抄録
Sodium carbonate, a primary standard of volumetric analysis, is used as a standard of hydrochloric acid and sulfuric acid in Japan, and is certified in different countries by using different drying conditions. The drying conditions for reference materials have a significant effect on the titration results due to changes in the acidimetric factor. The drying conditions for sodium carbonate were investigated by thermogravimetry-mass spectrometry (TG-MS), gravimetric titration, mass and pH measurements. In TG-MS analysis, about 29 mg of sodium carbonate was heated from room temperature to 700°C. The loss through heat was 2.1%, and the amount of water content detected was 1.8%. Most of the weight lost through heat was caused by releasing water, which was produced at about 100°C, and carbon dioxide was released at about 100°C and 600°C. The weight of sodium carbonate dried at different temperatures was decreased, which also indicates the decomposition of sodium carbonate under high-temperature conditions. Sodium carbonate samples dried at 100°C to 700°C for 1 h were gravimetrically cross-titrated with each other using 1.5 mol/L hydrochloric acid as an intermediate titrant. The effective purity of sodium carbonate was significantly increased at over 400°C. In addition, the pH value of carbonate-free solutions of sodium carbonate dried at different temperatures prepared by precipitation as a barium carbonate was increased, especially at over 500°C. It was concluded that suitable drying conditions were 200°C∼400°C, and the optimum condition was approximately 300°C for 2 h.
