Quaternary ammonium compounds, titration

Alkylsilyl sulphates were determined180 by potentiometric and conductiometric titration in anhydrous alcohols, ketones, methyl cyanide or nitromethane. Titration curves were the same for all solvents. In potentiometry, titration with sodium, potassium or lithium methoxide gave curves with one step titration with quaternary ammonium compounds gave curves with two steps. In conductiometry, titration with quaternary ammonium compounds gave curves with no inflection, whereas titration with alkali metal methoxides gave curves with a sharp break at the end-point. 

Background. Another form of colloid titration for estimating cationic (quaternary ammonium) compounds is based on a potentiometric procedure and utilizes an eleetrode that is specific to the titrating anionic surfactant chosen. The theory is that the cationic material complexes stoichiometrically with the anionic surfactant. 

Salts of weak acids such as carboxylates , picrates , or carbonates can be titrated with mineral acids. Ammonium halides can be converted into acetates by treatment with mercury(n) acetate in glacial acetic acid, and determined by subsequent titration with perchloric acid . Salts of strong acids, such as sulphates and halides, can be determined by measuring the anion, or alternatively the quaternary cation can react to form complexes which precipitate, or remain in aqueous solution, or have to be extracted into oi nic solvents. These complexes can be measured directly, or indirectly by measuring the excess reagent. In Table 13 appears a list of reagents used for the determination of quaternary ammonium compounds by complex formation. 

Metcalfe, L. D., R. J. Martin, A. A. Schmitz, Titration of long-chain quaternary ammonium compounds using tetraphenylboron, J. Am. Oil Chem. Soc., 1966,43, 355-357. 

Regarding titrants, cationic substances (for example, protonized alkaloids, compounds containing quaternary nitrogen, etc.) are usually titrated with sodium tetraphenylborate, the exact concentration of which is determined titrimetrically against a standard substance such as thallium(I) nitrate or pure copper(II) or nickel(II) salts in the presence of 1,10-phenanthroline. For titrations of anionics, substituted quaternary ammonium or pyridinium salts are applied. The procedures are simple and represent an ecologic alternative to so-called two-phase titrations. ... 

Acid compounds are usually titrated with sodium or potassium hydroxide solutions. Apart from some mineral compounds, the othCT compounds titrated are essentially carboxylic acids, phenols, imides, and quaternary ammoniums resulting from the protonation of amines. 

A completely new approach to the determination of small amounts of nitrogenous bases (including tropine alkaloids) in aqueous solutions such as eye-drops and injections has been published by Johnson and King. Precipitation with tetraphenylboron at pH 3-7 is used the excess of the latter is then determined by back-titration with a standard solution of quaternary ammonium salt to a visual end-point. Melting-points of the organic tetraphenylboron salts may be used in the identification of many of these compounds. An assay, including standardisation of reagents, can be completed in half an hour and the accuracy of the method was shown to compare very favourably with that of many of the official methods for the compounds examined. 

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