Hard water is a term used to describe water containing cations of calcium, magnesium, strontium, iron, manganese, etc. Since the amount of strontium, iron, and manganese in water is very low, calcium and magnesium cations are considered the main cause of water hardness.

The factor affecting the corrosion and sedimentation of water is alkalinity. For example, the alkalinity of water used in steam boilers must be high enough to prevent corrosion of the boiler walls.

TDS stands for total dissolved solids, which is the sum of the concentrations of all ions in water. Dissolved substances in water may be “organic” or “inorganic” in nature. Total dissolved solids (TDS) cannot be considered the main criterion for determining water quality and is more a measure of the degree of clarity of water.

Chemically, the alkalinity of water indicates its capacity to be neutralized by an acid. Neutralization of water means that the pH of the water reaches about 4.5. The alkalinity of natural waters is due to the presence of hydroxides, carbonates, and bicarbonates.

Chloride, in the form of the chloride ion (Cl-), is one of the most important mineral anions in water and wastewater. The salty taste varies with the chloride concentration and is dependent on the chemical composition of the water. Some waters containing 250 mg Cl-/L may have a detectable salty taste if the sodium cation is present.

pH measurement is one of the most important and common tests in water chemistry. Almost every step in water supply and wastewater treatment (e.g., acid-base neutralization, water softening, sedimentation, coagulation, disinfection, and corrosion control) depends on pH. pH is used in the measurement of alkalinity and carbon dioxide and many other acid-base equilibria. At a given temperature, the intensity of the acidic or basic activity of a solution is indicated by pH, or hydrogen ion activity.

Water hardness is caused almost entirely by calcium and magnesium ions. Other divalent and trivalent metals have a similar effect, but are not usually present in sufficient concentrations in drinking water to cause problems. High hardness prevents the formation of soap scum and can cause scale in water systems, especially boilers, cooling systems, and fresh water piping.

The electrical conductivity (EC) of water is a measure of the ability of water to conduct an electric current. This ability is directly related to the concentration of conductive ions in the water. These conductive ions are caused by the presence of inorganic substances such as chlorides, alkalis, carbonate compounds, and sulfides, and dissolved salts. Most metals are excellent conductors of electricity because of the large number of free electrons.