Standard Methods: 4500-P F: Phosphorus by Automated Ascorbic Acid Reduction
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Official Method Name
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4500-P F. Automated Ascorbic Acid Reduction 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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Automated Spectrophotometer |
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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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Ammonium molybdate and potassium antimonyl tartrate react with orthophosphate in an acid medium to form an antimony-phosphomolybdate complex, which, on reduction with ascorbic acid, yields an intense blue color suitable for photometric measurement. |
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Scope and Application
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Orthophosphate can be determined in potable, surface, and saline waters as well as domestic and industrial wastewaters over a range of 0.001 to 10.0 mg P/L when photometric measurements are made at 650 to 660 or 880 nm in a 15-mm or 50-mm tubular flow cell. Determine higher concentrations by diluting sample. Although the automated test is designed for orthophosphate only, other phosphorus compounds can be converted to this reactive form by various sample pretreatments described in Section 4500-P.B. |
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Applicable Concentration Range
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0.001 to 10.0 mg-P/L |
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Interferences
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As much as 50 mg Fe3+/L, 10 mg Cu/L, and 10 mg SiO2/L can be tolerated. High silica concentrations cause positive interference. In terms of phosphorus, the results are high by 0.005, 0.015, and 0.025 mg/L for silica concentrations of 20, 50, and 100 mg/L, respectively. Salt concentrations up to 20% (w/v) cause an error of less than 1%. Arsenate (AsO43-) is a positive interference. Eliminate interference from NO2- and S2- by adding an excess of bromine water or a saturated potassium permanganate (KMnO4) solution. Remove interfering turbidity by filtration before analysis. Filter samples for total or total hydrolyzable phosphorus only after digestion. Sample color that absorbs in the photometric range used for analysis also will interfere. |
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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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If dissolved phosphorus forms are to be differentiated, filter sample immediately after collection. Preserve by freezing at or below -10 degrees C. Add 40 mg HgCl2/L to the samples, especially when they are to be stored for long periods. Do not add either acid or CHCl3 as a preservative when phosphorus forms are to be determined. If total phosphorus alone is to be determined, add 1 mL conc HCl/L or freeze without any additions. Do not store samples containing low concentrations of phosphorus in plastic bottles unless kept in a frozen state because phosphates may be adsorbed onto the walls of plastic bottles. Rinse all glass containers with hot dilute HCl, then rinse several times in distilled water. Never use commercial detergents containing phosphate for cleaning glassware used in phosphate analysis. |
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Maximum Holding Time
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48 hours (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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See Section 4500-P B. |
