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Indicators
Definition
Indicators are the organic substances, the presence of very small amount of which
indicates the termination of a chemical reaction by a change of colour
...
Acid base indicators
2
...
Precipitation or adsorption indicators

1
...
Acid - Base indicators (also
known as pH indicators) are substances which change colour with pH
...


 Theory of indicators:
An indicator is a substance which is used to determine the end point in a titration
...
They change their colour within a certain pH range
...


Two theories have been proposed to explain the change of colour of acid-base
indicators with change in pH
...
Ostwald's theory:
 According to this theory:
(a) The colour change is due to ionization of the acid-base indicator
...

(b) The ionization of the indicator is largely affected in acids and bases as it is
either a weak acid or a weak base
...
Similarly if the indicator is a weak base, its ionization is large in acids and
low in alkalis due to common OH ions
...
The equilibria in the aqueous
solution may be written as:
HIn
→
H+
+
In¯
InOH
→
OH¯ +
In+
Unionized ionized
Color
color
 If the indicator is a free amine or substituted amine the equilibrium is:
In + H2O

OH¯ + HIn+

→

 Indicator-acids HIn dissociate in aqueous solution as follows:
HIn

H + + In¯

→

 Applying the law of mass action to this dissociation
[H+ ] [In- ]
―――――

= K a ………… (1)

[HIn]
From which
[HIn]
[H+ ] = K a -----------------------……………
...
Eq
...
(3)
[HIn]
 In this equation [HIn] represents the concentration of the undissociated indicator
molecule whose colour is called ‘acid colour’ while [In − ] denotes the concentration
of the indicator-anions, the colour of which is called ‘alkaline colour’
...
The indicator base may be characterized
similarly to the indicator acid
InOH ========== In+ + OH−
[In+ ] [OH- ]
------------------------------= K b………………
...
(5)
[H+ ] [InOH]
Kb
[H+ ]=

-------KW

[In+ ]
------------ …………… (6)
[InOH]

[InOH ]
pH=

PkW - Pkb + log

-------------…………………
...


2- Modern Quinoid Theory
 According to this theory
(a) The acid-base indicators exist in two tautomeric forms having different
structures
...
One form is termed benzenoid form and
the other quinonoid form
...
The color change in due to the
interconversation of one tautomeric form into other
...
Thus,
during titration the medium changes from acidic to alkaline or viceversa
...


Phenolphthalein has benziod form in acidic medium and thus, it is colourless while
it has quinonoid form in alkaline medium which has pink in color
...
The color of benzenoid form is yellow while that of quinoniod form is red
...
Redox indicators
A redox indicator (also called an oxidation-reduction indicator) is an
indicator which undergoes a definite color change at a specific electrode potential
...
Redox titration may involve the use of a redox indicator and/or a
potentiometer
...


Redox Indicator

color change

Diphenylamine

colorless blue

Diphenylamine sulphonate

colorless blue

Diphenyle benzidine

colorless reddish violet

Methylene blue
Starch

colorless blue
colorless blue

Example
One of the most common examples of such an indicator is the use of diphenylamine
in the titration of Fe2+ by dichromate ion in the acidic solution
...
Adsoption indicators
 Adsorption Indicators are substances that indicate an excess of a reactant in
argentometric titrations
...
A type of indicator used in reactions that involve
precipitation
...
when a chloride solution is
treated with a solution of silver nitrate ,the precipitated silver chloride adsorbs
chloride ion as they are in access
...


Example
 The yellow dye fluorescein is a common example, used for the reaction
NaCl (aq) +AgNO3(aq)

→ AgCl(s)+NaNO3(aq)

 As silver nitrate solution is added to the sodium chloride, silver chloride precipitates
...
At the end
point, no Cl− ions are left in solution and negative fluorescein ions are then
adsorbed, giving a pink colour to the precipitate
...


 Commonly used indicators for titration of chloride against silver nitrate are

 Fluorescein
It is a suitable for very dilute solution of chloride with silver nitrate in neutral or
fainty acidic solution
...


 Dichlorofluorescein
It is a suitable indicator for every dilute solution of chloride and works even in
the presence of acetic acid and weakly acid solution
...


 Eosin
It is employed for titration of bromide and iodide with silver nitrate in the
presence of acetic acid
...


 Di– iodo diethyfluorescein
It is better indicator for determination of iodides
...
It changes from red to blue red when an iodide is titrated with
silver nitrate
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 A weak acid versus a strong base
 A strong acid versus a weak base
...

 In order to choose a suitable indicator, it is necessary to understand the pH changes
in the above four types of titrations
...
The curve obtained by plotting
pH as ordinate against the volume of alkali added as abscissa is known as
neutralisation or titration curve
...
Each titration curve becomes almost vertical for some distance
(except curve 10
...
This region of abrupt change in pH
indicates the equivalence point
...


1
...
strong base:
pH curve of strong acid (say HCI) and strong base (say NaOH) is vertical over
almost the pH range 4-10
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3 to 10
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4-6
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2- 4
...


2
...
weak base:
pH curve of weak acid (say CH3COOH of oxalic acid) and strong base (say NaOH) is
vertical over the approximate pH range 7 to 11
...


3
...
weak base:
pH curve of strong acid (say HCl or H2SO4 or HNO3) with a weak base (say
NH4OH) is vertical over the pH range of 4 to 7
...


4
...
weak base:
pH curve of weak acid and weak base indicates that there is no vertical part and
hence, no suitable indicator can be used for such a titration
...

The colour of the indicator can be seen on the white solid
...

If a soil is found to be too basic, it can be partially neutralised by adding a weak
acid such as ammonium sulfate
...

Monitoring liquid wastes, such as those from photographic processing, as liquids
Mood lipstick
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