SiO_{3}^{—­} + 2H^{+} —­> [H_{2}SiO_{3}] —­> H_{2}O + SiO_{2}.]

[Note 4:  A portion of the fused mass is usually projected upward by the escaping carbon dioxide during the fusion.  The crucible must therefore be kept covered as much as possible and the lid carefully cleaned.]

[Note 5:  A gritty residue remaining after the disintegration of the fused mass by acid indicates that the substance has been but imperfectly decomposed.  Such a residue should be filtered, washed, dried, ignited, and again fused with the alkaline flux; or, if the quantity of material at hand will permit, it is better to reject the analysis, and to use increased care in grinding the mineral and in mixing it with the flux.]

DEHYDRATION AND FILTRATION

Procedure.—­Evaporate the solution of the fusion to dryness, stirring frequently until the residue is a dry powder.  Moisten the residue with 5 cc. of strong hydrochloric acid (sp. gr. 1.20) and evaporate again to dryness.  Heat the residue for at least one hour at a temperature of 110 deg.C. (Note 1).  Again moisten the residue with concentrated hydrochloric acid, warm gently, making sure that the acid comes into contact with the whole of the residue, dilute to about 200 cc. and bring to boiling.  Filter off the silica without much delay (Note 2), and wash five times with warm dilute hydrochloric acid (one part dilute acid (1.12 sp. gr.) to three parts of water).  Allow the filter to drain for a few moments, then place a clean beaker below the funnel and wash with water until free from chlorides, discarding these washings.  Evaporate the original filtrate to dryness, dehydrate at 110 deg.C. for one hour (Note 3), and proceed as before, using a second filter to collect the silica after the second dehydration.  Wash this filter with warm, dilute hydrochloric acid (Note 4), and finally with hot water until free from chlorides.

[Note 1:  The silicic acid must be freed from its combination with a base (sodium, in this instance) before it can be dehydrated.  The excess of hydrochloric acid accomplishes this liberation.  By disintegrating the fused mass with a considerable volume of dilute acid the silicic acid is at first held in solution to a large extent.  Immediate treatment of the fused mass with strong acid is likely to cause a semi-gelatinous silicic acid to separate at once and to inclose alkali salts or alumina.

A flocculent residue will often remain after the decomposition of the fused mass is effected.  This is usually partially dehydrated silicic acid and does not require further treatment at this point.  The progress of the dehydration is indicated by the behavior of the solution, which as evaporation proceeds usually gelatinizes.  On this account it is necessary to allow the solution to evaporate on a steam bath, or to stir it vigorously, to avoid loss by spattering.]

[Note 2:  To obtain an approximately pure silica, the residue after evaporation must be thoroughly extracted by warming with hydrochloric acid, and the solution freely diluted to prevent, as far as possible, the inclosure of the residual salts in the particles of silica.  The filtration should take place without delay, as the dehydrated silica slowly dissolves in hydrochloric acid on standing.]