Formazine
Standard synthetic colloid for calibrating turbidity measurements.
Formazine (also spelled formazin) is a heterocyclic polymer that forms when hexamethylenetetramine reacts with hydrazine sulfate. Its molecular structure is built from a tetrahedral cage-like unit—similar to adamantane—which links into a three-dimensional polymer network. Because it is very poorly soluble in water, synthesizing it directly in an aqueous solution by mixing its two highly soluble precursors yields small colloidal particles. These organic colloids scatter light in all directions, and the optical behavior of such suspensions depends on the size and size distribution of the particles. Since formazine is a stable synthetic material with a uniform particle size, it is widely used as a standard for calibrating turbidimeters and for checking the reproducibility of their readings. The use of formazine for this purpose was first suggested in 1926 by Kingsbury and colleagues, who needed a rapid way to standardize turbidity measurements of albumin in urine. The resulting unit is called the Formazin Turbidity Unit (FTU). A suspension made from 1.25 mg/L hydrazine sulfate and 12.5 mg/L hexamethylenetetramine in water has a turbidity of one FTU. In U.S. environmental monitoring, the standard unit is the Nephelometric Turbidity Unit (NTU), while the international standard unit is the Formazin Nephelometric Unit (FNU). The most generally applicable unit remains the FTU, though different measurement methods can yield quite different values when reported in FTU.
To prepare a formazine suspension for turbidity measurement, solutions of 10 g/L hydrazine sulfate and 100 g/L hexamethylenetetramine are mixed with ultrapure water. The mixture is left for 48 hours at 25 °C ±1 °C to allow the suspension to develop, producing a turbidity of 4000 NTU/FAU/FTU/FNU. This stock suspension is then diluted to match the instrument’s range. There is no straightforward conversion between FTU/FAU and NTU/FNU because the relationship depends on the optical characteristics of the particular matter in the sample. A common challenge in preparing formazine standards is achieving sufficiently reproducible and accurate results. Temperature during preparation is critical, as it affects particle size; uncertainties from temperature fluctuations are about 1.2% per °C. The purity of the water used is also important, since it must initially contain no colloidal particles.
- Composition
- heterocyclic polymer from hexamethylenetetramine and hydrazine sulfate
- Turbidity unit
- Formazin Turbidity Unit (FTU)
- Standard suspension for 1 ftu
- 1.25 mg/L hydrazine sulfate and 12.5 mg/L hexamethylenetetramine in water
- Preparation concentration
- 10 g/L hydrazine sulfate and 100 g/L hexamethylenetetramine yields 4000 NTU/FAU/FTU/FNU
- Preparation time and temperature
- 48 hours at 25 °C ±1 °C
- Temperature sensitivity
- 1.2% per °C
- Residual scatter of filtered water
- 0.02 FTU = 20 mFTU
Lore & Background
Formazine is synthesized directly in aqueous solution by mixing its two highly soluble precursors, hexamethylenetetramine and hydrazine sulfate. The hexamethylenetetramine tetrahedral cage-like structure, similar to adamantane, serves as a molecular building block to form a tridimensional polymeric network. Because formazine is very poorly soluble in water, it forms small colloidal particles that scatter light in all directions, with optical properties depending on particle size and size distribution.
The use of formazine as a turbidity standard was first proposed by Kingsbury et al. (1926) for the rapid standardization of turbidity measurements of albumin in urine. In the United States environmental monitoring, the standard unit is called Nephelometric Turbidity Units (NTU), while the international standard unit is Formazin Nephelometric Unit (FNU). The most generally applicable unit is Formazin Turbidity Unit (FTU), although different measurement methods can give quite different values as reported in FTU.
A general difficulty in preparing formazine standards is obtaining sufficiently reproducible and accurate results. Preparation temperature is essential because it affects particle size, with uncertainties of about 1.2% per °C. The purity of water used is also critical, as it cannot initially contain colloidal particles; filtered water has a residual scatter of about 0.02 FTU. Commercially available aqueous dispersions are often traceable to the EN ISO 7027 norm, but their shelf life does not exceed a few months (a few weeks if opened) due to Ostwald ripening and possible microbial development.
Reader's Guide
Formazine's significance lies in its role as a primary standard for turbidity measurement, a key water quality parameter. The article notes that a suspension of 1.25 mg/L hydrazine sulfate and 12.5 mg/L hexamethylenetetramine in water has a turbidity of one FTU. For calibration, a concentrated suspension is prepared by mixing 10 g/L hydrazine sulfate and 100 g/L hexamethylenetetramine with ultrapure water, left for 48 hours at 25 °C ±1 °C to develop a turbidity of 4000 NTU/FAU/FTU/FNU, then diluted to suit the instrument range. However, 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. The preparation temperature is critical, with uncertainties of 1.2% per °C, and water purity must be high to avoid residual scatter of about 0.02 FTU. The shelf life of formazine dispersions is limited to a few months (a few weeks if opened) due to ageing of colloidal particles via Ostwald ripening and potential microbial growth. These factors underscore the care needed in using formazine as a reproducible standard, despite its widespread adoption.
Did You Know?
- Formazine is produced by reacting hexamethylenetetramine with hydrazine sulfate.
- The preparation temperature affects formazine particle size, with uncertainties of 1.2% per °C.
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