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
Equipment and reagent requirements
- Peristaltic pump, variable speed, with pump tubing 1.52mm. i.d.
- Sample injector, fixed loop size, and syringe with luer tip.
- Connectors and PTFE tubing, 0.8mm i.d.
- Flow cell.
- Strip chart recorder.
- Phosphate standard. 1000 ppm.
- Ammonium molybdate, 0.02M in 2M nitric acid.
- Ascorbic acid, 0.5%
- Detergent sample(breeze) or other.
Determination of Phosphate by Flow injection analysis
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 %
Compare the manual procedure for phosphate determination (3) with the
What parameters could be varied to increase detection? Give the
rationale for your suggestions.
- Ruzicka, J. and Hansen, E.H. (1981) Flow Injection Analysis. Wiley
- Hansen, E.H. and Ruzicka, J. Chem Ed. 10, 677-80.
- Vogel A.I. (1982) A Textbook of Quantitative Inorganic Analysis,
Longmans, 3rd. Edition. 810.