GENERAL PROPERTIES: THE CORPUSCLES. 415 



concentration of the hydroxy] ions would be 1 X 10-6— this liquid would have an 

 alkaline reaction. Accepting the value of 1 X 10-7 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 hquids is sometimes 

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

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

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

 The hydrogen exponent is obtained by taking the logarithm of the number 

 expressing the concentration in hydrogen ions. Since, however, this con- 

 centration for the body liquids is always less than unity (normal) the loga- 

 rithm will have a negative characteristic. It proves more convenient in use to 

 employ the logarithm of the recipocal of the number expressing the concentra- 

 tion, since in this case it varies uniformly with the acidity or alkalinity. For 

 example, the concentration of hydrogen ions in the blood is 0.4 X lO-^ or 

 0.00000004. The logarithm of this number is 8.602. The logarithm of the 

 reciprocal, namely, n 4 y iq-t i^ 7.39, which is the hydrogen exponent of 

 the blood and is written pH = 7.39. The same expression, as a negative 

 logarithm, is obtained by extracting the logarithm in the usual way and then 

 subtracting the mantissa from the characteristic, 8 - .602 = 7.39. The 

 hydrogen exponent is defined therefore as the negative logarithm of the number 

 expressing the concentration in hydrogen ions, or, as originally defined by 

 Sorensen, as the logarithm of the reciprocal of the number expressing the con- 

 centration. In this nomenclature the hydrogen exponent of a neutral solution 

 is 7. Figures lower than this ex-press acid reactions and higher figures an alka- 

 line 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 alkahne it is necessary to 

 ascertain its concentration in hydrogen ions. Two methods are 

 employed 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 second method, 

 known as the colorimetric method, makes use of a number of so- 

 called indicators which give different colors according to the 

 degree of acidity. A series of indicators may be selected which 

 undergo a change in color at certain 'concentrations of hy- 

 drogen 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 making the most accurate 

 determinations of the normal reaction of the blood it is necessary 

 to examine it at the temperature of the body, and in an atmosphere 

 containing as much CO2 as is present normally in the tissues of 

 the body. The results of the measurements of the reaction of the 

 blood made with these precautions indicate that it has a hydrogen- 

 ion concentration, varying from 0.35 X 10"^ to 0.49 X 10"^, that 



* Consult Michaelis, "Die Wasserstofiionen Konzentration," Berhn 

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

 1917. 



