GB/T 7731.1-2021 in English

Technical points of cinchonine gravimetric method

Adopting the hydrofluoric acid-nitric acid dissolution system, after fluorine is driven out by sulfuric acid fuming, cinchonine and α-benzoin oxime double precipitants are used to ensure the complete precipitation of tungstic acid. Critical Control Points:

• Use 5g of anhydrous sodium carbonate
for the molten residue
• Settling time ≥4 hours
• Molybdenum interference needs to be corrected through Appendix A

Innovation of ammonium nitrate gravimetric method

The newly added ammonium nitrate method uses roasting oxidation pretreatment (800-850℃) and ammonium nitrate to increase the precipitate particle size, significantly improving the filtration efficiency:

• Ammonium nitrate addition amount 20mL (500g/L)
• Hydrofluoric acid desiliconization step cannot be omitted
• Parallel determination allowable difference ≤0.45%

Requirements for inter-laboratory comparison

According to the acceptance flow chart in Appendix B of the standard, when the difference between two measurement results exceeds the allowable difference in Table 2, it is necessary to:

1. Immediately check the validity period of the reagents (especially the shelf life of the cinchonine solution)
2. Recalibrate the temperature control system of the high-temperature furnace
3. Add a third measurement to take the median value

Safety Warning

The standard specifically emphasizes that hydrofluoric acid operations must be performed in a fume hood, and recommends:

• Equipped with calcium gluconate gel for emergency
• Check for scratches on the platinum dish before use
• Control the heating power during the smoking stage