57-37
QUATERNARY AMMONIUM COMPOUNDS, continued
Highly buffered samples or extreme sample pH may exceed the
buffering capacity of the reagents and require sample pretreatment.
Adjust the sample pH to between 3 and 5 by using a pH meter or pH
paper and adding, dropwise, an appropriate amount of acid or base such
as 1.0 N Sulfuric Acid Standard Solution or 1.0 N Sodium Hydroxide
Standard Solution. If significant volumes of acid or base are used, a
volume correction should be made by dividing the total volume (sample
+ acid + base) by the original sample volume and then multiplying the
test result by this factor.
After several samples have been analyzed, the sample cells may exhibit
a build-up of a pink or purple color. A rinse with 1.0 N Sodium
Hydroxide Solution followed by a Alconox detergent wash and
demineralized water rinse will eliminate the build-up when it occurs.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two prepared solutions,
using one DR/700, matched sample cells and two representative lots of
testing reagents. Testing 2.8 mg/L CTAB concentration samples, the
standard deviation was ±0.12 mg/L CTAB.
Testing zero concentration samples, the limit of detection was 0.11 mg/L
CTAB. The limit of detection was calculated as three times the standard
deviation when testing zero concentration samples (adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
SUMMARY OF METHOD
The test method makes use of a colorimetric chemistry in which a
quaternary ammonium compound reacts with an indicator to produce a
color change from pale pink to vivid purple. The test is conducted in a
stabilized, acid-buffered solution containing a masking agent to
eliminate potential interferences. This test is applicable to the
monitoring of QACs in swimming pools and cooling towers.
REQUIRED REAGENTS
Quantity
Description Per Test Unit Cat. No.
Q.A.C. Reagent 1 . . . . . . . . . . 2 pillows . . .50/pkg . . . . . . . . 24010-66
Q.A.C. Reagent 2 . . . . . . . . . . 2 pillows . . .25/pkg . . . . . . . . 24012-68
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