Usage of standard addition method
The standard addition method is often used in quantitative analytical chemistry. In this method, a standard is added directly to the aliquot of the analyte sample. The concentration of the analyte sample is kept constant throughout the process. Added standard concentration is varied to determine an unknown concentration of an analyte by using Beer-Lambert’s law.
Eppawala rock phosphate (ERP) is mined from Eppawala in Sri Lanka. In a dilute orthophosphate solution, ammonium molybdate reacts under acid conditions to form the heteropoly acid, molybdophosphoric acid. In the presence of vanadium, yellow vanadomolybdo-phosphoric acid is formed. This method is called the vanadomolybdo phosphoric acid method (VM method). The absorbance wavelength of the complex is in-between 460 – 480 nm. (Reference: Jeffrey G. H, Bassett J. Mendhan J.. Denney R. C.; “Vogel’s Textbook of Quantitative Chemical Analysis”, Fifth Edition (ELBS) Longman, pages 730, ISBN 0582251672).
Determination of the maximum wavelength (𝜆𝑚𝑎𝑥) for the citric-soluble phosphate
Determination of the citric-soluble phosphate.
| Trail | Added volume of Ammonium vanadate (cm3) | Added volume of Ammonium molybdate (cm3) | Added volume of Eppawala Apatite (cm3) | Added volume of Standard phosphate sol. (cm3) | Absorbance at 420 nm wavelength |
| 01 | 5.00 | 5.00 | 5.00 | 1.00 | 0.309 |
| 02 | 5.00 | 5.00 | 5.00 | 2.50 | 0.344 |
| 03 | 5.00 | 5.00 | 5.00 | 4.00 | 0.360 |
| 04 | 5.00 | 5.00 | 5.00 | 5.50 | 0.395 |
| 05 | 5.00 | 5.00 | 5.00 | 7.00 | 0.435 |
| 06 | 5.00 | 5.00 | 5.00 | 8.50 | 0.478 |
We can extrapolate the graph until y=0
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