EXPERIMENT STATION BULLETINS. G13 



tested. The basis of the method is found iu the fact that iudicators 

 change color not abruptly but through a definite range of hydrogen ion 

 concentration and that the range of reaction through which the change 

 of color is observable differs for different indicators. Thus by the 

 proper choice of indicators, any region of reaction from normal hj^dro- 

 gen ion concentration to normal hydroxyl ion concentration may be 

 studied. 



The choice of indicator is governed by its availability, its effective 

 range and the vividness of its color change; the selection of the standard 

 buffer solutions by the ease with vvdiich they can be accurately duplicated 

 and their range of reaction. 



At the time of the performance of their work, the selection of both 

 the indicator and standard made by Eastman and Hildebrand (15) 

 was probably the best one possible but since that time new indicators 

 and new standards have become available which surpass in excellence 

 those previously known. 



Their technic was as follows: 



''For the preparation of citrate solutions in two liter lots, dissolve 

 370 grams in 1500 cc. of water, and nearly neutralize with concentrated 

 ammonia solution. Cool to 20° and then add more ammonia from a 

 burette until a 10 cc. portion of the thoroughly stirred solution, with 

 a suitable quantity of rosolic acid, shows the same color in a Nessler 

 tube that the same amount of indicator gives with a 10 cc. portion of 

 a phosphate solution prepared as follows : 



Titrate a 25 cc. portion of an approximately 0.1 molar stock solution 

 of Na,HPOi (to which dilute HGl or NaOH has been added until 

 phenolphthalin is just colorless in the solution) with N/10 HCl and 

 methyl orange. To a fresh 25 cc. portion (neutral to phenolphthalin) 

 add 1/6 of the volume of HCl used in the previous titration. Stir 

 well and use for the color standard above. When the citrate is neutral- 

 ized, bring the solution to a specific gravity of 1.09 at 20° C." 



I\osolic acid was selected after the investigation of several iudicators 

 including alizarin, guaiacum tincture, neutral red, crj^anin, hematin and 

 azolitmus. It will be seen that this method, while making use of an 

 indicator comnnmly used in acidimetry, does so in a peculiar manner. 

 Hence it cannot be grouped with the other methods utilizing this prin- 

 ciple as was indicated above (p 7) and the general criticisms to which 

 methods of the other type are subject cannot be applied to this one. 

 On the other hand this method involves factors of importance which are 

 lacking in the other. Thus rosolic acid while unsatisfactory to many 

 when used in the ordinary fashion may be admirably suited to this new 

 form of technic. But preferable though it may be for this purpose in 

 comparison with the older indicators, it is quite inferior to tlie more 

 recently discovered ones of the sulphonphthalin series which in their 

 brilliancy of color and the distinctness of color changes surpass all 

 previoush' known substances. 



In a neutral solution Ch =Coh = lxlO~~ ' =Ph 7 where Ch represents 

 the concentration of hydrogen ions, Co that of hydroxyl ions and Pn 



the symbol used bv Sorensen (49) to indicate the log -p^ Since Eastman 



Ch- 

 and Hildebrand (15) have shown that in a solution of triammonium 



citrate Ch = 1x10 ~ 7.4 ^n indicator mtist be chosen whose maximum 



