The current standard method for measurement of turbidity depends upon instruments that employ the principles of nephelometry. In the instrument, a light source illuminates the sample and one or more photoelectric detectors are used with a readout device to indicate the intensity of scattered light at right angles to the path of the incident light. It is customary to use a particular formazin polymer suspension as a standard, the stock solutions for which should be prepared monthly. Commercially chapter 13 Turbidity
available preparations such as styrene divinylbenzene beads may also be used. Details are given in "Standard Methods." When using the formazin standard, 40 NTU are about equivalent to 40 Jackson candle turbidity units (JTU). Turbidities as low as o,02 NTU can be determined by this procedure providing that water with sufficiently low turbidity can be obtained for use in instrument calibration. Samples with turbidities greater than 40 NTU are diluted with turbidity-free water until values within the range of 30 to 40 NTU are obtained. The turbidity is then determined by multiplying the measured turbidity by the dilution factor. In this manner, a single instrument can be used to measure a very broad range of turbidity values. In addition, the instruments can be automated to provide a continuous reading of water turbidity for process control.
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