228 BERTHA VAN H. ANTHONY AND C. V. EKROTH 



These shades given differ more or less from the Standard 

 Method (1913). 



That no two people seem to titrate to exactly the same shade 

 has often been shown in our laboratory when a different worker 

 in the media room has attempted to set the final reaction of 

 some special medium. If the method of titrating is to be at 

 all accurate, it is necessary to assume a shade of pink for phenol- 

 phthalein. This necessity is brought out by such great dis- 

 crepancies between different workers titrating the same sub- 

 stance as are given by Clark (1915).*^ As a help, a practical 

 color scale may be of aid in determining the most suitable shade 

 for a certain kind of work and approximating it as closely as 

 possible. This should eUminate the personal factor to some 

 extent. 



As stated before, in our opinion, the correct shade for a deli- 

 cate end point in pale broth or other solutions with little color 

 is the first most delicate pink tinge observable throughout, 

 remaining at least one minute. 



With us, when titrating agar, a mixture of 5/20 red, 3/20 

 orange and 12/20 white on the color-top (see page 210) has 

 proved to be a desirable shade for the first and second titrations 

 of neutral agar;^" while the third titration, when the medium 

 comes out of the autoclave, should give (on the addition of 

 phenolphthalein) a shade consisting of 3/20 red, 3/20 orange 

 and 14/20 white on the color-top. These shades differ from 

 the ones given above both in the Standard Method of titrating 

 at boiling point (page 210) and our own definition (page 213) 

 but they have yielded very good results. However, it may be 

 as difficult to decide on an end point by means of a color-top 

 or scale as to imagine an end point from the descriptions of the 

 various writers. 



LITMUS 



Since any medium with meat infusion as a basis and peptone 

 added is a most complex mixture, no one indicator shows all 



1' (16) page 117. Such relatively great discrepancies are surprising especially 

 among the chemists. 



" The deeper color of agar as compared with the usual coloi of broth makes 

 necessary the use of some orange in this scale. 



