Physical And Chemical Characteristics

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The constituent physical and chemical characterizations to be discussed include the following: turbidity (physical), color (physical), taste (physical) temperature (physical), chlorides (chemical), fluorides (chemical), iron and manganese (chemical), lead and copper (chemical), nitrate (chemical), sodium (chemical), sulfate (chemical), zinc (chemical), biochemical oxygen demand (chemical), solids (physical), pH (chemical), chemical oxygen demand (chemical), total organic carbon (chemical), nitrogen (chemical), phosphorus (chemical), acidity and alkalinity (chemical), fats and oils and grease (chemical), and odor (physical). The characterization will also include surfactants (physical), priority pollutants (chemical), volatile organic compounds (chemical), and toxic metal and nonmetal ions (chemical). These constituents are discussed in turn in the paragraphs that follow.

2.1.1 Turbidity

Done photometrically, turbidity is a measure of the extent to which suspended matter in water either absorbs or scatters radiant light energy impinging upon the suspension. The original measuring apparatus that measures turbidity, called the Jackson turbidimeter, was based on the absorption principle. A standardized candle was placed under a graduated glass tube housed in a black metal box so that the light from the candle can only be seen from above the tube. The water sample was then poured slowly into the tube until the candle flame was no longer visible. The turbidity was then read on the graduation etched on the tube. At present, turbidity measurements are done conveniently through the use of photometers. A beam of light from a source produced by a standardized electric bulb is passed through a sample vial.

The light that emerges from the sample is then directed to a photometer that measures the light absorbed. The readout is calibrated in terms of turbidity.

The unit of turbidity is the turbidity unit (TU) which is equivalent to the turbidity produced by one mg/L of silica (SiO2). SiO2 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 other method of measurement is by light scattering. This method is used when the turbidity is very small. The sample "scatters" the light that impinges upon it. The scattered light is then measured by putting the photometer at right angle from the original direction of the light generated by the light source. This measurement of light scattered at a 90-degree angle is called nephelometry. The unit of turbidity in nephelometry is the nephelometric turbidity unit (NTU).

2.1.2 Color

Color is the perception registered as radiation of various wavelengths strikes the retina of the eye. Materials decayed from vegetation and inorganic matter create this perception and impart color to water. This color may be objectionable not for health reasons but for aesthetics. Natural colors give a yellow-brownish appearance to water, hence, the natural tendency to associate this color with urine. The unit of measurement of color is the platinum in potassium chloroplatinate (K2PtCl6). One milligram per liter of Pt in K2PtCl6 is one unit of color.

A major provision of the Safe Drinking Water Act (SDWA) is the promulgation of regulations. This promulgation requires the establishment of primary regulations which address the protection of public health and the establishment of secondary regulations which address aesthetic consideration such as taste, appearance, and color. To fulfill these requirements, the U.S. Environmental Protection Agency (USEPA) establishes maximum contaminant levels (MCL). The secondary MCL for color is 15 color units.

2.1.3 Taste

Taste is the perception registered by the taste buds. There should be no noticeable taste at the point of use of any drinking water.

The numerical value of taste (or odor to be discussed below) is quantitatively determined by measuring a volume of the sample A (in mL) and diluting it with a volume B (in mL) of distilled water so that the taste (or odor) of the resulting mixture is just barely detectable at a total mixture volume of 200 mL. The unit of taste (or odor) is then expressed in terms of a threshold number as follows:

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