45 



through a weighed Gooch crucible, wash with dilute ammonia, aud pro- 

 ceed as in the determination of magnesia, p. 41. 



The increase in weight represents that of the magnesium pyrophos- 

 phate. This weight, multiplied by 0.63976, will give the weight of the 

 phosphoric acid in the 4 grams of air-dried soil. (Fres., 134, 1, &, p, a.) 



Fresenius advises adding to the weight of the magnesium pyrophos- 

 phate 0.0018 grams, to compensate for the loss which results from the 

 feeble solubility of the ammonium magnesium phosphate in the wash 

 waters. 



DETERMINATION OF CHLORINE. 



Wash 10 grams of the air- dried soil on a filter, with boiling water, 

 using about 500 c. c. before testing a few drops of the wash water, 

 acidulated with nitric acid, with argentic nitrate to see that all the 

 chlorides are washed out. 



When all the chlorides are removed, concentrate the washings to 200 

 c. c., filter if necessary, and divide into two equal parts, determining 

 the chlorine volumetrically by means of a standard solution of argentic 

 nitrate, using potassium chromate as an indicator. The determination 

 is made by first adding to the 100 c. c. of solution, equal to 5 grams of 

 air-dried soil, 3 drops of a saturated solution of potassium chromate, 

 and then dropping in the silver solution from a burette, and noting 

 when the red color of silver chromate appears. The number of c. c. of 

 silver nitrate solution used, multiplied by the value of 1 c. c., will give 

 the amount of chlorine present in the 5 grams of soil. (Fres., 141, &, a.) 



PREPARATION OF THE ARGENTIC NITRATE SOLUTION. 



Dissolve 8.5 grams of pure argentic nitrate in 1 liter of distilled watery 

 1 c. c. of the solution is equal to 0.001775 grams Cl. To standardize 

 the solution, dissolve 1 gram of pure fused sodium chloride in 1 liter of 

 distilled water , take exactly 10 c. c. of the solution and dilute to 100 c. c. 

 with water, add 3 drops of a saturated solution of potassium chromate r 

 and drop in from a burette the silver solution until the red color of 

 silver chromate appears. The known quantity of chlorine in the 10 c. c. 

 of salt solution, divided by the number of c. c. of silver solution used, 

 will give the value of 1 c. c. of the latter. 



DETERMINATION OF THE CARBONIC ACID BY ABSORPTION. 



For this purpose an absorption apparatus, such as that described by 

 Fresenius, under the head of carbonic acid, may be used. (Fres., 139 e r 

 182.) 



This cousistsof: (1)A small tube filled with fused chloride of calcium, 

 to absorb the atmospheric moisture; this is connected by means of rub- 

 ber tubing to (2) the closed funnel, provided with a stop-cock, through 

 which the dilute hydrochloric acid is admitted to (3) the flask, holding 

 about 150 c. c. (4) The flask is connected by rubber tubing to an U-tube, 



