Standard Methods: 4500-SO32- B: Sulfite by Iodometry
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Official Method Name
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4500-SO32- B. Iodometric 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 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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An acidified sample containing sulfite (SO32-) is titrated with a standardized potassium iodide-iodate titrant. Free iodine, liberated by the iodide-iodate reagent, reacts with SO32-. The titration endpoint is signaled by the blue color resulting from the first excess of iodine reacting with a starch indicator. |
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Scope and Application
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The iodometric titration method is suitable for relatively clean waters with concentrations above 2 mg SO32-/L. |
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Applicable Concentration Range
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Above 2 mg-SO32-/L |
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Interferences
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The presence of other oxidizable materials, such as sulfide, thiosulfate, and Fe (II) ions, can cause apparently high results for sulfite. Some metal ions, such as Cu (II), may catalyze the oxidation of SO32- to SO42- when the sample is exposed to air, thus leading to low results. NO2- will react with SO32- in the acidic reaction medium and lead to low sulfite results unless sulfamic acid is added to destroy nitrite. Addition of EDTA as a complexing agent at the time of sample collection inhibits Cu (II) catalysis and promotes oxidation of ferrous to ferric iron before analysis. Sulfide may be removed by adding about 0.5 g zinc acetate and analyzing the supernatant of the settled sample. However, thiosulfate may have to be determined by an independent method (e.g., the formaldehyde/ iodometric method), and then the sulfite determined by difference. |
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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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Maximum Holding Time
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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