Weak base

A weak base is a base that upon dissolution in water, does not dissociate completely and the resulting aqueous solution contains hydroxide ions and the concerned basic radical in a small proportion, along with a large proportion of undissociated molecules of the base.

pH, Kb, and Kw

Bases range from a pH of greater than 7 (7 is neutral, like pure water) to 14 (though some bases are greater than 14). pH has the formula:

Since bases are proton acceptors, the base receives a hydrogen ion from water, H2O, and the remaining H+ concentration in the solution determines pH. Weak bases will have a higher H+ concentration because they are less completely protonated than stronger bases and, therefore, more hydrogen ions remain in the solution. If you plug in a higher H+ concentration into the formula, a low pH results. However, pH of bases is usually calculated using the OH concentration to find the pOH first. This is done because the H+ concentration is not a part of the reaction, while the OH concentration is.

By multiplying a conjugate acid (such as NH4+) and a conjugate base (such as NH3) the following is given:

Since then,

By taking logarithms of both sides of the equation, the following is reached:

Finally, multiplying throughout the equation by -1, the equation turns into:

After acquiring pOH from the previous pOH formula, pH can be calculated using the formula pH = pKw - pOH where pKw = 14.00.

Weak bases exist in chemical equilibrium much in the same way as weak acids do, with a base dissociation constant (Kb) indicating the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up:

Bases that have a large Kb will ionize more completely and are thus stronger bases. As stated above, pH of the solution depends on the H+ concentration, which is related to the OH concentration by the self-ionization constant (Kw = 1.0x10−14). A strong base has a lower H+ concentration because they are fully protonated and less hydrogen ions remain in the solution. A lower H+ concentration also means a higher OH concentration and therefore, a larger Kb.

NaOH (s) (sodium hydroxide) is a stronger base than (CH3CH2)2NH (l) (diethylamine) which is a stronger base than NH3 (g) (ammonia). As the bases get weaker, the smaller the Kb values become.[1]

Percentage protonated

As seen above, the strength of a base depends primarily on pH. To help describe the strengths of weak bases, it is helpful to know the percentage protonated-the percentage of base molecules that have been protonated. A lower percentage will correspond with a lower pH because both numbers result from the amount of protonation. A weak base is less protonated, leading to a lower pH and a lower percentage protonated.[2]

The typical proton transfer equilibrium appears as such:

B represents the base.

In this formula, [B]initial is the initial molar concentration of the base, assuming that no protonation has occurred.

A typical pH problem

Calculate the pH and percentage protonation of a .20 M aqueous solution of pyridine, C5H5N. The Kb for C5H5N is 1.8 x 10−9.[3]

First, write the proton transfer equilibrium:

The equilibrium table, with all concentrations in moles per liter, is

initial normality .2000
change in normality -x+x+x
equilibrium normality .20 -xxx
Substitute the equilibrium molarities into the basicity constant
We can assume that x is so small that it will be meaningless by the time we use significant figures.
Solve for x.
Check the assumption that x << .20 ; so the approximation is valid
Find pOH from pOH = -log [OH] with [OH]=x
From pH = pKw - pOH,
From the equation for percentage protonated with [HB+] = x and [B]initial = .20,

This means .0095% of the pyridine is in the protonated form of C5H5NH+.


Simple Facts

  • An example of a weak base is ammonia. It does not contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions.[4]
  • The position of equilibrium varies from base to base when a weak base reacts with water. The further to the left it is, the weaker the base.[5]
  • When there is a hydrogen ion gradient between two sides of the biological membrane, the concentration of some weak bases are focused on only one side of the membrane.[6] Weak bases tend to build up in acidic fluids.[6] Acid gastric contains a higher concentration of weak base than plasma.[6] Acid urine, compared to alkaline urine, excretes weak bases at a faster rate.[6]

See also


  1. "Explanation of strong and weak bases]". ChemGuide. Retrieved 2018-03-23.
  2. Howard Maskill (1985). The physical basis of organic chemistry. Oxford University Press, Incorporated. ISBN 978-0-19-855192-8.
  3. "Calculations of weak bases". Mr Kent's Chemistry Page. Retrieved 2018-03-23.
  4. Atkins, Peter, and Loretta Jones. Chemical Principles: The Quest for Insight, 3rd Ed., New York: W.H. Freeman, 2005.
  5. Clark, Jim. "Strong and Weak Bases."N.p.,2002. Web.
  6. Milne, M.D.; Scribner, B.H.; Crawford, M.A. (1958). "Non-ionic diffusion and the excretion of weak acids and bases". The American Journal of Medicine. 24 (5): 709–729. doi:10.1016/0002-9343(58)90376-0.
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