How to understand water quality based on the relationship between hardness and alkalinity?
Hardness and alkalinity in natural water are important indicators of water quality. Hardness mainly refers to the total concentration of calcium ions (Ca²⁺) and magnesium ions (Mg²⁺) in water, while alkalinity is mainly determined by bicarbonate ions (HCO₃⁻). The relationship between the two directly affects the composition of dissolved salts in water, and thus reflects the chemical properties of the water body. According to the concentration ratio of the two, it can be divided into the following three situations:
1. Alkalinity is greater than hardness (in mol/L)
When the HCO₃⁻ concentration in water is higher than half of the total concentration of calcium and magnesium ions (i.e. HCO₃⁻ > ½(Ca²⁺ + Mg²⁺)), it indicates that alkalinity significantly exceeds hardness. At this time, calcium and magnesium ions will preferentially combine with bicarbonate to form calcium bicarbonate (Ca(HCO₃)₂) and magnesium bicarbonate (Mg(HCO₃)₂). The remaining HCO₃⁻ combines with monovalent cations such as sodium (Na⁺) and potassium (K⁺) to form sodium bicarbonate (NaHCO₃) and potassium bicarbonate (KHCO₃).
In this case, the hardness in the water is entirely composed of bicarbonate, and there is no non-carbonate hardness (such as calcium and magnesium salts in the form of sulfates or chlorides). In addition, the difference between alkalinity and hardness corresponds to the bicarbonate concentration of sodium and potassium. This excess alkalinity is called "negative hardness" or "excess alkalinity", which means that there is extra bicarbonate in the water that is not consumed by calcium and magnesium.
2. Alkalinity equals hardness (in mol/L)
When the HCO₃⁻ concentration is exactly equal to half of the total concentration of calcium and magnesium ions (i.e. HCO₃⁻ = ½(Ca²⁺ + Mg²⁺)), the calcium and magnesium ions will all combine with bicarbonate to form calcium bicarbonate and magnesium bicarbonate. At this time, sodium and potassium ions cannot form bicarbonate, but combine with sulfate (SO₄²⁻) or chloride ions (Cl⁻) to form neutral salts such as sodium sulfate (Na₂SO₄), potassium sulfate (K₂SO₄), sodium chloride (NaCl) or potassium chloride (KCl).
In this case, there is neither excess alkalinity (negative hardness) nor non-carbonate hardness in the water. The hardness and alkalinity are perfectly matched, indicating that the concentrations of calcium and magnesium are just neutralized by bicarbonate, and other anions are only combined with sodium and potassium.
3. Alkalinity is less than hardness (in mol/L)
When the HCO₃⁻ concentration is less than half of the total concentration of calcium and magnesium ions (i.e. HCO₃⁻ < ½(Ca²⁺ + Mg²⁺)), the calcium and magnesium ions in the water cannot be completely neutralized by bicarbonate, and the remaining part will combine with other anions (such as sulfate or chloride ions) to form non-carbonate hardness. According to the difference in the ratio of calcium and magnesium, this situation can be further divided into two types:
(1) Calcium hard water (high proportion of Ca²⁺)
If the calcium ion concentration is high (½Ca²⁺ > HCO₃⁻), bicarbonate will preferentially combine with calcium to form calcium bicarbonate, and the remaining calcium ions will combine with sulfate to form calcium sulfate (CaSO₄). At the same time, since magnesium ions are not completely neutralized, they will exist in the form of magnesium sulfate (MgSO₄). At this time, there is obvious non-carbonate hardness in the water (such as calcium sulfate and magnesium sulfate), but magnesium bicarbonate (Mg(HCO₃)₂) does not exist at all.
(2) Magnesium hard water (Mg²⁺ accounts for a high proportion)
If the magnesium ion concentration is high (½Ca²⁺ < HCO₃⁻), bicarbonate first combines with calcium to form calcium bicarbonate, and the remaining part combines with magnesium to form magnesium bicarbonate (Mg(HCO₃)₂). Unneutralized magnesium ions will form magnesium sulfate (MgSO₄). In this case, there is both magnesium carbonate hardness (Mg(HCO₃)₂) and magnesium non-carbonate hardness (MgSO₄) in the water, but calcium non-carbonate hardness (such as calcium sulfate) completely disappears.
In either case, as long as the alkalinity is less than the hardness, non-carbonate hardness must exist in the water, and sodium and potassium bicarbonates (such as NaHCO₃) completely disappear.
Summary and analysis:
By comparing the relationship between hardness and alkalinity, we can clarify the composition and source of salts in water:
Alkalinity > Hardness: Calcium and magnesium in water exist in the form of bicarbonate, accompanied by sodium and potassium bicarbonate (negative hardness).
Alkalinity = Hardness: Calcium and magnesium bicarbonates are completely neutralized, and other salts are formed by sodium, potassium and sulfate or chloride ions.
Alkalinity < Hardness: Non-carbonate hardness must exist, and the specific composition depends on the concentration ratio of calcium and magnesium.