Total dissolved solids (TDS) comprise inorganic salts and small amounts of organic matter that are dissolved in water. TDS in drinking-water originates from natural sources, sewage, urban runoff and industrial wastewater. Concentrations of TDS in water vary considerably in different geological regions owing to differences in the solubilities of minerals.

A measure of water’s capability to pass electrical flow. This is directly related to the concentration of ions in the water. These come from dissolved salts and inorganic materials such as alkalis, chlorides, sulfides and carbonate compounds. The more ions that are present, the higher the conductivity of water. Pure water has a specific conductivity of 0.055 μS/cm at 25°C

pH is a measure of the hydrogen ion concentration of water. It is measured on a logarithmic scale from 0 to 14. A pH of 7 is neutral, greater than 7 is alkaline, and less than 7 is acidic.

Hard water requires more soap than soft water to obtain a lather. It can also cause scale to form on hot water pipes and fttings. Hardness is caused primarily by the presence of calcium and magnesium ions, although other cations such as strontium, iron, manganese and barium can also contribute.

A measure of the capacity of water to resist a change in pH that would tend to make the water more acidic.