Hach Method 10241

Spectrophotometric Measurement of Free Chlorine (Cl2) in 

Finished Drinking Water 
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Hach Company Method 10241

Revision 1.2

November 2015

Spectrophotometric Measurement of Free Cl2 in Finished Drinking Water 

1.0 Scope and Application

1.1 This method is for the determination of free chlorine (Cl2) in finished drinking water.

1.2 The method is applicable in the range from 0.1 to 4.5 mg/L Cl2. 

1.3 This method is equally effective in performance and use to SM 4500-Cl G for the 

purposes of regulatory compliance reporting of Cl2.

2.0 Summary of Method

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

free chlorine is immediately converted into 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.

3.0 Interferences

3.1 The items listed in the Interfering Substances table have been individually checked up to 

the given concentrations and do not cause interference. The cumulative effects and 

influence of other ions have not been determined. Measurement results can be verified 

using sample dilutions or standard additions.

Interfering substance Interference level (mg/L)

Ozone > 1 

Sulfide > 0.5

4.0 Safety

4.1 The toxicity or carcinogenicity of each reagent used in this method has not been precisely 

determined; however, each chemical should be treated as a potential health hazard. 

Exposure to these chemicals should be reduced to the lowest possible level. It is 

suggested that the laboratory perform personal hygiene monitoring of each analyst using 

this method and that the results of this monitoring be made available to the analyst.

4.2 Unknown samples may contain high concentrations of volatile toxic compounds. Sample 

containers should be opened in a hood and handled with gloves to prevent exposure.

4.3 This method does not address all safety issues associated with its use. The laboratory is 

responsible for maintaining a safe work environment and a current awareness file of 

OSHA regulations regarding the safe handling of any chemicals specified in this method. 

A reference file of material safety data sheets (MSDSs) should be available to all 
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personnel involved in these analyses. Additional information on laboratory safety can be 

found in Sections 16.3 and 16.4.

5.0 Equipment

Note: Brand names, suppliers, and part numbers are for illustrative purposes only. No 

endorsement is implied. Equivalent performance may be achieved using apparatus and 

materials other than those specified here, but demonstration of equivalent performance 

that meets the requirements of this method is the responsibility of the laboratory.

5.1 Sampling equipment

5.1.1 Sample collection bottles – Collect samples in chlorine demand-free glass bottles. 

6.0 Equipment for sample analysis

6.1 Hach Company DR 6000, DR 3900, DR 1900 spectrophotometer, or equivalent

6.2 Hach Company sample cells

6.3 Equipment for standard preparation

6.3.1 500 mL bottle or flask – Chlorine demand-free glass

6.3.2 Analytical balance.

7.0 Reagents and Standards

7.1 Deionized water – Water in which Cl2 concentration is below the detection limit of this 

method. Water prepared by passage of tap water through reverse osmosis and carbon 

filtration has been shown to be an acceptable source of reagent water.

7.2 Hach Company Monochlor F Reagent Pillows, Cat. No. 2802299, or equivalent

7.3 Hach Company Freechlor F Reagent Solution, Cat. No. 2964926, or equivalent

7.4 Hach Company Cl2 Ampule: 10 mL, 50-75 mg/L as Cl2 (Cat. No. 1426810) or equivalent

7.5 Initial precision and recovery (IPR) solution

7.5.1 Prepare 4 or more replicate 1.0-1.5 mg/L Cl2 IPR solutions by weight. Tare an empty 

500 mL bottle. Transfer the contents of a 10 mL standard ampule to the bottle (Sect. 

7.4). Record the weight. Identify the concentration of the standard from the label. Using 

the concentration and the transferred weight of the standard, calculate the final dilution 

weight as (W1)(C1)/(C2)=(W2). Dilute to the final weight with DI water. Assuming the 

concentration in the Standard ampule is 75 mg/L, the calculated final concentration = 1.5

mg/L Cl2.
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7.6 Method detection limit (MDL) solution

7.6.1 Prepare 7 or more replicate Cl2 MDL solutions by weight (approximately 0.20 mg/L). 

Tare an empty 500 mL bottle. Transfer 60.00 g of a prepared IPR solution to the bottle. 

Dilute to 450 g with DI water. Final concentration = 0.20 mg/L Cl2.

8.0 Sample Collection, Preservation and Storage

8.1 Samples should be collected in clean chlorine demand-free glass bottles.

8.1.1 Pretreat sample containers to remove chlorine demand. Soak containers in a weak bleach 

solution for at least 1 hr. Rinse fully with DI water.

8.2 Analyze samples immediately. 

9.0 Quality Control

9.1 Each laboratory that uses this method is expected to operate a formal quality assurance 

program (16.1). The minimum requirements of this program consist of an initial 

demonstration of laboratory capability and ongoing analyses of laboratory prepared water 

standards as a test of continued performance to assess accuracy and precision. 

Laboratory performance is compared to established performance criteria to determine if 

the results of analyses meet the performance characteristics of the method.

