Table of content
Acid Base Titration
Acid:
- Any substance which has a sour test.
- Its aqueous solutions turns litmus paper blue to red.
- Acid is a molecule or ion capable of donating hydrogen (proton or hydrogen ion H+).
- Acid has pH value less than 7.
- Examples: Hydrochloric acid (HCl), sulphuric acid (H2SO4).
Base:
- Any substance which has a bitter test.
- Its aqueous solutions turn litmus paper red to blue.
- Base of substance that produce hydroxide ion (OH-) in aqueous solution.
- Example: sodium hydroxide(NaOH), sodium bicarbonate (NaHCo3).
Definition - Acid base titration.
- An acid best titration is the determination of concentration of acid or base by exactly neutralizing the acid or base with an acid or base of known concentration. this allows for quantitative analysis for the concentration of an unknown acid or base solution.
- It is also known as neutralization titration.
- Example: HCl + NaOH —---> NaCl + H2O
Acid Base Theories:
There are five theories acid and base.
- Arrhenious theory.
- Bronsted Lowry theory.
- Lewis theory
- Usanovich theory
- Lux flood theory
1)Arrhenius Theory.
- Svante Arrhenious as a graduate student, introduced a radical theory in 1894 (for which received the Nobel prize).
- It defines an acid as a hydrogen compound ionizing in water to give hydrogen ions, and a base as a hydroxyl compound which gives hydroxide ions in water.
- The neutralization reaction between an acid and a base produces a salt and water only:
HA(acid) + BOH(base) <—----> BA(salt) + H2O(water)
Example:
HCl ( acid ) + H2O <—---> H+ + Cl-
NaOH (base) +H2O <—-----> Na+ + OH-
Nature of the water:
Nature of the water is neutral as per the Arrhenius concept and acts as a solvent.
Neutralization Reaction:
Acid and base will react with each other and neutralization reaction takes place; finally they will form a salt and water molecule after completion of the reaction.
Example:
In (Aqueous medium) ,
HCl + NaOH <—---->NaCl + H2O
(Acid) (Base) (Salt) (Water)
Advantage:
This concept explain the behaviour of and base practically.
Limitations:
- In this concept acid and base is defined in the aqueous medium only.
- This concept fails to explain the stability of the H ions.
- It does not explain the conjugate acid-base theory.
- It does not able to define those acids or base which does not contains H+ or OH- ions like SO2 , CO2 , etc.
2)Bronsted- Lowry Theory.
- In 1923, Bronsted and Lowry separately described a theory known as the Bronsted-Lowry Theory.
This theory states that:
- Acids are those compounds or species, which have the tendency to donate the protons (H+) in any type of solvent by any method.
- Example: H2SO4, HCl, Ch3COOH, NH4+, etc.
- Bases are those compounds or species, which have the tendency to accept the protons (H+) in any type of solvent by any method.
- Example: NaOH, KOH, OH-, CI- , Br-, F-, etc
Conjugate Acid-Base Concept: Always a pair of conjugate acid and base will be present.
Nature of the water:
Nature of the water is amphiprotic or amphoteric in nature,when sometimes water acts as an acid sometimes water acts as a base.
Advantages:
- This concept can explain the acid base in any type of solvent.
- This concept is able to explain the stability of the proton.
Limitation:
This concept is not able to define acid or base, where proton(s) are absent. ie. SO2, SO3 etc.
3)Lewis Theory:
- In 1923, G. N. Lewis also introduced the electronic theory of acids and bases. In the concept he has defined acid and base according to the electron pair donor acceptor concept.
- Bases are those species which have self tendency to donate the lone pair of electrons.
- Lewis bases are an nucleophiles.
- Example: NH3, RNH2, NH2, H2O, SO4, Cl-, Br-, F-, CN-, etc.
- Acids are those species which have self tendency to accept the lone pair of electrons.
- Lewis an acids are electrophiles.
- Example: AICl3, BCl3, SnCl2, CO2, SO H+, Ag+, Al2+, etc.
Nature of the water:
Water is a Lewis base.
Advantages:
- Defines acid and base without any type of solvent.
