LaMotte Pool Manager Water Quality Handbook - Manual (Page 15)

How do we test pH?
pH is one of the most vital tests
performed daily on pool water,
and it is also one of the most
simple to perform. A single
liquid or tablet indicator is used
to provide a distinct pH color
reaction which varies from
yellow to deep red. This
indicator (called Phenol Red)
measures pH from 6.8 to 8.4.
Most state public health codes require that pH color
comparator be suitable for reading to 0.1 pH unit.
Check local regulations to confirm that the on-site test
equipment is in compliance.
Testing pH is simple: the test tube or sample cell is
rinsed and filled to a specified line with sample water.
Phenol red liquid or tablet is added to the sample as
specified in the instruction. The sample is capped and
mixed. The resulting colored solution is compared to
color standards to determine the precise pH level.
Alternative methods of testing pH include two types of
instrumentation. One is an electronic colorimeter
which analyzes the color development with the phenol
red pH test by passing a light beam through the sample.
The amount of light that is absorbed by the reacted test
sample determines the result, which is indicated by a
meter display.
The pH meter relies upon a sensor called an electrode,
which is immersed in a water sample. The electrode
measures the electrical activity within the solution and
displays the pH value. Though the pH electrode needs
no phenol red reagent to provide a test result, it does
need constant calibration with specific pH solutions
(known as buffers) to assure accurate readings.
13