General Hardness (GH) measures the concentration of certain dissolved minerals in water. It represents the total amount of specific metal ions dissolved as water passes through soil and rock formations. GH is a fundamental indicator of water quality, particularly for biological and industrial applications. Understanding GH is important because the mineral content directly influences the water’s chemical behavior and its suitability for living systems.
The Chemical Basis of General Hardness
GH is chemically defined by the presence of divalent cations (positively charged ions with a valence of two). The majority of GH consists of dissolved Calcium (\(Ca^{2+}\)) and Magnesium (\(Mg^{2+}\)) ions. Water typically picks up these minerals by flowing over and dissolving common rocks like limestone and dolomite.
GH is distinct from total dissolved solids (TDS), which measures all dissolved substances, including salts and organic compounds. While Calcium and Magnesium are the primary determinants, other divalent ions like Iron and Manganese may contribute in some locations.
General Hardness Versus Carbonate Hardness
General Hardness (GH) is often confused with Carbonate Hardness (KH), sometimes called alkalinity. GH measures the concentration of metallic ions, specifically Calcium and Magnesium, which are necessary for biological processes and cause water to be “hard.”
KH measures the concentration of carbonate and bicarbonate ions (\(CO_3^{2-}\) and \(HCO_3^{-}\)). These ions function as buffers, allowing the water to neutralize acids and resist sudden changes in pH. This neutralizing capacity is distinct from the mineral content measured by GH.
Water can have a high GH but a low KH, or vice versa, depending on the source rock. For example, water high in Calcium Sulfate has high GH but low buffering capacity. GH represents the quantity of mineral content, while KH represents the water’s stability against acidity fluctuations.
Measuring and Classifying GH Levels
GH is quantified using several units, most commonly parts per million (ppm) or milligrams per liter (mg/L). These units are numerically interchangeable and represent the mass of hardness minerals per volume of water. The degree of General Hardness (dGH), also known as German hardness, is another widely used unit, especially in biological systems. One dGH is approximately equivalent to 17.9 ppm.
Water quality is classified based on these measurements:
- Soft water registers between 0 and 60 ppm.
- Moderately hard water falls between 61 and 120 ppm.
- Hard water is from 121 to 180 ppm.
- Very hard water is anything above 180 ppm.
Common methods for measuring GH involve using liquid titration kits or simple test strips that change color based on mineral concentration.
The Practical Impact of General Hardness
GH levels have significant implications for both biological and domestic systems. For aquatic life, the correct GH range is required for osmoregulation, the process by which fish regulate water and salt balance. Calcium and Magnesium are also necessary for bone development in fish and for molting in crustaceans like shrimp.
Plants rely on these minerals for growth; calcium aids in cell structure, and magnesium is a component of chlorophyll. If GH levels are too low, organisms can suffer from mineral deficiencies. Extremely high levels can create osmotic stress.
In household settings, high GH causes limescale, a hard deposit that forms inside pipes, water heaters, and appliances. This scale buildup reduces the energy efficiency of heating elements and decreases equipment lifespan. Hard water also reduces the effectiveness of soap because calcium and magnesium ions react to form soap scum.