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TURBIDITY
GENERAL (Information from HACH Company)

The American Public Health Association defines turbidity as expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample.

Turbidity of a liquid is important for many reasons, depending on its use. Turbidity in water can be caused by harmful organisms, particles that feed them, or particles that can shelter them from disinfection processes. To assure a safe drinking water supply, water treatment plants are required by law to maintain a uniform low turbidity of finished product. In other liquids, turbidity can be particulates detrimental to the end use, or perhaps by particles that are vital ingredients of a product. In either case, turbidity can be used as a quality control measure to monitor the efficiency of the treatment or manufacturing process.

Turbidimeters are calibrated in Turbidity Units (TU). Originally, one TU was equal to the turbidity caused by one ppm of suspended silica. The Nephelometric Turbidity Unit (NTU), the most common unit in use, provides reference to the nephelometric measurement technique. The NTU is now traced to the primary standard formazin. The NTU can differ from Jackson Turbidity Unit (JTU) used with the Jackson Candle Turbidimeter, which was      standardized using a suspension of clay or some other natural material.

Formazin is the primary turbidity standard. Formazin can be synthesized in the laboratory or purchased from Hach as a 4000-NTU standard. There are also alternative primary standards available in a ready-to- use form. The alternative standards are StablCal Stabilized Formazin Standards, which are available in values suitable for direct use, without preparation or dilution, and styrene divinylbenzene microspheres suspended in a pure aqueous medium. Of these standards, only formazin is synthesized from raw materials. All other turbidity standards are traced to formazin.

Secondary standards, once their values are determined by use of a primary standard, are used to verify turbidimeter calibration before day-to-day use. Secondary standards are not to be used to perform calibrations. Example: Secondary standards are metal oxides suspended in a gel matrix, glass rods or latex particles.

Three major features of a turbidimeter affect its response to a sample: the light source, the photodetectors and the sample cell (optical geometry). Different light sources have different spectral outputs, producing varying color intensity. Photodetectors also have different spectral characteristics; some are sensitive to near infrared while others have peak sensitivity to the ultraviolet band. In addition, optical geometry between detectors and the sample cell affects factors such as sensitivity and linearity. As a result, turbidimeters may respond      differently to a sample, even though they are calibrated on the same primary formazin standard.

The greatest misunderstanding in turbidity measurement concerns the natural desire to equate the quantity (mg/L or ppm) of a sample's suspended material with the sample's measured turbidity. Turbidity is a measurement of the light-scattering properties of a sample's particulates and particulate makeup and instrument sensitivity directly affect light scattering and its detection. The analyst must restrict direct correlation of turbidity and quantity of suspended matter to cases where several turbidity measurements are made in succession with the same (or same type of) instrument on samples with the same particulate makeup. Even in these cases many samples do not exhibit a linear relationship between ppm suspended matter and turbidity value. For example, a natural sample showing a turbidity of 500 NTU often shows a turbidiy of substantially more than 100 when diluted 5-to-1 with distilled water. This nonlinear response is due the multiple scattering and greater absorption occurring at the higher concentrations.

Generally, measurement of turbidity provides an immediate estimate of the relative quantity of suspended solids. For direct suspended solids measurement s for each type of turbidity material, turbidity readings must be supported by auxiliary measurements and a correlation (calibration) curve must be established.