Nonionic surfactants calibration standards

Transfer 20 mL of a sample containing not more than 4 mg nonionic surfactant to a beaker. Add 20 mL 8% NaCl solution. Rapidly add 10.0 mL potassium tetraiodomercurate reagent and let stand exactly 15 min. Read the turbidity promptly with a nephelometer and compare to a calibration curve prepared with standards of the same composition as the sample. 

Calibration is more complicated for nonionic than ionic surfactants. Most ionic surfactants have only one functional group per molecule. Thus calibration can be performed with any well-characterized standard compound, and the results for an unknown will still be fairly accurate. With nonionics, the color development is not only related to molar concentration, but is also a function of the ethoxy chain length and the particular hydrophobe. Thus, all of the spectrophotometric methods suffer from the disadvantage that, if an arbitrary standard must be chosen on which to base concentration measurements, large errors may creep in, since the standard will not yield the same molar calibration curve as the sample. 

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