20 BULLETIN 949, U. S. DEPARTMENT OF AGRICULTURE. 



INSOLUBLE RESIDUE. 



22 i To a 1-gram sample of cement shall be added 10 c. c. of water and 5 c. c. of 

 concentrated hydrochloric acid ; the liquid shall be warmed until effervescence ceases. 

 The solution shall be diluted to 50 c. c. and digested on a steam bath or hot plate 

 until it is evident that decomposition of the cement is complete. 



The residue shall be filtered, washed with cold water, and the filter paper and 

 contents digested in about 30 c. c. of a 5 per cent solution of sodium carbonate, the 

 liquid being held at a temperature just short of boiling for 15 minutes. The remaining 

 residue shall be filtered, washed with cold water, then with a few drops of hot hydro- 

 chloric acid, 1:9, and finally with hot water, and then ignited at a red heat and 

 weighed as the insoluble residue. 



(23) A permissible variation of 0.15 will be allowed, and all results in excess of the 

 specified limit but within this permissible variation shall be reported as 0.85 per cent. 



SULPHURIC ANHYDRIDE. 



(24) One gram of the cement shall be dissolved in 5 c. c. of concentrated hydro- 

 chloric acid diluted with 5 c. c. of water, with gentle warming; when solution is 

 complete, 40 c. c. of water shall be added, the solution filtered, and the residue washed 

 thoroughly with water. The solution shall be diluted to 250 c. c, heated to boiling, 

 and 10 c. c. of a hot 10 per cent solution of barium chloride shall be added slowly, 

 drop by drop from a pipette, and the boiling continued until the precipitate is well 

 formed. The solution shall be digested on the steam bath until the precipitate has 

 settled. The precipitate shall be filtered, washed, and the paper and contents placed 

 in a weighed platinum crucible and the paper slowly charred and consumed without 

 flaming. The barium sulphate shall then be ignited and weighed. The weight ob- 

 tained multiplied by 34.3 gives the percentage of sulphuric anhydride. The acid 

 filtrate obtained in the determination of the insoluble residue may be used for the 

 estimation of sulphuric anhydride instead of using a separate sample. 



(25) A permissible variation of 0.10 will be allowed, and all results in excess of the 

 specified limit but within this permissible variation shall be reported as 2 per cent. 



MAGNESIA. 



(26) To 0.5 gram of the cement in an evaporating dish shall be added 10 c. c. of 

 water to prevent lumping and then 10 c. c. of concentrated hydrochloric acid. The 

 liquid shall be gently heated and agitated until attack is complete. The solution 

 shall then be evaporated to complete dryness on a steam or water bath. To hasten 

 dehydration the residue may be heated to 150° or even 200° C. for one-half to one hour. 

 The residue shall be treated with 10 c. c. of concentrated hydrochloric acid diluted 

 with an equal amount of water. The dish shall be covered with the solution digested 

 for 10 minutes on a steam bath or water bath! The diluted solution shall be filtered 

 and the separated silica washed thoroughly with water. Five cubic centimeters of 

 concentrated hydrochloric acid and sufficient bromine water to precipitate any 

 manganese which may be present shall be added to the filtrate (about 250 c. c. ). This 

 shall be made alkaline with ammonium hydroxide, boiled until there is but a faint 

 odor of ammonia, and the precipitated iron and aluminum hydroxides, after settling, 

 shall be washed with hot water, once by decantation and slightly on the filter. Set- 

 ting aside the filtrate, the precipitate shall be transferred by a jet of hot water to the 

 precipitation vessel and dissolved in 10 c. c. of hot hydrochloric acid. The paper 

 shall be extracted with acid, the solution and washings being added to the main 

 solution. The aluminum and iron shall then be reprecipitated at boiling heat by 

 ammonium hydroxide and bromine water in a volume of about 100 c. c, and the 

 second precipitate shall be collected and washed on the filter used in the first instance 

 if this is still intact. To the combined filtrates from the hydroxides of iron and alumi- 



