1912] Lipman-Sharp : FhenoldisulpTionic Acid Method 23 



acid iTsed as the reagent. Their results indicate (1) that in 

 order to obtain the phenoldisulphonic acid free from the mono 

 and tri-phenolsulphonic acid a careful digestion of the phenol 

 and sulphuric acid under certain constant conditions must be 

 assured; (2) that the mono and tri-phenolsulphonic acids intro- 

 duce other colors which interfere with the readings in the colori- 

 meter; (3) that the tri-potassium salt of nitrophenoldisulphonic 

 acid gives the characteristic color employed in the determination 

 and should always be used as a standard; (4) that heating the 

 dry residue of nitrates even for several hours on the water bath 

 occasions no losses; (5) that aluminum cream is the best pre- 

 cipitating agent for organic matter of several used and occasions 

 no losses of nitrates; (6) that 2 c.c. of the phenoldisulphonic acid 

 should be used in uniform amounts in all determinations; (7) 

 that KOII was to be preferred to NaOH and NH^OH, as the 

 alkali employed; (8) that chlorides induced losses of nitrates; 

 (9) that carbonates and organic matter did likewise; (10) that 

 temperature, concentration, and length of exposure to reagent 

 greatly affect results; and (11) that there have been other minor 

 effects of iron, magnesium, and nitrites. 



Reference must also be made here to the brief investigation of 

 Stewart and Greaves^ pertaining to the effect of chlorine in de- 

 termining nitrates in soils, both because the work is recent and 

 because it is the only one published which is derived from re- 

 searches on soils. This investigation and those above reviewed 

 cover most completely the questions involved and reference will 

 be made in the discussion of our experimental work below to 

 those questionable points which were considered settled but which 

 our work shows M'ere far from being so. 



The Interference of Salts with the Nitrate Determination 



As has been above indicated the salt accumulations which 

 occur in the soils of California, Nevada, Utah, and other arid 

 or semi-arid regions frequently contain considerable quantities 

 of nitrates and the determination of the latter in the presence 

 of the "alkali salts" is, as has been found, frequently attended 



^ Ihkl., vols. 21, p. 922; 32, p. 630; 33, p. 366 

 ^ Ihid., vol. 32, p. 756. 



