Use this glossary of links to navigate to selected parts of this section of the Introductory Biochemistry module.
Almost all chemical reactions occur in solution. Water is the most common solvent for these reactions.
Water is also the universal solvent for all biological systems.
Water molecules can dissociate (at a very low rate) to form :
hydronium ions (H30 +) and
hydroxide ions (OH -)
Concentration of hydronium and hydroxide Ions
Equal concentrations of H3O + and OH - are formed each time a water molecule dissociates.
It can be shown experimentally that in pure water :
When the relative concentration of H3O + and OH - is equal in a solution it is described as neutral.
An acid is defined as a proton donor.
If a substance capable of dissociating to yield H + is added to water, the H + will combine with water molecules to form more H3O +.
The result will be that in this solution now :
The substance added which donated the H + is termed an acid and the aqueous solution
in which [H3O +] > [OH -] is termed acidic.
A base is defined as a proton acceptor.
If a substance dissociated to form OH - ions in water, they would combine with some of
the H3O + ions present, reducing their concentration.
The added OH - has accepted a proton from the H3O + .
This type of substance is termed a base, and a solution in which [H3O +] <[oh -] is termed basic.
Some bases accept a proton directly, rather than forming OH - .
The H3O + concentration has decreased.
It can be shown that the PRODUCT of the [ H3O +] and [OH -] is constant in aqueous solutions and is always equal to 10-14 mole/litre.
This means that the two components must change in opposite directions.
i.e. as [ H3O +] increases, [OH - ] must decrease.
This also means that if we know one concentration, we can calculate the other.
To avoid the problem of working with these unwieldy small numbers which express the concentration of [ H3O +] and [OH - ] a contracted scale is used to express the acidity or basicity of a solution.
This is termed the pH scale.
It is based on the exponential of the mole/litre concentration of [ H3O +] :
Occasionally the term pOH is used :
A low pH signifies an acidic solution.
A high pH signifies a basic solution.
The pH of some common substances is shown in the table
Acids and bases vary in strength. The term "strength" is a measure of how easily they ionise in water.
A strong acid or base ionises to a large extent and markedly changes the pH of the solution.
As an example, hydrochloric acid (HCl) is a strong acid. It dissociates almost completely in water.
Sodium hydroxide (NaOH) is a strong base.
This base ionises substantially contributing a large increase in OH - ions to the solution and raising the pH to a large extent.
On the other hand, a weak acid such as acetic acid ionises only slightly in water :
and a weak base such as ammonia accepts protons from the aqueous solution only to a small extent :
Neither change the pH of the solution to any great extent.
When an acid dissociates (donates a proton) the product is no longer an acid because it can no longer donate a proton.
What is formed is a base. It can accept a proton and reform the acid.
The product of the acid dissociation is the conjugate base of that acid.
Similarly when a base ionises (accepts a proton) it forms a conjugate acid.
These are called conjugate acid/base pairs.
The solution inside cells, intracellular fluid (ICF), and the solution outside cells, extracellular fluid (ECF), are aqueous solutions and maintain a very narrow [ H3O +] or pH range.
The body's metabolic processes function only within this narrow pH range.
Various metabolic and exogenous events can change the acid/base balance of the body. These changes are dealt with in various ways, one of which is the use of buffers.
Buffers are solutions which resist pH change.
A buffer consists of a mixture of a weak acid and its conjugate base.
The weak acid resists change from the addition of base (OH -).
The weak base resists change from the addition of acid (H +).
A mixture of acetic acid and sodium acetate is a good buffer.
When a strong base is added e.g. NaOH, the following reaction takes place :
The strong base has been converted to a weak base CH3COO - which will have very little effect on pH.
When acid is added e.g. HCl, it also is neutralised.
The strong acid has been converted to a weak acid CH3COOH which will have very little affect on pH.
The most widely used buffer system in the body is a mixture of carbonic acid (H2CO3) and its conjugate base bicarbonate ion (HCO3 -).
This topic will be discussed later during your study of the physiological mechanisms operating within the body to maintain blood pH within the very narrow limits of 7.35 -- 7.45 .
This completes the section on acids, bases and buffers.