528 CHAPTER XXV 



Insoluble Silica. — Mix the residue obtained in the determination of the 

 sand with four or five times its weight of fusion mixture, composed of 

 molecular proportions of sodium and potassium carbonates, and keep at a 

 red heat for half an hour after effervescence has ceased : dissolve out with 

 dilute hydrochloric acid, evaporate to dryness and determine the silica as 

 before. 



Iron, Alumina. — Mix the filtrates from the determinations of soluble 

 and insoluble silica ; evaporate to convenient bulk, add ammonia t.'il 

 alkaline, and heat till the solution smells only faintly of ammonia ; filteJ 

 while hot ; wash, dry, ignite, and weigh the precipitate as FegOg + AlgOg. 

 If there be present any large quantity of iron and alumina after decanting 

 off the supernatant liquid, the precipitate should be dissolved in hydrochloric 

 acid and re-precipitated. 



Lime. — Precipitate the lime in the filtrate from the iron and alumina 

 while boiling hot with ammonium oxalate ; allow to stand for six hours, and 

 filter, wash, dry and ignite the precipitate to constant weight and weigh as 

 CaO ; convert the lime to sulphate or carbonate by evaporation to dryness 

 with either sulphate or carbonate of ammonia and again ignite, and weigh 

 as CaSOj, or as CaCOg : CaCOg X 0-56 = CaO ; CaS04 X 0-4118 = CaO. 



Magnesia. — Precipitate the magnesia in the filtrate from the lime de- 

 termination as phosphate by the addition of sodium phosphate : agitate 

 the solution violently, and allow to stand for twelve hours ; filter, wash with 

 dilute ammonia, dry, ignite strongly and weigh as MggPgO, : Mg2P207 X • 3604 

 = Mg O. 



As magnesia is detrimental to the value of good limestone, Geerligs^^ 

 has given a scheme for its rapid estimation. Two grams are dissolved in 

 hydrochloric acid, evaporated to complete dryness, the residue brought 

 into solution with hydrochloric acid, boiled after the addition of a few drops 

 of nitric acid, and evaporated to small bulk. An excess of calcium carbonate 

 is added to precipitate iron and alumina, the precipitate filtered off, and the 

 filtrate collected in a flask to which an excess of lime water is added ; the 

 flask is filled nearly to the neck and set aside to settle ; the supernatant 

 liquid is decanted through a filter, and the precipitate washed by decantation. 

 The precipitate from the lime water contains the magnesia : it is dissolved 

 in hydrochloric acid, the lime precipitated as before by ammonium oxalate, 

 and the magnesia determined in the filtrate. 



The method of Sundstrom^^ for the rapid estimation of magnesia in lime- 

 stones is as follows : — 



Weigh out one grm. of material into a small dish, add about 100 c.c. 

 water and 25 c.c. of normal hydrochloric acid : heat to boiling, allow to cool 

 and titrate the excess of acid with normal caustic soda, thus obtaining the 

 quantity of acid required to neutralize the carbonates of lime and magnesia. 

 The lime is determined as usual and calculated to carbonate ; if the per- 

 centage of calcium carbonate be divided by five, the quotient will give the 

 number of c.c. of normal hydrochloric acid required to neutralize the calcium 

 carbonate. The difference between that found above, as necessary to 

 neutralize the lime and magnesia carbonates, and the calculated number of 

 c.c. necessar\^ for the lime alone, gives the number of c.c. requisite to neu- 

 tralize the magnesia carbonate ; this number, multiplied by 0-42, gives the 

 percentage of magnesia carbonate. 



