A DEFINITE H-ION CONCENTRATION 167 



The second method is most commonly called the colori- 

 metric method. The phenolphthalein method is also a colori- 

 metric method. The essential difference in the newer 

 method is that indicators are selected which show color 

 changes throughout a definite pH range and are used only 

 within this range and calculations can not be made from 

 the amounts of acid or alkali used to give these color changes 

 and applied outside their range any more than with phenol- 

 phthalein. The latter is a very good indicator for its jjTL 

 range, which is from pH8 to pH9.6. 





R 



Fig. 110. — Diagram of wiring for set up of potentiometer as illustrated 

 in Fig. 109. In case R2 (not shown in the photograph) is used the wiring 

 should be according to the dotted lines and not direct from S C to T S. 



D C, dry cells; R, rheostat; P, potentiometer; G, galvanometer; K, key; 

 S C, standard cell; T S, two way switch; C, calomel electrode; H, hydrogen 

 electrode; R2, extra resistance for protection of the standard cell; H C, 

 cylinder of compressed H (a hydrogen generator may be used) ; B, burette 

 for adding acid or alkali. 



It would be possible to select a series of indicators each 

 of which will show color changes some place between ^^HO 

 (normal HCl) and 2^H14 (normal NaOH). 



Clark and Lubs {Jour, Bad. II) have suggested the fol- 

 lowing series with jjK as indicated on page 168. 



Many of these are valuable for detecting changes in reac- 

 tion caused by organisms growing in culture media. It has 

 been stated that most bacteria require a medium with an 

 initial reaction in the neighborhood of neutral, pH7. The 

 table shows three indicators which cover this range: Brom- 

 cresol purple pH5.2-6.8, Bromthymol blue, 2^H6.0-7.6 and 

 Phenol red, pH6.8-8.4. 



