2.1 Theories of acid-base titration
- Indicators are the substance /chemicals which exhibit/show color change at a particular stage of the chemical reaction.
- This substance/chemical shows different colors at various PH.
- Weak acid /weak base indicators have different colours in their conjugate base and acid forms.
- Most indicators are available and commonly used in dilute solution/ liquid form.
- The selection of indicators depends on the type of titration.
- We select an indicator that will show distinct color change at pH close to the equivalence point.
Theories of indicators
There are two theories of indicators
- Ostwald theory
- Resonance theory
- This theory is given by w.Ostwald.
- This theory explains the behavior of indicators.
- According to this theory – the undissociated indicator acid-base has a color different than its ions.
Hln → H+ ln – (1)
- In an acid solution depression of ionization of indicator due to common Ion effects.
- In alkaline solution increase ionization of indicator.
Consider equation (1)
HIn → H+ + In–
By applying law of mass action
KIna = [H+] [In–]
[H+] = KIna × [HIn] / [In]
Taking log on both side,
Log [H+] = log KIna +log.[HIn] / [In–]
Taking negative log,
-log [H+] = -log KIna – log [HIn] / [In–]
PH = -log KIna + log [In–] / [HIn]
PH = pKIna + log [In–] / [HIn]
- KINa is a dissociation constant of indicator the color of the indicator depends upon the ratio of the concentration of ionized and unionized form and hence directly proportional to PH.
- The indicator should be such that the pH at the equivalence point Falls within the transition interval of the indicator.
- All the acid-base indicators which are commonly used are organic compounds
- The difference in the color of some compounds in acid and base medium is apparently due to the difference in the structure of the two forms.
- The Colour shown by the compound is associated with the compatibility of the compound to observe the visible light and this capability is related to the Electronic structure.
- Change in the electronic features will result in the absorption of different color components of light with a resultant color change.
Indicators used in various titration
Strong acid strong base titration e.g methyl orange, methyl red, phenolphthalein bromothymol blue, phenol red.
Weak acid strong base titration e.g phenolphthalein, thymolpthaline, thymol blue.
Strong acid weak base titration e.g Methyl orange, methyl red, bromophenol, bromocresol green,
Weak acid weak base titration e.g mix indicators.
According to the type of titration which is the appropriate indicator that gives color change at a defined pH range.
Acid Base Titration Notes
2.2 Classification of acid-base titration
Classification of acid-base titration
i) Strong acid strong base titration
When a strong acid and strong base are mixed together then reaction occurs are represented as follows
H+ + OH– ↔ H2O
It is a Reverse process of ionization of water
The calculation of titration curve is illustrated as consider the titration of 100ml, N HCL with N NaOH. The pH before titration commences is pH of N HCL
(H+) = 1N or PH = 0
After adding 1 ml of titrant the volume becomes 101 ml and it contains (100 – 1) = 99 ml equivalent of acid in 101 ml or 99/101 eq/liter
[H+] = 99/101
PH = -log (99/101)
PH= log (101/99) = log 1.020
Then after addition of 90ml titrant
(H+) = 10/190
PH = 2.2788
After addition of 99.9 ml of titrant
(H+) =0.1/ 199.9
PH = 3.3008
Then after the addition of a 100ml titrant, the pH will change sharply to the equivalence point,
Then after the addition of a 100ml of titrant
P[OH] = log(201/1)
p[OH] = 2.3032
PH = 14- pOH
PH = 11.6968
As the titration process, the pH Rises slowly, but near the equivalence point the change in pH is very sharp 3.3 (or 2.2988) to 7
so, with 1N solution in indicator which is effective in pH range 3 to 10.5 may be used
e.g phenolphthalein, methyl orange, methyl red, phenol red
Acid Base Titration Notes
ii) Weak acid with strong base titration
- Titration is represented by the following equations
- HA + OH ⇔ H2O + Awere HA is A weak acid and it Is in undissociated form,
- This titration is different from the first type of titration
i.e strong acid strong base titration
By three ways that ways of differences described as follows
- The H concentration is small at the start of the titration because of un dissociation but pH will be greater.
- In 0.1 NHCL (H+) = 0.1 and PH = 3.127
- In 0.1 N CH3 COOH is a weak acid and is poorly dissociated, hence it gives a smaller concentration of H+ ions.
- A solution is added to the weak acid the salt form produces a common Ion effect so that the dissociation of a weak acid is suppressed. e.g when NaOH is added to CH3COOH there is a formation of salt and the following equilibria exist.