9.1.1 The analyst shall make an initial demonstration of the ability to generate acceptable 

accuracy and precision with this method. This ability is established as described in 

Sections 9.2 and 9.3. The laboratory shall, on an ongoing basis, demonstrate through 

analysis of the ongoing precision and recovery sample that the analysis system is in 

control.

9.1.2 Accompanying QC for the determination of Cl2 is required per analytical batch. An 

analytical batch is a set of samples processed during a contiguous 8-hour period. Each 

analytical batch must be accompanied by an ongoing precision and recovery sample 

(OPR), matrix spike sample (MS), and matrix spike duplicate sample (MSD) resulting in 

a minimum of four analyses (1 OPR, 1 sample, MS, and MSD).

9.2 Initial demonstration of laboratory capability.

9.2.1 To establish the ability to detect Cl2 the analyst shall determine the MDL using the 

apparatus, reagents, and standards that will be used in the practice of this method. An 

achieved MDL less than or equal to the MDL in Section 13.0 is recommended prior to 

the practice of this method.

9.2.2 Prepare and measure seven replicates of the MDL standard (Sect. 7.6) according to the 

procedure in Section 11

9.2.3 Using the results of the set of seven analyses, compute the MDL using the following 

equation:
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MDL =

 

3.14

1

2

2







 

n

n

x

x

where:

n = Number of samples (7)

x = measured concentration of each sample

9.3 Initial precision and recovery (IPR) - To establish the ability to generate acceptable 

precision and accuracy, the analyst shall perform the following operations: 

9.3.1 Prepare and measure four samples of the IPR standard (Sect. 7.5.1) according to the 

procedure in Section 11.

9.3.2 Using the results of the set of four analyses, compute the average percent recovery (x) 

and the standard deviation of the percent recovery (s) for Cl2. Use the following equation 

for calculation of the standard deviation of the percent recovery: 

 

1

2

2







 

n

n

x

x

s

where:

n = Number of samples (4)

x = % recovery in each sample

9.3.2.1 Compare s and x with the corresponding limits for initial precision and recovery in Table 

1 (Sect. 17). If s and x meet the acceptance criteria, system performance is acceptable and 

analysis of samples may begin. If, however, s exceeds the precision limit or x falls 

outside the range for recovery, system performance is unacceptable. In this event, correct 

the problem, and repeat the test.

9.4 Ongoing precision and recovery (OPR) - To demonstrate that the analysis system is in 

control, and acceptable precision and accuracy is being maintained with each analytical 

batch, the analyst shall perform the following operations: 

9.4.1 Prepare a 1.0-1.5 mg/L recovery standard with each analytical batch as described in Sect. 

7.5.1 and measure according to the procedure in Section 11. Calculate the percent 

recovery and compare this value with the limits for ongoing recovery in Table 2 (Sect. 

17). If the percent recovery meets the acceptance criteria, system performance is 

acceptable. If the percent recovery falls outside the acceptance criteria, system 

performance is unacceptable. In this event, correct the problem, and repeat the test.
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9.4.1.1 Measure a field sample. After measuring the background concentration, spike the sample 

with a known concentration of Cl2. The spike concentration should be 1-5 times the 

background concentration, but still within the reporting range of the method. Prepare a 

duplicate of this spiked sample.

9.4.1.2 Measure the spike duplicates and calculate the spike recovery for each sample and the 

relative percent difference (RPD) between the two results.

Use the following equation to calculate the spike recovery:

Spike Recovery =

[Conc] − [Bkgd]

[Sp]

× 100

where:

[Conc] = the measured concentration of the spiked sample

[Bkgd] = the measured concentration of the un-spiked sample

[Sp] = the concentration of the spike

RPD =

|Conc1 − Conc2|

(

Conc1 + Conc2

2

)

× 100

where:

Conc1 = the concentration of the first spiked sample

Conc2 = the concentration of the second spiked sample

9.4.1.3 Compare the spike recoveries and RPD with the corresponding limits in Table 2 (Sect. 

17). If recoveries and RPD meet the acceptance criteria, system performance is 

acceptable and analysis of samples may begin. If recoveries or RPD fall outside the 

limits, system performance is unacceptable. In this event, correct the problem, and repeat 

the test.

9.4.1.4 The laboratory should add results that pass to IPR and previous OPR data and update QC 

charts to form a graphic representation of continued laboratory performance. The 

laboratory should also develop a statement of laboratory data quality for each analyte by 

calculating the average percent recovery (R) and the standard deviation of the percent 

recovery (sr). Express the accuracy as a recovery interval from R - 2sr to R + 2sr. For 

example, if R = 95% and sr = 5%, the accuracy is 85% to 105%. Control charts are 

acceptable for evaluating process control, but under no circumstances can the control 

limits be widened beyond those established in the acceptance criteria defined in Section 

13. 

