Post on 18-Jul-2015
Non-aqueous acid base titrimetry
Md : Obydulla Al MamunId:131-29-500
9th(B)Department Of Pharmacy
Daffodil International Universityalmamundcc94@gmail.com
Non-aqueous acid base titrimetry
Non- aqueous titrations are those in which thetitrations of too weakly acidic or basicsubstances are carried out using non-aqueoussolvents so as to get sharp end point.
Such titrations can also be used for the titrationof the substances not soluble in water.
The speed, precision and accuracy of the non-aqueous method are close to those of classicalacidimetric and alkalimetric titrations.
REASONS FOR PERFORMING NON – AQUEOUS ACID – BASE
TITRATION
Non – aqueous Acid – Base Titration is performed to eliminate 2 problems encountered during the aqueous titration of weakly acidic or weakly basic analyte by a Strong Acid or Strong Base Titrantrespectively.
The 2 problems are –
Interaction of the Titrant with H2O
Poor Solubility of Weakly Acidic (WA) or Weakly Basic (WB) Analyte in H2O
ACIDS & BASESAcids:
Arrhenius acid: Any substance that, when dissolved in water, increases the concentration of hydronium ion (H3O+)
Bronsted-Lowry acid: A proton donor
conjugate base
Lewis acid: An electron acceptor
Bases:
Arrhenius base: Any substance that, when dissolved in water, increases the concentration of hydroxide ion (OH-)
Bronsted-Lowery base: A proton acceptor
conjugate acid
Lewis acid: An electron donor
Solvents
Non-aqueous solvents are classified into the 4groups:
- Protophilic solvents
- Protogenic solvents
- Amphiprotic solvents
- Aprotic solvents
Protophilic solvents
Possess high affinity for proton
Weak acids are normally used as solute
Strong protophilic solvents convert weak acid
to strong acid-known as ’leveling effect’
Example: Liquid ammonia, amines, ether andketones
HA + S SH+ + A-
Weak Acid (appeared as strong acid)
Basic solvent
Solvated proton
Conjugated base of acid
Protogenic solvents
Acidic in nature
Readily donates protons
Strong protogenic solvents increase the
strength of weak bases
Such solvents exert a leveling effect on all
bases dissolved in them
Example: Anhydrous acid like hydrogenfluoride & sulfuric acid
B + H+ BH+
Weak Base (appeared as strong
base)
From solvent
Conjugated acid of base
Amphiprotic solvents
Combine protogenic and protophilic
properties of solvent
Able to both donate and accept proton
Example: Water, alcohol & weak organic acid
Acetic acid shows acidic property by releasingproton-
CH3COOH CH3COO- + H+
In presence of perchloric acid (strong acid)acetic acid shows basic property by acceptingproton and produce ‘onium’ ion-
CH3COOH + HClO4 CH3COOH2+ + ClO4
-
‘onium’ ion
APROTIC SOLVENTS
Chemically neutral substances
Virtually un-reactive
Do not cause ionization of solute
No reactions with acids and bases
Used to dilute reaction mixture
Example: Carbon tetrachloride,benzene, tolune.
NON AQUEOUS TITRATION OF WEAK ACIDS
Many weakly acidic substances (aicohol or aprotic solvent)can be titrated in an appropiate non aqueous solvents with a sharp end point .
Ex:- acidic halides, acids,amino acids, enols(barbiturates,xanthines), phenols, pyrroles sulphonamides etc,.
1) Solvents used in the titration of weak acids:Ex:-Ethylenediamine,n-butylamine,morpholine
2) Titrant used in the titration of weak acids:Ex:-sodium methoxide, lithium methoxide, potasium
methoxide ,tetrabutyl ammonium hydroxide etc.
3) Indicatores used in the titration of weak acidsEx:-azo violet.thymol blue,thymolphthalein.,O-Nitro aniline
NON AQUEOUS TITRATION OF WEAK BASES
Solvents used in the titration of weak
bases;
Neutral solvents :
Ex: - alcohol,chloroform,benzene,chlorobenzene
Acidic solvents:
Ex:-formic acid,glacial acetic acid,propionic acids
Titrant used in the titration of weak basess:
Ex:-Perchloric acid
Indicatores used in the titration of weak
bases
Ex:-oracat blue ,crystal
violet,naphtholbenzein(weak bases)
BASIS OF INDICATOR SELECTIONIndicator colour change, from acid
to alkali pKind
pH
range example of titration use
Methyl orange, (red ==> yellow) 3.7 3.1-4.4
weak base - strong acid
titration e.g. ammonia titrated
with hydrochloric acid
Bromophenol blue, (yellow ==>
blue) 4.0 2.8-4.6
weak base - strong acid
titration
Methyl red, (red ==> yellow) 5.1 4.2-6.3 weak base - strong acid
titration
Bromothymol blue, (yellow ==>
blue) 7.0 6.0-7.6
strong acid - strong base
titration e.g. hydrochloric acid
<=> sodium hydroxide titration
Phenol red, (yellow ==> red) 7.9 6.8-8.4
strong acid - strong base
titration e.g. hydrochloric acid
<=> sodium hydroxide titration
Thymol blue (base form), (yellow
==> blue) 8.9 8.0-9.6
weak/strong acid - strong base
titration
Phenolphthalein, (colourless ==>
pinky-red) 9.3
8.3-
10.0
weak acid - strong base
titration e.g. ethanoic acid
titrated with sodium hydroxide
Preparation of a 0.1 N solution:
When metal has dissolved, have to add sufficient methanol untilclear solution
Then add dry benzene slowly with continuous shaking until thesolution appears cloudy
Repeat the addition of methanol followed by benzene until 1 literclear solution has been prepared.
Mixture of 40 ml methanol and 50 ml drybenzene in Erlenmeyer flask
Add 4 gm of K or 2.3 gm of Na or 0.6 gm of Lito the flask (the metal should be freshly cut andhave to add slowly)
1. Minimum amount of methanol have touse to ensure clear solution.
2. Have to store in sodium free glass.
3.Have to protect it from atmospheric CO2.
Precaution:
Titrants are usually standardized by usingreference standard- benzoic acid.
0.5% thymol blue in anhydrous methanolused as indicator.
Dimethylformamide used as solvent forthe titration.
Standardization:
ADVANTAGES OF NON AQUEOUS SOLVENT OVER AQUEOUS SOLVENT:
1) Organic acids and bases that are insoluble in water are soluble in non-aqueous solvent.
2) Organic acid, which is of comparable strength to water, can be titrated easily in non-aqueoussolvent. Bases also follow the same rules.
3) A non-aqueous solvent may help two are more acids in mixture. The individual acid can giveseparate end point in different solvent.
4) By the proper choice of the solvents or indicator, the biological ingredients of a substance whether acidic or basic can be selectively titrated.
5) Non aqueous titrations are simple and accurate, examples of non aqueous titration are:Ephedrine preparations, codeine phosphate in APC, tetracycline, teramycin, Antihistamines andvarious piprazine preparations
DISADVANTAGES NON-AQUEOUS ACID BASE
TITRATION
Temperature ,moisture,co2 should be control.
Solvents are expensive.
Volatile solvents can polute environment
Indicator must be prepare in non aquous
medium.