Standard Methods: 4500-Cl B: Chlorine by Iodometry
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Official Method Name
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4500-Cl B. Iodometric Method I |
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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 Color Indicator |
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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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Chlorine will liberate free iodine from potassium iodide (KI) solutions at pH 8 or less. The liberated iodine is titrated with a standard solution of sodium thiosulfate (Na2S2O3) with starch as the indicator. Titrate at pH 3 to 4 because the reaction is not stoichiometric at neutral pH due to partial oxidation of thiosulfate to sulfate. |
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Scope and Application
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The iodometric method is suitable for measuring total chlorine concentrations greater than 1 mg/L in natural and treated waters. Not applicable to estuarine or marine waters. |
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Applicable Concentration Range
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Interferences
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Oxidized forms of manganese and other oxidizing agents interfere. Reducing agents such as organic sulfides also interfere. Although the neutral titration minimizes the interfering effect of ferric and nitrite ions, the acid titration is preferred because some forms of combined chlorine do not react at pH 7. Use only acetic acid for the acid titration; sulfuric acid (H2SO4) will increase interferences; never use hydrochloric acid (HCl). |
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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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