176 AGRICULTURAL ANALYSIS 



distilled water. To this end the flask should be attached to the 

 filter pump as shown in the figure. One hundred cubic centi- 

 meters of warm dilute sulfuric acid are placed in the funnel b, and 

 the stop-cock c opened. When the funnel is almost empty, the 

 solution which is to be reduced is transferred thereto. The solu- 

 tion should be hot, but not boiling. The vessel which held the 

 solution should be washed with dilute sulfuric acid, and this 

 added to the funnel again when it is nearly empty in such a way 

 as to wash it thoroughly and this should be followed with about 

 200 cubic centimeters more of warm dilute sulfuric acid, and 50 

 cubic centimeters of hot distilled water. In no. case is the funnel 

 allowed to become empty, and the stop-cock c is closed when 

 there is still a little of the wash-water left in the funnel above 

 fhe surface of the zinc. In this way air is prevented from pass- 

 ing into the reductor tube. Blank determination is made by 

 passing through the reductor a mixture containing 10 cubic 

 centimeters of strong phosphoric acid, 10 cubic centimeters of 

 dilute ammonia, and 50 cubic centimeters of water. This is 

 preceded and followed by the dilute acid as described above 

 The amount of potassium permanganate required to give this 

 blank a distinct color is subtracted from the amount required 

 to give the* same color to each reduced solution. To estimate 

 the value of the solution the weight of the iron wire used is 

 multiplied by the percentage of the iron in the wire and divided 

 by the number of cubic centimeters of potassium permanganate 

 in terms of metallic iron. The result is multiplied by the factor 

 0.88163 which is the ratio of molybdic acid to iron, and this 

 product by 0.01794 which is the ratio of phosphorus to molybdic 

 acid, and the result is the value of one cubic centimeter of the 

 permanganate solution in terms of phosphorus. The formula 

 of the reduced molybdic acid is given as Mo 24 O 37 . 



153. Calculating Results. To illustrate the method of calcu- 

 lating results Blair gives the following example: The weight 

 of the wire represented by 0.1745 gram requires 50 cubic centi- 

 meters of permanganate to give the required color. A blank 

 determination carried on as described above shows o.i cubic 

 centimeter of permanganate, so the quantity required by the 



