Turbidity is a measure of the cloudiness or haziness of a liquid. It is related to the amount of light scattered or absorbed (as opposed to being transmitted) by the sample. The primary standard for turbidity is a Formazin polymer suspension (Group No. 8330), which has a turbidity of 4000 NTU (Nephelometric Turbidity Units) by definition.
Lower-level turbidity standards may be prepared as linear dilutions of the 4000 NTU stock solution, but are less stable. For turbidities below 100 NTU, RICCA CHEMICAL COMPANY uses a proprietary blend of polymer microspheres with a range of particle sizes designed to mimic the particle size distribution in natural water samples.
All turbidity standards should be mixed gently before use to ensure an even distribution of the suspended particles. Overmixing can introduce bubbles or damage the suspended particles, resulting in erroneous readings.
There is no NIST standard for turbidity. All RICCA CHEMICAL COMPANY turbidity standards are traceable to EPA-recognized commercial primary standards.