LaMotte DPC-52 Water Testing Laboratory - Manual (Page 12)

ALKALINITY
Alkalinity refers to the amount of bicarbonates, carbonates and hydroxides present in
water. More specifically alkalinity is a measure of the buffering capacity of the water or
the ability of water to resist changes in pH. If the alkalinity is too low, the pH will
fluctuate erratically, while high alkalinity water may resist changes of pH altogether.
A low alkalinity water is indicated by great changes in pH values with small additions of
an acid or base. Under low alkalinity conditions, the pH is extremely difficult to
maintain. To raise the alkalinity to the desired range (usually between 80 - 125 ppm for
most pool waters) sodium bicarbonate (baking soda) is added. Sodium Carbonate (soda
ash) can also be used to raise the alkalinity; however, it may also raise the pH drastically.
Water with a high alkalinity resists changes of pH. Acid has to be added in order to
destroy the bicarbonates, carbonates, and hydroxides for the alkalinity to be lowered to
the desired range.
RANGE:
0 - 200+ ppm as Calcium Carbonate (CaCO
3
)
METHOD:
The alkalinity of pool water is determined by a titration
procedure involving the use of a standard acid that is titrated
to a Bromcresol Green-Methyl Red Indicator endpoint at
about pH 4.6.
INTERFERENCE:
Sample turbidity and color may interfere with the
determination of the endpoint. Cyanuric Acid may contribute
to the alkalinity of pool water. High chlorine values may cause
a yellow rather than red endpoint.
DIRECT READING
TITRATOR METHOD:
Carefully read the User’s Guide manual for use of the Direct
Reading Titrator before performing the titration procedure
described below. The titrator is calibrated in terms of
Alkalinity in parts per million Calcium Carbonate and each
minor division on the Titrator scale equals 4 ppm.
PROCEDURE
1. Fill the titration tube marked “ALK” (0644) to the line with the water sample to be
tested.
2. Add one Alk 1 Indicator Tablet (WL-T-2311) to the sample. Cap the tube and
shake to dissolve the tablet. A green-blue color will develop.
3. Fill the Direct Reading Titrator (0382) with Alk Titrant (WL-4493).
4. Titrate the test sample, while mixing, until the green-blue color changes to red. Read
the test result directly from the scale where the large ring on the Titrator meets the
Titrator barrel.
5. If the titration endpoint has not occurred when the plunger tip reaches the 200 ppm
mark, refill the Titrator and continue the titration. Be sure to include the original
amount added in the test result.
6. Results are expressed as Total Alkalinity in ppm Calcium Carbonate (CaCO
3
).
7. Use the alkalinity test result to read the Acid Demand Index on the next page.
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