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HYDROGEN ION CONCENTRATION AND TITRATABLE ACIDITY IN 
RELATION TO BACTERIOLOGICAL MEDIA. 
I. L. BALDWIN. 
It has long been recognized that the reaction of media is a very im- 
portant consideration in the cultivation of bacteria. The bacteriologist is 
confronted with the problem of determining or measuring this reaction in 
two instances. First, in the adjustment of the original reaction of the media 
and secondly in the measurement of acid production by bacteria. 
In the past two methods of measurement have been used, based on two 
different chemical phenomena. The older method of the two is the titra- 
tion of the media with a standard acid or basic solution, using phenolph- 
thalien as an indicator. In using this method media was almost universally 
made +1 to phenolphthalein, or 1% of normal acid was added after the neu- 
trality point was reached. This method was based on a measurement of 
the total acid or base in the solution. The newer method is a measure 
of the concentration of the free hydrogen ions in the solution. This may be 
accomplished by either the electrolytic or the colorimetric method. The 
electrolytic method is the more accurate of the two, but requires more 
time and complicated apparatus, to which the bacteriologist rarely has 
access. The colorimetric method, since the introduction by Clark and Lubs 
of a series of phthalein indicators whose sensitive ranges have been ac- 
curately determined, is accurate enough for bacteriological work, is applic- 
able to the solutions with which the bacteriologist is working and is quick 
and simple in operation. 
Since it is the concentration of free or disassociated hydrogen ions and 
not the total amount of acid present that affects the bacterial growth, it is 
readily seen that a method which measures hydrogen ion concentration is 
preferable to one which measures titratable acidity. Also it should be 
clearly understood that a determination of the titratable acidity gives 
no indication of the hydrogen ion concentration. A single example using 
two common acids will illustrate this point. Normal acetic acid has ten 
times the titratable acidity of tenth normal hydrochloric acid, yet tenth 
normal hydrochloric acid has about 22.4 times the hydrogen ion concen- 
tration of normal acetie acid. 
Another very serious source of error in the determination of the reaction 
of media by the old method of titrating with a standard solution lies in the 
buffer effect of various ingredients of the media. By the buffer effect of a 
substance we mean its ability to combine with an acid or base in the union- 
ized condition. Peptone, mainly due to the proteoses and phosphates which 
it contains, has a marked buffer effect. Thus the addition of an acid or a 
base to a peptone solution may change the hydrogen ion concentration but 
very slightly. Also the extent of the buffer effect of a peptone solution is 
dependent upon the brand of the peptone and the technic followed in making 
up the solution. As mentioned above, the buffer effect of peptone is largely 
due to its content of proteoses and phosphates, according to Kligler’s work 
