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C30F Analytical Chemistry Labscript

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Determination of phosphate by batch preparation
and rapid flow injection analysis

1. Introduction:

In manual procedures for phosphate analyses, the samples are prepared in defined volumes,(usually 25-50mL), with the same volumes of each reagent being added to each solution. A period of at least 1 hour is then allowed for the color to develop fully before measurement.

Similarly, the measurement stage usually requires the rinsing, filling and wiping of a cuvette; measurement of the signal; emptying and rinsing the cuvette. This process is repeated for each solution and normally requires about 1 to 2 min.

In flow injection analysis (FIA) of phosphate, the reagents are pumped continuously through a reaction manifold, into the which the samples are injected at regular intervals. The reagent between each injected sample provides the signal baseline, but is not used for reaction with phosphate and is thus wasted.

In this experiment, phosphate will be determined by batch development of the color using minimal sample and reagent volumes and the color intensities of solutions measured by rapid flow injection through a flow cell in a spectrophotometer. Distilled water will be used as the carrier.

2. Reagent and equipment requirements:

(A) Reagents:

(a) Stock solution of PO4 in distilled water: 50ug/mL.
(b) Potassium antimony tartrate: 1.3715 g in 50mL distilled water (Soln. A)
(c) Ammonium molybdate: 10g in 250mL (Soln. B)
(d) Ascorbic acid: 1.76g in 100 mL distilled water. Prepare fresh daily. (Soln. C)
(d) 5N sulphuric acid: 35mL conc. acid to 250ml with distilled water. (Soln. D)

(e) Mixed reagent: (Mix fresh daily, and in exact volume ratios):

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

(B) Equipment requirements:

(a) Peristaltic pump, variable speed, with pump tubing 2.75mm i.d.
(b) Sample injector, 60cm x 0.8mm i.d. and 1mL polyethylene syringe with luer tip.
(c) Tubing Connectors and 30cm PTFE tubing 0.8mm i.d. between injector and detector.
(d) Single-beam spectrometer, fitted with a 1cm path length flow cell,and a strip chart recorder.

3. Standard and Sample Preparation:

(A) PO4 standards: (25mL each):

(a) Use the 50ug/mL stock solution provided to prepare the following phosphate standard solutions, using distilled water as diluent: 0.25, 0.5, 1.0, 2.0. 4.0ug/mL.

(b) Pipette 5mL from each standard solution into clean dry test tubes, and add 0.8mL of mixed reagent. Stopper, mix well and allow to stand for 1h. for color to fully develop.
Prepare a reagent blank simultaneously.

(B) Sample solutions:

(a) Use the sample extracts prepared for the previous FIA experiment.

(b) Dilute each sample extract as recommended (see Demonstrator) with distilled water, in 25mL volumetric flasks. Prepare a sample blank simultaneously.

(c) Pipette 5mL from each sample and blank solution into clean dry test tubes, and add 0.8mL of mixed reagent. Stopper, mix well and allow to stand for 1h. for color to fully develop.

d) Record the time taken for this phase of the experiment.

4. Flow Injection analysis:

(a) Set detector wavelength at 825nm for phosphate determination and flow rate of distilled water of 5mL/min.

(a) Inject standard blank and then PO4 standard solutions in triplicate into FIA manifold and record peak heights.

(b) Inject sample blank and then sample solutions in triplicate and record signals.

(d) Record total analysis times for the measurement phase for all the standards, blanks and samples.

5. Calculations:

(a) Calculate the total time taken for the preparation and measurement of phosphate in the sample extracts.

(b) Plot peak heights vs phosphate concentrations of standards, or determine the linear regression equation and correlation coefficient for the calibration standards.

(c) Determine the sample solution PO4 concentrations, then convert to % P2O5 units.

6. Exercises:

Compare the previous FIA procedure used, and the procedure used in this experiment, in terms of:

(a) The values of % P2O5 in the sample.

(b) Time taken for the determination of phosphate in standards, samples and blanks.

(c) The reagent consumption for 10 samples, each analyzed in triplicate.

(d) List 3 other possible benefits of the procedure used in this experiment.

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