I am trying to determine the available phosphate content in a commercial fertilizer. I need a lab protocol (methodology) to carry out the test
The fertilizer material that isn't dissolved in water is then placed in an ammonium citrate solution. The amount of P dissolved in this solution is measured and expressed as a percentage of the total in the fertilizer material. Phosphate measured with this analytical procedure is referred to as citrate-soluble.
1. Sample Weigh out 1 gram (±0.001g) of the feed phosphate sample, ground to a granular size of less than 0.5mm. Place in a 250ml flask with sufficient space to allow for good agitation (ie Stohmann).
2. Reagent Prepare 200ml 2% (w/w) citric acid solution by dissolving pure citric acid (C6H8O7.H2O) in distilled water.
3. Procedure Add part of the reagent to the flask at ambient temperature (20°C). Shake vigorously to avoid the formation of lumps and to prevent any of the sample sticking to the walls of the flask. Add the remainder of the solution and stop up the flask.
Agitate for two hours in a rotary shaker at 35-40 revolutions per minute. Filter the resulting solution immediately through a paper filter free of phosphates into a dry volumetric flask, discarding the first 20ml of the filtrate.
Taking an appropriate aliquot part of the clear filtrate, use the gravimetric quinoline phosphomolybdate method to determine its phosphorus (P) content.
https://www.feedphosphates.org/index.php/guides/11-guides/19-inorganic-feed-phosphate-test-methods
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