Alkaline

Alkalinity is the name given to the quantitative capacity of an aqueous solution to neutralize an acid. Measuring alkalinity is important in determining a stream's ability to neutralize acidic pollution from rainfall or wastewater. It is one of the best measures of the sensitivity of the stream to acid inputs. There can be long-term changes in the alkalinity of streams and rivers in response to human disturbances.

Alkalinity is related to the pH of a solution (its basicity), but measures a different property. Roughly, the alkalinity of a solution is a measure of how "strong" the bases are in a solution, whereas the pH measures the "amount" of chemical bases. A good example is a buffer solution, which can have many available bases (high alkalinity) despite having only a moderate pH level.

Alkalinity roughly refers to the amount of bases in a solution that can be converted to uncharged species by a strong acid. The cited author, James Drever, provides an equation expressed in terms of molar equivalents, which means the number of moles of each ion type multiplied by (the absolute value of) the charge of the ion. For example, 1 mole of HCO₃¹⁻ in solution represents 1 molar equivalent, while 1 mole of CO₃²⁻ is 2 molar equivalents because twice as many H⁺ ions would be necessary to balance the charge. The total charge of a solution always equals zero.

Quoting from page 52, "Ions such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl ⁻, SO₄²⁻, and NO₃⁻ can be regarded as "conservative" in the sense that their concentrations are unaffected by changes in the pH, pressure, or temperature (within the ranges normally encountered near the earth's surface and assuming no precipitation or dissolution of solid phases, or biological transformations)."

On the left-hand side of the equation is the sum of conservative cations minus the sum of conservative anions. Balancing this on the right side is the sum of the anions that could be neutralized by added H⁺ ions (non-conservative anions) minus H⁺ ions already present, as indicated by the pH. All numbers are molar equivalents.

This right side term is called total alkalinity. It is, quoting Drever, "formally defined as the equivalent sum of the bases that are titratable with strong acid (Stumm and Morgan, 1981)". The listing of ions shown on the right in Drever was "m_HCO3⁻ + 2m_CO3²⁻ + m_B(OH)4⁻ + m_H3(SiO)4⁻ + m_HS⁻ + m_{organic anions} + m_OH⁻ - m_H⁺". Total alkalinity is measured by adding a strong acid until all the anions listed above are converted to uncharged species. The total alkalinity is not (much) affected by temperature, pressure, or pH, though the values of individual constituents are, mostly being conversions between HCO₃⁻ and CO₃²⁻.

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