Formazin is the standard used for measuring what parameter?

Formazin is a polymer compound that is widely used as a standard for measuring turbidity in water samples. Turbidity is a measure of the cloudiness or haziness of water caused by the presence of suspended particles, including sediments, organic matter, and microorganisms.

The reason for utilizing formazin as a standard is that it forms a stable suspension that can be accurately quantified, providing a reliable reference point for turbidity measurements. Laboratory instruments such as turbidimeters use formazin standards to calibrate their readings, ensuring accurate assessments of turbidity in various water sources.

In contrast, other parameters listed, such as pH levels, hardness, and chlorine residual, require different standards and methods for measurement. For example, pH is measured using pH indicators or meters, hardness is typically assessed through titration, and chlorine residual is evaluated using different chemical reagents suitable for chlorination levels. Thus, formazin's specific application to turbidity makes it the correct answer in this context.