632 AGRICULTURAL ANALYSIS 



fifth-normal solution of potassium cyanid which is prepared by 

 standardizing against a known weight of copper dissolved in 

 nitric acid, the method being the same as that described in the 

 following paragraph. 



Determination. Neutralize an aliquot portion of the nitric acid 

 solution used in Method I for total copper oxid with sodium car- 

 bonate and add a trifling excess of the carbonate ; add one cubic 

 centimeter of 0.960 specific gravity ammonia and titrate the dark 

 Hue solution to the disappearance of the blue color with stand- 

 ard potassium cyanid. 



Sand. Dissolve the sample used for moisture determination in 

 hydrochloric acid, filter, wash, dry and finally burn the filter 

 .and calculating the residue as sand. 8 



Sodium Sulfate, Treat the boiling filtrate from the deter- 

 mination of sand with a boiling solution of barium chlorid, al- 

 low to stand until the precipitate settles, leaving a clear solu- 

 tion ; filter, wash, dry and burn with the usual precaution used 

 in determining barium sulfate. Calculate the barium sulfate 

 found to sodium sulfate, since it is in this form that sulfuric acid 

 is supposed to be present. 



Acetic Acid. This figure is obtained by subtracting the sum of 

 the other constituents from 100. 



530. Discussion of Methods of Analysis of Paris Green. Hay- 

 wood has very carefully tested all three of the methods given 

 above for total arsenious oxid and has found that all of them 

 give excellent results. The methods have also been tested by the 

 Association of Official Agricultural Chemists with like results. 



The electrolytic method for total copper is of course a stand- 

 ard method, the accuracy of which has long since been deter- 

 mined. The thiosulfate method for total copper gives most ex- 

 cellent results when precautions are taken, but is a failure unless 

 certain precautionary details are followed to the letter. It is ab- 

 solutely necessary that there be only a very slight excess of ace- 

 tic acid present when the potassium iodid is added, otherwise 

 iodine is continuously set free and an end point can not be ob- 

 tained. It is also necessary that the chemist be very careful 

 8 Bureau of Chemistry, Bulletin 68, 1902 : 13. 



