1895.] PUBLIC DOCUMENT — Xo, 33. 301 



Experiment Station, published a bulletin in which he sug- 

 o-ests that the l:)est reagent for recognizing 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 will 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 grams 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 prepared, otherwise it will dissolve slowly, 

 if at all. The phosphate of iron was filtered and washed 

 quite thoroughly with water, and both filter and precipitate 

 brought into a beaker containing 400 cubic centimeters of 

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

 phoric acid. A gentle heat dissoh'es 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- 

 gallic acid, a dark wine color results. In order to recog- 

 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 1)oiling, filtered, a little of 

 the iron phos[)hate sohition 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 l)e 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 number of fertilizers 

 ofiicially collected in Massachusetts, liut 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. 



