Hanna Instruments HI 904 - Manual (Page 165)

17
TITRATION THEORY
Figure 7.
Figure 6.
Visual indicators such as Ferrion are also available. The oxidized and reduced form of the
indicator will have different colors and can be used to determine the end point.
Various reductants can be determined by titrants with oxidants such as potassium
permanganate, potassium chromate or iodine. Commonly used reductants that are used as
titrants include sodium thiosulfate, and ferrous ammonium sulfate.
As with Acid-Base titrations the potential changes dramatically at the equivalence point.
2.3 Titrations According to The Titration Sequence
2.3.1 Back Titrations
Back titrations are generally used when a reaction is too slow to be directly accomplished
during a “direct” titration, where the reaction goes to completion within a few seconds. In a
back titration, a large excess of a reagent is added to the sample solution, helping a slow
reaction to go to completion. The un-reacted, excess reagent is then titrated. The difference
in the total volume of the first reagent added and amount determined from the second
titration is the quantity of reagent required to complete the first reaction.
2.3.2 Multiple End Point Titrations
Under certain conditions, some titrations can exhibit more than one equivalence point and be
titratable to the individual end points to determine the concentration of each individual
component. Examples of these types of titrations include acid-base, where different strength
acid or bases are in a mixture; redox, where each species has a different reduction potential;
complexometric, where different species are separately titratable; and acid-base, using
polyprotic acids (the pK
a
of the different protons varies enough to separate them).
Figure 8 shows three different types of multiple end point titrations. “A” shows the titration of
a polyprotic acid. The different acid strengths of the first and second proton can be determined.
“B” illustrates a mixture of two different metal redox species, where the different redox
potentials allow the species to be separated. “C” is the titration of a solution containing
strong, weak, and very weak acids.