Delloyd's Lab-Tech Chemistry

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Determination of Phosphate by Flow Injection Analysis

Equipment:
Peristaltic pump, Sample loop and injection valve, Flow cell, pump tubing and connectors.

Apparatus:
Volumetric flasks 25ml, 250ml, Hotplate, Stopwatch.

Chemicals:
Ammonium molybdate, (NH4)₆ Mo₇O₂₄.4H₂O, FW = 1235.9
Potassium dihydrogen phosphate, KH₂PO₄, FW = 138
Ascorbic acid, FW = 176.12
Conc. nitric acid, HNO₃

Sample:
Detergent (Breeze) 0.5g, or Fertilizer 0.25g

Solutions preparation:
Prepare all reagents using distilled de-ionised water.

1. 0.02M ammonium molybdate (FW = 1235.90) in 2M nitric acid. (Method : weigh out 24.70 g of the salt, dissolve in about 800 mls distilled water, then add 122mls of conc. HNO₃ and dilute to 1 liter.)
2. 0.5% ascorbic acid : dissolve 1g in 200mls distilled water (prepare fresh daily)
3. 1000 ppm phosphate std. Use potassium dihydrogen phosphate. Dry in oven at 80°C to remove moisture. Dissolve 1.45g in 1 liter distilled water.

Analytical notes:

1. Sonicate all reagents before measuring.
2. Digest Fertilizer sample with 5ml conc. HNO₃.
3. Range of stds. 10, 20, 30, 40, and 50ppm phosphate.
4. Measure molybdenum blue complex at 800nm. or 820nm.
5. Sample preparation for breeze
Predigest 0.5g with 15mls conc HNO₃ in a 250ml beaker overnight. Add 100 mls distilled water and boil on hotplate for 10 mins. , cool, and filter into a 250 vol. flask and make up to the mark with distilled water. Make a 1 in 10 dilution for measurements. eg 2.5mls in 25mls.
6. Flush system with 4M H₂SO₄, then with distilled water.


Operation of Instrument:

Follow User Guide: obtain help and advice from Demonstrator or Technician



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