Standard Methods: 4500-Cl D: Chlorine by Amperometry
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Official Method Name
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4500-Cl D. Amperometric Titration Method |
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Current Revision
| Standard Methods Online |
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Media
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WATER |
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Instrumentation
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Titration with an 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 method is a special adaptation of the polarographic principle. Free chlorine is titrated at a pH between 6.5 and 7.5, a range in which the combined chlorine reacts slowly. The combined chlorine, in turn, is titrated in the presence of the proper amount of KI in the pH range 3.5 to 4.5. |
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Scope and Application
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The amperometric titration method is a standard of comparison for the determination of free or combined chlorine. It is affected little by common oxidizing agents, temperature variations, turbidity, and color. The method is not as simple as the colorimetric methods and requires greater operator skill to obtain the best reliability. Loss of chlorine can occur because of rapid stirring in some commercial equipment. Clean and conditioned electrodes are necessary for sharp end points. |
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Applicable Concentration Range
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Interferences
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Accurate determinations of free chlorine cannot be made in the presence of nitrogen trichloride, NCl3, or chlorine dioxide, which titrate partly as free chlorine. When present, NCl3 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. 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. 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. |
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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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Chlorine in aqueous solution is not stable, and the chlorine content of samples or solutions, particularly weak solutions, will decrease rapidly. Exposure to sunlight or other strong light or agitation will accelerate the reduction of chlorine. Therefore, start chlorine determinations immediately after sampling, avoiding excessive light and agitation. Do not store samples to be analyzed for chlorine. |
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Maximum Holding Time
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0.25 hour (See Section 1060) |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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