DETERMINATION OF FREE SILICA BY PHOSPHORIC ACID METHOD (I)

The phosphoric acid method for the determination of free silica mixed in silicates was improved in its reproducibility by applying the copper mantle as a heat radiator. The superiority of this method to the electric heater method was proved by the observa-tion of the dissolution velocities of several kinds of quartz samples using the two methods.The dissolution velocities were studied on quartz powders which were produced by three different methods by observing their dissolution in hot phosphoric acid using copper mantle and it was cleared that the surface of quartz powder was denaturalized by the grinding of quartz and this part was dissolved in phosphoric acid with far higher velocity than the original quartz. The appearances of these quartz were observed before and after the treatment with phosphoric acid under the optical microscope, and the denaturalized surface was demonstrated clearly by the phase contrast microscope with polarizer.The obtained value of non-soluble part in phosphoric acid had been taken as free silica but the observations in this experiment showed that the part of quartz denatura-lized during the grinding was soluble and this part was accounted as non-free silica.