An Improved Method for the
Analysis of Linear Alkylate Sulfonate
LAWRENCE K. WANG, Environmental Engineer
JOHN Y. YANG, Principal Chemist
Environmental Systems Department
Calspan Corporation
Buffalo, New York
MU HAO WANG, Research Fellow
Department of Civil and Environmental Engineering
Rutgers University
New Brunswick, New Jersey
INTRODUCTION
The wide usage of linear alkylate sulfonate (LAS) type detergents in both domestic and
industrial applications, together with the known toxicity of these pollutants, have made the
surveillance of anionic surfactant discharges in waste water effluents one of the most
important water quality safeguard requirements. The needed routine analysis task is
complicated by the acknowledged difficulty inherent in the determination of trace
quantities of anionic detergents in saline water.
The most commonly-used colorimetric procedure for LAS is the Methylene Blue
Method (1,2). This method depends on the formation of a blue-colored complex when
methylene blue reacts with LAS. The resulting complex is more soluble in chloroform than
in water, whereas LAS and methylene blue themselves are soluble in water and not in
chloroform. The intensity of the blue color in the chloroform layer as determined
spectrophotometrically is taken as a measure of the LAS content. However, it is known the
standard Methylene Blue Method cannot be used for the measurement of anionic
surfactant-type materials in saline waters (2).
A serious deficiency arises in the fact that this method will exhibit a positive
interference due to an electrolyte effect leading to the salting-out of methylene blue ion pairs
which become chloroform soluble. Thus, the Methylene Blue Method will give rise to
erroneous results when the salinity of the water samples is high.
Spectroscopic determination in conjunction with carbon adsorption (Carbon
Adsorption Method) is recommended by Standard Methods as an alternative for
determining the LAS of water and wastewater. This method is based on a preliminary
concentration of the pollutant by carbon adsorption followed by either ultraviolet or infrared spectrophotometric analysis. Although this method is more precise and reliable than the
Methylene Blue Method, it is rather tedious and requires sophisticated instrumentation not
normally available in the water and waste analysis laboratory. It is, therefore, not suitable
as a routine analysis technique. In addition, it has been noted in the Standard Methods that
the Carbon Adsorption Method is subject to interference and not applicable to the analysis
of sewage or industrial wastewaters.
For the study of marine ecology of marine pollution control, a reliable method for the
analysis of anionic surfactants, such as LAS or the like, is needed but is still absent (3).
Relevant data are presented here to introduce an improved two-phase titration method
which can be easily used in the field, and is applicable for analyzing the detergent content in
either fresh water samples or saline water samples.
Sodium tetraphenylboron, which was orginally used by Cross (4) for the identification
and determination of cationic surface-active agents, is examined as a titrant for anionic
surfactant analysis. This proposed method is based on an expected stoichiometric reaction
between the anionic surfactant and an added known cationic reagent. The excess cationic
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