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FIAlab Instruments, Inc.
14450 NE 29th Place STE 113
Bellevue, WA  98007 USA

Voice: 425-376-0450
Fax: 425-376-0451
fialab@flowinjection.com

              


              


Phosphate (Water Based Samples, Ortho Phosphate)
Phosphate (Bray Soil Extracts)
Phosphate (Mehlich Soil Extracts) 		  

FIAlab standard method for the phosphate assay in fresh/salt waters, Bray extracts, and Mehlich extracts  using the FIAlab-2500/2600/2700 system. Variations of this method for lower/higher concentration samples exist, please contact FIAlab for additional details.

Assay Typical Throughput Concentration Range Notes
Phosphate (Mid to High) 120 samples/hour 0.1 to 25 mg (P)/L 1 cm flowcell
Phosphate (Low) 60 samples/hour 0.01 to 2.5 mg (P)/L 10 cm flowcell
Phosphate (Ultra Low) 45 samples/hour 0.002 to 0.5 mg (P)/L 50 cm flowcell

 

Principle:

Soluble phosphorus is typically found in three forms: 1.) Orthophosphate (i.e. PO43-); 2.) Organo-phosphates (R-P04H-R’) and 3.) Condensed phosphates (metaphosphates, pyrophospates, and polyphosphates).  In this method, all three forms can be analyzed, but only reactive phosphorus can be directly determined without any pretreatment.  Reactive phosphorus is simply hydrolyzed orthophosphates.  Orthophosphates react directly with molybdate anions to form a yellow-colored phosphomolybdate complex.  This complex is then reduced by ascorbic acid to create a molybdenum blue species that has a broad absorbance range from 700nm to 900nm, making this method suitable for a variety of light sources and detectors.

 Comments: 

A heater set to 45 C is recommended for concentrations < 100 ppm.   The flowrate of the pump should be set to 50. The FIA LOV connections B should be bridged by a simple tubing (no Cadmium Column).   Recommended wavelengths:    860 nm for the primary and 490 reference.

For most Phosphate assays make the sample loop from three inches of green tubing (.03” ID).

Low Concentration Assays:  Use the 10 cm flowcell and sample loop to 18” of green (.03” ID) tubing.   

Ultra low Concentration Assays:  Use the 50 cm flowcell and sample loop to 36” of green (.03” ID) tubing.      

Adjustments to the selected wavelengths may be necessary to compensate for any natural color in the samples.  

Interferences:


The colorimetric molybdenum blue reaction is sensitive to changes in acidity, and works best in the pH 6-8 range.  The sample should not have a strong matrix absorbance at the selected detection wavelength.  Turbidity will cause high phosphate results and noisy data.  This can be a problem with acid digested samples.  High iron concentrations (>50 mg/L) will decrease the phosphate signal.  While the presence of As3+ can cause high phosphorus results.

Reagents:

Carrier: DI Water
1-Liter Degassed DI Water.    Matrix matching is required for salt water and soil extract samples.

Reagent 1:   6 mM Ammonium Molybdate.
 10.0 grams Ammonium molybdate tetra-hydrate [1235.81 FW]
 0.2 grams Antimony Potassium Tartrate half-hydrate [333.94 FW] catalyst

40 mL conc. H2SO4 acid (36N) ACS grade (Sigma-Aldrich 38,337-6)

1-Liter Degassed DI Water. Place acid into 800mL of DI water, mix and let cool to room temperature.  Add molybdate and antimony potassium tartrate and mix until dissolved.  Fill flask to the mark.  Transfer solution into a dark and airtight glass bottle for maximum longevity. This solution is stable for several weeks.

Reagent 2: Reagent Carrier Stream of 300mM Ascorbic Acid.
30 grams Ascorbic acid  [176.12 FW]
 1.0 grams Sodium dodecyl sulfate  [288.38 FW] surfactant (Sigma-Aldrich 436143-25G)
  

1-Liter Degassed DI Water. Place the ascorbic acid into a 1-liter volumetric flask and mix with 600 cc of DI water until dissolved.  Add the sodium dodecyl sulfate and mix slowly (prevent foaming) until dissolved.  Fill flask to the mark.  Transfer solution into an airtight light sensitive glass bottle for maximum longevity.  Minimize exposure to air and prepare fresh weekly since this solution is unstable.

 

 Standards:

100ml  ICPHO-100   (Phosphate standard)
 Source: 727-524-7732 - www.exaxol.com 

Please email or phone FIAlab Instruments for additional product information.
 Email: fialab@flowinjection.com, Voice: 425-376-0450 or 1-800-963-1101,  Fax: 425-376-0451