METHODS FOR MICEOSCOPIC STUDY OF BACTERIA 213 
is neutral in reaction, and the equivalent amount of CO2 is added to 
the plasma, and excreted as before. The reaction of the j)lasma, 
however, is practically unchanfjed so lon^ as there is NaHCO.i left 
in solution. 
Similarly, the cells of the body retain their proper reaction in spite 
of the constant formation of carbonic acid, and the incorporation of 
strong acids, as sulphuric acid, through the regulatory effect of a 
phosphate system. The following equations illustrate the method 
of action : 
Potassium dihydrogen-phosphate (a weakly acid salt) and disodium 
hydrogen phosphate (a weakly basic salt) in equimolar concentration 
create a solution of almost exact neutrality (pn = 6.979) when they 
are present in pure water to the extent of 6 cc. of a -^~ disodium hydro- 
gen phosphate and 4 cc. of a -§- potassium dihydrogen phosphate 
solution respectively. The reaction of the blood (pH 7.5) would be 
obtained by a mixture containing 8.5 cc. of the disodium hydrogen 
phosphate solution and 1.5 cc. of the acid potassium phosphate. The 
addition of sulphuric acid, or other strong acid, to such a mixture re- 
sults in a displacement of the phosphoric acid by sulphuric acid, 
according to the equation. 
2Na2HP04 + H2SO4 
Weakly basic salt. Strong acid. = 
NajSOi + 2NaH2P04 
Neutral .salt. Weakly acid salt. 
It will be seen that the neutralization of the sulphuric acid by the 
weakly basic salt results in the formation of a neutral salt, and the 
production of an equivalent amount of the weakly acid salt, NaH2P04. 
It is obvious that the acidity of the mixture cannot exceed that of 
XaH2P04 (about pn 5) until all the secondary phosphate is trans- 
formed to a primary phosphate by the addition of the strong acid 
as indicated above. This explains the remarkable fact that relatively 
large amounts of acid or alkali may be added to cultural media (which 
contain more or less of these bufl'er mixtures) without producing 
immediately detectable changes in reaction. Phosphate (buffer) 
mixtures are essential constituents of a majority of cultural media. 
Usually enough phosphate is present among the naturally occurring 
ingredients of cultural media to suffice both for the nutrition of the 
bacteria (which contain relatively high percentages of phosphorus 
(see page 57), and also to act as regulators of reaction within reasonable 
limits. Additional phosphate bufl'er solution (preferable to the 
COo 
XT TrA r\ buffer system) is advantageously added to media in which a 
JNaHLUs 
development of acid is to take place as a result of bacterial growth and 
metabolism. 
Determination of the Ilydrugen-ion Concentration of Media by the 
Use of Indicators and Standard Solutions of Known Hydrogen-ion Con- 
ce7itratio7i.— Indicators are substances which react in general like 
