USEPA Validation Study Report for Hach Method 10241 Free Chlorine

Spectrophotometric Measurement of Free Chlorine in Drinking Water

Submitted by:

Hach Company

5600 Lindbergh Drive

Loveland Colorado 80538

Cary Jackson 

and

Luke Johnson

November 2015
Hach Company Validation Study Report

Measurement of Free Chlorine in Water, Method 10241

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Background

Purpose of Study

The purpose of this study is to validate Hach Method 10241, Measurement of Free Chlorine

(Cl2) in Drinking Water for approval and use in USEPA regulatory water compliance and 

reporting programs.

Method Summary

An ammonia solution at a pH of 8.3 is added to a sample that contains free chlorine. The free 

chlorine rapidly reacts with ammonia to form monochloramine (NH2Cl). In the presence of a 

cyanoferrate catalyst, the monochloramine reacts with a substituted phenol to form an

intermediate monoimine compound. The intermediate couples with excess substituted phenol 

to form a green indophenol compound, which is proportional to the amount of free chlorine in 

the sample. A sample blank that contains Monochlor F Reagent corrects for background color 

from the reagent and sample. The measurement wavelength is 655 nm for spectrophotometers 

or 610 nm for colorimeters.

Method Justification

EPA reference method SM 4500-Cl G cannot be used to measure free chlorine in the presence 

of chloramines, manganese and other oxidants. Proposed Hach Method 10241 for the 

Spectrometric Measurement of Free Chlorine overcomes these interferences.

Scope of Method

Hach Method 10241 is applicable for use in drinking water. 

Study Design and Objectives

The design and objectives of this validation study are detailed in the Validation Study Plan1

appended to this report.

Study Implementation

Study Sponsor

The organizational sponsor responsible for managing this study is Hach Company.

 

1 Hach Validation Study Plan, March 2015
Hach Company Validation Study Report

Measurement of Free Chlorine in Water, Method 10241

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Study Participants

The participants, laboratories, and test matrices were selected from geographically diverse 

regions of the United States and are listed in the following table.

Treated Drinking 

Water Matrix 

Facility

Laboratory Source of Treated 

Test Water

Loveland, Colorado Hach Company Surface Water

Cedar Falls, Iowa Test America-Cedar Falls Surface Water

Brighton, Colorado TRE Environmental Strategies Ground Water

Sample Collection and Distribution

Samples from Cedar Falls, IA and Loveland, CO were taken directly from the laboratory drinking 

water tap and analyzed immediately per SM4500-Cl sampling and storage protocols. The 

sample from Brighton, CO was taken from the drinking water plant effluent and delivered to 

the TRE laboratory for immediate analysis. 

Analysis Description

The description of the type and number of analyses are described in the Validation Study Plan 

appended to this report.

Data Reporting and Validation

Data Collection, Laboratory Submission, and Validation

For each laboratory performing analyses, results from each test measurement were read from a 

DR1900 or DR6000 spectrophotometer with built-in calibration, and manually transferred to an 

electronic spread sheet. Each laboratory then submitted the electronic spread sheet to the 

Hach Study Director for review and statistical evaluation. 

Results

Summary results of the validation study are listed in Tables 1-5. Initial demonstration of 

capability is demonstrated by accuracy (bias) and precision data which are generated 

throughout the calibration range and at the minimum residual disinfection concentration in

several matrices. These data were collected at the Hach laboratory and are included in Table 1. 

The quantitation range and method reporting limits for this method are 0.1 – 4.5 mg/L Cl2. 
Hach Company Validation Study Report

Measurement of Free Chlorine in Water, Method 10241

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Table 1 – Initial Demonstration of Capability for Hach Method 10241

Parameter Concentration 

(mg/L)

Average Percent Recovery by Matrix

LIS 250 mg/L CO3

250 mg/L 

CaCO3 2.5 mg/L TOC

Method Blank 0 0.008 0.007 0.009 0.013

Average Percent Bias by Matrix

LIS 250 mg/L CO3

250 mg/L 

CaCO3 2.5 mg/L TOC

Accuracy 

(bias) 

0.05 13.3 20.0 20.0 6.7

0.2 10.0 11.7 0.0 5.0

2.0 4.7 1.3 5.2 6.5

4.0 6.2 1.8 3.2 4.3

Percent Relative Standard Deviation by Matrix

LIS 250 mg/L CO3

250 mg/L 

CaCO3 2.5 mg/L TOC

Precision 

0.05 13.3 0.0 0.0 10.8

0.2 5.6 2.6 8.7 0.0

2.0 0.3 0.3 0.6 1.4

4.0 0.4 0.4 2.7 1.1

Average Percent Bias by Matrix

Loveland DW Brighton DW Keokuk DW

Matrix Spike 

Accuracy 

(bias)

