TITRATION OF SURFACTANTS

The most useful methods for volumetric determination of ionic surfactants are based on titration of an anionic surfactant with a cationic surfactant or the titration of a cationic surfactant with an anionic surfactant. In general, equivalent results are obtained by pouring the sample into the buret and using it to titrate a precisely known quantity of a standard solution of opposite charge. Thus, the methods given below for determination of anionics and determination of cationics are to a great extent interchangeable with each other the reader should study both sections to appreciate the choices available for the analysis. 

Reiner Schulz has written a wonderfully clear book on surfactant titrations. Anyone able to read German is urged to study that work, which contains a wealth of practical knowledge (1). 

Titrimetric methods were the first procedures to be widely applied to the analysis of anionic surfactants and they remain very useful for assay purposes. Their speed and low cost make them especially suitable for quality control. Cross has written a thorough and recent discussion of the titration of anionic surfactants (2) 

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Most anionic surfactants are salts of moderately strong acids. As such, they can be titrated directly with base, provided that a suitable solvent system and visual or instrumental end point can be found. However, since anionic surfactants are generally found in the company of other ionic materials, some of them also acidic, direct acid-base titration is not used for most applications. For example, acid-base titration for assay of alkylarylsulfonate would risk high results because of titration of byproduct sulfate or other ions. Acid-base titration of LAS in a detergent formulation suffers from interference from such buffering compounds as sodium silicate and sodium tripolyphosphate. Titration with alkali is therefore limited to cases where the anionic surfactant can be isolated in pure form. 

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EXPERIMENTAL PROCEDURES AUTOMATED TURBIDIMETRIC TITRATION. A method for the automated aqueous turbidimetric titration of surfactants has been published (10) in which anionic surfactants are titrated against N-cetylpyridinium chloride to form a colloidal precipitate near the equivalence point. N-cetylpyridinium halides have a disadvantage in that they have the tendency to crystallise out of solution (15), consequently the strength of the solution may alter slightly without the knowledge of the operator, also the crystals suspended in solution may cause damage to the autotitrator. In view of these drawbacks hyamine was preferred as the titrant. 

Vytas, K. Kalous, L Jekova, J. Automated potentiometry as an ecologic alternative to two-phase titrations of surfactants. Egypt. J. Anal. Chem. 1997, 6, 107-123. 

Upon subsequent steps of titration of surfactant into the cell, the total concentration in the cell wiU increase, but since some of the detergent is removed after each titration step, the amount of surfactant in the cell after n steps of titration will be smaller than nvtDf After the first step of titration and removal of a volume v the amount of surfactant left in the cell, as given by Eq. (27), will be... 

Buschmann, N. Does the two-phase titration of surfactants require a mutagenic indicator J. Am. Oil Chem. Soc. 1995, 72,1243. 

Schulz, R., Titration of Surfactants and Pharmaceuticals (in German), Verlag fUr Chemische Industrie, Augsburg, 1996. 

Arens, M., J. G. A. Kooijman, H. P. Wingen, Potentiometric titration of surfactants—Determination of the active agent in quaternary fatty alkyl ammonium salts—Collaboration of the DGF. Communication 155 German standard methods for study of fats, fatty products, surfactants and related materials. Communication 121 Analysis of surface active materials XXVIII (in German), Fett Lipid, 1996,98,128-130. 

Concerning the determination of surfactants, in 1997 Vytras s group demrai-strated that carbon paste-based electrodes could also be used as potentiometric sensors to monitor titrations of surfactants.When compared to PVC and CW electrodes, CPEs had the advantages of very low Ohmic resistance, very short response time in addition to the ease of fabrication and regeneration as well as long functional lifetime. Jezkova et al. used perchlorate and fluoroborate ion-selective carbon paste electrodes for both direct potentiometric measurements and potentiometric titration of perchlorate or fluoroborate with 0.1 M CPC. The electrodes had a rapid response, low Ohmic resistance, and limits of detections and selectivity similar to the limits of commercial membrane electrodes. A carbon paste electrode incorporated with the ion association of CPC-thallium halo complexes was applied as indicator electrode with the potentiometric titration of surfactants. ... 

Fig. 10.2 Schematic diagram of a portable system manifold used for the potentiometric titration of surfactants in the field. (Reprint with permission from reference Copyright (2008) Elsevier...

A commercially available fluoroborate ion-selective indicator electrode was used for the potentiometric titration of surfactants and soaps.The titrants were HDTAC, hexadecylpyridinium chloride (HDPC), and diisobutylphenoxy-ethoxyethyI-(dimethyI)benzyIammonium chloride (DIPEBC). Similarly, Benoit et al. " titrated CTAB with sodium tetraphenylborate potentiometrically using a commercial CI04 selective electrode. The method proved to be precise exhibiting a relative standard deviation of 1.1 %. 

Buschmann N, Schultz R (1992) Development of an ion-sensitive electrode fin the titration of surfactants. Comun Jom Com Esp Dctcrg 23 323—328... 

Vytras K, Kalous J, Jezkova J (1997) Automated potentiometry as an ecologic alterative to two-phase titration of surfactants. Egypt J Anal Chem 6 107-128... 

Khaled E, Mohamed GG, Ali TA (2008) Disposal screen-printed carbon paste electrodes for the potentiometric titration of surfactants. Sens Actuators B Chem 135 74-80... 

Selig S (1980) The potentiometric titration of surfactants and soaps using ion-selective electrodes. Fres J Anal Chem 300 183-188. 

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