I02 CHEMICAL ANALYSIS OF SOIL 



minutes, allowed to cool a little, filtered, and washed 

 with hot water. The precipitate is dried in the oven 

 and strongly ignited, first over the Bunsen flame and 

 then by means of the blowpipe, and finally weighed as 

 CaO. 



84. Magnesium. 



The filtrate and washings from estimation of the 

 calcium are concentrated to about 150 c.c, allowed to 

 cool, and transferred to a conical flask of about 800 

 c.c. capacity. Sufficient sodium-phosphate solution 

 (NagHPO^) is added to precipitate the magnesium, and 

 then about 10 c.c. of strong ammonia. The liquid is 

 allowed to stand in the cold for twenty-four hours ; the 

 precipitate formed is filtered off and washed with 2 per 

 cent, ammonia solution until free from chlorides, dried, 

 and ignited, first at moderate heat and then very 

 strongly. The residue is magnesium pyrophosphate 

 (MggPgO^), and from its weight is calculated the per- 

 centage of MgO. 



85. Sulphuric Acid. 



For the determination of the sulphates, 200 c.c. of 

 the original hydrochloric acid extract are evaporated to 

 about 50 c.c, a little solid barium chloride added, and 

 the beaker heated on the water-bath for two or three 

 hours. After standing for another twelve hours the 

 small precipitate is filtered off, washed thoroughly with 

 hot water, dried, and ignited together with the filter. 

 The residue is BaSO^, and from its weight the percentage 

 of sulphuric acid is calculated as SO3. 



86. Nitrates. 



If it is desirable to estimate the nitrates in the soil, 

 the following method should be adopted. 



