174 PEECHLOKATES PHOSPHORIC ACID 



(Ulsch's method), or by treatment with a mixture of sulphuric acid, 

 salicylic acid and sodium thiosulphate (Forster's method). 



In many cases nitrate of soda is valued by " refraction," i.e., by 

 determining the total percentage amount of the impurities present. 

 The substances usually present in Chili saltpetre are moisture, in- 

 soluble matter, chlorine, sulphuric acid, magnesia and perchlorate. 

 These constituents are determined by the usual processes. A word or 

 two may be said about the determination of the perchlorate, a substance 

 whose presence and importance in nitrate has only recently been 

 realised. Five grammes of the dried sample are heated for a quarter 

 of an hour over a Bunsen lamp with 7 or 8 grammes of pure calcium 

 hydroxide in a covered crucible. The mass is then transferred to a 

 measuring flask, made up to 128 cc. with water, digested for an hour, 

 and filtered (3 cc. is taken as the volume of the undissolved matter , 

 so that 125 cc. of solution are really taken). To 100 cc. of the 

 filtrate (=4 grammes of the sample), dilute nitric acid is added until 

 exactly neutral and the chloride present is then determined by titra- 

 tion with silver nitrate. After deducting the chlorine present origin- 

 ally as chloride (determined by direct titration without treatment with 

 lime), the chlorine found is calculated to perchlorate. 1 



In the case of most manures the determinations of the various 

 constituents are made by the usual quantitative methods, but in some 

 cases shorter and easier, though perhaps less accurate, methods are 

 employed. A few of these rapid methods may be mentioned. 



Phosphoric Acid. In the presence of aluminium, iron and 

 calcium in ordinary analytical work it is generally considered neces- 

 sary to first precipitate the phosphoric acid with ammonium molyb- 

 date, wash the yellow precipitate until the above-mentioned metals 

 are removed, redissolve the precipitate in ammonia, and precipitate 

 with magnesia mixture. Instead of this somewhat tedious and costly 

 process, it is possible by the addition of citric acid to the original 

 solution, followed by magnesia mixture and ammonia, to keep the 

 iron, aluminium and lime in solution and obtain all the phosphoric 

 acid as magnesium ammonium phosphate. The method is only 

 suitable when the quantity of phosphoric acid is fairly large and that 

 of iron and aluminium not considerable. 



The phosphoric acid of a manure may be present in three states of 

 combination, as already stated : 



1. Water-soluble phosphates. 



2. Citrate-soluble phosphates. 



3. Insoluble phosphates. 



The methods of determining the first and third require little or no 

 explanation. For the second the residue after washing with water is 

 used. It is extracted with a solution of ammonium citrate of specified 

 strength, for a specified time, and at a specified temperature. 



The ammonium citrate solution employed is usually of specific 



1 Blattner and Brasseur, Chem. Zeitung, 1900, 767. 



