GENERAL PHYSICAL AND CHEMICAL PROPERTIES 25 



in virtue of their amphoteric character, to combine either with 

 acids or with bases, so that, when excess of base is added to blood, 

 the proteins act as acids, and neutralize the base; when excess of 

 acid is added, the proteins act as bases, and neutralize the acid. 

 (2) The equilibrium of certain of the inorganic constituents of the 

 blood (carbon dioxide, the carbonates, and the phosphates) is such 

 that even great variations in the concentration of any of these, 

 such as may normally occur, produce scarcely any effect upon the 

 concentrations of the hydrogen and hydroxyl ions. 



Thus, when phosphoric acid and sodium hydroxide are added to 

 water in certain proportions, and the solution placed under a certain 

 tension of carbon dioxide (which is kept constant), we get a more or 

 less accurate imitation of blood as regards the inorganic substances 

 concerned in the regulation of its reaction, sodium bicarbonate 

 (NaHCO 3 ) and disodium phosphate (NagHPC^) being present in the 

 solution as in blood. It is found that when the quantities are so chosen 

 that the H + concentration lies within the limits of variation of the 

 normal blood reaction, relatively large quantities of alkalies can be 

 added or withdrawn without causing much change in the H + concen- 

 tration. It can be shown both theoretically and experimentally that 

 precisely those weak acids present in blood (CO 2 , NaH 2 PO 4 ) require the 

 largest addition of alkali to alter the reaction to a given extent, and 

 are therefore particularly suited to give stability to the reaction. 

 Thus carbon dioxide requires twenty-four times, and monosodium 

 phosphate thirty-three times, as much alkali as an equivalent solution 

 of acetic acid to cause a given alteration of colour in rosolic acid 

 (E. Henderson). 



The so-called ' titratable ' alkalinity of blood or serum, measured by 

 the amount of standard acid which must be added before the colour of 

 the indicator used changes from alkaline to acid, bears no necessary or 

 fixed proportion to the actual alkalinity. When blood, for instance, 

 is titrated with hydrochloric acid, with methyl orange as indicator, at 

 the point where the red colour appears all the disodium phosphate and 

 sodium bicarbonate will have been changed into monosodium phos- 

 phate and carbon dioxide, all the alkali removed from combination 

 with proteins, a certain amount of acid -protein compounds formed, 

 and other minor reactions produced (Henderson). It is difficult to 

 correlate the quantity deduced from such a titration with any physio- 

 logical condition, although undoubtedly it bears some relation to the 

 acid-neutralizing power of the blood, and some relation to its real 

 reaction. Still, by titration information of value can be obtained 

 which is not yielded by the physico-chemical method in regard to the 

 potential ' acid capacity ' of the blood and its power of resistance against 

 acid-poisoning. 



What is estimated here is the quantity of acid required to satisfy the 

 proteins and to react with the carbonates and phosphates before that 

 concentration of hydrogen and hydroxyl ions just necessary to cause 

 the change of colour is established. This is not the same for different 

 indicators, since there is a certain minimum ratio in the concentration 

 of these ions at which each indicator turns in one or the other direction, 

 none turning precisely at the neutral point. Thus serum appears to be 

 acid when tested with phenolphthalein, and alkali must be added to 

 the serum before the pink colour indicating alkalinity is produced. 

 On the other hand, with litmus or methyl orange it gives the alkaline 



