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C30F Analytical Chemistry Experiments
FIA - Flow Injection Analysis

Determination of phosphate by Flow Injection Analysis

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Flow injection analysis

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Flow injection analysis (FIA) is a continuous flow non-segmented method of analysis(1). Standards and samples are injected into the continuously flowing stream of reagents, and this initiates continuous interaction between reagent and analyte in the injected plug. The sample plug on reaching the detector, which may be a spectrophotometric or electrochemical device, is detected as a peak with a sharp front and a tail. The degree of dispersion of the injected plug may be controlled by several means, including the flow dynamics of the system, reactor path length, and reagent flow rate. Thus the sensitivity of detection is maximized by optimizing factors such as reagent concentrations, pH, temperature, reactor path length, and reagent flow rate.

In this experiment a simple flow injection system will be used to determine the level of phosphate in powdered detergent (2). Polyphosphates in the detergent sample can be decomposed to elemental phosphate by boiling with acid,and precipitated silica removed by filtration.

Equipment and reagent requirements

  1. Peristaltic pump, variable speed, with pump tubing 1.52mm. i.d.
  2. Sample injector, fixed loop size, and syringe with luer tip.
  3. Connectors and PTFE tubing, 0.8mm i.d.
  4. Flow cell.
  5. Strip chart recorder.
  6. Detector.
  7. Phosphate standard. 1000 ppm.
  8. Ammonium molybdate, 0.02M in 2M nitric acid.
  9. Ascorbic acid, 0.5%
  10. Detergent sample(breeze) or other.

Determination of Phosphate by Flow injection analysis


Sample preparation:

Weigh accurately triplicate aliquots 0.05g of the detergent sample provided into a 500 ml flask, add 250 ml distilled water and 5 ml conc. nitric acid. Carry out a blank simultaneously. Boil with constant stirring on a hot plate for 10 minutes to decompose polyphosphates and precipitate silica. Cool and filter through Whatman #1 filter paper into a 250 ml volumetric flask and make up to the mark with distilled water washings of flask and filter. Mix thoroughly and transfer a 2 ml aliquot of each to 50 ml volumetric flasks. Make up to the mark with distilled water and mix thoroughly.

Prepare phosphate standards 10, 20, 30, 40, and 50 ppm by dilution of the 1000 ppm provided.

Flow injection Analysis

Check the system for leaks and set the pump flow rate as recommended. Pump the reagents through the system and ensure that no air bubbles remain in the system. Set the appropriate detector sensitivity, recorder sensitivity, chart speed, and baseline.

Record all parameters set.

Inject triplicate aliquots of each standard, allowing about 30 to 45 seconds after the complete emergence of each peak, before injecting another aliquot. Similarly, inject triplicate aliquots of diluted solutions of samples and blanks.

Measure and record the peak heights of the respective solutions. Plot a graph of peak height vs PO4 concentration and hence determine PO4 concentration in sample solutions.

Calculate the phosphate concentration of the detergent and express as % P2O5

Compare the manual procedure for phosphate determination (3) with the present procedure.

What parameters could be varied to increase detection? Give the rationale for your suggestions.


  1. Ruzicka, J. and Hansen, E.H. (1981) Flow Injection Analysis. Wiley Inter-science. 9-64.

  2. Hansen, E.H. and Ruzicka, J. Chem Ed. 10, 677-80.

  3. Vogel A.I. (1982) A Textbook of Quantitative Inorganic Analysis, Longmans, 3rd. Edition. 810.

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