OF VITAL PHENOMENA 49 



range of temperatures. All these corrections are combined in 

 the following table from Clark and Lubs : 



Correction in millivolts to be added to potentiometer reading, for 

 change in concentration of hydrogen due to barometric pressure, tempera- 

 ture and vapor tension of water. 



Barometer (mm) 20 



780 0.85 



770 1.02 



760 1. 19 



750 1.36 



740 i.53 



Determination of Hydrogen Ion Concentration by Means of 



Indicators 

 According to Ostwald, indicators are weak acids or bases whose 

 ions are not colored like the undissociated molecules. Indicators 

 may, however, be used in titration in non-aqueous solvents in 

 which dissociation is very slight. The theory of Stieglitz that 

 the change in color is due to intramolecular change, following 

 change in ionization, may account for the fact that the change 

 in color may not be instantaneous. It is possible that the amount 

 of hydration of the ions has an effect on their color, since the 

 presence of neutral salts affects the color of indicator solutions. 



The following indicators and PH at which they change are given by 

 Friedenrhal : 



Indicator PH Indicator PH 



Alizarin 6- 8, 1 1-15 Lacmoid 4-8' 



Alizarin blue S n-13, 7- 8 Magdalarot 3-4, 14-15.. 3- 4,14-15 



Alizarin green B 12-13 Mauvein 0-3, 13-15 



Alizarin Na sulfonate. 4-6 Methyl green 0-2,11-14 



Alkali green 3-7 Methyl orange 2-5, 14-15 



Azolilmin (litmus) 5-9 Methyl violet 0-3, 12-15 



Benzopurpurin B 0-5, 13-14 a-Napthol benzoin 9-1 1 



Bitteralmondoilgreen . . 0-2,11-14 Neutral red o- 1, 7-9 



Cochineal 4-6 p-Nitrophenol 5-7 



Congo red 3-5 Phenacetolin 3-6, 10-13 



Crocein 12-14 Phenolphthalein 8-10, 13-15 



Curcumein (Tumeric) . 13-15 Rosolic acid 6-8, 13-15 



Cyanin 6-8 Saf ranin o- 1, 14-15 



Dimethylamidoazobenzol 2- 4 Saurefuchsin (Acid 



Echtrot o- 1 fuchsin) 9-14 



Eosinnrorhyleneblue ... 0-3, 14-15 Tetrabromphenolphtha- 



Fluorescein 0-6 lein 8-9,12-15 



Gallein 1-6, 10-14 Thymolphthalein 10-n 



Guaiac 7-8 Tropaeolin 7-9 



Haematein 0-15 Tropaeolin o 11-13 



Heliantin I 11-13 Tropaeolin 00 0-3 



Heliantin II 12-14 Tropaeolin 000 11-13 



Iodeosin 0-15 Trinitro 'benzol 12-15 



