What does methylene blue active substance (MBAS) primarily identify?

Methylene blue active substance (MBAS) is primarily utilized to identify the presence of anionic surfactants, which are often found in detergents containing phosphates. When anionic surfactants are present in a water sample, they react with methylene blue, leading to a measurable color change that indicates their concentration. This method is particularly important in water quality testing since the presence of detergents can contribute to pollution and affect aquatic life by disrupting the surface tension of water.

In contrast, other options do not align with the role of MBAS. The presence of chlorides, heavy metals, and nitrates is assessed using different analytical methods that specifically target those substances. Chlorides can be measured using titration methods, heavy metals typically require atomic absorption spectrophotometry or inductively coupled plasma methods, and nitrates are often analyzed using colorimetric tests or ion chromatography. Thus, the specific association of MBAS with anionic surfactants in detergents makes it the appropriate answer.