Thermo Fisher Scientific C101 Colorimeter - Manual (Page 26)

Measurement: pH
C401/301/101 Colorimeter
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2.6.5 Chemistry
The monitoring and control of the pH of water is important in many
different applications. This test method provides a quick and accurate
method for measuring pH in the range pH5.9 - 8.2.
A precise amount of phenol red indicator is added to a sample of water.
The phenol red changes color from yellow at pH5.9 through to dark red at
pH8.2.
The amount of red color is related to its pH. The amount of 525nm light
absorbed by the red color is accurately measured by the meter and then
converted into the pH of the sample.