Page 1 of 4

Office of Ground Water and Drinking Water 

November 20, 2013 

MEMORANDUM 

SUBJECT: Basis for Expedited Approval of Palintest ChlordioX Plus Method for Determination of 

 Chlorine Dioxide and Chlorite 

FROM: Glynda Smith, Chemist, SRMD/TSC 

 Steve Wendelken, Chemist, SRMD/TSC 

TO: The Record 

Palintest’s ChlordioX Plus method measures chlorine dioxide and chlorite by amperometry using 

disposable sensors. Standard Method 4500-ClO2 E is an amperometric titration method that is currently 

approved in the regulations at 40 CFR 141.74(a)(2) and 40 CFR 141.131(c)(1) for determination of 

chlorine dioxide in drinking water; it is also cited in the regulations at 40 CFR 141.131(b)(1) for routine 

daily monitoring of the disinfection byproduct, chlorite. The approved method requires experienced 

analysts and is not portable which limits options for field and site testing. The Palintest ChlordioX Plus 

method is simple for operators to use and associated with a portable sensor driven instrument. 

In the Palintest ChlordioX Plus method, chlorine dioxide is reduced at the surface of the ChlordioX Plus 

sensor and the current produced is directly proportional to the chlorine dioxide level in the sample. 

Using a sample duplicate, chlorite can be determined by oxidation of iodide to iodine. 

In addition to the methodology, Palintest has developed an analytical standard for chlorine dioxide that 

can be used for preparing calibration solutions. The standard concentration needs to be confirmed 

using a UV-Vis spectrophotometer as described in the method, but it is simpler and safer than the 

acidified sodium chlorite approach described in the approved method. Performance of the chlorine 

dioxide standard has been demonstrated as described below. 

Palintest Chlorine Dioxide Analytical Standard Solution

In-house experiments were conducted to evaluate the Palintest chlorine dioxide analytical standard. 

Stock standards were prepared with the Palintest chlorine dioxide analytical standard as described in 

the Palintest literature. The concentrations of the stock standard solutions were evaluated by 

amperometric titration as described in the approved Standard Method 4500-ClO2 E. The stock 

standards were prepared at a nominal 80 mg/L chlorine dioxide level. Seven solutions were prepared 

and analyzed in duplicate. When the stock standards were prepared in glass containers, mean value of 

the replicate determinations was 81 mg/L chlorine dioxide with a standard deviation of 3.2. Preparation 

of the solutions in plasticware resulted in lower results – the mean value of the replicate determinations 

was 68 mg/L chlorine dioxide with a standard deviation of 4.9. Thus, laboratories can expect reasonably 

good results as long as the standard solutions are prepared in glass containers. The chlorine dioxide 

stock standard solutions were further evaluated and no residual chlorite was present, only 100% 

chlorine dioxide. Finally, storage stability of the stock standards was evaluated. Three individual stock 

standard solutions were evaluated after being stored 24 hours in amber glass bottles held at 4 oC. Mean 

recoveries after 24 hours relative to the freshly prepared stock solutions were 100% + 1.1%. Three 
Page 2 of 4

additional stock standard solutions were evaluated after being stored 72 hours in amber glass bottles 

held at 4 oC. Mean recoveries after 72 hours relative to the freshly prepared stock solutions were 101% 

+ 0.2%. 

Multi-Laboratory Method Validation

Three laboratories in public drinking water utilities that use chlorine dioxide disinfection participated in 

the method validation study. Two of the utilities use surface water as their source water while one of 

the utilities uses ground water under the influence of surface water. Two of the laboratories used the 

Palintest chlorine dioxide analytical solution for standards and fortifying samples; one laboratory chose 

to use the in-house chlorine dioxide generator used to treat their water supply. The laboratories 

evaluated precision and accuracy in fortified samples, comparing the results obtained with the Palintest 

ChlordioX Plus method against those obtained with the approved method (Standard Method 4500-ClO2

E). 

Note: In the original ATP application and validation study, Palintest proposed using the ChlordioX Plus 

method for determination of free chlorine along with chlorine dioxide and chlorite. In the validation 

study, only one of the laboratories performed the free chlorine determination correctly. As such, the ATP 

evaluation and approval through expedited action only apply to determination of chlorine dioxide and 

chlorite, not free chlorine.

Method performance was evaluated in various matrices: fortified reagent water, fortified high ionic 

strength water, and either fortified tap water or pretreated source water from the utility. The high ionic 

strength water was prepared by adding 1.40 g. of sodium bicarbonate, 1.48 g. of sodium sulfate, and 

1.54 g. of sodium chloride to one liter reagent water. This yields a solution that is approximately 1000 

mg/L of the ionic interferents. Dilution by 50% yields 500 mg/L in each sample that was evaluated.

