GENERAL PROPERTIES: THE CORPUSCLES. 417 



concentration of the hydroxyl ions would be 1 X 10~ 6 this liquid would have an 

 alkaline reaction. Accepting the value of 1 X 1Q- 7 as expressing the concentra- 

 tion of hydrogen ions in a neutral aqueous solution, it is found that blood has a 

 slightly alkaline reaction, since it has a hydrogen ion concentration less than 

 that of a neutral solution, namely, approximately 0.4 X 10~ 7 . In physiological 

 literature the concentration in hydrogen ions of the body liquids is sometimes 

 expressed, as above, in terms of the actual concentration in grams per liter, 

 blood being 0.00000004 gram, but sometimes also, for convenience sake, in 

 terms of what is known as the hydrogen exponent expressed by the symbol pH. 

 By hydrogen exponent is meant the logarithm of the number expressing the 

 concentration. Since these numbers are all negative, the logarithms are nega- 

 tive. In order to obtain positive logarithms, Sorensen, who introduced the 

 term, defined the hydrogen exponent as the logarithm of the reciprocal of the 

 number expressing the concentration. Thus in blood the concentration in 

 hydrogen ions is 0.4 X 10~ 7 ; the hydrogen exponent is the logarithm of 



, or the logarithm of 25,000,000, namely, 7.39. The same figures 



may be obtained by extracting directly the logarithm of 0.4 X 10~ 7 or 0.00000004 

 if it is remembered that in tables of logarithms the mantissa is always positive. 

 Thus, as usually expressed, the logarithm of 0.00000004 is 8.602, in which the 

 mantissa is positive. By adding algebraically 8 and +0.602 we obtain the 

 expression 7.39. In this nomenclature the hydrogen exponent of a neutral 

 solution is 7. Figures lower than this express acid reactions and higher 

 figures an alkaline reaction. Thus, pH of blood is 7.39, the pH of urine is 6, 

 the pH of gastric juice is 1.4, and so on. 



In accordance with these facts, it is evident that to determine 

 whether the blood is neutral, acid, or alkaline it is necessary to ascer- 

 tain its concentration in hydrogen ions. Two methods are em- 

 ployed in making these determinations.* One, the electrical 

 method, makes use of platinum electrodes covered with hydrogen 

 gas, and measures the difference in electric potential when one of 

 the electrodes is surrounded by blood and the other by a solution 

 of a known concentration in hydrogen ions; the other is the 

 method of selected indicators. A series of indicators may be 

 selected which undergo a change in color at certain concentrations 

 of hydrogen ions, and under suitable precautions the color change 

 given to these indicators by the blood may be interpreted in terms 

 of the hydrogen-ion concentration. In the first determinations of 

 the reaction of the blood made by the electrical method slightly 

 erroneous figures were obtained, owing to the fact that precautions 

 were not taken to examine the blood under its normal carbon-dioxid 

 tension. In the body the blood is surrounded by a medium con- 

 taining carbon dioxid under a certain pressure, for venous blood 

 about 5 per cent, of an atmosphere, or 40 mm. The carbon dioxid 

 gives the blood an acid reaction, hence if the blood is taken out of 

 the body and exposed to an atmosphere practically free from CO 2 , it 

 will tend to give off some of this gas and take on a slightly more al- 



* Consult Michaelis, "Die Wasserstoffionen Konzentration," Berlin, 

 1914, and McClendon, "Physical Chemistry of Vital Phenomena," Princeton, 

 1917. 



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