Standard Methods: 4500-ClO2 C: Chlorine Dioxide by Amperometry
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Official Method Name
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4500-ClO2 C. Amperometric Method I |
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Current Revision
| Standard Methods 21st Edition (2005); Standard Methods Online |
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Media
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WATER |
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Instrumentation
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Amperometer |
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Method Subcategory
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Inorganic |
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Method Source
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Citation
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Brief Method Summary
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The amperometric titration of ClO2 is an extension of the amperometric method for chlorine. By performing four titrations with phenylarsine oxide, free chlorine (including hypochlorite and hypochlorous acid), chloramines, chlorite, and ClO2 may be determined separately. The first titration step consists of conversion of ClO2 to chlorite and chlorate through addition of sufficient NaOH to produce a pH of 12, followed by neutralization to a pH of 7 and titration of free chlorine. In the second titration KI is added to a sample that has been treated similarly with alkali and had the pH readjusted to 7; titration yields free chlorine and monochloramine. The third titration involves addition of KI and pH adjustment to 7, followed by titration of free chlorine, monochloramine, and one-fifth of the available ClO2. In the fourth titration, addition of sufficient H2SO4 to lower the pH to 2 enables all available ClO2 and chlorite, as well as the total free chlorine, to liberate an equivalent amount of iodine from the added KI and thus be titrated. |
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Scope and Application
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The method is useful when a knowledge of the various chlorine fractions in a water sample is desired. It distinguishes various chlorine compounds of interest with good accuracy and precision, but requires specialized equipment and considerable analytical skill. |
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Applicable Concentration Range
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Interferences
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Nitrogen trichloride can titrate partly as free chlorine and partly as dichloramine, contributing a positive error in both fractions at a rate of approximately 0.1%/min. Some organic chloramines also can be titrated in each step. Monochloramine can intrude into the free chlorine fraction and dichloramine can interfere in the monochloramine fraction, especially at high temperatures and prolonged titration times. Free halogens other than chlorine also will titrate as free chlorine. Combined chlorine reacts with iodide ions to produce iodine. Interference from copper has been noted in samples taken from copper pipe or after heavy copper sulfate treatment of reservoirs, with metallic copper plating out on the electrode. Silver ions also poison the electrode. Interference occurs in some highly colored waters and in waters containing surface-active agents. Very low temperatures slow response of measuring cell and longer time is required for the titration, but precision is not affected. A reduction in reaction rate is caused by pH values above 7.5; overcome this by buffering all samples to pH 7.0 or less. |
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Quality Control Requirements
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See Section 4020 Quality Assurance/Quality Control. |
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Sample Handling
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Plastic or glass containers. Determine ClO2 promptly after collecting the sample. Do not expose sample to sunlight or strong artificial light and do not aerate to mix. Method can be performed on site, with prior calibration in the laboratory. Minimum ClO2 losses occur when the determination is completed immediately at the site of sample collection. |
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Maximum Holding Time
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0.25 hour |
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Relative Cost
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Greater than $400 |
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Sample Preparation Methods
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