Titrations According To The Titration Sequence - Hanna Instruments Hi934 Bedienungsanleitung

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 (see Figure 8).
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.
Visual indicators such as Ferroin are also available. The oxidized and reduced form of the indicator will have different
colors and can be used to determine the endpoint.
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 unreacted, 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 ENDPOINT TITRATIONS
Under certain conditions, some titrations can exhibit more than one equivalence point and be titratable to the individual
endpoints 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 of the different protons varies enough to separate them).
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Figure 7 Figure 8
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