GB/T 35069-2018Coke—Determination of phosphorus content—Reduced molybdophosphate spectrophotometric method (English PDF)

4.6 Sulfuric acid solution, 1+1

4.7 Nitric acid-bismuth nitrate solution, 10 g/L. Use nitric acid (1+3) to prepare it.

4.8 Sodium thiosulfate-anhydrous sodium sulfite solution; 100 mL of solution contains 0.2 g of sodium thiosulfate and 1 g of anhydrous sodium sulfite solution.

4.9 Ascorbic acid ethanol solution, 20 g/L. Weigh -take 20 g of ascorbic acid; dissolve it in 1 L of ethanol solution (1+1).

4.10 Ammonium molybdate-potassium sodium tartrate solution; 100 mL of solution contains 3.5 g of ammonium molybdate and 6.5 g of potassium sodium tartrate.

4.11 Phosphorus standard solution. weigh-take 0.4394 g of potassium dihydrogen phosphate that is previously dried at 105 °C to a constant weight. Use a proper amount of water to dissolve it; add 10 mL of nitric acid, then, transfer it into a 1,000 mL volumetric flask. Use water to dilute to the constant volume, then, mix it up. Transfer 25.00 mL into a 500 mL volumetric flask; use water to dilute to the constant volume, then, mix it up. 1 mL of this solution contains 5.0 µg of phosphorus.

7.3.2 Alkali melting decomposition method

Add the sample (please refer to 7.1) to a platinum crucible, which already has 3 g ~ 4 g of boric acid-anhydrous sodium carbonate flux (please refer to 4.5); mix it up. Then, cover it with 2 g ~ 3 g of boric acid-anhydrous sodium carbonate flux (please refer to 4.5). Place it into a 950 °C ~ 1,000 °C Muffle furnace; start high-temperature melting for 10 min ~ 15 min. Take it down to cool it down, then, use water to rinse the external wall of the platinum crucible. Place the platinum crucible into a 250 mL beaker which holds 50 mL of water. Take 10 mL of sulfuric acid solution (please refer to 4.6), then, heat it up, start leaching; clean the crucible. Continue the heating, till it boils, then, cool it down to room temperature. Transfer it into a 100 mL volumetric flask; use water to dilute to the constant volume, then, mix it up.

7.4 Color Development and Determination

Divide-take 10.00 mL of solution (please refer to 7.3), then, place it into a 50 mL volumetric flask. Add 15 mL of nitric acid-bismuth nitrate solution (please refer to 4.7), 5 mL of sodium thiosulfate-anhydrous sodium sulfite solution (please refer to 4.8), 10 mL of ascorbic acid ethanol solution (please refer to 4.9) and 5 mL of ammonium molybdate-potassium sodium tartrate solution (please refer to 4.10). After placing it for 1 min ~ 2 min, use water to dilute to the constant volume, then, mix it up. Place it at room temperature for over 15 min. On a spectrophotometer, use a 1 cm ~ 3 cm colorimetric cuvette; at the wavelength of 700 nm, take blank test solution as a reference. Measure the absorbance of the color developing solution; obtain corresponding phosphorus content through the calibration curve.

NOTE: after adding various reagents in turn, thoroughly mix it up.

7.5 Draw a Calibration Curve

Respectively transfer-take 0 mL, 0.50 mL, 1.00 mL, 2.00 mL, 4.00 mL, 6.00 mL and 8.00 mL of phosphorus standard solution (please refer to 4.11); place it in a group of 50 mL volumetric flasks; comply with the operation (in 7.4). Take phosphorus-free standard solution as a reference; take the measured absorbance as the y-coordinate; take the mass of phosphorus in the standard series solution as the x-coordinate to draw a calibration curve.

8.1 Calculation of Analysis Result

In accordance with Formula (1), calculate the content of phosphorus in coke; calculate by mass fraction; the numerical value shall be expressed in %.