Low 18.3 7.2 6.2

Mid 3.9 5.2 2.4

High 6.5 0.1 8.0

Percent Relative Standard Deviation by Matrix

Loveland DW Brighton DW Keokuk DW

Matrix Spike 

Precision

Low 1.8 3.0 1.7

Mid 1.8 1.4 0.6

High 3.0 0.8 0.9
Hach Company Validation Study Report

Measurement of Free Chlorine in Water, Method 10241

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 Table 2 – Loveland Colorado Tap Water Performance Results 

Method 

Detection 

Limit (mg/L)

Method 

Detection 

Limit (mg/L)

Low Ionic Strength 

Reagent Water

0.02 0.02

High Ionic Strength 

Reagent Water 0.02 0.02

Average 

Percent (%) 

Recovery

Percent Bias Standard 

Deviation

Average 

Percent (%) 

Recovery

Percent Bias Standard 

Deviation

Low Ionic Strength 

Reagent Water 97.5 2.5 0.8 96.8 3.2 0.8

High Ionic Strength 

Reagent Water 99.0 1.0 0.4 97.2 2.8 1.0

Average 

Percent (%) 

Recovery

Relative 

Percent 

Difference

Average 

Percent (%) 

Recovery

Relative 

Percent 

Difference

97.5 0.8 103 0.8

Initial Precision and Recovery Initial Precision and Recovery

Loveland Treated 

Tap Water

Matrix Spike Analysis Matrix Spike Analysis

Matrix

SM 4500-Cl G Hach Method 10241
Hach Company Validation Study Report

Measurement of Free Chlorine in Water, Method 10241

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Table 3 –Cedar Falls Tap Water Summary Results 

Method 

Detection 

Limit (mg/L)

Method 

Detection 

Limit (mg/L)

Low Ionic Strength 

Reagent Water

0.02 0.05

High Ionic Strength 

Reagent Water 0.03 0.03

Average 

Percent (%) 

Recovery

Percent Bias Standard 

Deviation

Average 

Percent (%) 

Recovery

Percent Bias Standard 

Deviation

Low Ionic Strength 

Reagent Water 94.3 5.7 0.4 93.8 6.2 0.3

High Ionic Strength 

Reagent Water 95.7 4.3 1.6 92.7 7.3 1.4

Average 

Percent (%) 

Recovery

Relative 

Percent 

Difference

Average 

Percent (%) 

Recovery

Relative 

Percent 

Difference

82.0 1.0 96.3 0.5

Matrix

Hach Method 10241

Initial Precision and Recovery Initial Precision and Recovery

Cedar Falls Treated 

Tap Water

Matrix Spike Analysis Matrix Spike Analysis

SM 4500-Cl G
Hach Company Validation Study Report

Measurement of Free Chlorine in Water, Method 10241

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Table 4 – Brighton Colorado Drinking Water Plant Summary Results

Development of QC Acceptance Criteria

Quality control acceptance criteria for method detection limit (MDL), initial precision and 

recovery (IPR), and matrix spike recovery are described in Table 5. Calibration verification is 

demonstrated through the accuracy (bias) data. These criteria are based on intra-laboratory 

data. 

Method 

Detection 

Limit (mg/L)

Method 

Detection 

Limit (mg/L)

Low Ionic Strength 

Reagent Water

0.02 0.09

High Ionic Strength 

Reagent Water 0.03 0.05

Average 

Percent (%) 

Recovery

Percent Bias Standard 

Deviation

Average 

Percent (%) 

Recovery

Percent Bias Standard 

Deviation

Low Ionic Strength 

Reagent Water 97.8 2.2 1.8 94.8 5.2 1.7

High Ionic Strength 

Reagent Water 94.8 5.2 2.8 92.8 7.2 0.9

Average 

Percent (%) 

Recovery

Relative 

Percent 

Difference

Average 

Percent (%) 

Recovery

Relative 

Percent 

Difference

92.0 1.3 102 2.0

Matrix

Brighton Treated 

Tap Water

Matrix Spike Analysis Matrix Spike Analysis

SM 4500-Cl G Hach Method 10241

Initial Precision and Recovery Initial Precision and Recovery
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Measurement of Free Chlorine in Water, Method 10241

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Table 5 – Quality Control Performance Criteria for Hach Method 10241

Data Analysis/Discussion

MDL Results

The low ionic strength MDL results for each of the three facilities ranged from 0.02 mg/L to 0.03

mg/L for EPA Reference Method SM 4500-Cl G and 0.02 mg/L to 0.09 mg/L for Hach Method 

