§ 104. MISCELLANEOUS ESTIMATIONS. 187 



of the soil for valuable elements of plant-food, treat 100 

 grms. of the soil with 200 c.c. of a hot solution contain- 

 ing in one litre 100 grms. of tartaric acid, 10 grms. of 

 oxalic acid, and ammonia in slight excess ; shake the mix- 

 ture frequently for 15 minutes, filter, and determine 

 aluminic and ferric oxide in a measured quantity of the 

 filtrate (§ 52). The oxalic acid in the solvent serves to 

 prevent lime from being taken up by the tartaric acid. 

 In order to prevent alumina also from being dissolved, 

 Midler recommends the use of Seignette salt instead of 

 ammonic tartrate. 



g. Ferrous oxide. — To determine this at least approx- 

 imately, pour 60 c.c. of hot concentrated hydrochloric 

 acid over 30 grms. of soil in a flask ; after having added 

 a few crystals of sodic carbonate, if the soil contains but 

 little carbonate, close the flask with a cork through which 

 passes a short tube bent at a right angle, put the flask in 

 an inclined position on the lamp-stand, and boil the mix- 

 ture some time. Add a considerable quantity of ammonic 

 chloride to the solution, whereby the tendency of the 

 ferrous oxide to absorb oxygen is very much lessened, 

 dilute with a large quantity of hot water, almost neutral- 

 ize the acid with ammonia, and precipitate the ferric 

 oxide in the solution with as little sodic acetate as possi- 

 ble (§ 93, -4, 1) ; filter the hot liquid rapidly through a 

 large, coarse filter, and wash the contents of the filter 

 several times with hot water; heat the* filtrate and wash- 

 ings to boiling, add some hydrochloric acid, oxidize the 

 ferrous oxide by the addition of a few crystals of potassic 

 chlorate, remove the lamp, and precipitate this solution 

 with sodic acetate, filter, wash, and weigh. The ignited 

 residue is ferric oxide, from which the corresponding 

 amount of ferrous oxide can be calculated. 



