of Phosphoric Acid. 339 



A dilute filtered solution of albumen, as we know, gives no 

 precipitate with the sokition of phosphoric acid. 



A short time since we were made acquainted, by Svanberg 

 and Struve, with an excellent reagent for phosphoric acid* in 

 the molyhdate of ammonia. This is so delicate in the detec- 

 tion of the smallest trace of phosphoric acid, and is capable 

 of showing its presence even in those compounds in which the 

 acid is discovered with difficulty or cannot be so at all, that 

 an important service has been rendered to analytical che- 

 mistry by the recommendation of this reagent. 



If a solution of the molybdate of ammonia be added to a so- 

 lution of any phosphate, and then so much muriatic, or what 

 is better nitric acid, that the precipitate which is formed at 

 first disappears again, the liquid immediately becomes yellow, 

 and deposits, even when the smallest quantity of phosphoric 

 acid is present, a yellow precipitate, which consists of molybdic 

 acid, but which is a different modification, and possesses dif- 

 ferent properties from the molybdic acid which is obtained 

 when phosphoric acid is not present. If the phosphoric 

 compound to be examined is insoluble in water, it is used 

 in solution in acids, especially nitric acid. The precipita- 

 tion is accelerated by heat. The yellow precipitate is soluble 

 in ammonia, as also in an excess of the phosphate. Hence 

 only very small quantities of phosphoric acid are most easily 

 detected in this manner; and it is quite possible that a larger 

 quantity might be overlooked, because in this case a very 

 large quantity of the molybdate is requisite to produce the 

 precipitate after saturation with nitric acid. 



The yellow precipitate can be readily recognised, even when 

 it is precipitated from a coloured liquid, as from a nitric solu- 

 tion of phosphate of copper, or from acid solutions of other 

 coloured phosphates. 



It must however be remarked, that c-phosphoric acid and 

 its salts only are able to produce this reaction. The other 

 modifications of phosphoric acid do not give the yellow preci- 

 pitate with the molybdate of ammonia, unless they have been 

 converted into the c-phosphoric acid by the nitric acid added. 

 As is well known, this often takes place slowly and incom- 

 pletely in the cold. Hence the pyrophosphate of soda may 

 be allowed to remain for a very long time in dilute solutions 

 with the molybdate of ammonia and free nitric acid, without 

 any effect being perceptible. But if the whole be made to 

 boil, a yellow liquid is instantly produced, and soon after- 

 wards a yellow precipitate. 



* Phil. Mag., S. 3, vol. xxiii. p. 524. 

 Z2 