Fortified matrix samples were evaluated at 0.5 mg/L and at 2.5 mg/L concentration levels with a 

minimum of seven replicates evaluated at each level. Table 1 shows the recovery and precision data 

obtained in fortified reagent water samples.

Table 1. Fortified Reagent Water

Sample Fortification

(mg/L)

Chlorine

Dioxide Chlorite

% Recovery %RSD % Recovery %RSD

Reagent Water (A) 0.5 104 9.0 90.0 4.0

Reagent Water (A) 2.5 100 2.0 100 4.8

Reagent Water (B) 0.5 106 5.3 89.4 4.0

Reagent Water (B) 2.5 101 2.6 94.3 3.8

Reagent Water (C) 0.5 101 4.5 105 12.5

Reagent Water (C) 2.5 98.9 2.7 90.3 12.7

A = Beaver Water District

B = City of Hamilton South Water Plant

C = Water One Water District
Page 3 of 4

Table 2 shows the recovery and precision data obtained in fortified high ionic strength water samples.

Table 2. Fortified High Ionic Strength Water

Sample Fortification

(mg/L)

Chlorine

Dioxide Chlorite

% Recovery %RSD % Recovery %RSD

Reagent Water (A) 0.5 91.4 3.8 107 5.0

Reagent Water (A) 2.5 98.8 2.8 91.1 3.1

Reagent Water (B) 0.5 100 5.0 83.7 3.5

Reagent Water (B) 2.5 99.9 3.5 83.8 6.3

Reagent Water (C) 0.5 108 14.2 88.3 7.8

Reagent Water (C) 2.5 102 5.9 91.5 8.7

A = Beaver Water District

B = City of Hamilton South Water Plant

C = Water One Water District

Table 3 shows the recovery and precision data obtained in fortified raw and finished waters.

Sample Fortification

(mg/L)

Chlorine

Dioxide Chlorite

% Recovery %RSD % Recovery %RSD

Finished Water (A) 0.5 98.0 4.1 93.7 4.2

Finished Water (A) 2.5 100 2.2 95.8 2.5

Raw Well Water (B) 0.5 N/A* ---- N/A* ----

Raw Well Water (B) 2.5 87.5 1.6 108 3.0

Raw Surface Water 

Influent (C) 0.5 91.1 5.0 90.8 20.6

Raw Surface Water

Influent (C) 2.5 97.3 2.7 87.4 5.8

A = Beaver Water District

B = City of Hamilton South Water Plant

C = Water One Water District

* The raw well water exhibited a high chlorine dioxide demand (~0.4 mg/L) and native chlorite level which resulted 

in low chlorine dioxide recoveries and high chlorite recoveries at the low level fortification concentration.

Finished tap water samples from each utility were fortified and samples were evaluated side by side 

using the ChlordioX Plus method and the approved amperometric titration method (Standard Method 

4500-ClO2 E). Tables 4-6 provide the results of the method comparison studies.

Table 4. Beaver Water District

Method Fortification

(mg/L)

Chlorine

Dioxide Chlorite

% Recovery %RSD % Recovery %RSD

ChlordioX Plus 2.5 99.8 4.5 93.6 3.6

SM 4500-ClO2 E 2.5 101 5.7 98.3 1.1
Page 4 of 4

Table 5. City of Hamilton South Water Plant

Method Fortification

(mg/L)

Chlorine

Dioxide Chlorite

% Recovery %RSD % Recovery %RSD

ChlordioX Plus 0.5 100 8.3 91.1 7.5

SM 4500-ClO2 E 0.5 87.7 4.0 106 6.5

Table 6. Water One Water District

Method Fortification

(mg/L)

Chlorine

Dioxide Chlorite

% Recovery %RSD % Recovery %RSD

ChlordioX Plus 2.24 86.1 32.0 98.7 11.6

SM 4500-ClO2 E 2.24 97.1 4.8 93.7 1.7

With the exception of the high variance associated with the Water One ChlordioX Plus chlorine dioxide 

determination, the comparative data are quite good. Water One means tested the results obtained with 

the two different methods and concluded that there was no statistical difference between the methods. 

Conclusion

Chlorine dioxide is a highly volatile disinfectant that is difficult for laboratories to handle and analyze. 

There are relatively few approved options available to laboratories for determining chlorine dioxide 

concentrations. The Palintest ChlordioX Plus method offers a simple alternative methodology for rapidly 

and accurately determining chlorine dioxide concentrations as well as its disinfection byproduct, 

chlorite. Sufficient validation data have been submitted and evaluated to conclude that the ChlordioX 

Plus method is equally as effective as the approved method, Standard Method 4500-ClO2 E, for 

determination of chlorine dioxide and chlorite levels in drinking water.