Formazine turbidity unit

Defining these units of measurement and establishing the formazin standard for turbidity measurement has not prevented the use of other units. For instance, nephelometric turbidity unit (NTU), identical to formazin turbidity unit (FTU) and equal to the FNU, are displayed by many types of commercial turbidimeters used in Romania. Some instruments also indicate Jackson turbidity units (JTU) or the silica unit (mg/1 Si02) [6], that are still found as references today. 

Turbidity is a measure of the light-transmitting properties of water. The measurement of turbidity is based on comparison of the intensity of light scattered by a sample to the light scattered by a reference suspension under the same conditions. The most widely used standards are prepared with hydrazine sulfate and hexamethylenetetramine in water, and give rise to the Formazin Turbidity Units (FTU). 

The unit of turbidity is the turbidity unit (TU) which is equivalent to the turbidity produced by one mg/L of silica (Si02). Si02 was used as the reference standard. Turbidities in excess of 5 TU are easily detected in a glass of water and are objectionable not necessarily for health but for aesthetic reasons. A chemical, for-mazin, that provides a more reproducible result has now replaced silica as the standard. Accordingly, the unit of turbidity is now also expressed as formazin turbidity units (FTU). 

The formazine turbidity unit is defined as the concentration of a standard suspension of formazine. Formazine is a polymeric reaction product of hydrazine sulphate and hexamethylenetetramine. The suspension formed by a reaction of 1.25 mg 1 of hydrazine sulphate and 12.5 mg 1 of hexamethylenetetramine has a turbidity of one formazine unit. Calibration by nephelometric or turbidimetric measurement is carried out so that the relation 1 FT = 1 mg 1 Si02 is valid. 

Unfortunately the EBC and the ASBC have adopted different scales. The EBC mix equal volumes of a 10% (w/v) solution of hexamethylene tetramine and a 1 % (w/v) solution of hydrazine sulphate. After 24 hr this stock suspension (10 ml) is diluted to 100 ml to give a solution which contains 100 EBC Formazin Haze units. In a similar manner the ASBC regard the haze formed from 0 0725 g hydrazine sulphate and 0 725 g hexamethylene tetramine diluted to 100 ml as equivalent to 10 000 Formazin Turbidity units. It must be remembered that haze is not a quantity of substance but an... 

ASBC Formazin Turbidity units = 145 EBC Fonnazin Haze units... 

Turbidity is caused by any solid material which is dissolved or suspended in a liquid. The intensity of light scattered by a sample is measured and compared with that measured for standard formazin suspensions, and expressed as nephelometric turbidity units (NTU). Colour is determined as the absorbance (measured spectrophotometrically at 400 nm) of the sample filtered through a 0.4S micron pore size membrane filter [12, 13]. 

Field meters use the nephelometric method for turbidity measurements and read turbidity in Nephelometric Turbidity Units (NTUs). A nephelometer measures the intensity of light scattered by a sample and compares it to the intensity light scattered by a reference standard. Sample turbidity is proportional to the intensity of the scattered light. The reference standard is a formazin suspension in water, which has a known turbidity value. 

To return to the two main different types of soft drinks one is a clear product, with virtually no or only very slight turbidity visible to the consumer the other type is a turbid product with different degrees of turbidity from slight haze to complete opacity. Figure 5.31 visualises the two categories by measuring the FNU value (Formazin Nephelometric Unit). 

Turbidity is determined by comparison with a standard of formazine using turbidimetry [14], or by nephelometry if mainly colloids are measured [13, 14]. The results are expressed by the formazine unit of turbidity (FT). 

The turbidity of this solution is approximately 00 FNU (formazine nephelo-meter units). The solution can be kept for weeks in the dark at 23 3 C. For calibration purposes the standard formazine solution is diluted to bring it within the measuring range of the water sample to be tested. 

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