Hanna Instruments HI 904, Uick Start Manual, Page: 163 / 180

15
TITRATION THEORY
Figure 5.
2.2.4 Complexometric Titrations
A complex is a species where a central metal ion is covalently bonded to one or more
electron donating groups called ligands. In a complexometric titration, metal ions are titrated
using a titrant that binds strongly to it. Often these titrants contain EDTA or CDTA, polydentate
ligands that form very stable coordination compounds with metal ions. The complexation
reaction must be fast in order to be useful for direct titration. Some metal ions react too
slowly with EDTA for a direct titration.
An indicator electrode that responds to the metal ion can be used to monitor the titration
progress. The titration curve will appear similar to a usual potentiometric titration. Complexation
indicators change color at the endpoint as all metal ions are “consumed”, or complexed by the titrant.
The titration curve will appear similar to a potentiometric titration, when using an indicator
electrode that responds to the metal ion (see Figure 5).
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 based 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.