FERTILIZERS. 7 



difficult to oxidize, and is more commonly less than an hour. In most instances 

 the use of potassium permanganate is quite unnecessary, but it is believed that in 

 exceptional cases it is required for complete oxidation, and in view of the uncer- 

 tainty it is always used. The potassium sulphid removes all the mercury from 

 the solution, and so prevents the formation of mercur-ammonium compounds 

 which are not completely decomposed by the sodium hydroxid. The addition 

 of zinc gives rise to an evolution of hydrogen and prevents violent bumping. 

 1'iwious to use the reagents should be tested by a blank experiment with sugar, 

 which will partially reduce any nitrates present that might otherwise escape 

 notice. 



(b) GUNNING METHOD. OFFICIAL. 



(Not applicable in the presence of nitrates.**) 



(1) PBEPAEATION OF REAGENTS. 



(a) Potassium sulphate. This reagent should be pulverized before using. 

 The other standard solutions and reagents used are the same as those de- 

 scribed under the Kjeldahl method (p. 5). 



(2) APPARATUS. 



The apparatus used is the same as that employed in the Kjeldahl method 

 (P. 6). 



(3) DETERMINATION. 



Place the substance to be analyzed in a digestion flask, employing from 0.7 

 to 3.5 grams, according to its proportion of nitrogen. Add 10 grams of pow- 

 dered potassium sulphate and from 15 to 25 cc. (ordinarily about 20 cc) of 

 sulphuric acid. Conduct the digestion as in the Kjeldahl process, starting with 

 a temperature below boiling point and increasing the heat gradually until froth- 

 ing ceases. Digest for a time after the mixture is colorless or nearly so, or 

 until oxidation is complete. Do not add either potassium permanganate or 

 potassium sulphid. Dilute, neutralize, distil, and titrate as in the Kjeldahl 

 method. In neutralizing it is convenient to add a few drops of phenolphtha- 

 leiu indicator, by which one can tell when the acid is completely neutralized, 

 remembering that the pink color, which indicates an alkaline reaction, is de- 

 stroyed by a considerable excess of strong fixed alkali. 



(c) KJELDAHL METHOD MODIFIED TO INCLUDE THE NITROGEN OF NITRATES. 



OFFICIAL. 



(1) PREPARATION OF REAGENTS. 



Besides the reagents given under the Kjeldahl method, there will be needed 

 (a) Zinc dust. This should be an impalpable powder. Granulated zinc or 

 zinc filings will not answer. 

 (&) Sodium thiosulphate. 

 (c) Commercial salicylic acid. 



(2) APPARATUS. 



The apparatus used is the same as in the Kjeldahl method, page G (2). 



(3) DETERMINATION. 



Place from 0.7 to 3.5 grams of the substance to be analyzed in a Kjeldahl 

 digestion flask, add 30 cc of sulphuric acid containing 1 gram of salicylic acid, 



a See page 5 footnote, for qualitative method for determining the presence of nitrates. 



