Page 295 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

pH units
Fig. 10.1

It is necessary to draw attention to the variable pH of water which may be
encountered in quantitative analysis. Water in equilibrium with the normal
atmosphere which contains 0.03 per cent by volume of carbon dioxide has a
pH of about 5.7; very carefully prepared conductivity water has a pH close
to 7; water saturated with carbon dioxide under a pressure of one atmosphere
has a pH of about 3.7 at 25 OC. The analyst may therefore be dealing, according
to the conditions that prevail in the laboratory, with water having a pH between
the two extremes pH 3.7 and pH 7. Hence for indicators which show their
alkaline colours at pH values above 4.5, the effect of carbon dioxide introduced
during a titration, either from the atmosphere or from the titrating solutions,
must be seriously considered. This subject is discussed again later (Section 10.12).

10.8 PREPARATION OF INDICATOR SOLUTIONS
As a rule laboratory solutions of the indicators contain 0.5-1 g of indicator per
litre of solvent. If the substance is soluble in water, e.g. a sodium salt, water is
the solvent; in most other cases 70-90 per cent ethanol is employed.
Methyl orange. This indicator is available either as the free acid or as the
sodium salt.
Dissolve 0.5 g of the free acid in 1 litre of water. Filter the cold solution to
remove any precipitate which separates.
Dissolve 0.5 g of the sodium salt in 1 litre of water, add 15.2mL of
0.1 M hydrochloric acid, and filter, if necessary, when cold.
Methyl red. Dissolve 1 g of the free acid in 1 litre of hot water, or dissolve in
600 mL of ethanol and dilute with 400 mL of water.