titrations
03/18/21DR NDUNGÚ KJ, PHARM ANALYSIS 1
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NON AQUEOUS TITRATION, Lecture notes of Pharmaceutical Analysis
Introduction Procedure Non aqueous solvents
Typology: Lecture notes
2020/2021
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titrations
Introduction
Solvents for Non- Aqueous
Titrations Indicators for Non- Aqueous
Titrations Titrations of Weak Bases
(^) Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end point when titrations are carried out in non- aqueous media. (^) An additional advantage is that many substances, which are insoluble in water , are sufficiently soluble in organic solvents to permit their titrations in these non-aqueous media.
Any acid, (HB) is considered to dissociate in solution to give a proton (H+) and a conjugate base (B-) (^) Where as any base (B) will combine with a proton to produce a conjugate acid (HB+) : HB ↔ H+^ + B- B+H+↔ HB+
The substances, which are either to weakly
acidic or too weakly basics to give sharp
end point in aqueous solutions, can easily
be titrated with accuracy in non-aqueous
solvent.
(^) 1) Organic acids and bases that are insoluble in water are soluble in non-aqueous solvent.
- Organic acid, which is of comparable strength to water, cannot be titrated easily non-aqueous solvent. (^) Bases also follow the same rules.
- A non-aqueous solvent may help two or more acids in mixture. The individual acid can give separate end point in different solvent.
1. Aprotic Solvents : Aprotic solvents include those substances, which may be considered chemically neutral, and virtually un-reactive under the conditions employed. (^) Carbon tetrachloride and toluene come in this group; they possess low dielectric constants, do not cause ionization in solutes and do not undergo reactions with acids and bases. (^) Aprotic solvents are frequently used to dilute reaction mixture.
(^) Protophilic solvents are the substances that possess a high affinity for protons. The over all reaction can be represented as: - HB+S ↔ SH+^ + B- The equilibrium in this reversible reaction will be generally influenced by the nature of the acid and the solvent. (^) Weak acids are normally used in the presence of strongly protophilic solvents as their acidic strengths are then enhanced and then become comparable to these of strong acids; this is known as the levelling effect.
(^) Amphiprotic solvents consist of liquids, eg water, alcohols and weak organic acids, which are slightly ionized and combine both protogenic and protophillic properties. Coz of being able to donate protons and accept protons. (^) Ethanoic acid displays acidic properties in dissociating to produce protons: CH 3 COOH ↔ CH 3 COO-^ + H+ (^) But in the presence of perchloric acid, a far stronger acid, it will accept a proton: CH 3 COOH + HClO 4 ↔ CH 3 COOH 2 +^ + ClO 4 - onium ion
(^) The CH 3 COOH 2 + (^) ion can very readily give up its proton to react with a base, so basic properties of a base is enhanced. So titrations between weak base and perchloric acid can often be accurately carried out using ethanoic acid as solvent. Water can act the same way H 2 O + H 2 O ↔ H 3 O+ + OH- hydroxonium ion base
i) CH 3 COOH + HClO 4 ↔ CH 3 COOH 2 +^ + ClO 4 - i) CH 3 COOH 2 +^ + B ↔ CH 3 COOH + BH During NAT water shud be avoided at all cost ie the solvent must b anhydrous, thus anhydrous acetic anhydride is used. CH 3 COOH 2 +^ ion combines with any water present to form acetic acid. CH 3 COOH 2 +^ is very unstable n thus dissociates readily than H 3 O+
H
3
C O
O
CH
3
O
(^) Determination in non-aqueous solvents is important for substances which may give poor end points in normal aqueous titration and for substances which are not soluble in water. (^) They are particularly valuable for determining the properties of individual components in mixtures of acids or mixture of bases. These differential titrations are carried out in solvents, which do not exert a leveling effect.
(^) Although indicators may be used to establish individual end points, as in traditional acid-base titrations, potentiometric methods of end point detection are also used extensively, especially for highly coloured solutions. Non aqueous titration have been used to (^) quantify the mixtures of primary, secondary and tertiary amines, (^) for studying sulphonamides, (^) mixture of purines (^) many other organic amino cpds and salts of organic acid.