Page 373 - The Veterinary Laboratory and Field Manual 3rd Edition

342 Susan C. Cork, Willy Schauwers and Roy Halliwell

acids and bases when the pH needs to be carefully controlled,
for example, when measuring enzyme activity.
A base is a substance that liberates hydroxide
–
ions (OH ) in solution and accepts a proton. The Indicators
common bases include oxides and hydroxides
of metals, such as sodium hydroxide (NaOH), Indicators are substances that change colour
potassium hydroxide (KOH) and so on. Alkaline or shades of colour at different pH values. For
solutions turn litmus blue and react with acids example, phenol red changes from yellow at
to form a salt and water. An acid is a substance pH 6.8 to a deep red at pH 8.4. Indicators are
+
that liberates hydrogen ions (H ) in a solution used to determine the pH of liquids and the
and donates a proton. When reacted with a base, ‘endpoint’ of acid-base titrations.
an acid produces a salt and water. Acids turn Neutral solutions have an equal concentra-
litmus red and react with carbonate (CO ) to tion of hydrogen ions and hydroxide ions, that
2–
3
produce carbon dioxide (CO ). is, pure water:
2
Most acids are corrosive, for example, sulph-
–
uric acid (H SO ), hydrochloric acid (HCl) and [OH ] + [H ] = H O
+
2 4 2
so on. Example:
Whether a solution is acidic or alkaline, there are
HCl (acid) + NaOH (base) = NaCl (salt) always both hydrogen ions and hydroxide ions
+ H O (water) present, in most cases the solution is described
2
by its [H ] where (H ) is the hydrogen ion
+
+
The strength of an acid or base is described by concentration.
its pK (pK = – log K) where K = ionization
10
+
+
constant. A strong acid or base has a low pK and pH = log [1]/[H ] In pure water [H ] =
10
a weak acid or base has a high pK. Concentrated 100 mmol/l
and low pK acids and bases can be corrosive and pH (pure water) = log /10 = 7
–7
very reactive and must be handled with care. The 10
pH of a solution will indicate whether or not it As outlined in Chapter 2 the pH scale is from
is acidic or basic, measuring pH is outlined in 0–14 with values > 7 indicating an alkaline
Chapter 2, in the section on buffers. solution and < 7, an acid solution at 25°C.
buffers techniques for preparing solutions

Buffer solutions contain a mixture of a weak acid A solution is composed of a solvent and a solute.
and a salt of a strong base, or a weak base and its The solvent is the ‘dissolving medium’ and the
salt with a strong acid. Owing to their composi- solute (that is, a chemical) is the substance dis-
tion, buffers are able to resist changes in pH. For solved. In a well-prepared solution there should
example, if a small amount of hydrochloric acid be an even distribution of the solute throughout
is added to a buffer solution the hydrogen ion the solvent. When preparing a solution decide
content does not increase very much because it whether the solution requires an accurate volu-
combines with the base in the buffer resulting metric preparation (for example, to prepare a
in only a slight decrease in pH. Buffers are used standard), or a less accurate method of prepara-
in clinical chemistry and in other disciplines tion (for example, to prepare a stain). For the

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