Tennessee Wastewater Operator Practice Exam

What chemical is used to standardize the acid titrant in the alkalinity test?

1. Hydrochloric acid

2. Sodium carbonate

3. Calcium carbonate

4. Potassium hydroxide

The correct answer is: Sodium carbonate

In the context of alkalinity testing, sodium carbonate is used to standardize the acid titrant because it serves as a primary standard. This compound is stable and has a known concentration, allowing for accurate calibration of the titrant solution. During an alkalinity test, a sample of water is titrated with a standard acid solution (the titrant), and sodium carbonate helps to ensure that the acid is accurately measured. The reaction of sodium carbonate with the acid provides a clear endpoint that can be observed, typically indicated by a color change due to a pH indicator. This makes it a reliable choice for standardization. Hydrochloric acid is the titrant rather than a standardizing agent in this scenario. Calcium carbonate is not commonly used for standardization in this context, as it does not provide the same stability and precision when creating a standard solution. Potassium hydroxide, while it is a strong base, does not function as a primary standard for acid titration since it is not acidic in nature and would not be used to standardize an acid titrant. Therefore, sodium carbonate is the appropriate chemical for standardizing the acid titrant in the alkalinity test, ensuring accurate measurement of the water's alkalinity.