LaMotte DPC-52 Water Testing Laboratory - Manual (Page 8)

COLORIMETRIC pH
The term pH is an index of the amount of hydrogen ions present in a substance. A high
pH means that the substance is alkaline, or basic, while a low pH means the water is
acidic.
Sanitation of pool water is best accomplished at a slightly basic pH (pH 7.2 - pH 7.8). If
the pool water is maintained under acidic conditions, the pool water may be corrosive
and may allow the chlorine to escape from the pool as a gas. A highly basic pool water
may also suppress the effectiveness of chlorine in the pool. The pH of pool water can be
decreased by the addition of acids, such as hydrochloric acid (muriatic acid) or sodium
bisulfate (dry acid), or increased by the addition of alkaline substances such as sodium
carbonate (soda ash) or sodium bicarbonate (baking soda).
RANGE:
Phenol Red, pH 6.8 - 8.2
METHOD:
The Phenol Red Indicator exhibits a specific color change over
a narrow pH range. The color changes are measured
colorimetrically.
INTERFERENCE:
Sample color and turbidity interfere with the colorimetric pH
measurement. Chlorine interference has been eliminated by
the addition of a chlorine inhibitor to the pH indicator.
PROCEDURE
1. Dispense the water sample into a clean colorimeter tube (0967), until the tube is
filled to the 10 mL line.
2. Use a 0.5 mL pipet (0369) to add exactly 0.5 mL of the *pH Indicator (WL-7027) to
the tube. Cap and mix.
3. With “pH” appearing on the meter display, insert the treated sample into the
colorimeter chamber. As soon as the display stabilizes, read the pH value.
NOTE: If the colorimeter reading is out of range in either direction (i.e., pH is
either less than 6.8 or greater than 8.2), adjust the pH by appropriate treatment and,
after one complete filter cycle, test again.
8