- Explained acid and base, which can not be defined by others.
4)Usanovich Theory:
- In year 1934, Usanovich modified the Lewis concept of acid and base by removing the acceptance or donation of electron pair.
- According to this theory "Acid is a chemical species that reacts with a base and gives cation or accepts anions or electrons".
- Examples: HCI, H2SO4, H+, Fe3+, Ce4+, etc.
- "Base is a chemical species that reacts with an acid and gives anions or electronsilute combines with cations".
- Examples: NaOH, NH3, CI-, Br-, F-, Fe2+, Ce3+, etc.
Nature of the water:
Nature of the water is amphiprotic or amphoteric in nature, when sometimes water acts as an acid sometimes water acts as a base.
5)Lux Flood Concept:
- Lux (1929) and supported by Flood (1947) first introduced the concept of acid-base reactions with respect to the oxide ion.
- According to this theory, bases are those species which can donate the oxide-ions.
CaO —---> Ca2+ + O2-
- Acids are those species which can accept the oxide-ions.
SO3 + O2- —---> SO42-
Nature:
As per Lux Flood concept ZnO is amphoteric in nature.
Limitations:
- Bases must contain oxide ion and acids must be able to accept the oxide ion.
Theories of acid base indicators
- An indicator is an Substance which is used in determinate the end point in a titration.
- In acid base titration, Organic substance (weak acid or weak base) or generally used as indicator.
- They change their colour within a certain pH range.
There are two types of theories of indicator.
- Ostwalt Theory.
- Quinoid Theory.
1)Ostwalt Theory:
- The first useful theory of indicator was suggested by W. Ostwald.
- According to this theory, the color change is due to ionization of the acid-base indicator. The unionized form has different color than the ionized form.
- The ionization of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionization is very much low in acids due to common H+ ions while it is fairly ionized in alkalies. Similarly if the indicator is a weak base, its ionization is large in acids and low in alkalies due to common OH- ions.
- Considering two important indicators phenolphthalein (a weak acid) and methyl orange (a weak base), Ostwald theory can be illustrated as follows:
- Phenolphthalein: It can be represented as HPh. It ionises in solution to a small extent as:
HPh <—--> H+ + Ph-
(Colourless) (Pink)
- Applying law of mass action, the law of mass action is the proposition that the rate of a chemical reaction is directly proportional to the product of the activities or concentration of the reactants.
Kph a = [H+][Ph-]/[HPh] —-----> (1)
[H+] = Kph a × [HPh] / [Ph-] —------>(2)
By taking - log on both side on equation (2),
-log[H+] = -log( Kph a × [HPh] / [Ph-] )
-log[H+] = -log Kp a + ( log[HPh]/[Ph-] )
pH = pKPh a - log( [HPh]/[Ph-] )
pH = pKph a + log( [Ph-]/[HPh] )
- The undissociated molecules of phenolphthalein are colorless while Ph- ions are pink in colour. In presence of an acid the ionization of HPh is practically negligible as the equilibrium shifts to left hand side due to high concentration of H+ ions.
- Thus, the solution would remain colorless. On addition of alkali, hydrogen ions are removed by OH- ions in the form of water molecules and the equilibrium shifts to right hand side. Thus, the concentration of Ph- ions increases in solution and they impart pink colour to the solution.
2)Quinonoid theory.
- The Ostwald theory has been revised and, the color changes are believed to be due to structural changes.
- According to his theory, the acid-base indicators exist in two tautomeric forms having different structures. Two forms are in equilibrium. One form is termed benzenoid form and the other quinonoid form.
- The two forms have different colors. The color change in due to the inter conversation of one tautomeric form into other.
- One form mainly exists in acidic medium and the other in alkaline medium.
- Thus, during titration the medium changes from acidic to alkaline or vice-versa. The change in pH converts one tautomeric form into other and thus, the color change occurs.
Example:
- This may be show by reference of phenolphthalein, in the presence of dilute alkali, the lactone ring of [A] opens to yield the triphenylcarbinol structure [B]. This undergoes loss of water to produce the resonating ion [C].
