Cary Spectrophotometer: User Guide
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Manual Information
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*(from a series of measurements over 8 h, it was found that results stabilized after 6.5 hr.)
Phosphate
Determination of dissolved phosphate, particularly in rapidly deposited organic carbon–rich sediments, is important in the shipboard analytical program. Phosphate concentrations may vary considerably, and it is therefore advisable to obtain a preliminary idea of the concentration ranges to be expected. This can most easily be accomplished by taking samples in the region of maximum alkalinities. Typically if alkalinities are >30 mM, dissolved phosphate concentrations may be >100 µM; thus, only very small sample aliquots will be needed to establish the concentration range.
This method is, in essence, the colorimetric method from Strickland and Parsons (1968) as modified by Presley (1971) for DSDP pore fluids. Orthophosphate reacts with Mo(VI) and Sb(III) in an acidic solution to form an antimony-phosphomolybdate complex. Ascorbic acid reduces this complex to form a blue color, and absorbance is measured spectrophotometrically at 885 nm.
It is important to note that the concentrations in the final test solution cannot exceed ~10 µM. Thus, for open-ocean (low sedimentation rate, low organic carbon) sediments, one might need to do the determination on 2 mL of sample (expected range 0–10 µM), but in typical continental margin settings, where concentrations can exceed 100–200 µM, a 0.1 or 0.2 mL sample aliquot must be used. The concentration range must be established prior to running samples, and it is highly advisable to make standards that cover the range of concentrations to be expected.
Note: Samples with high silica concentrations may give a false increase in measured concentration of phosphate (http://dx.doi.org/10.1007/BF02071829; S. Noriki, Silicate correction in the colorimetric determination of phosphate in seawater, 1983).
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In a 1000 mL volumetric flask, dissolve 0.102 g of antimony potassium tartrate trihydrate (KSbC4H4O7·3H- 3H2O) in ~600 mL of nanopure water. (If using antimony potassium tartrate hemihydrate [KSbC4H4O7·½H- ½H2O], dissolve 0.09 g.) Dilute to 1000 mL with reagent water.
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In a 1000 mL volumetric flask, dissolve 2 g of ammonium molybdate tetrahydrate ([NH4]6Mo7O24·4H- 4H2O) in ~600 mL of nanopure water. Dilute to 1000 mL with reagent water.
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In a 500 mL volumetric flask, shake 50 g of analytical-grade oxalic acid dihydrate [(C2H4O2)·2H- 2H2O] in 500 mL of nanopure water and allow to stand overnight. Let stand overnight. Decant saturated solution of oxalic acid from crystals before use.
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