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Marine Geochemistry - Laboratory Methods

Phosphate detection with Merck Photometer
method In acid solution and in the presence of molybdate-ions, ortho-phosphate forms a complex which is reduced to phosphorous-molybdenium-blue with ascorbic acid. Phosphate is detected photometrically at 820 nm using a portable photometer. (alternatively use Autoanalyzer-method)
setup
solutions  
reagent

Ammonium-molybdate solution (do not use metal spatula !):
Dissolve 115 mg KSb-tartrate (C4H4KO7Sb*0.5 H2O) in 400 ml pure water. Stir gently and add 35 mL concentrated sulfuric acid (H2SO4 , 97%). Add 3g ammonium-molybdate (NH4)2MoO4 , let it dissolve and fill up with pure water to 500 mL. Store in a brown bottle. This solution is stable for 1 week.
ascorbic acid solution Dissolve 5.5 g ascorbic acid(C6H8O6) in 400 mL pure water and add 30 mL acetone (C3H6O). Fill up to 500 mL. Store @ 4°C in a brown glass bottle.This solution is stable for 1 week.
standard stock 1000 ppm Phosphate Standard (Merck #19898)
work solution 10 ppm PO4: 0.5 mL Phosphate standard diluted to 50 mL with pure water. This solution is stable for 1 week.
dilution water and blank pure water or phosphate-free natural or artificial seawater depending on samples
work standards
concentration work PO4 solution dilution water
ppm mL mL
0.0 0.0 10
0.1 0.1 9.9
0.3 0.3 9.7
0.5 0.5 9.5
0.7 0.7 9.3
1 1 9
1.5 1.5 8.5
standards are stable for one day only
analysis conditions Add 50 µL reagent + 50 µL ascorbic acid solution to 1 mL sample in a micro cuvette. Shake and store dark for 10 min. Measure with Merck Photometer methods 74 or 241 (see below)
Measure blank first as Auto Null, then the work standards.
Method 74 displays the values in mg/L.
cross contamination If the samples contain sulfide the solution becomes brownish and cloudy when adding reagents and method 74 does not work. In order to eliminate sulfide, add 20 µL HCl (30%) to both samples and work standards and store cool for one day. If necessary drive out bubbles by treatment in ultrasonic bath for 5 min prior to measurement. Then add 50 µL reagent + 50 µL ascorbic acid solution. shake, wait for 10 min and measure extinction values with method 241at a wavelength of 820 nm. Calculate concentrations with excel worksheet.
tips + tricks

All bottles and vials should be glass. Do not use plastic unless absolutely necessary. Rinse all glassware with 1.5% HCl followed by pure water prior to use in order to reduce phosphate background from detergents. The ascorbic acid powder loses its reducing effect when has become real warm (i.e. during storage in a container) ! If you suspect this might happen, carry it in your hand luggage.

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