How to determine metal ion concentration using photometric method
EDTA Chelant Demand Photometric Method
Theory of Test
When a complexometric dye is introduced to an alkaline solution of magnesium chelate, a bluish- purple color results. The absorbance of this neutral solution is arbitrarily considered to be a zero chelant demand. Hardness and heavy metal cations may be present in feedwater react with the system to produce a transition of color from bluish purple to red. This color change is proportional to the concentration of precipitating cations present. The corresponding changes in absorbance are in accordance with Beer’s Law and are converted into parts per million concentrations of chelant demand by referring to a standard curve.
Apparatus Required
Filter photometer complete with assorted laboratory plasticware.
Chemicals Required
Chelant Demand Reagent P1
Chelant Demand Reagent P2
Chelant Demand Reagent P3
Chelant Demand Reagent P4
Calcium Standard Solution, 50 ppm as CaCO3
Procedure for Test
(use plasticware only. Do not use glass except for optical cell)
Transfer 100 ml of sample to a 250-ml beaker by means of a plastic graduate. Pipet 1.0 ml of chelant demand reagent P1 into the beaker. Stir the solution with a plastic stirring rod and wait one minute. Pipet 1.0 ml of chelant reagent P2. Again stir and wait one minute. Pipet 1.0 ml of chelant reagent P3. Stir solution and wait one minute. (Solution should be at room temperature). Rinse the optical cell twice with chelant blank reagent. Shake the cell unit it is free of excess water. Carefully fill a 20x20 mm optical cell with a portion of this prepared solution and use it to set a photometer equipped with a 640 nm filter to zero absorbance.
Return the contents of the cell to the beaker containing the prepared solution.
Pipet 5.0 ml of chelant demand reagent P4 into the beaker and stir the solution. Fill the optical cell with this solution and measure the absorbance on the photometer.
Calculations of Result
The chelant demand concentration in parts per million is obtained by reference to the prepared chelant demand curve. Note; the range of the test is 0 to 8 ppm as chelant demand. One part per million of chelant demand is equivalent to one-tenth part per million (0.1) of hardness as calcium carbonate.
Limitations of Test
The ions normally present in boiler feedwaters suitable for chelant treatment do not interfere with this test.
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