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Turbidity Standards – Quick Reference Guide

Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.


Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of the container if a liquid sample is left to stand (the settable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid.

Drinking water standards

Governments have set standards on the allowable turbidity in drinking water. In the United States, systems that use conventional or direct filtration methods turbidity cannot be higher than 1.0 nephelometric turbidity units (NTU) at the plant outlet and all samples for turbidity must be less than or equal to 0.3 NTU for at least 95 percent of the samples in any month. Systems that use filtration other than the conventional or direct filtration must follow state limits, which must include turbidity at no time exceeding 5 NTU. Many drinking water utilities strive to achieve levels as low as 0.1 NTU.

The standard used for measuring turbidity is Formazine –

Formazine is a suspension made by mixing together solutions of 5 g/L hydrazine sulfate and 50 g/L hexamethylenetetramine with ultra pure water. The resulting solution is left for 24 hours, at 25°C ±3°C, for the suspension to develop. This produces a suspension with a turbidity value of 4000 NTU/FAU/FTU/FNU. This is then diluted to a value to suit the instrument range. There is no satisfactory relationship between FTU/FAU and NTU/FNU because this will depend on the optical characteristics of the particular matter in the sample. It is well known that when preparing formazine standards, great care is often needed to obtain reproducible and accurate results. It is extremely important to observe the prescribed preparation temperature, because it affects the particle size of the formazine particles perceptibly.

Types of Formazine Standards –

  • StablCal Turbidity standards can be used in all turbidimeters- they are accepted for calibration bu USEPA and are stabilised to provide true Formazine standard dilutions. Formazine is stable at 4000NTU’s.
  • Gelex Standards are used for Hach 2100P/Q  portable Turbidimeters- The vials are sealed and cover the ranges 0-10 NTU, 10-100 NTU and 100-1000 NTU.