pKa

(noun)

A quantitative measure of the strength of an acid in solution; a weak acid has a pKa value in the approximate range −2 to 12 in water and a strong acid has a pKa value of less than about −2.

Related Terms

Examples of pKa in the following topics:

  • Acid Dissociation Constant (Ka)

    • The logarithmic constant (pKa) is equal to -log10(Ka).
    • The larger the value of pKa, the smaller the extent of dissociation.
    • A weak acid has a pKa value in the approximate range of -2 to 12 in water.
    • Acids with a pKa value of less than about -2 are said to be strong acids.
    • What is the pKa for acetic acid?

  • Strong Acids

    • More precisely, the acid must be stronger in aqueous solution than a hydronium ion (H+), so strong acids have a pKa < -1.74.
    • An example is hydrochloric acid (HCl), whose pKa is -6.3.
    • p-Toluenesulfonic acid is an example of an organic soluble strong acid, with a pKa of -2.8.

  • The Henderson-Hasselbalch Equation

    • The Henderson–Hasselbalch equation connects the measurable value of the pH of a solution with the theoretical value pKa.
    • The Henderson–Hasselbalch equation mathematically connects the measurable pH of a solution with the pKa (which is equal to -log Ka) of the acid.
    • The equation can be derived from the formula of pKa for a weak acid or buffer.
    • With a given pH and known pKa, the solution of the Henderson-Hasselbalch equation gives the logarithm of a ratio which can be solved by performing the antilogarithm of pH/pK­a:
    • What is the pH of a buffer solution consisting of 0.0350 M NH3 and 0.0500 M NH4+ (Ka for NH4+ is 5.6 x 10-10)?

  • The Acid Dissociation Constant

    • The logarithmic constant, pKa, which is equal to −log10 (Ka), is sometimes incorrectly referred to as an acid dissociation constant as well.
    • Smaller Ka values yield larger pKa values.
    • Therefore, the larger the value of pKa, the smaller the extent of dissociation.
    • A weak acid has a pKa value in the approximate range of -2 to 12 in water.
    • A pH indicator is a weak acid or weak base that changes color in the transition pH range, which is approximately pKa ± 1.

  • Oxoacids

    • These acids can be arranged in order of their pKa values and, by extension, their relative strengths:
    • HOCl pKa = 7.5 < HOBr pKa = 8.6 < HOI pKa = 10.6
    • Recall that smaller values of pKa correspond to greater acid strength.
    • Consider the family of chlorooxoacids, which are arranged below in order of pKa values:
    • HOClO3 pKa = -8 < HOClO2 pKa = -1.0 < HOClO pKa = 1.92 < HOCl pKa = 7.53

  • Important Reagent Bases

    • In the following table, pKa again refers to the conjugate acid of the base drawn above it.
    • Its basicity and nucleophilicity may be modified by steric hindrance, as in the case of 2,6-dimethylpyridine (pKa=6.7), or resonance stabilization, as in the case of 4-dimethylaminopyridine (pKa=9.7).

  • Nucleophilicity & Basicity

    • Basicity may be related to the pKa of the corresponding conjugate acid, as shown below.
    • Learning the pKa values for common compounds provides a useful foundation on which to build an understanding of acid-base factors in reaction mechanisms.
    • Thus, 2,2,2-trifluroethoxide (pKa 12) is a weaker base and nucleophile than ethoxide (pKa 16).
    • Two common examples of this exception, called the alpha effect, are hydroxide ion (pKa 15.7) compared with hydroperoxide ion (pKa 11.6), and ammonia (pKa 9.3) compared with hydrazine (pKa 8.0).

  • Other Reactions

    • This delocalization substantially reduces the basicity of these compounds (pKa ca. –1) compared with amines (pKa ca. 11).

  • Binary Acids

    • The acid strength increases as the experimental pKa values decrease in the following order:
    • HF (pKa = 3.1) < HCl (pKa = -6.0) < HBr (pKa = -9.0) < HI (pKa = -9.5).

  • Acid-Base Reactions

    • Stronger acids have smaller or more negative pKa values than do weaker acids.
    • If the pH is lowered by two or more units relative to the pKa, the acid concentration will be greater than 99%.
    • On the other hand, if the pH (relative to pKa) is raised by two or more units the conjugate base concentration will be over 99%.
    • A useful rule here is: pKa + pKb = 14.
    • A useful table of pKa values in DMSO solution has been compiled from the work of F.G.