Hanna Instruments HI 904, Uick Start Manual, Page: 164 / 180

16
TITRATION THEORY
Titration of Acids
Weak acids with pK
a
’s up to about 11 can be titrated in non-aqueous solvents. These include
carboxylic acids, enols, phenols, imides, sulfonic acids, and inorganic acids. Water or lower
alcohols are suitable for titrating medium to strong acids (pK
a
less than 5). Titrating a weaker
acid with a strong base titrant requires a solvent less acidic than water or ethanol/methanol.
Solvents such as acetone, acetonitrile, t-butyl alcohol, dimethlyformamide, isopropanol and
pyridine have been found to work well for acid-base titrations of strong, medium and weak
acids/bases. Titrants include alcoholic potassium hydroxide and various sodium or potassium
alkoxides in a 10:1 mixture of benzene/methanol. The best titrants are quaternary ammonium
hydroxides (such as tetrabutylammonium hydroxide) due to good solubility of tetraalkylammonium
salts of the titrated acids and the clean potentiometric titration curve obtained (see Figure 6).
Titration of Bases
Weak bases with pK
b
’s up to about 11, which do not ionize with water, can be titrated in non-
aqueous solvents. These bases include aliphatic and aromatic amines, basic nitrogen
heterocycles, alkali metal and amine salts of acids, and many other organic basic compounds.
Titrating a weak base with a strong acid titrant requires a basic solvent that is as weak as
possible. Water and alcohols allow the titration of medium strength bases such as aliphatic
amines (pK
b
= 4 to 5), but not the titration of weaker bases such as pyridine (pK
b
= 8.8).
Glacial acetic acid works well for weak bases and has been used extensively. Less basic solvents
such as acetone, acetonitrile, and nitromethane extend the range of titrable compounds.
The endpoint for non-aqueous titrations are usually determined potentiometrically using a
pH glass electrode, a modified calomel or double junction reference electrode with a low-flow
rate reference junction. Good potentiometric titration curves are obtained in most solvents,
except those with very low dielectric constants such as benzene, chloroform and others,
when high electrical resistance of the solvent causes unstable potentials.
2.2.7 Precipitation Titrations
Precipitation titrations allow for faster analysis compared to the old gravimetric analysis,
where a precipitate is formed, filtered, dried and weighed to analyze a compound. Typically
silver halides, silver thiocyanate and a few mercury, lead, and zinc salts are titrated using this
method. The chemical reactions must form an insoluble salt and precipitate out quickly in
order to be analyzed by this method. When the reaction is not quick, a back titration can be
used. A measured excess of the precipitating reagent (titrant) is added to force the reaction
to occur, and then unreacted titrant is then titrated with a standard solution of another reagent.
2.2.8 Redox Titrations
There are a number of oxidation-reduction reactions that can be used to determine unknown
concentration by titration. If the reaction goes to completion, is fast and has an analytical
signal available to follow it, a titration can be performed. The term “fast” means that each
addition of titrant is reacted completely and the sensing electrode is able to detect the change
in solution in less than one second.
Redox titrations are potentiometric titrations where the mV signal from a combination ORP
(redox) electrode (usually with a platinum indicator electrode) is used to follow the reaction
of oxidant/reductant. The electrode potential is determined by the Nernst equation and is
controlled by the oxidant reductant ratio.