Non-Aqueous titrations

That is, the perchloric acid acts as a monoprotic acid and 1 mole of perchloric acid is equivalent to 1 mole of basic drug. The derivation of the equivalent and the calculations required are the same as for their aqueous counterparts. 

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Non-aqueous titrations have been used to quantify mixtures of primary, secondary and tertiary amines,5 for studying sulphonamides, mixtures of purines and for many other organic amino compounds and salts of organic acids. 

The various relationships concerning the interconversion between un-ionised and ionised or different resonant forms of indicators referred to in Section 10.7 apply equally well to those indicators used for non-aqueous titrations. However, in this type of 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. The appropriate colour corresponds to the inflexion point of the titration curve (see Section 15.18). 

The majority of non-aqueous titrations are carried out using a fairly limited... 

As indicated in Section 2.4 the strength of an acid (and of a base) is dependent upon the solvent in which it has been dissolved, and in Sections 10.19-10.21 it has been shown how this modification of strength can be used to carry out titrations in non-aqueous solvents which are impossible to perform in aqueous solution. Potentiometric methods can be used to determine the end point of such non-aqueous titrations, which are mainly of the acid-base type and offer very valuable methods for the determination of many organic compounds. 

Table 15.6 Some common potentiometric non-aqueous titrations...

Potentiometric titrations - continued EDTA titrations, 586 neutralisation reactions, 578, 580 non-aqueous titrations, 589, (T) 590 oxidation-reduction reactions, 579, 581, 584 precipitation reactions, 579, 582 Potentiometry 548 direct, 548, 567 fluoride, D. of, 570 Potentiostats 510, 607 Precipitants organic, 437 Precipitate ageing of, 423 digestion of, 423... 

After the acetylation, the analytical finish is not based on non-aqueous titration, but consists in acid titration after complete conversion of the anhydride with water and by comparison with a blank determination. 

Whereas in many instances potentiometric non-aqueous titrations of acids can show anomalies24 depending on the type of solvents and/or electrodes (owing to preferential adsorption of ions, ion pairs or complexes on the highly polar surface of the indicator electrode, or even adherence of precipitates on the latter), conductometric non-aqueous titrations, in contrast, although often accompanied by precipitate formation30, are not hindered by such phenomena sometimes, just as in aqueous titrations, the conductometric end-point can even be based on precipitate formation34. 

According to Harlow73, it is nevertheless one of the least unstable quaternary ammonium hydroxides (see under practice of non-aqueous titration, p. 281). 

Neutralization with KOH in aqueous THF gave the desired poly(styrene-b-potassium methacrylate) (S-b-MA.K). The carbonyl band in the IR spectrum is replaced with a strong, broad carboxylate absorption centered near 1566 cm 1 (Figure 2c). The carboxylate and potassium contents were assayed by non-aqueous titration and ICP, respectively. The resulting values of 0.91 meq COj /g and 0.98 meq K/g indicate essentially quantitative conversion to the potassium methacrylate. S-b-MA.K obtained in this manner is easily dissolved in solvents such as THF and dichloromethane, in contrast to the... 

A non-aqueous titration method was developed (25,26) for the determination of basic compounds with thio(-S-) and mercapto (-SH-) groups. The reaction of the S group with Hg (0Ac)2 in acetic acid makes this possible. Methimazole and other compounds were all titrated with HCIO in acetic acid using gentian violet as indicator. 

Non-aqueous titration of tablet extracts with sodium methoxide(27) or lithium methoxide(l)(2) has been reported. 

The determination of neomycin by non-aqueous titration has been described by Penau et all2l. Neomycin base is allowed to react with standardised perchloric acid the excess acid is then back-titrated with potassium hydrogen phtha-late using crystal violet as indicator. To determine the neomycin content of the sulphate salt the same authors precipitated the sulphate with benzidine before reacting the neomycin with perchloric acid. The amount of benzidine required to precipitate the sulphate is calculated from the sulphate content which is itself determined by titration with sodium hydroxide. 

In order to overcome these shortcomings the non-aqueous titrations were introduced. 

Non-aqueous titrations have the following advantages, namely ... 

Weak bases which have KZ> values less than 10-6 can be titrated satisfactorily by non-aqueous titrations. The reason being that in aqueous medium and at higher K/> values (> 10 6) the solvent water competes progressively with the basic species in solution for the proton of the solvent. 

The concepts of the Lowry-Bronsted theory may explain the various reactions that take place during many non-aqueous titrations. Thus, an acid is a proton donor and a base is a proton acceptor. Therefore, when an acid HA undergoes dissociation it gives rise to a proton and the conjugate base A of the acid ... 

In short, it is possible to titrate mixtures of two or three components selectively with a single titration by wisdom of the right choice of solvent for the non-aqueous titrations. 

A number of indicators stated below are commonly used in non-aqueous titrations. It is, however, necessary to mention here that the same indicator must be used throughout for carrying out the standardization, titration and neutralization of mercuric acetate solution. 

Assays of various pharmaceutical substances either in pure form or in dosage form may be assayed successfully by non-aqueous titrations. For the sake of convenience these typical titrations can be categorized into two broad groups, namely ... 

Table 5.1 Acidimetric Assays Non-aqueous Titrations with Perchloric Acid using Various Indicators...

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