456 AGRICULTURAL ANALYSIS 



394. Hooker's Method. The quantitative action in this method 

 depends upon the deep green coloration given by nitric acid, when 

 dissolved in sulfuric acid and carbazol. 77 Other oxidizing bodies, 

 such as iron, chlorin, bromin, chromic acid, etc., give the same 

 reaction, but not in such a prominent manner. Such bodies, with 

 the exception of chlorin and iron, are not often found in the 

 solutions with which the agricultural chemist is occupied. In 

 the application of the process, iron, if present, in quantities greater 

 than one-tenth part per one hundred thousand, must be removed. 

 Chlorids, also, even when present in very small quantities, inter- 

 fere with the delicacy of the reaction and must be removed. Eas- 

 ily destructible organic matter tends to lower the result, but not 

 materially, unless present in large excess. Calcium carbonate and 

 sulfate, soda, and other alkalies, in the quantities in which they are 

 usually present in such solutions, do not affect the result. 



The following reagents are required: 



1. Concentrated sulfuric acid. 



2. An acetic acid solution of carbazol ; diphenylimid, 



3. A sulfuric acid solution of carbazol. 



4. Standard solutions of potassium nitrate. 



5. A solution of aluminum sulfate. 



6. A solution of silver sulfate. 



1. The sulfuric acid used for all purposes in the process 

 should be entirely free from nitrogen oxids. It may be read- 

 ily tested by dissolving in it a small quantity of carbazol. If the 

 solution be at first golden-yellow or brown, the acid is suffi- 

 ciently pure; if it be green or greenish, another and better sam- 

 ple must be found. It is essential also that the specific gravity 

 of the acid be fully 1.84, and it is well to ascertain that this is 

 really the case. 



2. The acetic acid solution is prepared by dissolving six-tenths 

 gram of carbazol in about 90 cubic centimeters of strongest 

 acetic acid, by the aid of gentle heat. It is allowed to cool, and 

 is then made up to 100 cubic centimeters by the further addition 



71 American Chemical Jourual, 1889, 11 : 249. 



