Free Chlorine
© Hach Company, 2002–2003, 2007. All rights reserved. Printed in the U.S.A. 3/07 3ed
Page 98 50081_Chlorine_Free_Forward.fm
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Lower Limit
Titration curve end points are not as sharp as those for Total Chlorine. Therefore, the
determination of Free Chlorine concentrations below 0.1 mg/L becomes problematic. As
a result, the lower limit for the AutoCAT 9000 has been arbitrarily set to 0.1 mg/L for
experimental reasons.
Safety
Use good safety practices and laboratory techniques throughout the procedure. Consult
the Material Safety Data Sheet (MSDS) for specific reagent(s) information.
Summary of Method
This procedure follows method number 4500-Cl D. b. in “Standard Methods for the
Examination of Water and Wastewater” (20th Edition). This procedure permits
determination of the Free Chlorine concentration in a water sample. Free chlorine
corresponds to that portion of chlorine existing either as elemental chlorine (Cl
2
),
hypochlorous acid (HOCl), or hypochlorite (OCl
–
) ion.
In the procedure, the sample pH is adjusted to pH 7 by addition of Phosphate buffer. The
sample solution is then titrated amperometrically with standard phenylarsine oxide
solution.
Required Reagents
Description Qty. required per test Unit Cat. No.
Phenylarsine Oxide (PAO) Titrant, 0.00564 N............................................varies................ 1000 mL ...............1999-53
Phosphate Buffer, pH 7, w/dropper .............................................................1 mL................. 100 mL ...............21553-32
Required Apparatus
Beaker, Glass, 250-mL ................................................................................1 ....................... each......................500-46H
Cylinder, Graduated, 250 mL ......................................................................1 ....................... each.........................508-46
Magnetic Stir Bar, Teflon
®*
coated.............................................................1 ....................... each.....................50085-00
Optional Reagents
Chlorine Standard Solution, Voluette
®
Ampules, 10-mL ...........................varies................ 16/pkg.................14268-10
Dilution water, organic-free ........................................................................ varies................ 500 mL ...............26415-49
Optional Apparatus
Mini-printer, thermal, w/cable............................................................................................... each.....................A70P021
Table 3 Accuracy Assessments
Sample Spike Addition Average % Recovery
% Range of
Recovery
Drinking Water
Free Cl
2
at 0.060 mg/L 92.6 (n=4) 87.5–98.0
Free Cl
2
at 0.220 mg/L 89.7 (n=6) 89.2–92.7
Free Cl
2
at 0.530 mg/L 91.0 (n=4) 85.4–93.5
*Teflon is a Registered Trademark of EI DuPont Company.
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