CONSTITUTION, ETC., OF PORTLAND CEMENT. 581 



and redissolving in water the small amount of silica is filtered out, 

 weighed, and corrected by HF1 and H 2 S0 4 .* The filtrate is reduced 

 by zinc, or preferably by hydrogen sulphide, boiling out the excess of 

 the latter afterward while passing C02 through the flask, and tritrated 

 with permanganate. f The strength of the permanganate solution 

 should not be greater than .0040 grains Fe 2 0s per c.c. 



CaO: To the combined filtrate from the Al 2 03 + Fe 2 03 precipitate 

 a few drops of NH40H are added, and the solution brought to boiling. 

 To the boiling solution 20 c.c. of a saturated solution of ammonium 

 oxalate is added, and the boiling continued until the precipitated 

 CaC 2 04 assumes a well-defined granular form. It is then allowed to 

 stand for 20 minutes, or until the precipitate has settled, and then filtered 

 and washed. The precipitate and filter are placed wet in a platinum 

 crucible, and the paper burned off over a small flame of a Bunsen burner. 

 It is then ignited, redissolved in HC1, and the solution made up to 100 

 c.c. with water. Ammonia is added in slight excess, and the liquid 

 is boiled. If a small amount of Al 2 0s separates, this is filtered out, 

 weighed, and the amount added to that found in the first determina- 

 tion, when greater accuracy is desired. The lime is then reprecipitated 

 by ammonium oxalate, allowed to stand until settled, filtered, and 

 washed, J weighed as oxide by ignition and blasting in a covered 

 crucible to constant weight, or determined with dilute standard per- 

 manganate.! 



MgO: The combined filtrates from the calcium precipitates are 

 acidified with HC1, and concentrated on the steam-bath to about 150 

 c.c., 10 c.c. of saturated solution of NaCNH^HPC^ are added, and 

 the solution boiled for several minutes. It is then removed from the 

 flame and cooled by placing the beaker in ice-water. After cooling, 

 NH 4 OH is added drop by drop with constant stirring until the crystal- 

 line ummonium magnesium orthophosphate begins to form, and then 

 in moderate excess, the stirring being continued for several minutes. 

 It is then set aside for several hours in a cool atmosphere and filtered. 



* This correction of A1 2 O 3 , Fe 2 O 3 , for silica should not be made when the HF1 

 correction of the main silica has been omitted, unless that silica was obtained 

 by only one evaporation and nitration. After two evaporations and nitrations 

 1 to 2 mg. of SiO 2 are still to be found with the Al 2 O 3 .Fe 2 O 3 . 



t In this way only is the influence of titanium to be avoided and a correct 

 result obtained for iron. 



J The volume of wash-water should not be too large, vide Hillebrand. 



The accuracy of this method admits of criticism, but its convenience and 

 rapidity demand its insertion. 



