The fact that the density of the carboxylic group plays a role in the Bathochromic titration is evident in that the shifts are much smaller in magnitude when the concentration of the surfactant is decreased to about 20 ppm (Figure ). From the midpoint of such a titration, the pKa of the carboxylic acid is found to be about 7.5, which is larger than that (5.2) expected of this organic acid in its native state. Similar behavior, i.e., the shift of the pKa to greater than 5.2 (Figure ), has been reported in the past with the so called "contact angle titration experiments" in which water drops of various pH were deposited on a carboxylic acid functional surface of polyethylene. C) A titration plot is obtained by plotting the primary absorption peak as a function of pH. D) UV absorption spectra of a mixture of dodecanoic acid and Brij-35 (Polyoxyethylene (23) Lauryl Ether), where the concentration of a non-ionic surfactant (Brij 35) is kept at a constant value (50 ppm), whereas the concentration of DDA is increased from 0 to 50 ppm. E) The ratio of Brij 35 to DDA is altered by keeping the total concentration to 100 ppm. F) The ratio of peak heights at  = 210 nm and  = 220 nm decrease till the concentration of DDA reaches to 30 ppm, beyond which this ratio changes only slowly.