text: For turbidity measurement, a formazine suspension is prepared by mixing solutions of 10 g/L hydrazine sulfate and 100 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 straightforward relationship between FTU/FAU and NTU/FNU because it depends on the optical characteristics of the particular matter in the sample. A general difficulty encountered for preparing formazine standards is to obtain sufficiently reproducible and accurate results. The preparation temperature is essential because it affects the size of the formazine particles. Uncertainties related to temperature fluctuations are of the order of 1.2% per °C.
  The purity of the water used in the preparation of the formazine dispersion is also important as it cannot initially contain colloidal particles. Experience shows that water filtered as required has a residual scatter of about 0.02 FTU = 20 mFTU (inherent brightening effect). This has to be taken into account during calibration and for the detection of very low turbidity levels. The commercially available aqueous dispersions of formazine standard are often traceable according to the EN ISO 7027 norm. The shelf life of formazine dispersions does not exceed a few months (a few weeks if the bottle has been opened) as their characteristics evolve with time due to the ageing of the colloidal particles (Ostwald ripening: change in their size distribution and in their number due to their coalescence/aggregation) and the possible development of micro-organisms such bacteria, microscopic fungi, and yeast.