The determination of the specific surface of barium sulphate: A comparison of results by different methods

Abstract

A description is given of the pipette and centrifuge methods of determining grain-size distributions by sedimentation, and results are presented for a number of barium sulphate powders. From the sizedistribution curves, the specific surfaces are calculated and compared with values obtained by air permeability, dye adsorption and a light-extinction method. The results for the dye-adsorption and air-permeability methods are shown to be in good agreement with each other, but do not always agree with the values calculated from the size distribution. It is suggested that this is due to aggregates, instead of discreet particles, settling in the sedimentation analysis, and this view is supported by electron-micrographs. The failure of the optical method is attributed to thc fineness of the powders.Barium sulphate is used in the photographic industry under the name of ' baryta ' or ' blancfure ' for coating paper before the application of the photographic-emulsion coat. The baryta, which is in the form of an aqueous slurry, is applied to the paper in admixture with gelatin, certain dyes, etc. A knowledge of the average particle size and of the particle-size distribution of the baryta is of great importance to the technician, as many of the coating properties, e.g. viscosity, surface quality of the coating, etc., are functions of the particle size.A study has been made of various methods for determining the particle-size distribution and the average particle size of such powders, and the results obtained by these methods have been compared.The powders which were to be examined consisted mainly of particles within the size range of 5 p to o * I p. Many methods, of different degrees of reliability, are available for particle-size measurement in this size range and, of these, the following were chosen for examination: (I) Sedimentation analysis (pipette).(3) Air permeability. 14) Dye adsorption.