DISCRETE ANALYZER CHEMISTRY o-Phosphate (PO 4 -3 ) Method
Methods Referenced • Standard Methods 4500-PO 4
-3 E — Automated Ascorbic Acid Reduction Method
• EPA Method 365.1 Rev 2.0 — Colorimetric, Automated, Ascorbic Acid Method Range 0.005 – 1.0 mg P/L Reagent Composition Two reagents are used for the PO 4 -3 method: • Reagent 1, SEALAnalyticalAQ PO4 Ascorbic is 15 g/L Ascorbic acid in DI water. • Reagent 2, SEALAnalyticalAQ PO4 Color Rgt is made from two separate sub-solutions combined in dilute sulfuric acid: Sub-solution 1: 3 g/L Potasssium antimonyl tartrate in DI water Sub-solution 2: 40 g/L Ammonium molybdate in DI water
Final Concentrations are as follows: SEALAnalyticalAQ PO4 Ascorbic 15 g/L Ascorbic acid SEALAnalyticalAQ PO4 Color Rgt
0.225 g/L Potassium antimonyl tartrate 8.8 g/L Ammonium molybdate 3.25 N Sulfuric acid
Chemical Reactions Ammonium molybdate and antimony potassium tartrate react in an acid medium with dilute solutions of phosphorus to form an antimony-phospho-molybdate complex. This complex is reduced to an intensely blue-colored complex by ascorbic acid. The color is proportional to the phosphorus concentration. Note: The stoichiometry of these reaction is still in question, but the paper, The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box by EA Nagul, goes into more detail about the formation of the blue color used for this analysis. This paper gives the following equations and mentions that the presence of antimony speeds up the formation of the blue color. PO 4 -3 + 12MoO 4 -2 + 27H + H 3 PO 4 (MoO 3 ) 12 + 12H 2 O H 3 PMo(VI) 12 O 40 + Reductant [H 4 PMo(VI) 8 Mo(V) 4 O 40 ] -3
Note: We recommend using ICPPO41 to make your standards for this method. The solution is certified as the Phosphorous concentration, so no conversions are necessary — just dilute to your working concentrations and start testing!
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