6.2 Calculable Quantitation Methods
267
strumental (optical, electro analytical and radiometric) indicators also exist that
monitor changes in the ingredients of the titrimetric reaction via continual
measurements of a physico-chemical property of A, R or B (see Box 6.11).
__ Box6.10
Visual Indicators in Titrimetries
Visual indicators allow one to identify the end-point of a titration and hence to stop the
addition of the titrant solution from the burette. As a rule, visual indicators are chemical substances (Ind) that are added at low concentrations to solutions containing the analyte. No
self-indicating titrimetric system need be added to the titrand. Several typical examples
follow.
TYPE 1. Self-indicating systems. These require no addition of Ind. One case in point is permanganimetry for the determination of reducing analytes (e.g. Fe 1 +, H 2 0 2 , n in an acid
medium. As the titrant is added, its deep violet colour gradually disappears; when the end -
poin t is reached, the addition of a further drop of Mn04" gives a persistent pink colour.
TYPE 2. Addition of chemical substances interacting with the analyte. Before the titration
proper, a small proportion of the analyte (<0.1%) interacts with the indicator to form a
chemical species IndA (colour 2). During titration, the rema ining analyte interacts with R. At
the end-point, R displaces the amount of analyte associated to Ind, which is released and
changes the colour from 2 to 1 (Ind):
IndA + R ~ Ind
colour 2
colour 1
Examples:
• Titration of Mg 2+ with EDTA. The indicator (neT) forms a chelate with Mg 2+. At the endpoint, neT, which has a different colour, is released.
• Titration of HCI with NaOH in the presence of phenolphthalein (phen) as indicator. At the
beginning of the titration, Phen is associated to protons (PhenH) and colourless. A slight
excess of NaOH at the end-point releases Phen, which gives its red-violet colour to the
solution.
TYPE 3. Addition of chemical substances interacting with the reagent. The indicator, Ind
(colour 1), is added at the start of the titration. Because it does not interact with the analyte,
the colour is preserved throughout the titration. At the end-point, a small excess of R causes
an abrupt colour change:
Ind + R ~ IndR
colour 2
colour 1
Examples:
• Titration of CI - with Hg (II). The indicator, diphenylcarbazide, forms an orange-coloured
chelate with the metal cation (R).
267
strumental (optical, electro analytical and radiometric) indicators also exist that
monitor changes in the ingredients of the titrimetric reaction via continual
measurements of a physico-chemical property of A, R or B (see Box 6.11).
__ Box6.10
Visual Indicators in Titrimetries
Visual indicators allow one to identify the end-point of a titration and hence to stop the
addition of the titrant solution from the burette. As a rule, visual indicators are chemical substances (Ind) that are added at low concentrations to solutions containing the analyte. No
self-indicating titrimetric system need be added to the titrand. Several typical examples
follow.
TYPE 1. Self-indicating systems. These require no addition of Ind. One case in point is permanganimetry for the determination of reducing analytes (e.g. Fe 1 +, H 2 0 2 , n in an acid
medium. As the titrant is added, its deep violet colour gradually disappears; when the end -
poin t is reached, the addition of a further drop of Mn04" gives a persistent pink colour.
TYPE 2. Addition of chemical substances interacting with the analyte. Before the titration
proper, a small proportion of the analyte (<0.1%) interacts with the indicator to form a
chemical species IndA (colour 2). During titration, the rema ining analyte interacts with R. At
the end-point, R displaces the amount of analyte associated to Ind, which is released and
changes the colour from 2 to 1 (Ind):
IndA + R ~ Ind
colour 2
colour 1
Examples:
• Titration of Mg 2+ with EDTA. The indicator (neT) forms a chelate with Mg 2+. At the endpoint, neT, which has a different colour, is released.
• Titration of HCI with NaOH in the presence of phenolphthalein (phen) as indicator. At the
beginning of the titration, Phen is associated to protons (PhenH) and colourless. A slight
excess of NaOH at the end-point releases Phen, which gives its red-violet colour to the
solution.
TYPE 3. Addition of chemical substances interacting with the reagent. The indicator, Ind
(colour 1), is added at the start of the titration. Because it does not interact with the analyte,
the colour is preserved throughout the titration. At the end-point, a small excess of R causes
an abrupt colour change:
Ind + R ~ IndR
colour 2
colour 1
Examples:
• Titration of CI - with Hg (II). The indicator, diphenylcarbazide, forms an orange-coloured
chelate with the metal cation (R).