Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C4 pH AND ITS CONTROL

Definition of pH The acidity or alkalinity of a reaction mixture is most important. It can control
the rate of reaction, the nature of the species present and even subjective proper-
ties such as taste. The original definition of pH (Sorensen, 1909) related it to the
concentration of hydrogen ions. Two facts should be recognized. First, like
many ions in solution, the hydrogen ion does not exist in aqueous solutions as a
‘bare’ H+species, but as a variety of hydrated ions, such as H 3 O+. Second, the
determination of pH is often carried out by methods that measure the activity of
the hydrogen ions, a(H 3 O+)

a(H 3 O+) =c(H 3 O+)g± or pH =-log[a(H 3 O+)]

where c(H 3 O+) is the molar concentration of hydrogen ions, and g±is the mean

ionic activity coefficient of the ions in solution (see Topic C1).

At low concentrations (<10-^2 molar), g± is close to 1, and the difference

between concentration and activity is small for uni-univalent electrolytes.

Key Notes

Since the concentrations of ions in aqueous solution vary over an

enormous range from greater than 1 molar down to 10-^14 molar or less, it

is convenient to use a logarithmic function to describe them. For

hydrogen ions in aqueous solution, denoted by H 3 O+, often called the

hydronium ion, this is defined as:

pH =-log(a(H 3 O+)) - log(c( H 3 O+)/mol dm-^3 )

Pure water contains equal amounts hydrogen ions and hydroxyl ions

OH-, at a concentration of 10-^7 molar. Therefore, the pH of neutral water

is -log (10-^7 ) =7. Acids have pH values less than 7, down to less than 1,

while for alkalis, the pH is greater than 7, up to 13 or more.

For many analytical measurements it is necessary to control the pH so

that it varies as little as possible. Solutions that resist changes in pH are

known as buffer solutions. They usually consist of a mixture of a weak

acid and its salt for lower pH values, or a weak base and its salt for

higher pH values.

pH may be estimated approximately using visual indicators, but more

accurate determination requires the use of pH meters. These must be

calibrated using standard buffers.

This is most usually achieved with buffers. Enzyme reactions,

electrophoretic separations and also spectrometric and electrochemical

determinations may all require pH control.

Related topics Titrimetry I: acid-base titrations (C5)

Definition of pH

The pH scale

Buffers

pH measurement

pH control