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C30F Analytical Chemistry Experiments
FIA Batch analysis

Determination of phosphate by rapid flow injection analysis

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Flow injection Batch analysis


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Introduction

The determination of phosphate in samples can be done manually, by ion chromatography, flow injection analysis, or other instrumental techniques.

In manual procedures, the samples are prepared in defined volumes, usually 25ml, with the same volumes of each reagent being added to each solution. A period of at least 2 hours 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 mins.

In normal flow injection analysis, the sample or standard solution is injected into the reaction manifold, in which the reagents are continuously pumped.

In this experiment, phosphate will be determined by batch development of the color, and the color intensities of solutions measured by rapid flow injection through a flowcell in a spectrophotometer.

Reagents

(1) Stock solution of PO4 in distilled water: 50ug/ml.

(A) Potassium antimony tartrate: 1.3715 g in 50ml distilled water (Soln. A)
(B) Ammonium molybdate: 10g in 250ml (Soln. B)
(C) Ascorbic acid: 1.76g in 100 ml distilled water. Prepare fresh daily. (Soln. C)
(D) 5N sulphuric acid: 35ml to 250ml with distilled water (Soln. D)

(2)Mixed reagent: (Mix fresh daily)
50ml D + 5ml A + 15ml B + 30ml C

Standard and Sample Preparation

(a) PO4 standards: (mot more than 25ml each)

(1) Use the 50 ug/ml stock solution provided to prepare the following phosphate standard solutions, using distilled water as diluant:
0.5 1.0, 2.0. 4.0, and 6.0 ug/ml.

(2) Pipette 5ml from each standard solution into clean dry test tubes, and add 0.8ml of mixed reagent.Stopper and allow to stand for about 1 hr. for color to fully develop.

(b)Sample solutions:

(1) Dissolve triplicate aliquots (0.100g) of salt provided in 100 ml volume distilled water
(2) Dilute 0.5 ml of each solution to 50 ml with distilled water (100x dilution)
(3) Pipette 5ml of each solution into clean dry test tubes, and add 0.8ml mixed reagent
(4) Stopper and allow color to develop

(c) Blanks:

Prepare a standard blank using 5ml distilled water with 0.8ml mixed reagent, and prepare a sample blank using 5ml sample and 0.8ml mixed reagent
Allow to stand alongside the other solutions.

Record the time taken for this phase of the experiment.

Flow Injection analysis

(1) Use pump tubing about 2.75 mm id. Set flow of distilled water carrier to 5ml / min.
(2) Injection loop: attach 60cm x 0.8mm id PTFE tubing to loop injector.
(3) Manifold connection: approx. 30cm length of 0.8mm id PTFE tubing between injector and flow cell.

Wavelength of measurement set at 825nm for phosphate determination

(a) Inject standard blank and then PO4 solutions into FIA manifold and record peak heights and the maximal absorbance readings for each solution

Inject sample blank and then sample solutions

Record total analysis times for all analyses

(b) Plot peak heights or absorbance values (after blank correction) vs PO4 standard concentrations

(c) Determine the sample solution PO4 concentrations as ug/ml, then calculate the PO4% in the salt provided

Use the salt formula weight (Na2HPO4 = 141.96) to calculate the percentage purity of the salt provided

Provide answers to the following:

(1) Explain the need for standard and sample blanks

(2) Explain the rationale of this experiment, compared to the FIA procedure used previously for phosphate determination

(3) What possible advantages does this experiment possess over the previous FIA procedure used, in time and reagent consumption?



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