INDICATORS 135and the new pH value is 4.735, which is thus not materially changed
from its original value.
If we add Ice. of liV NaOH to 1 liter of pure water we obtain
a solution 0.001 N in OH~ ions with a pH of 11. If on the other
hand we add 1 cc. of 1 iV NaOH to 1 liter of the buffered solution
the reaction
OH" + HAc -» H 2 O + Ac"
decreases [HAc] by 0.001 and increases [Ac~] by 0.001 so that
[H+] = 0.000018 X = 0.0000176 =and the new pH value is 4.7 55, which again is not materially changed.
From the above equation (1) it is apparent that the maximum
, „.. ..,,•!! ,i e added weak acid
buffering action is obtained when the ratio of added salt of that acid
is unity, also that a numerically greater pH (lower [H+]) can be
obtained by lowering this ratio and a lower pH by increasing this
ratio. However, to obtain an essentially greater pH value than
5 it would be more advantageous to use a weaker acid and its salt
for the buffer.
The above reasoning applies to weak bases and their salts which
may be advantageously used as buffers to establish pH values be-
tween 7 and 11.
It goes without saying that, although absolutely pure water
has a pH value of 7, the pH value is subject to a variation of 1 or 2
units through almost unavoidable contamination from glass or
from NH 3 , H 2 S, or CO 2 from the air, and that to maintain an
unfluctuating value of 7 a buffer must be used.INDICATORS
Indicators are highly colored substances which change in color
at certain pH values. The color is so intense that the amount
of indicator needed is so small that it itself has practically no
effect in altering the pH value. There are a great many such in-
dicators, and it is possible to choose a list covering the whole range
of pH values. Thus, if a solution is colorless it is possible to apply
to different portions of it the successive indicators in this list, and
when the one is found which shows its color change in this solu-