50 PHYSICAL CHEMISTRY 



The following indicators were found by Sorercsen (1909) to be least 

 affected by proteins, but some control is necessary if the protein content 

 of the unknown solution is considerable. 



Indicator PH 



Methyl violet ' 0.1— 32 



Mauvein 0.1 — 2.9 



(Methyl red) o — Benzolcarbonicacid azo dimethyl anilin 4.2 — 6.3 



p — Nitro phenol 5 — 7 



Neutral red 6.8— 8 



Rosolic acid 6.9 — 8 



(Tropaeolin 000) ,p — Benzolsulfonicacidazo-a-napthol 7.6 — 8.9 



a— Napthol phthalein 7-3— 8.7 



Fhenolphthalein • ■ 8.3 — 10 



Thymolphthalein 9.3 — 10.5 



The following indicators were studied by Clark and Lubs (1915) : 



Indicator PH 



Monomethyl red 4.25 — 6 



Monoethyl red 4.25 — 6 



Diethyl red 4.5 —6-5 



Monopropyl red 4.25 — 6.25 



Dipropyl red 4.5 — 6.5 



Dimethyl-a-naptlhylamine red 5. — 6.75 



Phenolsulpfaonephthalein 6.5 — 8.5 



o — Cresolsulphonephthalein 6.5 — 8.5 



Thymolsulphonephthalein 8. — 9.75 



a — Naptholsulphonephthalein 7.5 — 9. 



Tetrabromphenolsulphonephthalein 3.5 — 4.5 



Dibromthymolsiulphonephthalein 6. — 7.25 



The color change and PH of solutions of some useful indi- 

 cators were shown in a colored chart. In order to find the PH 

 of an unknown solution, pour it into a test tube of 1 cm bore, 

 add 0.1 cc of the indicator solution, and while looking down into 

 it against a white background, compare it with the chart. If 

 the test tube is 2 cm bore, .4 cc of indicator solution should be 

 used. (McClendon, 1916, b; also in Tortugas Vol., Carnegie 

 Ins. Wash., in press.) 



As shown by Sorensen (1909) some indicators are more sensi- 

 tive than others to neutral salts, proteins and protein decomposi- 

 tion products. Congo red and litmus are therefore to be 

 especially shunned in biological work. For solutions containing 

 large amounts of proteins, or having a color of their own, the 

 method of Rowntree, Marriott and Levy, 1915, is useful, pro- 

 vided that obvious extra precautions are recognized. The fluid 

 to be tested is placed in a dialyzing capsule of collodion or parch- 

 ment paper impermeable to protein, and introduced into a test 

 tube containing some neutral (boiled) distilled water or salt solu- 

 tion. The test tube is stoppered to prevent the loss of CO., and 



