Phosphate-P Determination Using Ion Exchange
Add 5g soil and a cleaned resin bag
to 50ml deionized water in a 125ml flask. Shake the suspension at an oscillation
rate of about 100rpm for 17 to 24 hours, keeping a consistent time between
batches. Remove and thoroughly rinse the resin bag with deionized water.
[Remember to weigh some soil into a tin for drying if moisture content is needed.]
Transfer the resin bag to a clean 125ml flask and add 40ml of 0.5M HCL (42ml HCL/L) to elute the phosphate. Shake for 1 hour. Pour the acid and resin bag into a funnel on top of a 50ml volumetric flask. Using a squirt bottle filled with 0.5M HCL, rinse the resin bag while keeping an eye on the volumetric mark, and bring to volume. Stopper the volumetric flask and mix thoroughly. Pour about 3.5ml into a sample cup and cap it.
New resin bags (1g anion+1g cation) should be rinsed with DI water, then soaked in 0.5M HCL for about 20 minutes, then rinsed thoroughly with water and placed into 2M NaCl overnight. Then rinse and place into fresh NaCl. Repeat until the pH of the sodium chloride doesn’t change. After a final rinse with DI water, the resin bags are ready for use.
Reference; SSSA Book Series 5: Methods of Soil Analysis, Part 3 – Chemical Methods. D.L. Sparks, Ed. 1996 pages 898 – 899.