158 



PHYSIOLOGICAL CHEMISTRY 



gives a mixture with an hydrogen ion concentration of i X io~ J 

 (exponent: 5.0). The mixtures are most satisfactory through the 

 ranges where the hydrogen ion concentration changes most gradually, 

 that is, through the flatter portions of the curves. 



The amounts of indicator solutions and their strengths to be used 

 in the determinations of hydrogen ion concentrations in 10 c.c. portions 

 of unknown solution are indicated below. 



1. Alizarin yellow R (p- 

 nitrobenzene-azo-salic- 

 ylic acid) 



2. Azolitmin (litmus) 



3. Cochineal 



4. 2, 5-dinitro-hydroquinone 



5. Mau vein 



6. Methyl orange 



7. Methyl red 



8. Methyl violet 



9. Neutral red 



10. p-Nitrophenol 



11. Phenolphthalein 



12. Rosolic acid 



13. Thymolphthalein 



14. Tropaeolin O 



15. Tropaeolin OO 



1 6. Tropaeolin OOO 



INDICATOR SOLUTIONS 



Drops Preparation of solution 



10-5 o. i gram to iqoo c.c. water. 



Aqueous solution. 



Alcoholic solution. 



5-2 i gram to 1000 c.c. alcohol. 



8-1 o . 5 gram to 1000 c.c. water. 



5-3 o.i gram recrystallized salt to 1000 c.c, 



water. 



4-2 Saturated solution in 50 per cent, alcohol. 

 8-1 0.5 gram to 1000 c.c. water. 



20-10 o . i gram in 500 c.c. alcohol, and 500 c.c. water 

 20-3 0.4 gram to 60 c.c. alcohol, 940 c.c. water. 



20-3 0.5 gram to 500 c.c. alcohol, 500 c.c. water. 



15-6 o . 4 gram to 400 c.c. alcohol, 600 c.c. water. 

 10-3 0.4 gram to 500 c.c. alcohol, 500 c.c. water. 

 10-5 o . i gram to 1000 c.c. water. 

 5-3 Of recrystallized salt, o.i gram to 1000 c.c 



water. 

 10-4 o.i gram to 1000 c.c. water. 



Determination of Hydrogen Ion Concentration. Introduce 10 c.c. portions 

 of the unknown solution into a series of test-tubes of similar diameter and oi 

 clear glass. Test first with litmus paper which changes at about the neutral 

 point. According to whether the reaction is acid or basic to litmus test other indi- 

 cators on the acid side such as p-nitrophenol, methyl orange and tropaeolin OO, 

 or on the basic side as phenolphthalein. Select an indicator which gives with the 

 solution neither its maximum acid or maximum basic color. Note from the chart 

 through what range this indicator exhibits its characteristic change of color. 

 Then to 10 c.c. portions of standard solutions of known hydrogen ion concentra- 

 tion (furnished by the instructor), which cover approximately the same range as 

 the indicator add exactly the same number of drops of indicator solution as was 

 added to the standard. Compare colors of unknown and standards until one is 

 found which matches and which consequently possesses the same hydrogen ion 

 concentration. If the unknown is so strongly acid or basic that none of the indi- 

 cators mentioned can be used directly it will be necessary to dilute it with 10 or a 

 greater number of volumes of water before testing further. 



In case the unknown solution is slightly colored the standards should like- 

 wise be brought to the same tint by the addition of some coloring agent as Bis- 

 marck brown, methyl orange, methyl violet, etc., before making the comparison. 



For applications of the indicator method for the determination of hydrogen ion 

 concentration to biological fluids see chapters on the quantitative analysis of 

 blood (XVI) and urine (XXVI). 



Comparison of H Ion Concentration and Titratable Acidity. i. Determine 



