IV
VOLUMETRIC ANALYSIS (VA)
VA is a method of quantitative analysis which depends
on the accurate measurement of the volumes of 2
solutions which react completely. Concentration of 1
solution is accurately known (the standard solution)
and this solution is used to determine the concentration
of another solution.
2 types of VA :
1. Acid-base titration
2. Redox titration
IV
VOLUMETRIC ANALYSIS (VA)
A.
Apparatus
Volumetric flask
Wash bottle
Glass dropper
Weighing bottle
B.
Retort stand
Burette with clip
Pipette & Pipette filler
Conical flasks
Recording of Results
1. Mass of substance: 2 dec. places (e.g. 1.65, 11.42 g)
2. Volume of pipette: 1 dec. place (e.g. 10.0 & 25.0 cm3)
3. Numerical answers: 3 significant figures where
appropriate (e.g. 0.0126, 4.89  10-3, 7.30)
4.
Burette
readings:
2 decimal
Accurate
readings
should
be withinplaces
0.10 cm3.
(e.g. 0.00,
4.70,
21.35, 18.70 cm3)
Reading
2 is not
accurate.
Accurate
Titration number
Rough
1
2
3
Final reading
25.00
49.80
25.00
28.75
Initial reading
0.20
25.00
0.00
4.00
24.80
24.80
25.00
24.75


Vol. Of HCl used
/cm3
Summary 25.0 cm3 of the borax solution required
________
24.78 cm3 of the HCl for complete reaction.
Average of the accurate readings ()

C.
Acid-Base Titrations
1. Acid-Base Titration Curves
Strong Acid: HCl, H2SO4, HNO3
Weak Acid: CH3COOH, (COOH)2
Strong Base: NaOH, KOH, Ca(OH)2
Weak Base: NH3 (aq)
The equivalence point of a titration is reached when
the reactants have just reacted or neutralised with
each other according to the stoichiometric ratio
given by the balanced equation of the reaction
concerned.
The end-point of a titration is reached when the
indicator in the titration has just changed its colour.
The indicator has a colour halfway between its
extreme colours at the end-point.
There are 4 types of acid-base titration reactions.
i.e. HA + BOH  H2O + BA
Strong acid – strong base titration curve
14
equivalence point
12
10
pH
8
6
4
2
5
10
15
20
Vol. of base used / cm3
Strong acid – weak base titration curve
14
equivalence point
12
10
pH
8
6
4
2
5
10
15
20
Vol. of base used / cm3
Weak acid – strong base titration curve
14
equivalence point
12
10
pH
8
6
4
2
5
10
15
20
Vol. of base used / cm3
Weak acid – weak base titration curve
14
equivalence point
12
10
pH
8
6
4
2
5
10
15
20
Vol. of base used / cm3
2. Acid-Base Indicators
Common acid-base indicators: (a) methyl orange
(b) phenolphthalein
Indicator
methyl
orange
phenolphthalein
0
1
2 3
4
5
red-orange
change
(orange)
colourless
6
7 8
9 10 11 12 13
yellow
change
(pink)
red
A suitable indicator for use in an acid-base titration
should change its colour in the pH range that includes
the pH of the solution at the equivalence point.
Table: Choice of Acid-Base Indicators
Type of Acid-Base Titration
Suitable Indicator(s)
Strong Acid-Strong Base
Methyl orange,
phenolphthalein
Strong Acid-Weak Base
Methyl orange
Weak Acid-Strong Base
phenolphthalein
Weak Acid-Weak Base
No suitable indicator
Strong acid – strong base titration curve
14
equivalence point
12
10
pH
phenolphthalein
8
6
methyl orange
4
2
5
10
15
20
Vol. of base used / cm3
Strong acid – weak base titration curve
14
equivalence point
12
10
pH
8
6
methyl orange
4
2
5
10
15
20
Vol. of base used / cm3
Weak acid – strong base titration curve
14
equivalence point
12
10
pH
phenolphthalein
8
6
4
2
5
10
15
20
Vol. of base used / cm3
Weak acid – weak base titration curve
14
equivalence point
12
10
pH
8
6
4
2
5
10
15
20
Vol. of base used / cm3
E.g. (a)
8.58 g of solid Na2CO3. nH2O was dissolved in water & then
made up to 250 cm3 solution in a volumetric flask. 10.0 cm3
of the solution required 24.00 cm3 of 0.100 mol dm-3
hydrochloric acid for complete neutralisation.
(i) Cal. the no. of moles of hydrochloric acid reacted.
(ii) Calculate the number of moles of Na2CO3 in the
10.0 cm3 of Na2CO3 solution.
(iii) Det. the no. of mole of Na2CO3 present in 250 cm3
of the solution.
(iv) Hence, determine the value of n in Na2CO3 . nH2O.
E.g. (b)
A solid mixture containing sodium chloride and calcium
hydroxide has a mass of 3.00 g. 100 cm3 of 0.510 mol dm-3
nitric acid was added to the mixture. The resulting solution
required 12.50 cm3 of 0.880 mol dm-3 KOH(aq) for
neutralisation in a titration.
(i) Write balanced equations for all reactions.
(ii) Calculate the number of moles of calcium hydroxide
in the solid mixture.
(iii) Find the percentage by mass of calcium hydroxide in
the solid mixture.