The pH test is a fairly straightforward one.  It is based on phenol red solution which turns different
colors based on the pH of the water.

There are a couple of interferences to be aware of in the pH test. 

High chlorine levels can cause the test to come out wrong.  If the chlorine level is above 5.0
ppm free available chlorine, then you will need to put one drop of R-0007 (chlorine neutralizer) into the sample before you add the pH drops.

High bromine ion concentration in the water can also affect the test.  If the spa is on bromine
or if you have used a bromine salt type algaecide, then it may cause your phenol red to turn to bromthyol blue.  This might cause a purplish reading on the pH
test even though you have a low pH. 

Acid and Base Demand

The acid and base demand portions of the pH test are designed to help you make the proper adjustments to
the pH of the water.  After you have done the pH test, you keep the sample (don’t pour it out yet) and add either base demand or acid demand solution to
determine how much soda ash or acid to add to the pool water to bring the pH to the desired level.

If you are trying to increase the pH of the pool water, then you add base demand solution to the
water drop by drop until the desired color is reached.  Count the number of drops it takes to reach that desired color and refer to the charts to see how much
soda ash to add.

If you are trying to decrease the pH of the pool water, then you add acid demand solution to the
water drop by drop until the desired color is reached.  Count the number of drops it takes to reach that desired color and refer to the charts to see how much
acid to add.