Nitroanilines base strength

In the 1930s, Louis Hammett and his students measured the strengths of very weak acids and bases, using a weak reference base (B). such as p-nitroaniline (p/C, = 0.99), whose base strength could be measured in aqueous solution. 

Activity coefficients of the free and protonated form of B and the activity of the proton cannot be determined directly, however, their combination Uh-Vhii /vh can be calculated from the known value of K by experimentally measured ratio [B]/[BH+], which can be done by spectrophotometry. Values of K can be obtained from dilute solutions (yBH+=yB=yH+=l)-Hammett proposed to use electroneutral bases of various strength (nitroanilines) as indicators for nonaqeous solutions... 

The Kamlet-Taft [Ka 76] scale of solvent basicity makes use of the magnitudes of the solvatochromic shifts of the longest wave band in the UV-visible spectrum of p-nitroaniline referred to that of N,N-diethyl-p-nitroaniline. The primary amino group of former compound interacts with Lewis bases, the latter one does not. So the difference between the solvatochrom shift in the spectra of these two compounds depends on the strength of the interaction between the Lewis base and p-nitroanilin. 

Hammett proposed to use electroneutral bases of various strengths (nitroanilines) as indicators for nonaqueous solutions ... 

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