Ibiid.J PUBLIC DOCUMENT — No. 33. 301 



Experiment Station, pnl)lishc(l a l)iillctiu in -which he sug- 

 o'csts that the best reaijent for reco<rni/inij the tannic acid is 

 a phosphoric acid solution of phosphate of iron. He states 

 that, if leather be present in the substance examined, a 

 purple color Avill soon appear if a few drops of this solution 

 be added to the alkaline solution of the leather extract. The 

 phosphoric acid solution of phosphate of iron was prepared 

 as follows : ten irrams of ferric chloride were dissolved in 

 water, and sodium phosphate added till all the iron was 

 precipitated as phosphate of iron. The phosphate of iron 

 must be freshly })repared, otherwise it will dissolve slowly, 

 if at all. The i)hosphate of iron was filtered and washed 

 quite thoroughly witli water, and both filter and precipitate 

 brouoht into a beaker containing 400 cubic centimeters of 

 water, to which had been added 40 grams of glacial phos- 

 phoric acid. A gentle heat dissolves the iron phosphate 

 quite readily. 



If a drop of pyrogallic acid is added to water, the solution 

 made slightly alkaline with ammonia and then a cubic cen- 

 timeter of the iron phosphate solution added, a dark purple 

 color appears. If tannic acid is substituted for the pyro- 

 o-allic acid, a dark wine color results. In order to recoo-- 

 nize leather in a mixture, a small amount (one gram) of the 

 substance supposed to contain it is placed in a beaker with. 

 30-40 cubic centimeters of water, a few drops of sulphuric 

 acid added, the liquid brought to boiling, filtered, a little of 

 the iron phosphate solution added, and the solution then 

 made slightly alkaline with ammonia. If leather is present, 

 a dark purple to wine color will soon appear. 



Should leather be present in a mixed fertilizer containing 

 soluble phosphate of lime, the latter will of course be pre- 

 cipitated on the addition of ammonia, but this in no way 

 interferes with the color reaction. The writer examined 

 during the summer of 1893 quite a numl)er of fertilizers 

 officially collected in Massachusetts, but in no case was 

 leather to be detected. When, however, ten per cent, of 

 leather was added to a mixed fertilizer, and then tested 

 with the phosphate of iron solution, the dark color, due 

 to the presence of tannic or gallic acids, very distinctly 

 appeared. 



