ESTIMATION OF PHOSPHORIC ACID 167 



tion is first established. The burette is quickly read and a few 

 tenths more alkali added to be certain that the unit point has not 

 been missed. On cloudy days the light may be concentrated by a 

 lens, for which a flask filled with clear water conveniently serves. 

 Each cubic centimeter of the soda-lye corresponds, in the first ti- 

 tration, to 7.1 and in the second to 3.55 milligrams of phosphoric 

 acid. 



148. Estimation of Phosphoric Acid in the Presence of a 

 Large Excess of Iron. The volumetric method given below, due 

 to Emmerton, depends upon the precipitation of a phosphomolyb- 

 date, of constant composition, in the presence of a large excess 

 of iron, as in the analysis of iron and steel and iron ores. 25 The 

 molybdenum trioxid obtained is reduced by zinc to Mo 12 O 19 . 

 The action of permanganate on this compound is shown in the 

 following equation : 



5Mo 12 O 10 -f i7(K 2 OMn 2 O 7 )=6oMoO 3 +i7K,O+34MnO. 

 Seventeen molecules of permanganate are equal to 60 mole- 

 cules of molybdenum trioxid. The iron or steel is dissolved in 

 nitric acid, evaporated to dryness, heated, and redissolved in hy- 

 drochloric acid, then treated again with nitric acid and evaporated 

 until a clear and concentrated solution is obtained free from hy- 

 drochloric acid. 



The solution obtained is diluted to 40 cubic centimeters with 

 water and washed into a 400 cubic centimeter flask, making the 

 total volume about 75 cubic centimeters. Add strong 

 ammonia, shaking after each addition, until the mass sets to a 

 thick jelly from the ferric hydroxid. Add a few more cubic 

 centimeters of ammonia and shake thoroughly, being sure the 

 ammonia is present in excess. Add next nitric acid gradually, 

 with shaking, until the precipitate has all dissolved ; add 

 enough more nitric acid to make the solution a clear amber color. 

 The volume should now be about 250 cubic centimeters. Bring 

 the solution to 85 and add at once 40 cubic centimeters of 

 molybdate solution of the following strength : Dissolve 100 grams 

 of molybdic acid in 300 cubic centimeters of strong ammonia and 

 loo cubic centimeters of water, and pour the solution into 1250 



25 Blair, Chemical Analysis of Iron, Second Edition, 1891 : 95. 



