Victor R. Deitz

Solubility product phenomena in hydroxyapatite-water systems

Activity products were determined from solubility measurements for a number of commercial and laboratory-prepared samples of basic calcium phosphate. The solid—water ratio was varied several fold, with and without the addition of sodium chloride or potassium hydroxide. The activity product calculated for hydroxyapatite was not constant but varied widely with the ratio of solid—water. The solubility was less when more of the solid had been dissolved. An increase in ionic strength by “spectator” ions, such as NaCl, leads to a decrease in solubility. This “salting out” effect observed by addition of NaCl at constant slurry density is not general. The increase in ionic strength resulting from the use of different slurry densities alone enhanced the solubility, which is the more common “salting in” effect. The molar ratio of calcium to total phosphate in the extracts was not constant at the hydroxyapatite level (1.667). It approached this value as a large fraction of the material dissolved. A calculation of the fraction of solids dissolved under limiting conditions showed that at high solid—water ratio far less than 1% of the material was dissolved at the lowest solid—water ratio the solution corresponded approximately to a crystal layer of one unit cell thickness. It was possible to reconcile these observations with the formation of a surface complex derived from the hydrolysis of the PO4≡ ions in the terminal planes of the crystal. The release of ions by this reaction and a simultaneous formation of the surface complex, Ca2(HPO4)(OH)2, account for the apparent “solubility” of hydroxyapatite. Published results are discussed in terms of this concept.

The surface composition of hydroxylapatite derived from solution behavior of aqueous suspensions

Successive extracts of a precipitated hydroxylapatite were made with water and each extract was analyzed for calcium, phosphate, sulfate, and pH. The sulfate impurity rapidly decreased to a non-detectable quantity. Calcium and phosphate were released in concentrations decreasing with successive extracts and in a mole ratio that depended on the proportion of solid and water used (slurry density) and the number of water extracts. The total ions in the water extracts was expressed as the fraction of the hydroxylapatite contained in the boundary layer and this fraction never exceeded a monolayer of 1 unit cell depth. The results are compatible with the formation of a surface complex having the composition Ca2(HPO4) (OH)2, this being a product of hydrolysis of the boundary planes of hydroxylapatite. The complex has a small solubility that yields Ca, HPO4, and OH ions in the water extracts and a value for pKc has been calculated.

Color and turbidity of sugar products.

Publisher Summary In an optical study of sugar products in solution, it is essential to differentiate among the color, the spectrophotometric data, and the evaluation of the colorant. The term “color” is used solely as an expression of visual appearance. A uniform nomenclature for spectrophotometric data is adopted by the sugar industry. Sugar products darken in processing and during storage at elevated temperatures, and this color change is of concern to practically all other industries that use these products directly or indirectly. Application to the problems of the sugar industry is of a direct nature. There is an inherent purity of raw-sugar crystals, and the surface adsorption plays a subordinate part in the cause of the original color of the raw sugar. Optical evaluation of sugar impurities is of limited value, when confined to the visible spectrum. Data in the ultraviolet region are more directly associated with molecular structure.

Introduction of Water Vapor into Vacuum Systems and the Adsorption by the Walls

A technique is described whereby precise amounts of water vapor can be introduced into evacuated glass–Kovar stainless-steel systems. The adsorption of water vapor by the walls of a typical vacuum system maintained at 26.8°C is reported in the range 0.03–0.20 relative pressure. A continuous dynamic behavior was observed over time intervals up to 4000 min, where time zero corresponds to the initial contact of the walls with the water vapor. At a constant contact time, the pressure dependence of each isochrone followed a conventional Type II physical adsorption isotherm and a linear B.E.T. plot was observed in the range 0.04 and 0.18 relative pressures. It is quite feasible to follow the adsorption of water vapor volumetrically on a variety of solids at all pressures above an absolute pressure of 0.1 Torr due to the good reproducibility of the wall adsorption, provided the isochronal nature of the adsorption process is recognized.