358 DETEKMINATION OF PHUSI'IIORIC OXIDE. 



The reagents required are : 



(i) Stilphate-molyhdate reag^ent, — This is prepared by dis- 

 solving 300 grams of ammonium molybdate in i litre of water, 

 and pouring the solution in a thin stream into i litre of nitric acid, 

 sp. gr. 1 .36, in which 100 grams of ammonium sulphate have been 

 dissolved. The mixture is allowed to stand in the dark for 48 

 hours, the clear solution decanted or filtered ofif, and kept in 

 brown glass stoppered bottles in the dark. 



(2) Sulphuric-nitric acid mixture. — 30 c.c. of sulphuric acid, 

 sp. gr. 1 .84, are added to i litre of nitric acid, sp. gr. i .20. 



(3) Nitric acid, sp. gr. i .20. 



(4) Sodium nitrate, a 3 per cent, solution. 



The precipitation is carried out as follows : — 



10 to 25 c.c. of the solution to be analysed, corresponding to 

 0.1 to 0.25 gram of the original substance, are placed in a 

 beaker of about 250 c.c. capacity. If the solution is made with 

 nitric acid, sufficient of the sulphuric-nitric acid mixture, and if 

 the solution is made with sulphuric acid, sufficient of the nitric 

 acid, sp. gr. i .20, is added to bring the volume of the liquid in the 

 beaker up to exactly 50 c.c, i.e., if 10 c.c. of a solution made with 

 nitric acid is taken, 40 c.c. of the sulphuric-nitric acid must be 

 added, or if 25 c.c. of a solution made with sulphuric acid be 

 taken, 25 c.c of nitric acid, sp. gr. 1.20, must be added. The 

 solution is heated on a wire gauze until the first appearance of 

 bubbles, the beaker is removed from the flame and rotated for a 

 few seconds to prevent the sides of the beaker from being over- 

 heated. 50 c.c. of the molybdate reagent are then added rapidly, 

 without allowing it to come in contact with the sides of the beaker. 

 The beaker is allow to stand for a few minutes, not more than 

 five, and then rapidly rotated for about half a minute. The 

 mixture is allowed to stand for 2 to 18 hours, in practice over- 

 night is convenient, and filtered off at the pump through a Gooch 

 crucible fitted with a circular disc of filter paper. The precipitate 

 is washed with the 3 per cent, sodium nitrate solution until the 

 washings are free from acid. The precipitate, together with the 

 filter paper is transferred to the beaker in which the precipitation 

 took place, by means of the sodium nitrate solution, and dissolved 

 in a slight excess of standard sodium hydroxide solution. About 

 0.5 c.c. of phenolphthalein solution (i per cent.) is added, and 

 the excess of sodium hydroxide titrated back with standard acid. 

 Hydrochloric, nitric, or sulphuric acid may be used. 



The reaction between the sodium hydroxide and ammonium 

 phosphomolybdate is usually expressed by the equation : 



(NH,),P,0,. 24M0O3 -^ 46NaOH = 2(NH,),HPO, 

 + (NH;),MoO, + 23Na,MoO^ -f 22H.,6. 



According to this equation each cubic centimetre of tenth 

 normal sodium hydroxide corresponds to 0.0003089 gram of 



