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.