Use of p‐nitrophenol for specific surface measurement of granular solids and fibres

Abstract

AbstractEarlier work describing the use of p‐nitrophenol (PNP) in measuring specific surface of finely divided solids is supplemented here by a description of recent investigations which enable the usefulness of the method to be more clearly defined. Normally PNP is used in aqueous solution, but for some materials liable to dissolve in, swell in, or react chemically with, water, e.g. sugar, certain inorganic oxides and textile fibres, an organic solvent is used. The method is suitable for a wide variety of solids, both non‐porous and porous, provided they either form a hydrogen bond with PNP or have aromatic nuclei. Thus, with appropriate solvents, oxides, textile fibres with polar molecules and carbons can be used, but not, for example barium sulphate. With porous solids the results may or may not reveal a lower accessible surface than the area measured by nitrogen, depending on the pore size distribution. With porous solids the method enables a rapid estimate of the relative proportion of small and large pores to be made. Also, it can be used to measure the external specific surface only, as distinct from the total surface of some porous granular solids and fibres. Normally PNP appears to be adsorbed flatwise from water or organic solvents with an effective molecular area of 52.5 Å2. (An earlier value for this area given for special cases of adsorption from non‐aqueous solutions is now shown to be invalid). In some cases on polar inorganic solids it is adsorbed end‐on with an effective area of 25 Å2. The mode of adsorption is indicated by the type of isotherm.