10241. The high ionic strength matrix MDL results ranged from 0.02 mg/L to 0.03 mg/L for EPA 

Reference Method SM 4500-Cl G and 0.03 mg/L to 0.05 mg/L for Hach Method 10241. The 

pooled MDL results for EPA Reference Method SM 4500-Cl G were determined to be 0.02 mg/L 

for low ionic strength matrix water and 0.02 mg/L for high ionic strength matrix water. The 

pooled MDL results for Hach Method 10241 were determined to be 0.05 mg/L for low ionic 

strength matrix water and 0.03 mg/L for high ionic strength matrix water. The regulatory limit

established by USEPA for this validation study is 0.2 mg/L as this is the minimum residual 

disinfectant concentration in water entering the distribution system as defined in the drinking 

water regulations at 40 CFR 141.72(a)(3).

Low Ionic Strength 

Reagent Water

High Ionic Strength 

Reagent Water

Initial Precision and 

Recovery Analysis

Average 

Percent (%) 

Recovery

Percent Bias

Lower Limit 

of % 

Recovery

Upper Limit 

of % 

Recovery

95% 

Precision of 

Recovery

Average 

Percent (%) 

Recovery

Percent Bias

Lower Limit 

of % 

Recovery

Upper Limit 

of % 

Recovery

95% 

Precision of 

Recovery

Low Ionic Strength 

Reagent Water 96.5 3 89.4 104 0.02 95.2 5 89.5 101 0.02

High Ionic Strength 

Reagent Water 96.5 4 88.3 105 0.04 94.2 6 84.8 104 0.02

Average 

Percent (%) 

Recovery

Percent Bias

Lower Limit 

of % 

Recovery

Upper Limit 

of % 

Recovery

95% 

Precision of 

Recovery

Average 

Percent (%) 

Recovery

Percent Bias

Lower Limit 

of % 

Recovery

Upper Limit 

of % 

Recovery

95% 

Precision of 

Recovery

90.5 10 66.8 114 0.05 100 0 89.9 110 0.06

Matrix

SM 4500-Cl G Hach Method 10241

Initial Precision and Recovery (IPR) Initial Precision and Recovery (IPR)

Drinking Water 

Matrix Analysis

Matrix Spike Recovery Matrix Spike Recovery

Method Detection Limit (MDL) 

mg/L

Method Detection Limit (MDL) 

mg/L

0.02

0.02

0.05

0.03
Hach Company Validation Study Report

Measurement of Free Chlorine in Water, Method 10241

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Initial Precision and Recovery Results

The IPR average recovery results for low ionic strength matrix water for each of the three 

facilities ranged from 94% to 98% for EPA Reference Method SM 4500-Cl G and 94% to 

97% mg/L for Hach Method 10241. The IPR average recovery results for high ionic strength 

matrix water ranged from 95% to 99% for EPA Reference Method SM 4500-Cl G, and 93% to 

97% for Hach Method 10241. The pooled low ionic strength matrix recovery results for EPA 

Reference Method SM 4500-Cl G ranged from 89.4% to 104% with a 95% precision of 0.02 and 

89.5% to 101% with a 95% precision of 0.02 for Hach Method 10241. The pooled high ionic 

strength matrix recovery results for EPA Reference Method SM 4500-Cl G ranged from 88.3% to 

105% with a 95% precision of 0.04 and 84.8% to 104% with a 95% precision of 0.02 for Hach 

Method 10241.

Matrix Spike Results

The average matrix spike recovery results of the three facilities ranged from 82.0% to 97.5% for 

EPA Reference Method SM 4500-Cl G and 96.3% to 103% for Hach Method 10241. The relative 

percent difference for matrix spike recoveries ranged from 0.8% to 1.3% for EPA reference 

method SM 4500-Cl G and 0.5% to 2.0% for Hach method 10241. The pooled matrix spike 

results of recovery ranged from 66.8% to 114% for EPA Reference Method SM 4500-Cl G and 

89.9% to 110% for Hach Method 10241. The 95% Precision of Recovery results for the EPA 

Reference Method 4500-Cl G and Hach Method 10241 were determined to be 0.05 and 0.06 

respectively.

Conclusion

A validation study was conducted to compare the performance of Hach Method 10241 for the 

measurement of Cl2 in water to that of the EPA Reference Method SM 4500-G. The results of 

the study indicate that Hach Method 10241 is equally effective to that of the EPA Reference 

Method SM 4500-Cl G for Safe Drinking Water Act monitoring and compliance reporting 

programs.