122 BACTERIOLOGY 



known hydrogen-ion concentration, it may be used as a 

 standard for the determination of unknown reactions. 

 Results obtained by such a procedure, the phosphate solu- 

 tion of Sorensen being employed as the standard of com- 

 parison, are as follows: 



RESULTS WITH PHENOL RED. 



Acid tubes. Alkaline tubes. 



Drops of phenol Drops of phenol 



red solution. red solution. pH. 



9 1 = 6.9 



8 2 = 7.2 



7 3 7.5. 



6 4 =7.7 



5 5 = 7.9 



4 6 = 8.1 



Outline of procedure -and equipment needed: Apparatus 

 and chemicals: 



(a) Clean test-tubes of approximately the same diameter. 



(b) A 5 c.c. and a 1 c.c. volume pipette. 



(c) A medicine dropper drawn out to a fine point. 



(d) A burette. 



(e) Indicator solution: 0.01 per cent, phenol red in 



distilled water. 

 ; (/) /TT KOH. ' 

 (g) HC1 or H 2 SO 4 . 



(h) Test-tube rack, double row of holes. 

 Preparation of Standard Color Series. Twelve test-tubes 

 are placed in two rows of six. Into each tube of one of 

 the rows five (5) c.c. of dilute alkali are placed (the ^ 

 KOH solution may be used). Into each tube of the other 

 row five (5) c.c. of very dilute acid are placed (1 drop of 

 concentrated HC1 or H 2 SO 4 in 100 c.c. distilled water). 



Into the six acid tubes 9, 8, 7, 6, 5, 4 drops, respectively, 

 of the indicator solution are placed. Into the six correspond- 