10.0 Calibration and Standardization

10.1 The Hach DR series spectrophotometers have a built-in calibration that is automatically 

initiated when the Indophenol Free Cl2 procedure is selected through the instrument

interface. No further initial calibration is required. However, the instruments have the 

capability of developing a user-calibration. See manufacturer’s manual for instructions.
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10.2 Calibration Verification

10.2.1 To verify that the instrument is measuring Cl2 properly, analyze 0.20 mg/L (Sect. 7.6.1)

and 1.5 mg/L (Sect. 7.5.1) Cl2 standards. Results should be within 15 percent of the 

actual value. Perform this calibration verification daily while instrument is in use. If the 

calibration verification standard result is outside the limit, it is unacceptable. In this 

event, correct the problem, and repeat the test.

11.0 Procedure

11.1 Instrument Setup – follow the instrument manufacturer’s instructions for instrument

setup.

11.2 Fill two sample cells with 10 mL of sample, one cell will be the sample and one will be 

the blank. 

11.3 Add 5 drops of Freechlor F Reagent to the sample cell.

11.3.1 Stopper the sample cell and invert to mix.

11.4 Add the contents of one Monochlor F Reagent Powder Pillow to both cells.

11.4.1 Stopper the cells and shake for ~20 s to dissolve the reagent.

11.5 Allow the samples to react for 5 min.

11.6 Invert the cells to mix.

11.7 Insert the blank cell into the spectrophotometer and zero.

11.8 Insert the sample cell into the spectrophotometer and read. Results will display in 

mg/L Cl2.

12.0 Data Analysis and Calculations

12.1 Cl2 concentration is calculated automatically against internal instrument calibration.

13.0 Method Performance

Performance of the method was demonstrated in multi-lab studies comparing the method 

against EPA Reference Method SM 4500-Cl G. The method was evaluated in a low ionic 

strength reference matrix as well as multiple geographically diverse finished drinking 

water samples obtained from both surface water and ground water sources.

Validation Results Section Limit 

Method Detection Limit 9.2 0.05 mg/L Cl2

Initial Recovery Range 9.3 89.5% - 101%
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Initial Precision 95% 9.3 0.02

Matrix Recovery Range 9.4 89.9 – 110%

Matrix Recovery Precision 95% 9.4 0.06

14.0 Pollution Prevention

14.1 Follow guidelines in Section 15.

15.0 Waste Management

15.1 It is the laboratory’s responsibility to comply with all federal, state, and local regulations 

governing waste management, particularly the hazardous waste identification rules and 

land disposal restrictions, and to protect air, water, and land by minimizing and control 

all releases from fume hoods and bench operations. Compliance with all sewage 

discharge permits and regulations is also required.

15.2 For further information on waste management, consult “The Waste Management manual 

for Laboratory Personnel”, and “Less is Better: Laboratory Chemical Management for 

Waste Reduction”, both available from the American Society’s Department of 

Government Relations and Science Policy, 1155 16th Street N.W., Washington, D.C. 

20036.

16.0 References

16.1 “Protocol for the Evaluation of Alternate Test Procedures for Organic and Inorganic 

Analytes in Drinking Water,” USEPA, EPA-815-R-15-007, February 2015.

16.2 40 CFR 136, Appendix B.

16.3 “OSHA Safety and Health Standards, General Industry,” (29 CFR 1910), Occupational 

Safety and Health Administration, OSHA 2206 (Revised, January 1976)

16.4 “Safety in Academic Chemistry Laboratories,” American Chemical Society, Committee 

on Chemical Safety, 3rd Edition, 1979.

16.5 “Water Analysis Handbook,” Hach Company, 8

th Edition, 2013.
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17.0 Tables

17.1 Acceptance Criteria for Performance tests – The QC performance criteria for this method 

were performed with a Hach Company DR6000 Spectrophotometer and Indophenol Free 

Cl2 Reagents.

Table 1. Initial Precision and Recovery Acceptance Criteria

Parameter Acceptance Criteria

Relative Standard Deviation ≤ 10%

Percent Recovery Range 100 ± 15%

Table 2. Ongoing Precision and Recovery Acceptance Criteria

Parameter Acceptance Criteria

Lab Fortified Blank Recovery 100 ± 15%

Sample Matrix Spike Recovery 100 ± 15%

Sample Matrix Spike RPD ≤ 10%
18.0 Glossary of Definitions and Purposes

The definitions and purposes are specified to this method but have been conformed to common 

usage as much as possible.

18.1 Units of weight and measure and their abbreviations

18.1.1 Symbols

oC: degrees Celsius

18.1.2 Alphabetical characters

mg/L: milligram per liter

18.2 Definitions, acronyms, and abbreviations

18.2.1 MDL: Method detection limit

18.2.2 IPR: Initial precision and recovery

18.2.3 OPR: On-going precision and recovery

18.2.4 MS: Matrix spike

18.2.5 MSD: Matrix spike duplicate

18.2.6 LIS: Low ionic strength, deionized water