GB/T 5195.1-2017Fluorspar—Determination of calcium fluoride content—EDTA titration method and distillation-potentiometric titration method (English PDF)

3.1 Principle

The sample is leached with dilute acetic acid containing calcium, filtered, decomposed by one of the following two methods.

3.2 Reagents

Unless otherwise specified in the analysis, only use approved analytical reagents and distilled water of grade 3 or higher that meets the requirements of GB/T 6682 or water of equivalent purity.

3.2.1 Sodium carbonate-boric acid mixed flux. take two parts of anhydrous sodium carbonate and one part of boric acid; grind and mix it uniformly.

3.2.2 Hydroxylamine hydrochloride.

3.2.3 Absolute ethanol.

3.2.8 Potassium hydroxide solution, 200 g/L.

3.2.9 Magnesium sulfate solution, 5 g/L.

3.2.10 Triethanolamine, 1 + 2.

3.2.11 Calcium fluoride standard solution, 0.0015601 g/mL

Weigh 1.0008 g of calcium carbonate ( > 99.99%) that has been pre-dried at 105 °C ~ 110 °C for 2 h and placed in a desiccator to cool to room temperature in a 250 mL beaker; cover a watch glass; slowly add 25 mL of hydrochloric acid (3.2.5). After the calcium carbonate is dissolved, add 100 mL of water; heat to boil it; drive off the carbon dioxide; cool to room temperature. Transfer the solution into a 500 mL volumetric flask; use water to dilute to the mark; mix it uniformly. 1.00 mL of this calcium standard solution is equivalent to 0.0015601 g calcium fluoride.

3.2.13 Mix the indicator, weigh 0.20 g of calcein (C30H26N2O13), 0.12 g of thymolphthalein (C28H30O4), and 20 g of anhydrous potassium sulfate in a mortar; grind them fine and evenly; transfer to a suitable container; place it at 105 °C ± 5 °C to dry it for 1 h and cool it down. Store in a mouth-ground bottle for later use.

3.4 Sample preparation

3.4.1 Prepare specimen according to GB/T 22564

3.4.2 The specimen shall be processed to a particle size less than 0.063 mm, dried at a temperature of 105 °C ± 5 °C for 2 h, placed in a desiccator to cool to room temperature.

3.5 Analytical procedures

3.5.1 Number of determinations

For the same specimen (3.4.2), make at least 2 independent determinations.

3.5.2 Sample amount

Weigh 0.50 g of specimen, accurate to 0.0001 g.

3.5.3 Blank test

Do a blank test with the sample.

3.5.4 Sample decomposition

3.5.4.1 Alkali fusion

4.1 Principle

In the presence of perchloric acid, the fluorine in the sample is separated by distillation with water vapor at 135 °C ± 2 °C through a temperature control device; the distillate is absorbed by sodium hydroxide solution; the fluoride ion selective electrode is used as the indicator electrode; the lanthanum nitrate standard titration solution is used to titrate the amount of fluorine in the distillate, to calculates the mass fraction of calcium fluoride.

4.2 Reagents

Unless otherwise specified in the analysis, only use approved analytical reagents and distilled water of grade 3 or higher that meets the requirements of GB/T 6682 or water of equivalent purity.

4.2.1 Potassium permanganate, crystalline.

4.2.3 Ethanol or isopropanol.

4.2.4 Perchloric acid, rho = 1.67 g/mL.

4.2.5 Perchloric acid, 1 + 6.

4.2.6 Sodium hydroxide solution, 40 g/L, stored in a plastic bottle.

4.2.8 Buffer solution, pH 6.5

Dissolve 140 g of hexamethylenetetramine in about 800 mL of water; use perchloric acid (4.2.5) to neutralize to a pH of 6.5 ± 0.2; use water to dilute it to 1000 mL; if necessary, adjust the pH to 6.5.

4.2.9 Phenolphthalein solution, 5 g/L, prepared with 95% ethanol (volume fraction).

4.3 Device

4.3.1 General laboratory equipment and gas generation and absorption devices

4.3.2 The distillation device consists of a steam generator, a distillation flask, a condenser tube, two electric heating jackets, a contact thermometer with a relay, as shown in Figure 1, which can control the temperature of the distillation flask to 135 °C ± 2 °C.

4.3.3 Magnetic stirrer

4.3.4 pH meter, equipped with glass electrode and saturated calomel electrode.

4.3.5 Burette, 20 mL, graduation interval 0.02 mL

4.3.6 Fluoride ion selective electrode

4.3.7 Reference electrode, saturated calomel electrode or other types of electrodes.

4.5.5 Potentiometric titration

Pipette 50.00 mL of test solution from a 500 mL volumetric flask in a 250 mL beaker; add 10 mL of buffer solution (4.2.8) and 60 mL of ethanol (or isopropanol) (4.2.3). Put the rotor of the magnetic stirrer into the beaker; place the beaker on the stirrer; insert the fluorine ion selection electrode (4.3.6) and the reference electrode (4.3.7); connect the potentiometer (4.3.8). Start the magnetic stirrer; use lanthanum nitrate standard titration solution (4.2.7) to titrate it; the titration speed is not more than 3.0 mL/min; record the volume of the titration and the corresponding potential reading. When the potential change speed increases, titrate at a slower speed. Determine the end point of the titration from the drawn or recorded titration curve.

4.6 Calculation and presentation of results

4.7 Precision

The difference between two independent measurement results under repeatability conditions is not greater than the repeatability limit r specified in Table 2.