2. Acid- Base Titration
• Titration is a procedure for determining the volume of two solution (acid &
base) which will exactly neutralize one another. By using a solution of known
concentration, (standard solution) the concentration of another solution can
be determined.
• An acid-base titration is a procedure used in quantitative chemical analysis
to determine the concentration of either acid or a base.
• The equivalence of an acid-base titration is the point at which there are
equal amounts (in moles) of H3O+ and OH- in titration flask.
3. • End point of titration – the point in a titration at which the indicator
changes colour.
• The indicator should change colour sharply at the equivalence point.
• At the end point of the titration,
• all the acid has been neutralized by the alkali
• the solution in the conical flask contain salt and water only.
Fig.: Titration apparatus
titrant
analyte
4. • Indicator is a substance that is generally added to the solution in the
receiving vessel which undergoes some sort of color change when
reaction is over.
• Acid base indicators are weak organic acids that dissociate slightly
in aqueous solutions to form ions.
• The indicators can change color because their ions have colors that
are different from undissociated molecule.
Acid- Base Indicators
5. Qualitative chemical analysis:
Determine the chemical compositionin a sample using chemical reactions and
interactions.
Example: Test for halides with Beilstein test (green flame ifhalides are
present).
Quantitative chemical analysis:
Determine the content in a sample of one or more chemical substances using
chemical reactions and interactions.
Example: Vitamin C content in orange juice by titration.
Sample:
A small representative portion of the entity that should be analyzed.
Example: Orange juice
6. Analyte:
The chemical substance to be determined with the analysis.
Example: Vitamin C.
Matrix:
Everything in the sample besides the analyte(s).
Example: Water, pulp, sugar, ... in vitamin C analysis of orange juice.
Titrant:
Solution of known concentration which is added to the sample (titration)
and reacts with the analyte. The analyte content is calculated from the
consumption of the titrant.
7. Acid- base reactions Example
Strong acid with strong base HCl and NaOH
Strong acid with weak base HCl and NH3
Weak acid withstrong base CH3COOH and NaOH
Weak acid withweak base CH3COOH and NH3
8. Indicator pH range
Colour change
Acid Alkali
Methyl orange 3.2-4.2 Red Yellow
Bromothymol blue 6.0-7.6 Yellow Blue
Phenolphtalein 8.2-10.0 Colorless Pink
9. Acid Base pH at equivalence
point
Indicators
Strong Strong = 7 (neutral) Methyl orange
Phenolphthalein
Strong Weak < 7 (acidic) Methyl orange
Weak Strong > 7 (basic) Phenolphthalein
Weak Weak pH depend on Ka and
Kb of acid & base
conc.
-
10. A titration curve is a plot of pH vs. the amount of titrant added. Typically
the titrant is a strong (completely) dissociated acid or base. Such curves
are useful for determining endpoints and dissociation constants of weak
acids or bases.
11. H2O (l) + NaCl(aq)
Base at the
end of the
titration
Acidic at the
start of the
titration
The titration curve for strong acid-strong base titration
12. 1. The pH starts out low, reflecting the high [H3O+] of the strong acid
and increases gradually as acid is neutralized by the added base.
2. Suddenly the pH rises steeply. This occurs in the immediate vicinity of
the equivalence point. For this type of titration the pH is 7.0 at the
equivalence point.
3. Beyond this steep portion, the pH increases slowly as more base is
added.
13. CH3COONa (aq) + H2O(l)
At
equivalenc
e point (pH
> 7):
The titration curve for weak acid-strong base titration
14. 1. The initial pH is higher.
2. A gradually rising portion of the curve, called the buffer region,
appears before the steep rise to the equivalence point.
3. The pH at the equivalence point is greater than 7.00.
4. The steep rise interval is less pronounced.
16. 1. The initial pH is above 7.00.
2. A gradually decreasing portion of the curve, called the buffer region,
appears before a steep fall to the equivalence point.
3. The pH at the equivalence point is less than 7.00.
4. Thereafter, the pH decreases slowly as excess strong acid is added.
17. Types of Titrations End Point pH Range Suitable Indicators
Strong Acid-Strong Base 3 – 10 Any Indicator
Weak Acid-Strong Base 7 – 11 Phenolphthalein,
thymol blue
Strong Acid-weak Base 3 – 7 Methyl orange, methyl
red
Weak Acid-Weak Base - -
Choosing a Suitable Indicator
• Choose an indicator which the end point pH range lies on the steep part of the
titration curve.
• This choice ensures that the pH at the equivalent point will fall within the
range over which indicator changes color
Table: pH Ranges for Indicator
18. What is the colour of the solution when 3 drops of the below
indicators are added separately to water (pH = 7) ?
Indicator pH range colour change
Phenolphthalein 8.2 – 10.0 Colourless reddish pink
Methyl orange 3.2 – 4.2 Red Yellow
Bromothymol
blue
6.0 – 7.6 Yellow Blue
phenol red 6.8 – 8.4 Yellow red
19. Titration is an analytical method which is widely used in many different
segments and for many different samples.
Chemical industry: Water content in solvents, acid value in resins and
polymers, hydroxyl value in polyols, surfactant content in raw materials,
…
Food and beverage: Acid and vitamin C content in fruit juices, chloride
content in spicy sauces, nitrite content in food products.
Electroplating: Nickel content in electroless bath, gold and silver
content in alloys, chromium content in electroplating baths, …
Application
20. • Pharmaceutical: Water contents in raw products, chondroitin sulfate
sodium in tablets, purity and content of different active pharmaceutical
ingredients like benzylnicotinate, clotrimazol and diclofenac sodium.
• Petrochemical: Acid and base number in oils, mercaptane sulfur
content in kerosene, chloride content in motor oil.
• Environmental: Total hardness of water, copper content in aqueous
solution, residual chlorine in bleach, fluoride content in water.
Application
