Determination of Phosphorus in Iron. 183 



flask and cork each time, it will do no harm if some of the 

 precipitate still adheres to the sides of the flask ; for the flask, 

 cork and crucible are all finally washed with ammonia. The 

 asbestus felt was transferred to a 100^"^ flask or narrow based 

 Erlenmejer. The precipitation flask and cork were thoroughly 

 washed with a mixture of S^'^ of ammonia and 10'^'^ of water, 

 and the washings were allowed to rinse the sides of the per- 

 forated crucible — standing on a small funnel — and so to run 

 into the 100'^^ flask. Twenty-five cubic centimeters of strong 

 hydrochloric acid were added and, where the phosphorus does 

 not exceed -0060 grra., 0*5 grm. of potassium iodide; but, 

 where more than -0060 grm. of phosphorus is present, a little 

 more potassium iodide is theoretically and practically needed. 

 Experience has shown that the iodide present should not exceed 

 the amount theoretically necessary by more than a half gram. 



The liquid was boiled down from a total volume of ^O*^*^ to 

 just 25^*^, a volume easily marked by two strips of paper pasted 

 on opposite sides of the flask. If the solution is boiled far- 

 ther, the molybdic acid is likely to be reduced beyond the 

 degree of oxidation indicated by the symbol Mo^O^. The 

 residue was neutralized with acid sodium carbonate, according 

 to one or the other of the methods described below, and 

 treated with an excess of standard iodine. 



For a trap to prevent loss during boiling, a 

 two-bulbed drying tube was cut off an inch and 

 a half below the largest bulb and this end fitted 

 loosely into the neck of the flask, as shown in 

 the accompanying figure. The addition of two 

 or three little pieces of pumice stone, previously 

 boiled in hydrochloric acid, insures quiet boiling. 



While still acid the reduced solution is not 

 appreciably reoxidized in the air, but when once 

 made alkaline it is very easily changed. There- 

 fore great care must be taken either to have 

 the iodine present before the solution is made alkaline ; or 

 else to stir up the solution as little as possible while adding the 

 alkalies and to draw in the iodine solution immediately. 



The safer method is to put the reduced solution, with one 

 gram of tartaric acid, to prevent the precipitation of the 

 molybdenum, and an excess of the iodine solution, amounting 

 to at least 8% into a liter flask. This flask was fitted with a 

 paraffined rubber stopper, carrying a funnel with a stop-cock 

 for admitting slowly the acid sodium carbonate to effect neu- 

 tralization, and a Will and Yarrentrapp trap, filled with a solu- 

 tion of potassium iodide, to catch the iodine thrown off during 

 the effervescence. 



