In chemistry, a weak base is a chemical base that does not ionize fully in an aqueous solution. As bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH level compared to strong bases. Bases range from a pH of greater than 7 (7 is neutral, like pure water) to 14 (though some bases are greater than 14). The pH level has the formula: A chemical substance is any material substance used in or obtained by a process in chemistry: A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions. ... Acids and bases: Acid-base reaction theories pH Self-ionization of water Buffer solutions Systematic naming Electrochemistry Acid-base extraction Acids: Strong acids Weak acids Mineral acids Organic acids Bases: Strong bases Weak bases Organic bases edit In chemistry, a base is most commonly thought of as a substance that... ... Drinking water This article focuses on water as we experience it every day. ... Protonation is the addition of a proton (H+) to an atom, molecule, or ion. ... The correct title of this article is . ... It has been suggested that this article or section be merged into base (chemistry). ...

Since bases are proton acceptors, the base receives a hydrogen ion from water, H₂O, and the remaining H⁺ concentration in the solution determines the pH level. 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 level results. However, the pH level of bases is usually calculated using the OH^- concentration to find the pOH level first. This is done because the H⁺ concentration is not a part of the reaction, while the OH^- concentration is. // Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... In chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. ...

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

Since K_w = [H₃O ⁺ ][OH ⁻ ] then,

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

logK_a + logK_b = logK_w

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

pK_a + pK_b = pK_w = 14.00

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

Weak bases exist in chemical equilibrium much in the same way as weak acids do, with a Base Ionization Constant (K_b) (or the Base Dissociation Constant) indicating the strength of the base. For example, when ammonia is put in water, the following equilibrium is set up: Chemical equilibrium is the state in which the concentrations of the reactants and products have no net change over time. ... A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissolved HA still remains. ...

Bases that have a large K_b will ionize more completely and are thus stronger bases. As stated above, the pH of the solution depends on the H⁺ concentration, which is related to the OH^- concentration by the Ionic Constant of water (K_w = 1.0x10^-14) (See article Self-ionization of water.) 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 K_b. The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H3O+) and a hydroxide ion (OH-): The reaction is also known as the autoionization or autodissociation of water. ...

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 K_b values become. The pie-chart representation is as follows: Diethylamine is a secondary amine with the molecular structure CH3CH2NHCH2CH3. ...

• purple areas represent the fraction of OH- ions formed
• red areas represent the cation remaining after ionization
• yellow areas represent dissolved but non-ionized molecules.

Percentage protonated

As seen above, the strength of a base depends primarily on the pH level. 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 level 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.

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 K_b for C₅H₅N is 1.8 x 10^-9.

First, write the proton transfer equilibrium:

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

Substitute the equilibrium molarities into the basicity constant
Assume that x << .20.
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 C₅H₆N⁺.

Examples

• Alanine, C₃H₅O₂NH₂
• Ammonia, NH₃
• Methylamine, CH₃NH₂
• Pyridine, C₅H₅N

Other weak bases are essentially any bases not on the list of strong bases. Alanine (Ala, A) also 2-aminopropanoic acid is a non-essential α-amino acid. ... Ammonia is a compound with the formula NH3. ... Methylamine is a simple primary amine with a formula of CH3NH2. ... Pyridine is a chemical compound with the formula C5H5N. It is a liquid with a distinctively putrid odour. ... It has been suggested that this article or section be merged into base (chemistry). ...

External links

• http://wine1.sb.fsu.edu/chm1046/notes/AcidBase/WeakBase/WeakBase.htm
• http://www.chemguide.co.uk/physical/acidbaseeqia/bases.html
• http://bouman.chem.georgetown.edu/S02/lect16/lect16.htm
• http://www.psigate.ac.uk/newsite/reference/plambeck/chem1/p01154.htm

References

• Atkins, Peter, and Loretta Jones. Chemical Principles: The Quest for Insight, 3rd Ed., New York: W.H. Freeman, 2005.