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Title:Assay of following compounds by non-aqueous titration(using perchloric acid as a titrant): i)Nicotinamide ii)Pyridoxine hydrochloride Mol.Wt.122.1 Mol.Wt.205.6
Theory: Substances which are either too weakly basic or too weakly acidic to give sharp end points in aqueous solution can often be titrated in non-aqueous solvents. These substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end points when titrations are carried out in non-aqueous media. An additional advantage is is that many substances substances which are insoluble in water are sufficiently soluble in organic solvents to permit their titration in these non-aqueous media. The Lowry Bronsted theory can explain the reactions which occur during many non-aqueous titrations. According to the theory an acid is a proton donor and a base is a proton acceptor.
HB acid
+
-
proton
base
H +B
An acid may be either an electrically electrically neutral molecule, such as HCl, or a positive charged cation such as + C6H5 NH3 . A base may be either an electrically neutral molecule, such as C6H5 NH2, or a negatively charged anion, such as Cl . Substances which are potentially acidic can can function as acids only in the presence of a base to which they can donate a proton. Conversely, basic properties do not become apparent unless an acid also is present. Hence, the ability of substances to act as acids or bases will depend very much upon the nature of the solvent system which is used. Types of solvents : Aprotic solvents: Carbon tetrachloride, benzene, toluene etc. Protophilic solvents: Ammonia, amines, ketones etc. Protogenic solvents: Anhydrous acids such as hydrogen fluoride, sulphuric acid etc. Amphiprotic solvents-: Water, acetic acid, alcohols etc.
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Amphiprotic solvents have both protogenic and protophilic properties. Acetic acid is frequently used solvent in non-aqueous titration of basic substances. Acetic acid dissociates to produ ce protons: -
CH3COO H
+
CH3COO + H
Here, the acetic acid is functioning as an acid. If a very strong acid such as perchloric acid is dissolved in acetic acid, the latter can function as a base and combine with proton donated by the perchloric acid(strong acid) to form and ‘onium; ion HClO4
+
H + ClO4
CH3COO H + H
-
+
+
CH3COOH2 (onium ion)
+
The CH3COOH2 so formed (as an acid) can readily give its proton to base(Nicotinamide). A weak base will, therefore, have its basic properties enhanced, and a consequence titration between weak bases and perchloric acid can readily be carried out using acetic acid as a solvent. H2 NCO C5H4 N (Nicotinamide) +
-
CH3COOH2 + CH3COO H2 NCO C5H4 N 122.1g
+
+ CH3COO H
-
H2 NCO C5H4 NH + CH3COO
2CH3COO H
= 1000ml of 1M perchloric acid
H2 NCO C5H4 N 0.01221g= 1ml of 0.1M perchloric acid,
Perchloric acid in non aqueous media is used in the assay of alkali metal salts of organic acids, primary , secondary and tertiary amines, amino acids. It is also equally applicable to amine and other organic base salts with organic acids. The halogen acid salts of bases or the halide ions chloride, bromide and iodide are too weakly basic to react quantitatively with acetous perchloric acid. Addition of mercuric acetate(which is undissociated in acetic acid solution) to a halide salt replaces the halide ion by an equivalent quantity of acetate ion, which is a strong base in acetic acid. 2 C8H11 NO3,HCl(Pyridoxine HCl)
2C10H16 NO -
(CH3COO)2Hg (undissociated) + 2Cl +
-
2CH3COOH2 + 2CH3COO
+
-
HgCl2 (undissociated) + 2CH3COO
4CH3COOH
C8H11 NO3,HCl 205.6g = 1000ml of 1M perchloric acid 0.02056 g C8H11 NO3,HCl= 1ml of 0.1M perchloric acid, Indicators:
-
+ 2Cl
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In non-aqueous titration the colour change exhibited by an indicator at the end point is not always the same for different titrations as it depends upon the nature of the titrand to which it has been added. The colour corresponding to the correct end point may be established by carrying out a potentiometric titration while simultaneously observing the colour change of the indicator. Typical indicator solutions for non-aqueous titrations are the following: Crystal violet(0.5%w/v in G.acetic acid ), methyl red(0.2%w/v in dioxan), 1-naphthol benzein(0.2%w/v in G.acetic acid), oracet bule B(0.5%w/v solution in G. acetic acid) etc. The most used indicator is acetous crystal violet which gives violet colour with substances which behave as bases in acetic acid solution. Addition of acetous perchloric acid changes it to greenish-blue (emerald green)at end point. In the acidic condition generated by acetous perchloric acid the colour of indicator becomes greenish-yellow. Addition of 2-3 drops of the indicator is sufficient to indicate the end point of titration.
Procedure: i)For Nicotinamide: Weigh accurately about 0.25 g dried sample, dissolve in 20 ml of previously neutralised anhydrous glacial acetic acid to crystal violet solution, heating slightly if necessary. Add 5 ml of acetic anhydride. Titrate with 0.1 M perchloric acid until greenish-blue(emerald green) colour appears. Carry out three sets of titration. The first titration is for the approximation of the end point.
ii)For Pyridoxine hydrochloride: Weigh accurately about 0.15 g, dissolve in a previously neutralised mixture of 5 ml of anhydrous glacial acetic acid and 6 ml of mercuric acetate solution. Titrate with 0.1 M perchloric acid, using crystal violet solution as indicator, until a emerald green colour is produced. 1 ml of 0.1 M perchloric acid is equivalent to 0.02056 g of C8H11 NO3,HCl. ii)Pyridoxine HCl Wt. of Initial burette Final burette Total vol. of
Calculation: i)Nicotinamide: S1= Vol. of titrant x factor of titrant x0.01221 g x 100 % = Wt.of dried sample in g S2= Vol. of titrant x factor of titrant x0.01221g x 100 % = Wt.of dried sample in g
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Mean assay % of Nicotinamide(On dried basis) = ii)Pyridoxine hydrochloride S1= Vol. of titrant x factor of titrant x0.02056 g x 100 % = Wt.of dried sample in g S2= Vol. of titrant x factor of titrant x0.02056 g x 100 % = Wt.of dried sample in g Mean assay % of Pyridoxein hydrochloride (On dried basis) =
Nicotinamide
Pyridoxine HCl
