90 METHODS OF DETERMINING ALKALI 



and making alkaline with ammonia. Chlorides, when 

 present in considerable quantities, interfere quite markedly 

 with the determination of nitrates and must be previously 

 removed. This is best accomplished by means of silver 

 sulphate free from nitrates. This can be added in the 

 solid form, thus avoiding dilution of the original solution. 

 The silver sulphate is tested for nitrates by treating some 

 of the solid salt with the phenoldisulphonic acid reagent, 

 diluting with water and adding ammonia water. No 

 yellow color should be produced. The silver sulphate as 

 found in the market frequently contains nitrates in amounts 

 sufficient to vitiate all results, and it is, therefore, advis- 

 able to prepare it specially for this work. 



"The presence of some kinds of organic matter also in- 

 terferes seriously w r ith the determination of nitrates by 

 this method. In some cases it is the foreign color only 

 which is produced by the strong acid, but often the action 

 is of more vital importance, as a considerable loss of nitrates 

 occurs, possibly due to oxidation of the organic matter 

 by the nitrate instead of the nitration of the phenoldisul- 

 phonic acid. In some cases it is advisable to reduce the 

 nitrates to ammonia by means of the copper-zinc couple. 

 The ammonia is distilled off and determined colorimetric- 

 ally. The ammonia originally present in the solution 

 must be determined separately and deducted. Nitrites 

 are likewise reduced to ammonia and must be allowed for 

 if present. 



" Analytical Process. Evaporate 50 cc, or other 

 convenient quantity, depending upon the amount of 

 nitrate present, to dryness in a porcelain dish on a water 

 bath, removing the dish as soon as it is completely dry. 

 Add 1 cc. of the phenoldisulphonic acid reagent and stir 

 thoroughly with the rounded end of a glass rod so as to 



