Scale of Hammett

The scale of acidity of Hammett (also called function of Hammett) is a general Graphique making it possible to position the ones with respect to the others the various couples acido - basic and the spheres of activity of various the Solvant S used in inorganic Chimie. It proves very useful to compare the properties of various solvents. Two different solvents are different by their dielectric Permittivité µr, and the variation of pKa (which characterizes the Acidité) between two solvents of the solution obtained depends only on the variation of µr: D (pKa) = K * D (1/µr) In the diagram of Hammett, one carries in X-coordinate 100* (1/µr) or µr is the permittivity of solvent I considered, and in ordinates the pKa of the sphere of activity of solvent. The sphere of activity is then represented by a vertical line segment length proportional to pKi. The origin of the arbitrarily selected scale of the PKa, corresponds to the couple H3O+/H2O in water. The position of each solvent in ordinates is carried out by using the property of couples HB+/B to have a pKa independent of µr and to be represented by the horizontal ones. Their eigenvalue in a given solvent HS (in general NH4+/NH3) is then used to find the origin of the pKa in this solvent.

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