Infrared titration of aqueous NaOH by aqueous HCl

The titration of NaOH by HCl is achieved through infrared spectral measurem ent of the various mixtures. Factor Analysis (FA) is used to separate the s pectra. Four principal spectra were necessary to perform the analysis: pure water, 2.09 M HCl, 2.23 M NaOH, and 2.60 M NaCl solvated water. Each of th e 18 experimental spectra obtained was determined to be a linear combinatio n of these principal spectra. The composition of the solutions retrieved fr om the IR analysis is identical to what was determined by the mass balance. The mean activity coefficient of HCl was calculated using the concentratio n of HCl obtained by IR and the activity obtained by pH measurements. It wa s found that the mean activity coefficient increased by a factor greater th an 2 in the presence of 1 M NaCl. Because the water molecules associated to the ionic pair H+/Cl-, as observed by IR, an unmodified by the presence of the salt, such an increase cannot be attributed to a modification of the s trong association of the water molecules to the pair of ions.