11. Acid/Base Titration – The Basics
a) The Technique
i) The Purpose – to find out concentration of an unknown acid or base.
ii) Titration – means to slowly and accurately add known [acid] to an unknown
[base] (or vice versa) using a buret, until base is exactly
neutralized (“equivalence point”).
iii) Equivalence Point – when proportion of acid = proportion of base described
by the reaction.
Example: NaOH + HCl  NaCl + H2O
1
:
1
1 mol base = 1 mol acid at equivalence point
Example: 3NaOH + H3PO4  Na3PO4 + 3H2O
3
:
1
3 mol base = 1 mol acid at equivalence point
b) Indicators
i) What is an “Indicator”?
 purpose is to tell us when we are at the equivalence point
 it is a weak acid/base
ii) How Do They Work?
 since it is a weak acid/base, it is in equilibrium:
HInd(aq) + H2O(l)
Acid form
H3O+(aq) + Ind-(aq)
Base form
 if there is an excess of H3O+ at the equivalence point, the equilibrium
will shift to the acid form of the indicator.
 if there is an excess of base at the equivalence point, the equilibrium will
shift to the base form of the indicator
 the key to indicators is that their acid and base forms have different
colours!
Indicator Name
Phenolphthalein
Methyl Orange
Bromothymol Blue
Bromocresol Green
Thymol Blue
Acid Form
Colourless
Red
Yellow
Yellow
Yellow
Base Form
Pink
Yellow
Blue
Blue
Blue
 see p.335 Hebden and Data Booklet for others.
iii) End Point
 the point at which an indicator is exactly halfway through its colour
change and [HInd] = [Ind-]
 thus, for the equilibrium for an indicator at the End Point:
Ka = [H3O+][Ind-] = [H3O+]
[HInd]
Ka for that indicator = [H3O+] present
- log Ka = - log [H3O+]
pKa of that indicator = pH of the solution
 the point is, you must choose an indicator that will have an End Point
near the Equivalence Point of that acid/base titration.
Example: The pH at the equivalence point is 4.5
An appropriate indicator will change colour around 4.5
Bromocresol green would be a good choice.
iv) Ka of Indicators
Example: What is the Ka value for Bromothymol blue indicator?
Bromothymol blue indicator has a pH range of 6.0 – 7.6
Midpoint of colour change = pka = pH = 6.8
Ka = antilog(-6.8) = 1.6 x 10-7 = 2 x 10-7
v) Universal Indicators
 a mixture of several indicators that has several colour changes
over a large pH range.
 useful to get an approximate pH of an unknown solution
c) Accuracy
i) Volume
 all titrations are repeated until two volumes are within  0.1 mL
 discard results from any titration that is outside this range
 use average of best results in calculations
Example: Volume of HCl added
1st titration
2nd titration
3rd titration
41.75 mL
41.32 mL
41.34 mL
Thus, volume HCl added = (41.32 + 41.34)/2 = 41.33 mL
ii) Standard Solutions
What is it?
A solution with a very accurately known concentration
How do we make one?
 Use a very pure (99.9%) substance and dissolve an accurate mass in
water. Called a Primary Standard
Acidic Primary Standard
oxalic acid
H2C2O42H2O
Basic Primary Standard
sodium carbonate
Na2CO3
 Titrate a solution with a primary standard to accurately find the
concentration of the solution.
Do Questions: #108-120 page 162-163; #121-123 page 165