## Hach Method 10241

Spectrophotometric Measurement of Free Chlorine ( Cl 2 

## ) in

Finished Drinking Water1 Hach Company Method 10241

Revision 1 .2

November 201 5

Spectrophotometric Measurement of Free Cl 2 in Finished Drinking Water

1.0 Scope and Application

1.1 Th is method is for the determination of free chlorine ( Cl 2 ) in finished drinking water

.

1.2 The method is applicable in the range from 0.1 to 4.5 mg/L Cl 2

.

1.3 This method is equally effective in performance and use to SM 4500

- Cl G for the

purposes of regulatory compliance reporting of Cl 2

.

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 t he 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 S ubstances 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 cur rent 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 all2

personnel involved in these analyses. Additional information on laboratory safety can be

found in Sections 1 6 .3 and 1 6 .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 39 00, DR 19 00 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 g lass

6. 3 .2 Analytical balance

.

7.0 Reagent s and Standards

7.1 Deionized water

– Water in which Cl 2 concentration is below the detection limit of this

method. W ater 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 Cl 2 Ampule : 10 mL, 50

- 75 mg/L as Cl 2 (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 Cl 2 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 (W 1 )(C 1 )/(C 2 )=(W 2 ). Dilute to the final weight with DI water. Assuming the

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

mg/L Cl 2

.3

7.6 Method detection limit (MDL) solution

7.6 .1 Prepare 7 or more replicate Cl 2 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 = 0.2 0 mg /L Cl 2

.

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 (1 6 .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

Section s 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 Cl 2 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 Cl 2 the analyst shall determine the MDL using the

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

achieved MDL less than or equal to the MDL in Section 1 3 .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:4 MDL =

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 Cl 2 . Use the following equation

for calculati on of the standard deviation of the percent recovery:

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 fall s

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 a nalytical batch as described in Sect.

7.5.1 and measure according to the procedure in Section 11

. C alculate the percent

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

17). If the percent recovery meets the acceptan ce 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.5

9 .4.1.1 Measure a field sample. After measuring t he background concentration, spike the sample

with a known concentration of Cl 2 . 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 ]

− [ B k gd ]

[ 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 = | Conc 1 − Conc 2 | ( Conc 1 + Conc 2 2 ) × 100

where:

Conc 1 = the concentration of the first spiked sample

Conc 2 = 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 an alysis 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 labor atory 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

calcula ting 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

accept able 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 Cl 2 procedure is selected through the instrument

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

capability of developing a user

- calibration. See manufacturer’s manual for instructions.6

10. 2 Calibration Verification

10. 2.1 To verify that the instrument is measuring Cl 2 properly, analyze 0.2 0 mg/L (Sect. 7.6.1)

and 1.5 mg/L (Sect. 7.5.1) Cl 2 s tandard s . 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 cell s

.

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

.

1 1

. 7 Insert the blank cell into the spectrophotometer and zero

.

1 1

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

mg /L Cl 2

.

1 2 .0 Data Analysis and Calculations

1 2 .1 Cl 2 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 Detec tion Limit 9.2 0.05 mg/L Cl 2

Initial Recovery Range 9 .3 89.5 %

- 101 %7

Initial Precision 95% 9 .3 0.02

Ma trix Recovery Range 9.4 89.9

– 110 %

Matrix Recovery Precision 95% 9.4 0. 06

1 4 .0 Pollution Prevention

1 4 .1 Follow guidelines in Section 1 5

.

1 5 .0 Waste Management

1 5 .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 Re lations and Science Policy, 1155 16 th Street N.W., Washington, D.C.

20036.

1 6 .0 References

1 6 .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 CF R 136, Appendix B.

1 6 .3 “OSHA Safety and Health Standards, General Industry,” (29 CFR 1910), Occupational

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

1 6 .4 “Safety in Academic Chemistry Laboratories,” American Chemical Society, Com mittee

on Chemical Safety, 3 rd Edition, 1979.

16.5 “Water Analysis Handbook,” Hach Company

, 8 th Edition, 2013

.8

17 .0 Tables

1 7 .1 Acceptance Criteria for Performance tests

– The QC performance criteria for this method

were performed with a Hach Company DR 60 00 Spectrophotometer and Indophenol Free

Cl 2 R eagent s

.

Table 1. Initial Precision and Recovery Acceptance Criteri a

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%1 8 .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.

1 8 .1 Units of weight and measure and their abbreviations

18.1.1 Symbols

o C : 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