

2.7.1 Repeatability

The absolute difference between two independent test results obtained under repeatability conditions, within the range of mean values given below, shall not exceed the repeatability limit (r) in more than 5 % of cases. The repeatability limit (r) is obtained by linear interpolation from the data in Table 2.

Table 2 - Repeatability limits: wSiO2 0.10 %, r 0.03 %; wSiO2 1.30 %, r 0.14 %; wSiO2 2.49 %, r 0.17 %; wSiO2 6.57 %, r 0.23 %; wSiO2 10.33 %, r 0.28 %.

2.7.2 Reproducibility

The absolute difference between two independent test results obtained under reproducibility conditions, within the range of mean values given below, shall not be greater than the reproducibility limit (R) in more than 5 % of cases. The reproducibility limit (R) is obtained by linear interpolation from the data in Table 3.

Table 3 - Reproducibility limits: wSiO2 0.10 %, R 0.04 %; wSiO2 1.30 %, R 0.16 %; wSiO2 2.49 %, R 0.32 %; wSiO2 6.57 %, R 0.33 %; wSiO2 10.33 %, R 0.34 %.

3.1 Principle

The test portion is fused with potassium hydroxide and leached with water. In a strongly acidic solution and a saturated potassium chloride medium, potassium fluoride reacts with silicate ions to form a potassium fluorosilicate precipitate. The potassium fluorosilicate is hydrolysed in hot water and titrated with sodium hydroxide standard solution using phenol red as indicator.

3.2 Reagents

Unless otherwise stated, only reagents of recognised analytical grade and grade 2 water shall be used in the analysis.

3.2.1 Potassium hydrogen phthalate (C8H5KO4, primary standard reagent).

3.2.2 Potassium hydroxide.

3.2.3 Potassium chloride.

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

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

3.2.6 Anhydrous ethanol.

3.2.7 Potassium fluoride solution (300 g/L).

3.2.8 Potassium chloride solution (50 g/L): dissolve 50 g of potassium chloride (3.2.3) in 500 mL of water, then add 500 mL of anhydrous ethanol (3.2.6) and mix.

3.2.9 Sodium hydroxide standard titration solution [c(NaOH) = 0.1 mol/L].

3.2.9.1 Preparation: dissolve 4 g of sodium hydroxide in water that has been boiled and cooled, dilute to 1 000 mL with this water and mix.

3.2.9.2 Standardisation: weigh three portions of 0.600 0 g of potassium hydrogen phthalate (3.2.1) (previously dried to constant mass at 105 °C to 110 °C and cooled to room temperature in a desiccator) into 250 mL conical flasks, add 100 mL of hot water previously boiled to remove carbon dioxide and dissolve completely; after cooling, add 5 drops of phenolphthalein ethanol solution (3.2.10) and titrate with the sodium hydroxide standard titration solution (3.2.9) until the solution turns pink and the colour persists for 30 s. Carry out a blank test at the same time.

The actual concentration of the sodium hydroxide standard titration solution is calculated using formula (2):

c = m / (M1 × (V3 - V2) × 10^-3) (2)

where:

c - concentration of the sodium hydroxide standard titration solution, in moles per litre (mol/L);

m - mass of potassium hydrogen phthalate, in grams (g);

M1 - molar mass of potassium hydrogen phthalate (C8H5KO4), 204.2, in grams per mole (g/mol);

V3 - volume of sodium hydroxide standard titration solution consumed in titrating the potassium hydrogen phthalate solution, in millilitres (mL);

V2 - volume of sodium hydroxide standard titration solution consumed in titrating the blank test solution, in millilitres (mL).

3.2.10 Phenolphthalein ethanol solution (10 g/L).

3.2.11 Phenol red ethanol solution (1 g/L).

3.3 Sample

The particle size of the sample shall not exceed 0.074 mm.