1. HC
CHEMISTRY HC
CHEMISTRY
CHEMISTRY IN SOCIETY
(E) CHEMICAL ANALYSIS

2. HC
CHEMISTRY HC
CHEMISTRY
(E) CHEMICAL ANALYSIS
Volumetric Titration

3. (E) CHEMISTRY IN SOCIETYHC
CHEMISTRY HC
CHEMISTRY
(E) CHEMISTRY IN SOCIETY
Volumetric Titration
After completing this lesson you should be able to :
Chemical analysis - Volumetric analysis.
Standard solution
A solution of accurately known concentration is known as a standard solution.
Redox Titration
Volumetric analysis involves using a solution of accurately known concentration in a quantitative reaction to determine the concentration of another substance.
Redox titrations are based on redox reactions. Substances such as potassium manganate(VII), which can act as their own indicators, are very useful reactants in redox titrations.
The volume (usually measured to one decimal place) of reactant solution required to complete the reaction is determined by titration. Data from concordant titres is used to calculate an average volume used and hence the concentration of a solution. Volumes within 0·2 cm3 are considered to be concordant at Higher. The end-point is the point at which the reaction is just complete. An indicator is a substance which changes colour at the end-point.
The concentration of a substance can be calculated from experimental results by use of a balanced equation.

4. Volumetric titrations
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A titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution. Typically, the titrant (the know solution) is added from a buret to a known quantity of the analyte (the unknown solution) until the reaction is complete. 4

5. Volumetric titrations
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H2SO4 + 2NaOH → Na2SO4 + 2H2O
20 cm3 of sodium hydroxide solution was neutralised by 15cm3 of
0.1mol l-1 sulphuric acid.
Calculate the concentration of the sodium hydroxide solution.
0.15 mol l-1 5

6. Volumetric titrations
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What volume of potassium hydroxide, KOH, concentration 2 mol l-1 is
required to neutralise 50cm3 of H2SO4, concentration 0.5 mol l-1 ?
0.025 Litres 6

7. COPY
Redox Titrations
Titration is a technique for measuring the concentration
of a solution. A solution of known concentration is used to
work out the unknown concentration of another solution.
Redox titrations involve solutions of reducing and
oxidising agents.
At equivalence-point of a redox titration precisely enough
electrons have been removed to oxidise all of the reducing
agent. 7 7

8. Vitamin C - Redox titration8

9. COPY
Vitamin C (ascorbic acid) is an important component of our diet. In its absence the protein, collagen, cannot form fibres properly and this results in skin lesions and blood vessel fragility.
Although vitamin C occurs naturally in many fruits and vegetables many people take vitamin C tablets to supplement their intake.
Vitamin C can undergo a redox reaction with iodine in which the vitamin C is oxidised and the iodine molecules are reduced
I2(aq) + 2e I-(aq) 9

10. Redox Titrations, Vitamin C
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Redox Titrations, Vitamin C
I2 (aq) + 2e-  2I - (aq)
C6H8O6 
C6H6O H+ (aq) e-
reduction
oxidation 
Blue/Black (in the presence of starch)
I2 (aq) + C6H8O6
colourless
C6H6O H I- (aq)
Iodine concentration is known (in the burette)
acts as an oxidising agent.
Vitamin C, the unknown concentration (in the conical flask)
is a reducing agent.
Starch is added to show when the end-point is reached. 10 10

11. Redox titrations
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From National 5 you should be familiar with volumetric titration questions.
The same procedures can be used with a balanced redox equation 11

12. Volumetric titrations
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Examples for you to try
1) What volume of sulphuric acid, concentration 2 mol l-1 is required to neutralise 25cm3 of KOH, concentration 4 mol l-1?
2) If 50cm3 of KOH solution is neutralised by 17.8cm3 of H2SO4 (2 mol l-1), what is the concentration of the alkali?
3) What volume of HCl, concentration 1.0 mol l-1 is required to neutralise 100cm3 of NaOH solution concentration 0.5moll-1?
25 cm3
1.42mol l-1
50cm3 12

13. Redox titrations
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The same procedures can be used with a balanced redox equation
Permanganate ions react with hydrogen peroxide in acidic solution.
2MnO4- + 6H+ + 5H2O2 → 2Mn2+ + 8H2O + 5O2
25cm3 of hydrogen peroxide solution reacted with 16cm3 of permanganate
solution, concentration 0.1 mol l-1.
Calculate the concentration of the hydrogen peroxide solution.
0.16 mol l-1 13

14. Redox titrations COPY Try the following example
The overall equation for the reaction of I2 with SO32- ions is:
I2 + SO H2O → 2I- + SO H+
Calculate the volume of iodine solution (concentration 0·5mol 1-1) needed to completely react with 50cm3 of sodium sulphite solution of concentration 0·2mol 1-1.
2. The overall equation for the reaction of Fe3+ ions with I- ions is:
2Fe I- → I Fe2+
Calculate the volume of iodide solution (concentration 0·2mol 1-1) needed to completely react with 25cm3 of iron (III) nitrate solution of concentration 0·1mol 1-1.
20cm3
12.5cm3 14

