【Daily Question】Chemical Analysis Version, 2019.9.23

To standardize 0.100 mol/L hydrochloric acid with reference anhydrous sodium carbonate, (B) is the appropriate indicator to use. A. Bromocresol green-methyl red B. Phenolphthalein C. Thymol blue D. Xylenol orange. It is recommended to use phenolphthalein as the reagent. In the initial hydrochloric acid solution, the acidic environment causes phenolphthalein to remain colorless; when the titration endpoint is reached, the solution is still colorless. If a slight excess of reagent is used, the hydrolysis of carbonate ions in sodium carbonate makes the solution alkaline, causing it to turn pink – at this point, the titration should be stopped. Phenolphthalein is a chemical product in the form of a crystalline powder that is almost insoluble in water. Its characteristic is that it is colorless in acidic and neutral solutions, and purplish-red in alkaline solutions. It is often used by people to test acidity and alkalinity.