APPENDIX 631 



settle over night, 200 cc. of the clear supernatant solution drawn off, and 

 the iron, aluminum, and phosphorus precipitated by adding ammonium 

 hydroxid to the boiling solution. If there is not enough iron present to 

 give a very decided brown color to the precipitate, a little ferric chlorid 

 should be added before precipitation to insure complete removal of the 

 phosphorus from solution. The precipitate is filtered off, washed 

 5 times, dissolved with warm dilute nitric acid, evaporated, and heated 

 on the steam bath to dehydrate the silica, taken up with strong nitric 

 acid, heated, then diluted, and the silica filtered off. The filtrate is 

 evaporated to about 5-10 cc., care being taken that it does not go to 

 dryness, as alumina and some silica are almost sure to separate out and 

 cause trouble. It is then completely neutralized with ammonia, cleared 

 up with nitric acid, approximately i g. of crystalline ammonium nitrate 

 added, and the phosphorus precipitated at 4o-5o with 15 cc. ammo- 

 nium molybdate solution, allowing it to stand on the water bath at this 

 temperature for one to two hours, stirring occasionally for the first 15 

 or 20 minutes. It is then allowed to stand at room temperature over 

 night, the precipitate filtered off through a double filter and washed 

 with a tenth-normal solution of ammonium nitrate until free from 

 molybdic acid and finally twice with cold distilled water. 1 It is then 

 removed together with the filter paper to a beaker, dissolved with a 

 measured excess of standard potassium hydroxid solution, and the 

 excess titrated back with standard nitric acid . 



A very convenient strength of potassium hydroxid solution is .83236 g. 

 KOH per 100 cc. One cubic centimeter is then equivalent to 0.2 mg. 

 of phosphorus. 



The nitric acid should be made equivalent in strength to the potassium 

 hydroxid, and with these strengths of solutions, duplicates should check 

 within 0.2 cc. 



A blank determination must be run, using no soil, and a correction 

 made for the phosphorus found in the reagents. 



(Ammonium molybdate solution is made by dissolving 100 g. molybdic 

 acid in 400 cc. NH 4 OH (sp. gr. .96) and adding very slowly to 1250 cc. 

 HNO 3 (sp. gr. i. 20), keeping the solution cool and well stirred.) 



Total potassium. This test is carried out as given on page 147, Bulle- 

 tin 105, Bureau of Chemistry, Department of Agriculture. One gram 

 of soil, one gram of ammonium chlorid, and eight grams of calcium 



1 Molybdic oxid is often precipitated if the first few washings, while iron is still 

 present, are done with either water or ammonium nitrate solution. This may be 

 prevented by washing two or three times, until free of iron, with ammonium nitrate 

 containing a little of the ammonium molybdate solution. 