- Phenolphthalein has benzenoid form in acidic medium and thus, it is colourless while it has quinonoid form in alkaline medium which has pink colour.
- Another example is methyl orange has quinonoid form in acidic solution and benzenoid form in alkaline solution. The color of benzenoid form is yellow while that of quinoniod form is red.
classification of acid base titrations
The acid base titration maybe classified on the basics of titration curve.
- Titration of strong acid with strong base.
- Titration of strong acid with weak base.
- Titration of weak acid with weak base.
- Titration of weak acid with strong base.
Strong acid: A strong acid dissociate or ionizes completely in an aqueous solution to form hydronium ion (H3O+).
HCl + H2O —---> H3O- + Cl-
Example: HCl, H2SO4, HNO3.
Week base: A base does not dissociate completely in an aqueous solution to form a hydroxide ion(H3O+).
Example: NH4OH, NH3.
1)Titration of strong acid which strong base.
- Let, hydrochloric acid(HCl) is a strong acid and sodium hydroxide(NaOH) is a strong base.
- For simplicity of calculation consider the titration of 100 mL of 1 M hydrochloric acid with 1 M sodium hydroxide solution. The pH of 1 M hydrochloric acid is O. When 50 mL of the 1 M NaOH have been added, 50 mL of unneutralised 1 M HCl will be present in a total volume of 150ml.
↑
Click the image for better view
Upon the addition of 100 mL of base, the pH will change sharply to 7, i.e. the theoretical equivalence point.pH during titration of 100 mL of HCl with 0.1 M NaOH of equal concentration.
↑
Click the image for better view
- So from the table pH change is very short 4.3 at 99.9 to 9.7 at 101.1. so infection range is formed from pH 4.3 to 9.7.
- With 0.01M solutions, the ideal pH range is still further limited to 5.5 to 8.5; such indicators as methyl red, bromothymol blue, or phenol red will be suitable.
- The titration error for methyl orange will be 1-2 per cent.
2)Titration of strong acid with weak base.
- Aqueous ammonia solution is a weak base titrated with hydrochloric acid is strong acid.
NH4OH + HCl —---> NH4Cl + H2O
- When strong acid is added to a week base, salt is formed which is further hydrolysed.
NH4+ + H2O —---> NH4OH + H+
- So due to hydrolysis of salt and formation of H+ ion pH of the solution at equivalence point is less than 7.
- If 100ml of 0.1 m of NH4OH is titrated against 0.1 m HCL
- Infection range is formed ph3 to 6.5 . So methyl red ( pH range 4.8 to 6) is an indicator of choice for this titration.
3) titration of weak acid and strong base.
- Acetic acid (week acid) is titrated with strong base sodium hydroxide( NaOH).
CH3COOH + NaOH —---> CH3COONa + H2O
- When strong acid is added to weak base it from Salt which suppress dissociation of weak acid and decrease H+ ion concentration result increase in pH.
- Due to hydrolysis of salt the equivalent point is more than pH7.
- So from the table at equivalence point the pH change is very sharp 7.7 at 99.9 to 10.0 at 100.2 . This for 0.1 M solution inflection range is from pH 7.7 to 10.0 .
- So any indicator have pH range between this inflection range can be used for this titration.
- Phenolphthalein and thymolphthalein, or thymol blue has pH range 8.2 to 10 . so, this indicator are choice for this titration.
↑
Click the image for better view
↑
Click the image for better view
4) Titration of weak acid and weak base.
- Titration of acetic acid ( weak acid ) with ammonium hydroxide ( weak base ).
CH3COOH + NH4OH —----> CH3COONH4 + H2O
- Salt formed during reaction is further hydrolysis.
CH3COONH4 + H2O —---> CH3COOH + NH4OH
- If 100 ml of 0.1 M NH4OH solution is titrated with 0.1 M CH3COOH.
- The infection ranges between pH 6 to 8.
- Each sharp end point can not be detected by simple indicator. so , mixed indicator is used neutral red-methylene blue mixed indicator may be uesd in this titration.
---------------X---------------
Comments
Post a Comment