32 AGRICULTURAL ANALYSIS 



the sample is digested with 10 grams of potassium sulfate and 

 30 cubic centimeters of sulfuric containing one gram of salicylic 

 acid, and three grams of zinc sulfid. The heat is kept down until 

 frothing ceases, and then the mass kept in gentle ebullition until 

 clear. The distillation is accomplished with the usual precau- 

 tions. The voorhees process is superior to that recommended 

 by Winton in adding the potassium sulfate at the beginning of 

 the combustion. 



332. Official Gunning Method Modified to Include the Nitro- 

 gen of Nitrates. 27 In a digestion flask holding from 250 to 500 

 cubic centimeters place from 0.7 to 3.5 grams of the substance 

 to be analyzed, according to the amount of nitrogen present. 

 Add from 30 to 35 cubic centimeters of salicylic acid mixture; 

 namely, 30 cubic centimeters of sulfuric to one gram of salicylic 

 acid, shake until thoroughly mixed, and allow to stand from five 

 tc 10 minutes, with frequent shaking; then add five grams of 

 sodium thiosulfate and TO grams of potassium sulfate. (It is 

 suggested that after adding the sodium thiosulfate, the solution 

 be heated for five minutes, cooled and the potassium sulfate 

 added. This variation prevents foaming). Heat very gently 

 until frothing ceases, then strongly until nearly colorless. Dilute,, 

 neutralize, and distil as in the gunning method. 



DETERMINATION OF NITROGEN IN DEFINITE FORMS OF 



COMBINATION 



333. Introductory Considerations. In the foregoing pages 

 has been given a summary of the methods most in vogue for the 

 estimation of nitrogen in fertilizers and fertilizing materials. 

 There are many cases in which the analyst may have to deal 

 with a definite chemical compound, and where a modified or 

 shorter method may be used. There are other cases in which 

 the nitrogen may be present in two or three definite forms, as in 

 artificially mixed fertilizers, and where it is desirable to show 

 the proportions in which the various forms are present. For 

 these reasons it is necessary to be able to use methods by which 

 the percentage of nitrogen in its various forms may be relatively,. 



n Bureau of Chemistry, Bulletin 107, 1907 : 8. 



