Formazine (formazin) is the polymer produced when hydrazine sulfate and hexamethylenetetramine are reacted. A suspension of 1.25 mg/L hydrazine sulfate and 12.5 mg/L hexamethylenetetramine in water has a turbidity of one Formazin Turbidity Unit (FTU).
For turbidity measurement, a formazine suspension is made by mixing solutions of 5 g/L hydrazine sulfate and 50 g/L hexamethylenetetramine with ultrapure 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. Errors caused by temperature variations are on the order of 1..2% per °C. Another critical variable is the purity of the water used. Experience has shown that water filtered as directed will have a residual scatter of about 0.02 FTU = 20 mFTU (inherent brightening effect). This should be taken into account during calibration and in the detection of very low turbidity levels.
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