Speciation of ions refers to the changing concentration of varying forms of an

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

as the pH of the

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Soluti ...

changes.{{cite book, last1=Butler, first1=James N., title=Ionic Equilibrium, date=1998, publisher=Wiley, location=New York, isbn=0-471-58526-2 The pH of a solution of a monoprotic weak acid can be expressed in terms of the extent of dissociation. After rearranging the expression defining the

acid dissociation constant In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction :HA ...

, and putting pH = −log₁₀ ⁺ one obtains :pH = p''K''_a – log ( H ⁻) This is a form of the Henderson-Hasselbalch equation. It can be deduced from this expression that * when the acid is 1 % dissociated, that is, when H ⁻= 100, pH = p''K''_a − 2 * when the acid is 50 % dissociated, that is, when H ⁻= 1, pH = p''K''_a * when the acid is 99 % dissociated, that is, when H ⁻= 0.01, pH = p''K''_a + 2 It follows that the range of pH within which there is partial dissociation of the acid is about p''K''_a ± 2. This is shown graphically at the right. A practical application of these results is that the ''pH transition range'' of a

pH indicator A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Hence, ...

is approximately p''K''_a ± 1; the colour of the indicator in its acid form is different from the colour of the conjugate base form. In the transition range both forms are in equilibrium, so the colour is intermediate. Outside the transition range the concentration of acid or conjugate base is less than 10 % and the colour of the major species dominates. A ''weak acid'' may be ''defined'' as an acid with p''K''_a greater than about −2. An acid with p''K''_a = −2 would be 99 % dissociated at pH 0, that is, in a 1 M HCl solution. Any acid with a p''K''_a less than about −2 is said to be a ''strong acid''. Strong acids are said to be fully dissociated. There is no precise p''K''_a value that distinguishes between strong and weak acids because strong acids, such as

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

, are associated in very concentrated solution. Calculation of the species concentrations for a polyprotic acid is more complicated unless the p''K'' values are separated by four or more, because three or more species may co-exist at a given pH. The example of

citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...

is shown at the right. The pH regions in which the species exist overlap extensively since the difference between successive p''K''_a values is small. A large number of computer programs for the calculation of equilibrium species concentrations have been published. Most of them can handle much more complicated equilibria than acid-base equilibria in solution. For details concerning general purpose programs see computer programs for calculating species concentrations in

chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the sy ...

References

Acid–base chemistry

See also

References