is stoppered with a cork which is protected from the solution by cellophane, 

 and then inverted several times until thoroughly mixed. It is then labelled 

 with the name of the indicator, the volume, and the concentration of the 

 indicator solution used, and the pH of the buffer. Tubes must be freshly 

 prepared each summer, after thorough washing, though labels may be reused 

 without removal if they have been protected by Scotch Tape or label varnish. 

 Stability is also enhanced by storage in the dark, but in any case, the 

 sets are good only for three months at the most. 



A set is issued in a test tube rack with an empty, matched tube and 

 a dropping bottle of the indicator solution. To avoid errors in the volume 

 of indicator used, the same bottle should be used in making the standard 

 tubes, since this will insure that the drops are all from the same dropper 

 and, therefore, of identical size. 



To use the sets, the proper indicator is first determined by adding 

 a drop of various indicators to small amounts of the unknown. The empty 

 tube of the proper set is then filled to the mark with the unknown solu- 

 tion, and five drops of the indicator solution added. After thorough mix- 

 ing, the color of the unknown tube is matched with those of the standards, 

 using a comparator block. Note that the most commonly used indicator is 



Phenol Red, since it exhibits its maximum color gradation in the range of 

 physiological pH's. 



TABLE OF INDICATORS 



In each case, add 0.25 ml. (5 drops) of 0.04% indicator solution to 

 10 ml. of the solution to be tested. 



Indicator 



*Thymol Blue 



Brom Phenol Blue 



Brom Cresol Green 



Chlor Phenol Red 



Brom Cresol Purple 

 Brom Thymol Blue 



Phenol Red 



Cresol Red 



Meta Cresol Purple 



Thymol Blue 



pH Range 



1.2-2.8 



3.0-4.6 



3.8-5.4 



4.8-6.4 



5.2-6.8 

 6.0-7.6 



6.8-8.4 



7.2-8.8 



7.4-9.0 



8.0-9.6 



ml. of 0.05N 

 NaOH per gram dye 



43 

 30 

 29 



47 

 37 



Color Change 



Red — Yellow 

 Yellow— Blue Violet, 

 Yellow — Blue 

 Yellow — Red 

 Yellow — Violet 



32 Yellow - Blue 



57 Yellow — Red 



5 3 Yellow - Red 



53 Yellow — Purple 



43 Yellow — Blue 



*Thymol Blue has two ranges, one acid and one alkaline. In the acid 



range, use twice as much indicator as usual, that is, 10 drops (0.5 ml. ) 

 of 0.04% solution per 10 ml. of unknown. 



38- 



