Base (chemistry) - Bases and PH

Bases and PH

The pH of an aqueous sample (water) is a measure of its acidity. In pure water, about one in ten million molecules dissociate into hydronium ions and hydroxide ions according to the following equation:

2H₂O(l) ⇌ H₃O+(aq) + OH−(aq)

The concentration, measured in molarity (M or moles per litre), of the ions is indicated as and ; their product is the dissociation constant which has the value of 10−14. The pH is defined as −log ; thus, pure water has a pH of 7. (These numbers are correct at 25 °C and are slightly different at other temperatures.)

A base accepts protons from hydronium ions, or donates hydroxide ions to the solution. Both actions will lower the concentration of hydronium ions, and thus raise the pH. By contrast, an acid donates protons to water or accepts OH−, thus increasing the concentration of hydronium and lowering the pH.

For example, if 0.1 mol (4 g) of sodium hydroxide (NaOH) is dissolved in water to make 1 litre of solution, the concentration of hydroxide ions becomes = 0.1 mol/L. As the ionic product remains a constant value, = 1×10−14/ = 1×10−13 mol/L, and pH = −log 10−13 = 13. The base dissociation constant, K_b, is a measure of basicity. It is related to the acid dissociation constant, K_a, by the simple relationship pK_a + pK_b = 14, where pK_b and pK are the negative logarithms of K_b and K_a, respectively.

Alkalinity is a measure of the ability of a solution to neutralize acids to the equivalence points of carbonates or bicarbonates.