What is the primary use of lead acetate paper in environmental testing?

Lead acetate paper is primarily used to detect hydrogen sulfide gas due to its chemical properties. When lead acetate paper is exposed to hydrogen sulfide, it undergoes a chemical reaction that results in a darkening of the paper, indicating the presence of this gas. This method is highly effective for monitoring air quality, especially in environments where hydrogen sulfide emissions might occur, such as in sewage treatment plants or near geothermal areas.

The other options, while relevant to environmental testing, do not apply to lead acetate paper. For example, determining the presence of lead in water typically involves more complex analytical procedures like atomic absorption or inductively coupled plasma methods. Measuring pH levels generally requires specific pH indicators or probes that directly respond to the acidity or alkalinity of a solution. Analyzing soil composition involves different methods, such as spectroscopy or chemical extraction processes, rather than using lead acetate paper. Thus, the unique application of lead acetate paper for detecting hydrogen sulfide makes it the correct choice in this context.