Hanna Instruments HI 903 - Manual (Page 206)

14
TITRATION THEORY
carboxylic acids, enols, phenols, imides, sulfonic acids, and inorganic acids. Water or lower
alcohols are suitable for titrating medium to strong acids (pKa 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 pKb’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 (pKb
= 4 to 5), but not the titration of weaker bases such as pyridine (pKb = 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.