Hardness is a measure of the soap consuming capacity of water. The term comes from an expression of how difficult or “hard” it is to wash clothes with the water. When soap is mixed with hard water, the minerals combine with the soap and form a solid precipitate. This decreases the cleaning efficiency of the soap and forms soap scum. As more soap is added, solids continue to form until the minerals are depleted. When the minerals are no longer available, the soap forms a lather and works as a cleaning agent.

In general, hard water forms solid deposits comprised of mainly calcium and magnesium salts and can damage equipment, while soft water may be corrosive and therefore, it is important measuring and knowing levels of hardness in your process water to maintain the delicate balance between scaling and corrosivity.

While some hardness may be acceptable in certain water quality applications, others require zero hardness to prevent scaling and damage to equipment. Therefore, water softening by either precipitation or ion exchange is often necessary to remove hardness. To optimize these processes, it is sometimes important to monitor calcium and magnesium levels separately, along with total hardness.