15. Redox titrations
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Try the following example
a) Write the equation for the overall reaction between acidified dichromate ions and bromide ions.
b) Calculate the volume of bromide solution, concentration 0.5 mol l-1 that would react completely with 50cm3 of 0.2 mol l-1 solution of acidified dichromate.
Cr2O H+ + 6Br- → 2Cr3+ + 7H2O + 3Br2
120cm3 15

16. Redox titrations COPY a) Write the ion-electron equation for the
Try the following example
a) Write the ion-electron equation for the
i) reduction of dichromate ions
ii) oxidation of sulphite ions
b) Write the equation for the overall reaction between dichromate ions and sulphite ions.
c) Calculate the volume of dichromate solution 0.5 mol l-1 that would react completely with 30cm3 of 0.25 mol l-1 solution of sulphite ions
Cr2O H+ + 6e- → 2Cr3+ + 7H2O
SO H2O → SO H+ + 2e-
Cr2O H+ + 3SO H2O → 2Cr3+ + 7H2O + 3SO H+
5cm3 16

17. COPY
Redox titrations
A 50·0cm3 sample of contaminated water containing chromate ions was titrated and found to require 27·4 cm3 of 0·0200 mol l–1 iron(II) sulphate solution to reach the end-point. The redox equation for the reaction is: 3Fe2+(aq) + CrO42–(aq) + 8H+(aq) → 3Fe3+(aq) + Cr3+(aq) + 4H2O(l) Calculate the chromate ion concentration, in mol l–1, present in the sample of water. Show your working clearly.
Concentration of CrO42- = 0·00365 (mol l-1 ) 17

18. Complete Redox titration – potassium permanganate and iron(ii) sulphate. Can also investigate the iron ion content of iron supplement tablets. 18

19. MnO4- + 8H++ 5Fe2+ → Mn2+ + 4H2O + 5Fe3+
Redox titrations
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MnO4- + 8H++ 5Fe → Mn2+ + 4H2O + 5Fe3+
8.25g of an iron (II) salt was dissolved in 250 cm3 of pure water cm3 aliquots were
pipetted from this stock solution and titrated with a standard solution of 0.02 mol l-1
acidified potassium permaganate solution.
The titration values obtained were cm3, cm3 and cm3.
Calculate the average titration volume.
(ii) What is meant by a standard solution?
(iii) Calculate the number of moles of potasssium permaganate which reacted with the 25.0cm3 aliquot.
(iv) Calculate the number of moles of iron (II) in the 25cm3 aliquot.
(v) Calculate the number of moles of iron in the original sample.
(vi) Calculate the total mass of iron in the original sample of the iron(II) salt.
(vii) calculate the % iron in the salt.
(viii) Why is the potassium permanganate solution acidified? 19

20. COPY
Redox Titrations
Aim : To estimate the iron(II) content of an iron tablet, a tablet is first dissolved in distilled water. This solution is then titrated against previously standardised potassium manganate(VII) solution. The reaction is represented by the equation:
5 Fe2+ (aq) + 8H+ (aq) + MnO4- (aq)  5 Fe3+ (aq) + Mn2+ (aq) + 4H2O(l)
purple
colourless

21. Redox Titrations COPY 1. Find the mass of one iron tablets.
5 Fe2+ (aq) + 8H+ (aq) + MnO4- (aq)  5 Fe3+ (aq) + Mn2+ (aq) + 4H2O(l)
purple
colourless
1. Find the mass of one iron tablets.
2. Crush the tablet in a mortar and pestle. Transfer all the ground material to a beaker where it is dissolved in about 100 cm3 of distilled water.
3. All of this solution (including washings) is transferred to a 250 cm3 volumetric flask and the solution made up to the mark with deionised water. The volumetric flask should be stoppered and inverted several times. This is the solution containing iron(II) ions. 21

22. RedoxTitration 1 2 3 4 COPY What to do: Carefully fill the burette
with potassium permanganate 0.002moll-1 1
Carefully pipette exactly
25 ml of iron (II) sulphate
into the conical flask. 2
Add the permanganate until a
permanent purple colour appears
in the conical flask. 3
A rough titration is done first to give a rough
equivalence-point (end-point), then repeated
more accurately to give concordant results. 4

23. 5 Fe2+ (aq) + 8H+ (aq) + MnO4- (aq)  5 Fe3+ (aq) + Mn2+ (aq) + 4H2O(l)
purple
colourless

24. Questions relating to the experiment
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Questions relating to the experiment
In this experiment why is dilute sulfuric acid added to the permaganate solution?
Why are burette readings taken from the top of the meniscus?
How is the end-point of the titration detected?
Why is a rough titration carried out?
Why is more than one titration carried out subsequently?