*3.2.5 Chlorine residual*

*Water Quality - Science, Assessments and Policy*

chemical reactions and biological activities [10].

using phenolphthalein as an indicator [10, 20].

in the following equation, the possibility of OH<sup>−</sup> and HCO3

OH<sup>−</sup>

+ HCO3

0.02 N) using selective indicators (methyl orange or phenolphthalein).

possible because they react together to produce CO3

Acidity is the measure of acids in a solution. The acidity of water is its quantitative capacity to neutralize a strong base to a selected pH level [10]. Acidity in water is usually due to carbon dioxide, mineral acids, and hydrolyzed salts such as ferric and aluminum sulfates [10]. Acids can influence many processes such as corrosion,

Carbon dioxide from the atmosphere or from the respiration of aquatic organisms causes acidity when dissolved in water by forming carbonic acid (H2CO3). The level of acidity is determined by titration with standard sodium hydroxide (0.02 N)

The alkalinity of water is its acid-neutralizing capacity comprised of the total of all titratable bases [10]. The measurement of alkalinity of water is necessary to determine the amount of lime and soda needed for water softening (e.g., for corrosion control in conditioning the boiler feed water) [22]. Alkalinity of water is mainly caused by the presence of hydroxide ions (OH<sup>−</sup>), bicarbonate ions (HCO3<sup>−</sup>),

<sup>−</sup> <sup>→</sup> CO3

Alkalinity is determined by titration with a standard acid solution (H2SO4 of

The high levels of either acidity or alkalinity in water may be an indication of industrial or chemical pollution. Alkalinity or acidity can also occur from natural sources such as volcanoes. The acidity and alkalinity in natural waters provide a buffering action that protects fish and other aquatic organisms from sudden changes in pH. For instance, if an acidic chemical has somehow contaminated a lake that had natural alkalinity, a neutralization reaction occurs between the acid and alkaline substances; the pH of the lake water remains unchanged. For the protection of aquatic life, the buffering capacity should be at least 20 mg/L as calcium

Chloride occurs naturally in groundwater, streams, and lakes, but the presence of relatively high chloride concentration in freshwater (about 250 mg/L or more) may indicate wastewater pollution [7]. Chlorides may enter surface water from several sources including chloride-containing rock, agricultural runoff, and

Chloride ions Cl<sup>−</sup> in drinking water do not cause any harmful effects on public health, but high concentrations can cause an unpleasant salty taste for most people. Chlorides are not usually harmful to people; however, the sodium part of table salt has been connected to kidney and heart diseases [25]. Small amounts of chlorides

Sodium chloride may impart a salty taste at 250 mg/L; however, magnesium or calcium chloride are generally not detected by taste until reaching levels of

are essential for ordinary cell functions in animal and plant life.

<sup>2</sup><sup>−</sup>), or a mixture of two of these ions in water. As stated

<sup>2</sup><sup>−</sup> ions:

<sup>−</sup> ions together are not

2− + H2O (8)

*3.2.2 Acidity*

*3.2.3 Alkalinity*

carbonate.

*3.2.4 Chloride*

wastewater.

and carbonate ions (CO3

**10**

Chlorine (Cl2) does not occur naturally in water but is added to water and wastewater for disinfection [10]. While chlorine itself is a toxic gas, in dilute aqueous solution, it is not harmful to human health. In drinking water, a residual of about 0.2 mg/L is optimal. The residual concentration which is maintained in the water distribution system ensures good sanitary quality of water [11].

Chlorine can react with organics in water forming toxic compounds called trihalomethanes or THMs, which are carcinogens such as chloroform CHCl3 [11, 22]. Chlorine residual is normally measured by a color comparator test kit or spectrophotometer [10].
