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(TOPICS 8 AND 18) SUMMARY
Indicators signal the endpoint of a titration by changing colour. Indicators used in acid-base
titrations are usually organic dyes and are mostly weak acids (or weak bases) where the acid
and the conjugate base of the indicator are different colours. Consider phenolphthalein, HIn, as
an example a weak organic acid, Ka = 1.0 x 10-9, in which HIn is colourless and its conjugate
base, In- is pink:
+
[H ][In- ]
�
HIn (aq) H+ (aq) + In- (aq); Ka = = 1.0 x 10-9 at 25oC
[HIn]
An indicator changes colour over H" 2 pH unit range. Phenolphthalein, is colourless below pH
H" 8, pink above 10 and end point colour change occurs at pH = 9. The end point for any
indicator depends on its Ka value - the pH at the end point equals the pKa of the indicator.
In carrying out an acid-base titration, select an indicator that changes colour at or near the
equivalence point of the reaction at which equal quantities of the acid and base have been
added i.e. pKa of (weak acid) indicator should be close to pH of solution at equivalence point.
Titration curves
Acid-base Titration Summary:
(a) Strong acid + strong base: solution at equivalence point, pH = 7.0; large pH change; can
use phenolphthalein (pH range 8.3  10); or methyl red: (4.2  6.3); or methyl orange (pH
range: 3.1  4.4).
(b) Weak acid + strong base: solution at equivalence point, pH > 7; use phenolphthalein (pH
range 8  10)
(c) Strong acid + weak base: solution at equivalence point, pH
range: 3.1  4.4)
(d) Weak acid + weak base: solution at equivalence point, pH about 7; gradual change in pH;
no suitable color indicator; record change in pH using a pH meter and obtain point of
inflection for equivalence point.
Methods of Determining Ka of a Weak Acid or a Weak Base
1. Determine pH of a solution of known concentration of the weak acid. Knowing pH,
calculate [H+] = x.
HX (aq) H+ (aq) + X (aq)
[initial]: 0.10  
Assume x is negligible;
[eq.]: 0.10  x x x
i.e., x
+ -
[H ][X ]
x2 = x2 ; if x = 1.0 x 10 pH, then Ka can be calculated.
Ka = =
[HX] 0.10 - x 0.10
2. Can obtain Ka or Kb from titration curve by the half-neutralization method:
(N.B. Shading indicates Ch 18 (AHL) material.)
� IBID Press 2007 4 [ Pobierz całość w formacie PDF ]