By determining the concentration of D2O in D2O/H2O liquid mixtures, it has been possible to develop a method to obtain the concentration of hydroxyl groups on the surface of silica gel. The results of 4.4 ± 0.2 –OH groups per 100 Å2 of surface are in good agreement with the findings of other workers.
Results are presented which indicate the effect of pH and flotation-collector type upon the flotation rates of natural rutile particles. Above pH -2.5, flotation rate decreases with increasing pH for anionic collectors and increases with increasing pH for a cationic collector. These results are in agreement with expectations from the known electrokinetic properties of the mineral. Also, for a fixed pH, the rate of flotation is influenced by the length of the carbon chain associated with the collecting ion, and, for any one collector, by its concentration in solution. Below pH -2.5, the flotation rate with anionic collectors decreases with fall in pH, whereas with the cationic type a small increase in rate is shown as pH1 is approached.
Gels were prepared by adding sodium silicate solution to sulphuric acid until a certain pH was reached, and then the products were aged, washed and dried under controlled conditions. The surface properties of the dry gels were studied by measuring adsorption isotherms of nitrogen at – 195·8° and nitrous oxide at — 78·5°. The pH reached came within the range 3·7–5·8, as this corresponded to a decrease in setting time from over an hour to a few seconds. For gels prepared over this pH range, the isotherms of the two gases also changed profoundly, both sets showing the same general pattern. In one preparation the temperature was varied, and in another the ageing period was reduced to the minimum; in neither case could any difference in the adsorption of nitrous oxide be detected. Activation, by soaking in very dilute sulphuric acid, was shown to be more important for a gel prepared at the higher‐pH end of the range.
Surface areas have been calculated from both sets of isotherms by the Brunauer, Emmett and Teller (B.E.T.) method. Good agreement is obtained, in some cases, by assuming that each nitrous oxide molecule occupies an area of 20·4 Å2 in the close‐packed monolayer, and taking the usual value of 16·2 Å2 for the nitrogen molecule. The values of residual water content and B.E.T. surface area both decreased substantially over the pH range, but there appears to be no simple relationship between them.
Adsorption isotherms of water vapour have been determined at 25° on some laboratory and commercial samples of silica gel. A McBain‐Bakr type of sorption balance, which included a ‘Pyrex’ glass spring of high sensitivity, was employed.The Brunauer, Emmett & Teller (B.E.T.) method is used to compare the water isotherms with those of nitrogen, previously determined on the same samples. It appears likely that water does not form a close‐packed monolayer.
Silica gels were prepared by adding sodium silicate to acetic acid, or acetic acid‐sodium acetate buffer, until the pH reached 4.64, the addition of silicate being made rapidly (over 1–2 min.) or slowly (over about 30 min.). Adsorption isotherms of nitrogen were determined on the outgassed samples at − 195.8° and used for the calculation of surface areas and pore volumes.
The gels prepared either by rapid addition of silicate to acid, over the concentration range 2–30%, or from the buffer (initial pH 3.78), were all microporous and had very similar surface properties. On the other hand, the surface properties of those gels prepared by the slow addition of silicate depended on the concentration of acid used, the surface area of the sample prepared from 30% acetic acid being half that of the one from 10% acid. It is suggested that this effect is associated with the loss of some of the smaller primary particles, which occurs as the concentration of polysilicic acid is increased and the pH allowed to change slowly during the reaction.
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