Hanna Instruments HI934 Bedienungsanleitung Seite 124

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2.2.5. ION SELECTIVE TITRATIONS
The most popular ion selective titration is an acid-base titration. The hydrogen ion concentration is specifically measured
and monitored during the titration process to locate the equivalence point. Using an ion selective electrode (ISE) as the
indicator electrode, the potentiometric signal (in mV) is used to directly follow a specific ion's concentration (or activity).
Examples of ISE titrations include titrating fluoride with an aluminum titrant using a fluoride ISE, chloride with silver
nitrate using a chloride ISE, sodium with a sodium ISE, etc. The equivalence point can be determined by plotting the
mV value vs. the amount of titrant added.
2.2.6. NON-AQUEOUS SOLVENT ACID-BASE TITRATIONS
Non-aqueous solvents must be used to titrate very weak acids and bases due to the inherent leveling effect water has on
all acids and bases dissolved in it. A wide variety of weak acids and bases can be titrated using non-aqueous solvents.
Mixtures of acids or bases can often be individually analyzed in a single sequential titration.
2.2.6.1. TITRATION OF ACIDS
Weak acids with pK 's up to about 11 can be titrated in non-aqueous solvents. These include carboxylic acids, enols,
a
phenols, imides, sulfonic acids, and inorganic acids. Water or lower alcohols are suitable for titrating medium to strong
acids (pK less than 5). Titrating a weaker acid with a strong base titrant requires a solvent less acidic than water
a
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 7)
2.2.6.2. TITRATION OF BASES
Weak bases with pK 's up to about 11, which do not ionize with water, can be titrated in non-aqueous solvents. These
b
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 = 4
b
to 5), but not the titration of weaker bases such as pyridine (pK = 8.8). Glacial acetic acid works well for weak bases
b
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 using 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 titrated with a standard solution of another reagent.
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