CH3COOH→H+ + CH3C00 – (poorly dissociation)
CH3COONa →Na+ + CH3COO – (strongly dissociation)
- In the titration of HCl with NaOH, PH at the equivalence point is 7 but if a weak acid like CH3COOH is taken pH at the completion of titration is more than 7.
CH3COONa + H2O → CH3COOH + Na+ + OH–
Due to the hydrolysis of West minus is greater than h + in the solution is alkaline basic
e.g its pH is greater than 7.
iii) Weak base with strong acid titration
- Here we consider the titration of 0.1 M and 0.1 N aq. Ammonia solution with 0.1 M and 0.1 N HCl solution.
- The reaction involved
NH4OH + HCl → NH4Cl +H2O
- The salt NH4Cl is hydrolysis and NH4OH + H2O→ NH4OH + H+
- Due to the formation of H+Ion, the pH at the equivalence point will be slightly on the acid side as shown by the following calculate.
- Suppose we titrate 50 ml of 0.1 M NH4OH with 0.1M HCl
- The pH of the equivalence point will be
PH = 1/2 PKW -1/2 PKb – 1/2 log C
KW = 10 -14 , 90 pkw = log 10-14 =pkw = -14.00
kh= 1.8 × 10 -5 , pkb =4.74
c = 0.05, log c =log 0.05 50 log c = 2.70
1/2 pkw =7.0, 1/2 pkb =2.37
iv) weak acid and weak base titration
Let Us consider the titration of 0.1 M CH3COOH with 0.1N NO4OH
CH3COOH + NH4OH →CH3COONH4 + H2O
The salt formed, CH3COONA undergoes hydrolysis
CH3COONH4 +H2O → CH3COOH +NH4OH
The disadvantage in such first titration is that the inflection of the pH neutralization curve is very small near the equivalence point.
2.3 The theory involved in the titration of strong, weak, and very weak acids and bases
Theories of titration
- Arrhenius Theory (H+ and OH–)
- Bronsted – Lowry Theory
- Lewis Theory
- Arrhenius introduce a radical theory in 1894
- Arrhenius receive a Nobel Prize for this theory
- According to this theory an acid that ionizes partially or completely in water to give h ions which associated with the solvent (which associated with the solvent to give hydronium ions) H2O.
HA + H2O → H3O + A –
- A base ionizes in water to give hydroxyl ions
- Weak (partially ionized) bases generally ionize as follows
B + H2O ↔ BH+ + OH–
- While strong bases such as metal hydroxide (NaOH) dissociated as
M(OH)n → Mn+ + nOH–
Theory restricted to solvent because in this theory limited to water as a solvent.
Acid Base Titration Notes
Bronsted – Lowry Theory
- Bronsted – Lowry separately describes the cherry in 1923.
- According to this theory, an acid is any substance that can denote a Proton and a base is any substance that can accept a Proton.
- thus we can write a half-reaction.
Acid =H+ + Base
- The acid and base of a half-reaction are called conjugate pairs.
- Free Proton does not exist in solution and there must be a Proton acceptor (base) before a Proton alone or (acid ) will release its proton.
- This theory is related to taking and giving electrons.
- The theory was introduced by G.N Lewis in 1923.
- According to this theory, base donates electron pair and acid accept electron pair
- The base contains oxygen/nitrogen as the electron donor, for example, Ether.
- Thus non-hydrogen-containing substances are introduced as acids. e.g aluminum chloride.
- This theory is useful to describe the indicator color change in the non-protonic system Exhibiting acid-base reaction.
2.4 Neutralization curve
- Strong Acid/Base
A strong acid is completely dissociated into its components ions in dilute aqua solution strong base is also completely dissociated into its components science. e.g HCl, NaOH, KOH, etc.
- Weak Acid
A weak acid is one that sexy video is completely dissociated. The extent to which it dissociated is characterized by Ka.
- Weak Base
The weak base does not ionize fully in an aqua solution the extent of dissociation of the associated is characterized by kb
- Buffer Solution
It is a solution of substance or a mixture of a substance that helps in maintaining and establishing PH.
If we study the changes in the H ion concentration during the course of titration we get a clear idea about the mechanism of the neutralization process.
The pH value of greatest importance is the one near the equivalence point as it helps it in selecting an indicator that will give the smallest titration error.
The curve obtained by plotting PH as ordinate against the percentage of acid remaining during titration is known as the neutralization curve or more generally titration curve.
written by: Ms. Mayuri Lendave (Asst. Professor- Pharmaceutical Analysis)