580 



CEMENTS, LIMES, AND PLASTERS. 



Silica. The residue, without further heating, is treated at first with 

 5 to 10 c.c. of strong HC1, which is then diluted to half strength or less, 

 or upon the residue may be poured at once a larger volume of acid of 

 half strength. The dish is then covered and digestion allowed to go 

 on for ten minutes on the bath, after which the solution is filtered and 

 the separated silica washed thoroughly with water. The filtrate is 

 again evaporated to dryness, the residue, without further heating, 

 taken up with acid and water, and the small amount of silica it con- 

 tains separated on another filter-paper. The papers containing the 

 residue are transferred wet to a weighed platinum crucible, dried, ignited, 

 first over a Bunsen burner until the carbon of the filter is completely 

 consumed, and finally over the blast for fifteen minutes and checked 

 by a further blasting for ten minutes or to constant weight. The silica, 

 if great accuracy is .desired, is treated in the crucible with about 10 c.c. 

 of HF1 and four drops of H 2 SO4 and evaporated over a low flame to 

 complete dryness. The small residue is finally blasted for a minute 

 or two, cooled, and weighed. The difference between this weight and 

 the weight previously obtained gives the amount of silica.* 



A1 2 3 and Fe 2 3 : The filtrate, about 250 c.c. from the second 

 evaporation for Si0 2 , is made alkaline with NH 4 OH after adding HC1, 

 if need be, to insure a total of 10 to 15 c.c. strong acid, and boiled to 

 expel excess of NH 3 , or until there is but a faint odor of it, and the pre- 

 cipitated iron and aluminum hydrates, after settling, are washed once 

 by decantation and slightly on the filter. Setting aside the filtrate, 

 the precipitate is dissolved in hot dilute HC1, the solution passing into 

 the beaker in which the precipitation was made. The aluminum and 

 iron are then precipitated by NH 4 OH, boiled and the second precipi- 

 tate collected and washed on the same filter used in the first instant. 

 The filter-paper, with the precipitate, is then placed in a weighed platinum 

 crucible, the paper burned off and the precipitate ignited and finally 

 blasted 5 minutes, with care to prevent reduction, cooled and weighed 

 as Al 2 3 + Fe 2 03.t 



Fe 2 3 : The combined iron and aluminum oxides are fused in a 

 platinum crucible at a very low temperature with about 3 to 4 grams 

 of KHSC>4, o r > better, NaHSC>4, the melt taken up with so much dilute 

 H 2 S04 that there shall be no less than 5 grams absolute acid and enough 

 water to effect solution on heating. The solution is then evaporated 

 and eventually heated till acid fumes come off copiously. After cooling 



* For ordinary control work in the plant laboratory this correction may, perhaps, 

 be neglected; the double evaporation never. 



t This precipitate contains TiO 2 , P 2 O 6 , Mn 3 O 4 . 



