386 



STATE BOARD OF AGRICULTURE. 



were too small to cause any appreciable error in detenninations made 

 with them. 



In order to test the correctness of this assumption several actual deter- 

 minations were made. The solutions used were R II, C./ M IV. and 

 I III. They were carefully diluted to a specific gravity of 1.09 and the 

 determinations were all made at one time so that there could be no 

 variation due to tem])erature of the bath. Determinations were made on 

 two sam]>les of fertilizer, one containing a small ix^rcentago of insoluble 

 P^Og and the other a large percentage. The results are given in Table 

 III: 



TABLE III. 



An inspection of these results shows a marked relationship between 

 the reaction of the ammonium citrate solution and the amount of phos- 

 phorus pentoxide extracted. 



These variations amounting to 1.99% in one case and 1.62% in the 

 other are certainly significant. The results prove that perfect neutrality 

 of the citrate solution is of the utmost importance in securing consistent 

 results as the above differences are of suflQcient magnitude to make the 

 method extremely uncertain. 



CONDUCTIVITY METHOD: 



The above mentioned method proposed by Hall and Bell was found 

 to be much more satisfactory'. 



It depends upon the fact that when different quantities of alkali are 

 added to an acid solution (or vice versa) the electrical conductivity or 

 resistance of the solution will vary with each addition, and if these 

 resistances or conductivities are plotted against the amounts of alkali 

 or acid added, there is a sharp change in the direction of llio curve at 

 the neutral [)oint. This proi)erty has been made use of for the titration 

 of various liquids where color changes were ditficult to observe and has 

 given very good results. 



As many cliPmists have had no cxpcrienco in determining electrical 

 conductivity, tlic :ij»paratus and method of procedure are taken up here 

 in. detail. 



•Neutralized by the conductivity method. 



