Particulate organic phosphorus in seawater
Source: Marine Biogeochemistry - Practical Course in Biological Oceanography, University of Kiel, Germany, 2005.
Organic phosphorus compounds are converted to orthophosphate (i.e. inorganic dissolved phosphate) by cooking with potassium peroxydisulphate. The inorganic dissolved phosphate is then measured colorimetrically, following Grasshoff (1983). During cooking with peroxydisulphate, some chloride is oxidised to chlorine. Because it may interfere with the measurement, this chlorine is then reduced by ascorbic acid (added in surplus, before the addition of the mixed reagent; see below).
Interferences
The corresponding reaction with silicic acid (up to 200 μm l-1 Si) (see 4.5) does not occur at a pH below 1.0. The reagents are designed for seawater in such a way, that the pH adjusts itself around 1.0.
If the samples, already mixed with the reagents, are allowed to stand for more than 30 minutes AsO43- reacts slowly and a heteropoly acid forms. However, the arsenate content in seawater amounts only to about 0.03 μm l-1 As.
Hydrogen sulphide in concentrations above 60 μmol l-1 H2S-S (= 2 mg S2- (sulphide)) disturbs the analysis. In such samples the sulphide has to be oxidized with bromine water and the surplus bromine is driven out by airflow prior to analysis.
Calibration
From the stock solution (10 μmol ml-1 PO43--P) a calibration series of 0, 1 and 2 μmol l-1 PO43- is set up. With a pipette 0.1 or 0.2ml are transfered into a 1 litre measuring flask and filled up to 1 litre with deionized water. For the 0 μmol l-1 PO43- standard only deionized water is used.
The calibration series is treated as described in the chapter “Execution of the Determination”.
Range: 0 – 10 μmol l-1 PO43--P (corresponding to 0 – 0,35 μmol/35 ml)
Precision: ± 0.02 μmol l-1 PO43--P (corresponding ± 0,0007 μmol/35 ml)
Procedure
A GF/F filter (blank or sample) is transferred to a 60ml Duran glass bottle and 35 ml of deionized water are added to this. Next, 2ml potassium peroxidisulphate solution are added, the bottle is closed and shaken. Samples are placed in an autoclave, or a household pressure cooker filled 2 cm high with deionized water, and cooked for 30 min at 121 C (at 15 psi), then cooled to room temperature. Next, 1 ml of ascorbic acid solution is added to each sample. Shake and wait for 1 min before adding 1 ml of mixed reagent II. 12 ml of this blue colored solution are transferred into centrifuge tubes and centrifuged at high speed for 10 min. Following centrifugation, a sample of the cleared solution is transferred to a cuvette and the absorption is measured at 882 nm against deionized water.
Reagents
4.5 M Sulphuric acid: Slowly add 250 ml concentrated H2SO4 (98%) to about 750 ml deionized water and make up to a volume of 1000 ml.
(the mixture must be cooled – wear safety glasses!!!!)
Ammonium heptamolybdate: Dissolve 9.5 g (NH4)6Mo7O24*4H2O in about 50 ml deionized water and fill up to 100 ml. Store in polyethylene bottles.
Potassium antimonyltartrate: Dissolve 3.25 g of potassium antimonyltartrate K(SbO)C4H4O6*0.5H2O in 100 ml deionized water.
Mixed-reagent II: Slowly add 125 ml 4.5 M H2SO4 to 47 ml ammonium molybdate solution, then 5.2 ml potassium antimonyltartrate solution and 73 ml deionized water (total volume of 250ml)
Potassium peroxydisulphate: 30ml of 4.5 M sulphuric acid are given to 200 ml deionized water. Then add 10 g of potassium peroxydisulphate (reagent remains stable during about 1 week)
Ascorbic acid: Dissolve 14 g ascorbic acid (C6H8O6) in 200 ml deionized water (at 4°C durable for 30 days)
Stock solution: Dissolve 1.361 g dried potassium hydrogen phosphate (KH2PO4) in deionized water and fill up to 1 000 ml (= 10 μmol ml-1)
Calculation of concentrations with a calibration series
Calibration series 0, 1 and 2 μmol l-1 (= 0,035 und 0,070 μmol/35 ml) 3–fold determination
Examples
Ø EBl ~ 0.015 absorption of reagent blank
Ø ESt1 ~ 0.115 absorption of Standard 0,035 μmol / 35ml PO43—P
Ø ESt2 ~ 0.215 absorption of standard 0,070 μmol / 35 ml PO43—P
