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R9 Laboratory SOP 401
Nephelometric Turbidity Measurement
Summary
This method describes the procedure for measuring turbidity in water samples using a Hach Model 2100N Laboratory Turbidimeter. The system is calibrated with four primary turbidity standards plus a blank. The calibration is checked using two secondary standards prior to measuring samples. The method is based on light scattering of a sample compared to that of reference standards. The nephelometric instrument light source illuminates the sample and the intensity of the scattered light is measured at right angles to the path of the incident light for samples <40 NTU or by ratioing multiple detectors for samples >40 NTU.
This Standard Operating Procedure (SOP) applies to all drinking water samples submitted for turbidity analysis and to determine whether drinking water samples submitted for trace metal analysis require digestion, and is also applicable for measurement of turbidity in other types of water samples, such as ground and surface waters, submitted under other EPA programs.