Delloyd's Lab-Tech Chemistry


LABGUIDE


DETERMINATION OF PHOSPHATE BY FLOW INJECTION ANALYSIS



Equipment

FIA peristaltic pump, Injection valve, 1 ml grad. syringes, Eppendorf pipette, Flow-cell, UV-VIS spectrophotometer, recorder.

Apparatus

Pump tubing 2.75 mm id, PTFE tubing 0.8 mm id, test tubes 10 ml, test tube rack, 50 ml volumetric flasks.

Chemicals

Potassium dihydrogen orthophosphate, (KH2PO4) FW = 136.09
Di sodium hydrogen phosphate (Na2 HPO4) FW = 141.96
Potassium antimony tartrate, FW = 324.92
Ammonium molybdate, (ammonium molybdate(V1)tetrahydrate, FW = 1235.86
Ascorbic acid, FW = 176.12
Sulphuric acid.

Sample
1.0g of a phosphate salt sample: disodium hydrogen phosphate (Na2HPO4 FW = 141.96)  or
Fertilizer sample (0.1 g)

Solutions Preparation

  • 50 ppm stock phosphate solution: dilute from 1000 ppm stock prepared as follows: weigh out accurately 1.43 g of dried KH2PO4 and dissolve in one liter distilled water. Pipette 2.5 mls into a 50 ml volumetric flask and make up to the mark with distilled water.
  • Soln. A = potassium antimony tartrate: 1.3715 g in 50ml distilled water.
  • Soln. B = ammonium molybdate: 10 g in 250 ml distilled water.
  • Soln. C = ascorbic acid: 1.76 g in 100 ml distilled water (prepare fresh daily)
  • Soln. D = 5N sulphuric acid: 35 ml to 250 ml distilled water.
Mixed reagent (mix fresh daily):

50ml D + 5ml A + 15ml B + 30ml C

Analytical notes:

  • All glassware used in this experiment must be acid washed (1 % nitric acid bath) and dried thoroughly (glassware oven) before use!
  • Use eppendorf pipette with plastic tips to measure exactly 0.8 mls of mixed reagent.
  • Color development: color develops fully within an hour. There is no need to leave samples and standards overnight for color development.
  • 50 ppm phosphate standard: dry salt in oven @ 80°C, cool in desiccator and then weigh out the required amount.
  • Sample: weigh out 1.00g of salt in triplicate in 100 ml volume solutions. Dilution is x 100 ( dilute 0.50ml in 50ml volumetric flask)
  • Measure @ 830 nm or 825 nm.
  • Blank: 5ml distilled water and 0.8ml mixed reagent.
  • Pump: flow rate of distilled water = 5ml per minute (Pump setting = 500)
  • Tubing: single piece pump tubing = 2.75 mm id. for distilled water.
    Injector loop: 60cm x 0.8mm id PTFE tubing.
    Manifold loop: 30cm x 0.8 mm id PTFE tubing from injector to flow cell.
Operation of instrument:

Follow User guide or get help from Technician or Demonstrator


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