Acid Base Indicators
Universal Indicator gives us rough values for the p H of a
solution. A p H meter provides more accurate readings.
Measuring the concentration of an acid and alkaline solution is
a routine task:
Titration measuring how much of a reagent of known
concentration is needed to completely react with another
reagent.
Many dyes interact with acids and alkalis. Their molecular
structure can be modified by changes in p H so that they
change colour.
The p H affects the colour of some dyes in dramatic ways. The
dyes, sometimes mixtures, used to monitor these chemical
changes are called indicators.
They usually change colour over a p H range of between 1 and
two p H units, with a recognised endpoint somewhere in the
middle. The end-point is the point between two colours which
is the most clearly seen difference between the two extremes
of its colour.
Bromothymol blue is yellow in acidic solutions and blue in
alkaline. The colour change takes place from p H 6.0 p H 7.6 and
the end-point occurs when the p H is 7.0.
The colours, ranges and end-points of indicators vary
considerably.
Phenolphthalein is colourless in p H of less then 8.2 and doesn’t
reach its final pink colour until p H 10. H+ and OH- ions have a
considerable effect on the molecular structure.
Different indicators need to be used to identify the end point
between acids and alkalis. (see bromothymol blue)
Neutralisation is the reaction between equal amounts of
hydrogen and hydroxide ions to form water
Strong acids and bases are those that are totally ionized when
dissolved in water.
In water
LiOH(s)
Li+ (aq) + OH- (aq)
In water
HCl(g)
H+(aq) + Cl- (aq)
Weak acids and bases do not totally ionize when dissolved in
water, In fact they may hardly ionize at all. When it comes to
donating protons weak acids are very limited.
CH3COOH
H+(aq) + CH3COO-(aq)
Weak proton donor!
Weak bases are pretty feeble at accepting protons.
You need to know whether you have a weak/strong acid/base
before you can monitor its progress with a suitable indicator.
Strong acid-strong base
Note the sharp fall in the graph tiny additions of H+ from the
acid have a dramatic effect on the p H. The midpoint of this
steep slope corresponds to a p H of 7.
Q1. identify the indicators that could be used for strong acidstrong base titrations and which ones that couldn’t. (see table)
Strong acid with a weak base
0.1M nitric acid with ammonium hydroxide
Methyl Orange is suitable as the sudden decrease occurs in the
range that methyl orange changes colour 3.2-4.4.
Q2. What other indicators could be used?
Weak acids and strong bases
Eg.Benzoic acid with potassium hydroxide
Weak acid with weak base
The shape of the shows that there is no significant p H range, in
which a small amount of one reagent produces a sharp change.
In this circumstance no indicator would be suitable because the
range of change is too large.
Q3. Suggest a suitable indicator to find the endpoint of the
reactions:
(a) 0.0500mol/dm3 nitric acid and 0.0500mol/dm3 aqueous
ammonia
(b) 2.00mol/dm3 sodium hydroxide and 1.00mol/dm3
sulfuric acid
(c) 0.00500mol/dm3 potassium hydroxide and aspirin (2ethanoyloxybenzoic acid) with a Ka 3.0x 10-4 mol/dm3
Recap
In acid base titrations the equivalence point is a point where
enough alkali has been added from a burette to react with all
the acid in the conical flask. Or vica versa! Reactions that have a
1:1 stoicheometry that means an equal number of moles of
acid and alkali. For a 2:1 stoicheometry that means the ration
of moles required for equivalence is also 2:1.
The p H at the equivalence is not necessarily p H7. This is the
point in the reaction that a salt is made and it depends on the
strength of the acid and alkali. The indicator shows when the
equivalence point has been reached.
Indicators are weak acids and the colour of the conjugate base
is different from the weak acid.
Hind
Colour 1
H+ + Indcolour 2
The acid form of the indicator is red (HIn) and its conjugate
base (In-) is yellow red in acid and yellow in base.
If we add acid to the solution there is an excess of H+ and the
position of the equilibrium moves to the left to remove some of
the H+ ions. The indicator will be red.
If we add alkali the position of the equilibrium moves to the
right. This will replace some of the H+ ions lost as they are
neutralized by the OH- ions added. The indicator will be yellow.
Kind = [H+][Ind-]
[Hind]
 When acid is added eqm driven to the left, indicator
appears as colour 1
 When alkali is added, OH- ions react with the H+ ions from
the indicator and eqm is driven to the right indicator
changes to colour 2
 The colour at equivalence point appears when
[Hind]=[Ind-]
At this point
Kind =[H+] or p H =p Kind
The eyes can see either colour 1 or 2 if at least 10% of the
species is present in the mixture.
Summary of graphs
Answers
Q1. The slope of the graph is steep over the range 3.5-10.0 so
any indicator with a colour change range over this range is
suitable bromocresol green, methyl red, bromothymol blue
phenolphthalein suitable others not.
Q2. The slope is steep over 7.0- 2.0 therefore methyl yellow,
methyl orange, bromophenol blue, bromocresol green , methyl
red and possibly bromothymol blue.
Q3.
(a) Strong acid-weak base; methyl orange/ bromophenol
blue
(b) Strong acid weak base: bromocresol green, methyl red,
bromothymol blue, phenolphthalein
(c) Weak acid- strong base : phenolphthalein