582 



CEMENTS, LIMES, AND PLASTERS. 



The precipitate is redissolved in hot dilute HC1, the solution made up 

 to about 100 c.c., 1 c.c. of a saturated solution of Na(NH4)HP04 added, 

 and ammonia drop by drop, with constant stirring until the precipitate 

 is again formed as described and the ammonia is in moderate excess. 

 It is then allowed to stand for about 2 hours when it is filtered on a 

 paper or a Gooch crucible, ignited, cooled, and weighed as Mg 2 P 2 O7. 



K 2 and Na 2 O: For the determination of the alkalies, the well- 

 known method of Prof. J. Lawrence Smith is to be followed, either 

 with or without the addition of CaC0 3 with NHC1 4 . 



80s: One gram of the substance is dissolved in 15 c.c. of HC1, 

 filtered and residue washed thoroughly.* 



The solution is made up to 250 c.c. in a beaker and boiled. To the 

 boiling solution 10 c.c. of a saturated solution of BaCL 2 is added slowly, 

 drop by drop, from a pipette and the boiling continued until the pre- 

 cipitate is well formed, or digestion on the steam-bath may be sub- 

 stituted for the boiling. It is then set aside overnight, or for a few 

 hours, filtered, ignited, and weighed as BaS04. 



Total sulphur. One gram of the material is weighed out in a large 

 platinum crucible and fused with Na 2 COs and a little KNOs, being care- 

 ful to avoid contamination from sulphur in the gases from source of 

 heat. This may be done by fitting the crucible in a hole in an abestos 

 board. The melt is treated in the crucible with boiling water and the 

 liquid poured into a tall, narrow beaker, and more hot water added 

 until the mass is disintegrated. The solution is then filtered. The 

 filtrate contained in a No. 4 beaker is to be acidulated with HC1 and 

 made up to 250 c.c. with distilled water, boiled, the sulphur precipitated 

 as BaS04 and allowed to stand overnight or for a few hours. 



Loss on ignition. Half a gram of cement is to be weighed out in 

 a platinum crucible, placed in a hole in an asbestos board so that about 

 three fifths of the crucible projects below, and blasted 15 minutes, prefer- 

 ably with an inclined flame. The loss by weight, which is checked by 

 a second blasting of 5 minutes, is the loss on ignition. 



Note. Recent investigations have shown that large errors in results 

 are often due to the use of impure distilled water and reagents. The 

 analyst should, therefore, test his distilled water by evaporation and 

 his reagents by appropriate tests before proceeding with his work. 



* Evaporation to dry ness is unnecessary, unless gelatinous silica should have 

 separated and should never be performed on a bath heated by gas, vide Hillebrand. 



