Chemistry - Titration and indicators
Titration and indicators
Many common consumer products are analysed by titration.
The concentrations of acetic acid CH3COOH, in vinegar and ammonia in
domestic cleaning agents may be determined by acid-base titrations.
The levels of alcohol CH3CH2OH in wine and sodium hypochlorite NaOCl in
bleach may be determined by redox titrations.
The acid or base content of a particular solution is determined by
reacting a sample of the solution with a standard solution of a base or
acid respectively. As in all titrations, this involves adding one solution,
from a burette, to a measured volume (aliquot) of the other solution.
The ethanol content of wine is determined by oxidising the CH3CH2OH, hence
a suitable oxidant is used. Determining the hypochlorite content of a
bleach requires that the hypochlorite ion ClO be reduced, so a suitable
reductant is used.
The completion of a titration is usually identified by a defined colour
Acid-base titrations use indicators such as methyl orange or
phenolphthalein, which display significantly different colours in acidic
and basic solutions. The transition from one colour to the other, known as
the endpoint, allows us to identify when a titration reaction is complete.
Redox titrations can be self-indicating as a distinct colour change
accompanies particular changes in oxidation state. Manganese, for example,
appears purple in its +7 oxidation state, MnO4(aq), but colourless in its
+2 oxidation state, Mn(aq). The endpoint of a reaction in which acidified
KMnO4(aq) is added from a burette may be identified when a faint
pink-purple colour persists in the titration flask.
Redox titrations involving the conversion of I(aq) to I2(s) or vice versa
may make use of starch as an indicator because it produces a distinct
blue/black colour in the presence of molecular iodine.
Indicators used in titrations are chosen so that the visual endpoint of
the reaction matches as closely as possible the equivalence point.
The equivalence point of a reaction is the point where stoichiometric
amounts of both reactants have been added to the titration flask.
Consider the determination of the accurate concentration of an aqueous
solution of hydrochloric acid via titration against a standard solution of
sodium carbonate according to the equation:
The equivalence point occurs when for every 1 mol Na2CO3 present in the
aliquot of Na2CO3(aq) used, exactly 2 mol HCl has been added.
The endpoint generally occurs fractionally after the equivalence point,
depending on the pH at which the indicator used changes colour.
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