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Continuous flow determination of phosphate using Flow cell
and peristaltic pump

1. Introduction:

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 a sample to be provided (2).

Phosphates in the sample will be extracted with nitric acid,and the phosphate determined by reaction with molybdate reagent in a FIA manifold. The blue color of the phosphomolybdate complex will be measured by visible spectrophotometry, using a flow cell fitted into the sample compartment. The signals generated will be recorded on a strip chart recorder as a series of peaks, the heights of which will be used to determine the phosphate concentrations of the sample solutions.

2. Experimental Procedure:

(A) Equipment and reagent requirements:

(a) Peristaltic pump, variable speed, with pump tubing 1.52mm i.d.
(b) Sample injector, fixed loop length, and syringe with luer tip.
(c) Tubing Connectors and PTFE tubing, 0.5mm i.d.
(d) Single-beam spectrometer, fitted with a 1cm path length flow cell, and a strip chart recorder.
(e) Phosphate standard (1000ug/mL).
(f) Ammonium molybdate (0.02M in 2M nitric acid).
(g) Ascorbic acid, 0.5%

(B) Procedure:

(a) Sample preparation:

(i) Weigh accurately in triplicate about 0.1g aliquots of the ground sample provided, into boiling tubes.

(ii) To each sample tube and one blank tube, add 5mL conc. nitric acid.

(iii) Place tubes on a heating block set at 135-1400C and let reflux for 2h, swirling every 15-20 min. to avoid bumping.

(iv) Cool, add 10mL distilled water and mix well.

(v) Filter through Whatman #1 filter paper into 100mL volumetric flasks. Make up to the mark with distilled water washings of flasks and filters.

(vi) Dilute each sample extract with distilled water as recommended (see Demonstrator).

(b) Flow injection analysis:

(i) Prepare 25mL each of phosphate standards (10, 20, 30, 40, and 50ug/mL respectively), by appropriate dilutions of the 1000 ug/mL phosphate standard provided.

(ii) Check the system for leaks and set the pump flow rate and wavelength for phosphate detection as recommended.

(iii) Pump the reagents through the manifold and ensure that no air bubbles remain in the system. Set the appropriate recorder sensitivity, chart speed and baseline. Record all parameters set.

(iv) Inject the sample blank in triplicate and record the signals generated. Inject each standard solution in triplicate, allowing about 15-20 sec. between each peak, to allow the baseline to be re-established between successive peaks.

Ensure that for triplicate injections of a phosphate standard, the peaks heights should not vary by more than 10% of each other.

(v) Similarly inject diluted solutions of samples.

(vi) Measure and record the peak heights of the respective solutions and correct for the blank values.

3. Calculations:

(i) Plot a graph of peak height vs phosphate concentration, or calculate the linear regression equation and correlation coefficient for the calibration data.

Hence determine PO4 concentration in diluted sample solutions.

Calculate the phosphate concentration of the sample provided and express as mean % P2O5 and standard deviation of analysis.

(ii) Compare the manual procedure for phosphate determination (3) with the present procedure, in terms of:

- reagent consumption per 10 samples, each analysed in triplicate.
- time taken for such analyses

4. References:

(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.

Anal-Chem Resources
Chem. Dept. UWI. St. Augustine Campus