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Troubleshooting for Total Phosphorous and Ortho‐Phosphate Colorimetric
Testing for Discrete and Segmented Flow AnalyzersSarah Leibenguth
Technical Sales & Support Chemist
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Topics for Discussion
• Method Principle• Test Procedures• Digestion Protocol
• Digestion Reagents• Digestion pH• Temperature• Digestion Block• Reconstitution
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Topics for Discussion
• Colorimetric Detection• Reagents• Interferences• Riley Ratio• pH and Matrix Matching
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Method Principle and Approval
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Method Principle
• TP and oP Colorimetric Reaction
• Ammonium Molybdate• Orthophosphate + Molybdate→ Phosphomolybdic Acid
• Antimony Potassium Tartrate• Catalyst for Phosphomolybdic Acid
• Ascorbic Acid• Phosphomolybdic Acid + Ascorbic Acid → Blue Phosphomolybdic Complex
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Test Procedures
• Ortho‐Phosphate• EPA 365.1, version 2 (1993)• Standard Methods 4500‐P F (18th,19th,20th)
• Total Phosphorous• EPA 365.1, version 2 (1993) • Standard Methods 4500‐P B, F (18th,19th,20th)
• Total Kjeldahl Phosphorous• EPA 365.4
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• Reactive Phosphorous• Available oP for Colorimetric Detection
• Prior to Hydrolysis or Digestion• Filtered or Non‐Filtered Samples• Total Reactive or Dissolved Reactive
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• Acid‐Hydrolyzable Phosphorous• Pre‐Treated by Acid Hydrolysis at Boiling Temperature
• Filtered or Non‐Filtered Samples• Total Acid‐Hydrolyzable or Total Dissolved Acid‐Hydrolyzable
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• Total Phosphorous• Digestion Pre‐Treatment
• Organically Bound Phosphorous Converted to oP• Filtered or Non‐Filtered Samples• Total Phosphorous or Total Dissolved/Soluble Phosphorous
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Total Phosphorous Digestion Protocol
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Total Phosphorous Digestion
• Persulfate Digestion• Hot Plate or Block Digestor• Autoclave
• Kjeldahl Digestion• High Temperature Block Digestor
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Total Phosphorous Persulfate Digestion
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TP Persulfate Digestion
• Persulfate Digestion Reagent• Potassium Persulfate• Ammonium Persulfate
• Easier to Dissolve • Sulfuric Acid• Prepare Combined Reagent Immediately Prior to Use
• Digestion pH• Persulfate Concentration• Acidic Digestion • Alkaline Digestion
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TP Persulfate Digestion
• Temperature• Hot Plate or Block Digestor
• Boil 30 min or 5 ‐10 mL Final Volume• Cool and Reconstitute• Block Stability
• Autoclave• Approximately 120°C • Heat for 30 min
• Reconstitution• Not Necessary for Autoclave! • Accurate Volumes • Block Digestor Automation
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Total Phosphorous Kjeldahl Digestion
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TP Kjeldahl Digestion
• TKP Digestion Reagent • Sulfuric Acid• Potassium Sulfate• Heat to 380°C• Catalyst
• Digestion pH• Incomplete Digestions• Sample Acidity Level
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TP Kjeldahl Digestion
• Evaporation Step in Protocol• Reduces Sample Volume Prior to Temperature Increase• Decreases Potential for Splatter• Final Volume Consistency• Example:
Step to 160°C and Hold for 30 minStep to 190°C and Hold for 30 minApproximately 5 mL Final VolumeHeat to 380°C and Hold for 30 min
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Reconstitution
• Cooling• Acid Volume Remaining in Digestion Tube• DI Water Addition• Cool Digestion Tubes• Reconstitute
• Dispense Accurately• Bottle Top Dispenser or Auto‐Pipette• Samples and Standards Same Final Volume• Volumetric Digestion Tubes
• Vortex Mixer
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Catalysts
• Mercury• Documentation• Colorless Digest• Preparing Digestion Reagent• Toxic• Waste Disposal Considerations
• Copper• Substitute for Mercury Catalyst• Blue/Green Final Digest• Less Toxic
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TP and Ortho‐Phosphate Colorimetric Reaction
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Colorimetric Reaction
• Reagents
• Stability• Storage• Preparation• Additional Considerations
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Reagents
• ACS Reagent Grade Chemicals
• Purifying System for Reagent Water
• Acid Rinse Reagent and Standard Containers
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Reagents
• Ammonium Molybdate• Remake Monthly • Stored in Plastic Bottle• Store in the Refrigerator• Prone to Precipitate and Turbidity• Dry Chemical White in Appearance• No Green Coloration • Prepared Reagent is Colorless
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Reagents
• Sulfuric Acid• Stored Indefinitely at Room Temperature• Cool Prior to Final Dilution
• Antimony Potassium Tartrate• Remake Monthly for Best Results• Stored in Glass Container• Store in the Refrigerator• Expired Reagent Concerns
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Reagents
• Prepared Color Reagent • Add Reagents in Specific Order
• Sulfuric Acid• Antimony Potassium Tartrate• Ammonium Molybdate
• Turbidity • 1 – 3 Week Stability• Store in Refrigerator in Plastic Bottle• 4 Hour Stability if Ascorbic Acid Added
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Reagents
• Ascorbic Acid Reagent• Stable 1 Week• Prepare Daily for Best Results• Stored in the Refrigerator• Discard if Yellow• Expired Reagent Concerns
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Interferences
• Nitrite and Hexavalent Chromium• Low Concentration Readings
• Arsenate• Positive Interference• Similar Colorimetric Reaction
• Salt Concentrations• Less Than 1% Error for Salt Concentrations up to 20%(w/v)
• Turbidity • Filtration if Applicable
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Interferences
• Iron, Copper, and Silica• High Silica Concentrations May Cause Positive Interference • 50 mg Fe3+/L, 10 mg Cu/L and 10 mg SiO2/L Tolerated• High Iron Concentrations May Cause Precipitation of Phosphorous
• Surfactants in SFA Reagents• SDS or Triton• No BRIJ!
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Riley Ratio
• Acidic Environment Needed to Form Phosphomolybdic Acid• The [ H+] : [ Mo ] Ratio for Stable and Rapid Results• Sample pH Changes Riley Ratio• Final Sample Digest Changes Riley Ratio • Method Adjustments for Acidified or Digested Samples
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pH and Matrix Matching
• Sample Matrix and Carrier Solution
• Acidified Samples• Saline Samples• Digested Samples• Standards and Control Treatment
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