560 



STATE BOARD OF AGRICULTURE. 



and Lubs (2). Ten cc. portions are placed in the 2.8x16 cm. test tubes with 

 the same amount of indicator that was placed in each 10 cc. of the broth 

 solution and sealed. In this way the standards •will keep for a considerable 

 period. The standards with the following readings are used in this work, 

 pH 7.4, 7.2, 7.0, 6.8, 6.6, 6.4, 6.2, 6.0, and 5.8. These standards may be 

 prepared in any chemical laboratory, or if one has not the facilities for mak- 

 ing may be obtained from Hj^nson, Wescott and Dunning of Baltimore at 

 a reasonable price. 



Comparators. Colorimetric procedures in experimental and analytical 

 chemistr}^ have grown in favor during the past few years until at the present 

 time many determinations are quickly made by colorimetric methods which 

 required tedious gravimetric technique a few years ago. 



To meet this growing demand several types of comparators and color- 

 imeters have been devised during the past few years. Most of these are 

 not suitable for the determination of pH where the standards are in test 

 tubes and need not be considered in connection witli this work. 



In using the technique of the colorimetric hydrogen ion determination to 

 read the approximate pH of solutions in test tubes the writer found the 

 method of changing standards and unknowns in the comparator so tedious 

 and time consuming that the desired number of readings could not be made 

 each hour. With the idea of finding a more convenient comparator or of 

 devising a means which would make the reading of a greater number of de- 

 terminations possible, a study was made of the types in which unknowns in 

 test tubes were compared with standards in test tubes. As none of the com- 

 parators in use were found to meet the requirements of this work the fol- 

 lowing arrangement, a modification of the comparator of Hurwitz, Myer 

 and Ostenberg was designed. (Fig. 1). 



Fig. No. 1 A comparator designed for examining a large number of samples in a short time. 



Two sliding test tube racks are constructed and so arranged in the com- 

 parator that the one carrying the standard tubes B (Fig. 2) can be slid back 



