200 PRINCIPLES OF GENERAL PHYSIOLOGY 



(CO. 2 ) system, having a total concentration in CO., of decimolar strength, which 

 corresponds very closely to that of blood. Let us see what change is necessary 

 to raise the H* ion concentration from 0*5 x 10~~ to 1'Ox 17"", keeping, for ease 

 of calculation, the total CO 2 constant hy dilution. In the manner described 

 above we have 



0-5x 10-^3 x 10- 7 x 



i.e., (H 2 CO 3 ) = 0-912 molar and (NaHCO 3 ) = 0'088 molar, together (H molar. 



From the previous calculation, we have, for 1 x 10~~, a value for the ratio of 



O~=-T, so that the concentration of NaHCCX in this case must be 0-046 molar. 

 3-7o 



The difference between this and the value for 0'5 x 10- 7 is 0'088 - 0-046 = 0-042 gram- 

 molecules of NaHCO 3 or CO 2 . This shows that nearly half as much CO., as the 

 bicarbonate present is required in order to produce a change of hydrogen ion so 

 small as that from 0"5 x 10"" to 1 x 10~", which is about what would be produced 

 by the addition of O'OOl gram-molecule of hydrochloric acid to 10,000 litres of 

 water. 



A similar calculation can be made of the amount of bicarbonate required to 

 reduce the hydrogen ion concentration from 0-5 x 10"" to 0*2 x 10"". Thus : 



That is, 0-228 molar in bicarbonate; and 0-228-0-088 = 0-140 molar, or nearly 

 twice as much, alkaline salt must be added as that originally present. 



The phosphate equilibrium can be treated in the same -way, so that we can 

 understand the great capacity of blood and cells to preserve an almost complete 

 neutrality. 



The results of the preceding calculations may be further realised in the 

 following way. In a bicarbonate system with a constant pressure of CO.,, in order 

 to change an acidity of 0-0000002 molar into an alkalinity of the same value, an 

 extremely small change, it is necessary to add a volume of decinorrnal sodium 

 hydroxide nearly equal in volume to the solution itself. On account of the 

 importance of the question, another example may be given (see L. J. Henderson, 

 1913, pp. 147-152). Consider 1 kg. of CO 9 dissolved in 100 litres of water and that 

 sodium hydroxide is added in quantities of 50 g. at a time. Before any addition, 

 the hydrogen ion concentration is about 10~ 4 , or about 1,000 times that at 

 neutrality. The addition of 50 g. of NaOH reduces this to 50 times that 

 at neutrality. After the addition of 200 g. more, the H- ion concentration is 

 only 10~ 6 , merely 10 times that at neutrality, although there are still present 

 682 g. of free CO 2 . An acidity of this order is produced by the addition of 

 only 0-004 g. of hydrochloric acid to 100 litres of pure water. We can continue to 

 add NaOH without causing any change, more than just perceptible, until 450 g. 

 more have been added. When 700 g. in all have been added, the reaction is 

 practically that of pure water, and a further 50 g. may be added without any 

 greater change in the H- ion concentration than from 0'9 x 10~" to 0'6 x 10~", and 

 in the OH' ion concentration from 1*1 x 10~" to 1'7 x 10~", although in pure water 

 one ten -thousandth part of the amount would reduce the H- ion concentration from 

 1-1 x 10~ 7 to 0-1 x 10~ T and raise that of the OH' ions from M x 10~ 7 , to 12 x 10~ 7 . 

 The same amount (50 g.) added to pure water would raise the OH' ion concentra- 

 tion to 120, 000 x 10~ 7 . 



Suppose now that we take a case which is analogous to that of the blood 

 of air-breathing animals. The state of affairs will be found to be still more 

 striking. In the experiment described by L. J. Henderson (1913, pp. 149-151) 

 we take a solution of 1 kg. of sodium bicarbonate in 100 litres of water and 

 allow it to attain equilibrium with an unlimited atmosphere containing 1 g. 

 of CO., per litre. Let hydrochloric acid be added in small portions at a 

 time, constantly shaking the solution so that there shall always be equilibrium 

 with the CO., in the gas phase. Further, let the temperature be such that 



