Because of the wide variety of materials that cause turbidity in natural waters, it has been necessary to use an arbitrary standard. The original standard chosen was
1 mg Si02/L = 1 unit of turbidity and the silica used had to meet certain specifications as to particle size.
Standard suspensions of pure silica are not now used for measuring turbidity. They were used originally to calibrate the Jackson candle turbidimeter, the former standard instrument for turbidity measurement. This was a rather crude instrument in which the turbidity of a suspension was measured by the depth of suspension through which the outline of a flame from a standard candle just disappeared. The Jackson candle turbidimeter was removed as a standard procedure from the 17th edition of "Standard Methods" as it has generally been replaced in practice by more reliable, sensitive, and easier to use instruments that depend upon the principle of nephelometry. Also, silica as a standard reference material has been replaced by standardized preparations of formazin polymer. The for-mazin suspensions were first calibrated against the Jackson candle turbidimeter, and thus there is some relationship between turbidity measurements by the Jackson candle turbidimeter and nephelometry. However, the Jackson candle turbidimeter measures the interference to light passage in a straight line while nephelometry measures the scattering of light from particles. Because of the basic difference in the phenomena measured, results from the two different procedures on different suspensions can vary widely. In order to avoid any confusion this may cause, turbidity measurements by the standard nephelometry procedure are now reported in NTU.
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