2O8 AGRICULTURAL ANALYSIS 



181. Remarks on the Conduct of the Direct Precipitation 

 Method. The citric acid extract of the basic slag changes by 

 long standing, so far as the external appearance is concerned, 

 very little. It remains for days either completely clear or only 

 slightly turbid, without the production of any precipitate. In 

 spite of this, however, important changes go on in relation to the 

 application of the direct precipitation method, which consist in 

 the fact that any silicic acid in the extract passes over into a pre- 

 cipitable condition upon the addition of ammonia or ammoniacal 

 citrate solution. The precipitability of the silicic acid increases 

 from hour to hour, and it is therefore necessary to precipitate the 

 filtrate extract immediately, or at longest, within an hour. 



The precipitability of silicic acid is greatly increased by heat. 

 The shaking apparatus is, therefore, to be supplied with a water 

 bath by which the mixtures during the summer time may be 

 cooled. 



The precipitability of silicic acid is very little immediately after 

 the citric acid solution of the phosphoric acid is made. The more, 

 therefore, the precipitation of the phosphoric acid with the mag- 

 nesia mixture is hastened, the more certainly is avoided any con- 

 tamination of the precipitate with silicic acid. The precipitation 

 of the phosphoric acid is also hastened as follows : 



a. If the ammoniacal citrate solution is not added first, and 

 then the magnesia mixture, but the mixture of both is added to 

 the citric acid extract ; 



b. The iron-citrate-magnesia mixture is poured into the citric 

 acid extract in the shaking apparatus, which is already in active 

 movement ; 



c. The shaking apparatus is to be placed in the shortest pos- 

 sible time at its maximum vibration of from 250 to 300 vibra- 

 tions per minute. 



The precipitability of the silicic acid is heightened through a 

 lack of iron in solutions which are rich in silicic acid and poor in 

 iron, therefore a pure precipitate is obtained only when the iron- 

 citrate-magnesia mixture is employed. 



182. The Conduct of the Molybdate Method. a. Great care must 



