CWEA Grade 2 Lab Analyst Practice Test

Which analyte requires filtration immediately?

Nitrite DO Sulfate Phosphate

Correct Answer:

Phosphate

Explanation:

Phosphate is the one that needs filtration right away because the analysis typically targets the dissolved orthophosphate fraction, not what’s attached to or trapped in particulates. Filtering immediately removes particulates and prevents phosphate from adsorbing onto solids, precipitating, or being altered by microbial activity, which would change the measured concentration if you waited. This ensures you’re measuring the dissolved portion that the standard method is designed to quantify. In contrast, DO is measured directly in the sample (often with prompt preservation methods), and while nitrite and sulfate can be filtered in some methods, immediate filtration isn’t a universal requirement for their routine determinations.

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