618 STATE BOARD OF AGRICULTURE. 



Azolitmin as an iudicator for this purpose is however much inferior 

 to phenol red, occupying an intermediate i>osition between coralline, 

 which is poorer, and rosolic acid which is somewhat better. 



In all of the above work it Avill be observed that N/10 NH^OH was 

 used Avhereas in previous investigations it has been customarj^ to work 

 with much stronger soUitions such as N/5 or N/2 with a corresponding 

 diminution of the sensitiveness. 



II. THE RELATION BETWEEN COMPOSITION AND REACTION. 



The use of McCandless' method (36) (37) of preparing the reagent 

 has been restricted to the preparation of the solution of the normal 

 salt because in this solution alone is the ratio of ammonia to citric acid 

 definitely fixed. In such a solution the ratio NH3 : CoHgO^ is 1 : 3.7G6. 

 Attempts have been made to fix the composition of a truly neutral solu- 

 tion but without much success and as Hand pointed out, ''any analytical 

 method in a large measure leaves the question in its former condition, 

 because we must first prepare the neutral solution before we can ascer- 

 tain the precise amounts of citric acid and of ammonia that will re- 

 produce it."^ 



McCandless (37) acting in accordance with instructions of the A. O. 

 A. C. attempted to determine the relation between the reaction and 

 compositions of several solutions sent to him by testing them for neutral- 

 ity to coralline and also determining the ratios of ammonia to citric 

 acid in them. Of nine solutions submitted, three proved to be neutral 

 to the indicator used. The ratios in these three were respectively 

 1 : 3.803, 1 : 3.81G and 1 : 3.808, giving an average of 1 : 3.809. 



A solution prepared by Rudnick and Latshaw (45) and carefully 

 neutralized to litmus had a ratio of 1 : 3.748. 



The development of the methods for determining the concentration 

 of hydrogen ions in solutions gave us the first method for accurately 

 preparing a truly neutral solution the determination of whose com- 

 position would serve for its reproduction. Using the colorimetric 

 method outlined above but with some variations in technic we prepared 

 and analyzed a number of such neutral solutions. The actual procedure 

 used in their preparation was as follows : 



One hundred ten gram portions of citric acid were weighed into 700 

 c.c. flasks, each dissolved in 75 c.c. of water and mixed with concentrated 

 ammonia to bring the reaction up to 6.G-G.8. For the final neutraliza- 

 tion, each solution was transferred to a 500 c.c. graduate, diluted to 

 a density of 1.11-1.10 and mixed with a measured quantity of indicator. 

 The same volume of standard phosphate solution having a Pji of 7.0 

 was placed in a similar graduate and mixed with the same amount of 

 indicator. 2N NH^OH was then added to the citrate solution until its 

 color checked that of the standard solution whose volume was equalized 

 with distilled water. The reaction was finally checked by removing a 

 few c.c. of the citrate, diluting and checking against some of the stand- 

 ard in the comparator. The neutralized solutions were then returned 



"Bull. 132, Bur. of Chem. p. 9. 



