Lect. 6 method of quantitative analysis
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1) The document discusses three methods for quantitative analysis in polarography: the calibration curve method, standard addition method, and pilot ion/internal standard method. 2) The calibration curve method involves creating a curve by measuring the current-voltage curves and wave heights of standard solutions and then determining the concentration of an unknown from that curve. 3) The standard addition method measures the wave heights of an unknown solution before and after adding a known amount of standard solution to calculate the unknown concentration using the Ilkovic equation. 4) The pilot ion method measures the relative wave heights of the unknown ion and a known "pilot ion", allowing the unknown concentration to be calculated using a ratio equation involving the two wave heights
Lect. 6 method of quantitative analysis
- 1. Dr. Y. S. THAKARE M.Sc. (CHE) Ph D, NET, SET Assistant Professor in Chemistry, Shri Shivaji Science College, Amravati Email: yogitathakare_2007@rediffmail.com SEM-III PAPER-X ANALYTICAL CHEMISTRY –I THERMAL AND ELECTROANALYTICAL METHOD UNIT- IV Method of Quantitative Analysis in polarography 16-March-21 1
- 2. OR Pilot Ion method OR Direct Comparison OR Calibration Curve method 16-March-21
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- 4. Quantitative Analysis: Ilkovic Equation 16-March-21
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- 7. (1) Calibration Curve Method OR Direct Comparison OR Standard Curve method In this method, the standard solutions of the test ion are prepared under the same conditions having the same composition of the supporting electrolyte and the suppressor. The current-voltage curves are recorded for each solution. The heights of the wave are measured and plotted as a function of the concentration. Current-voltage curve of the unknown solution is now recorded and wave height measured exactly in the same manner as the standard solution. 'The concentration is measured from the graph. Quantitative Analysis : What To Do? 16-March-21
- 8. OR Direct Comparison OR Standard Curve method Quantitative Analysis : What To Do? 16-March-21
- 9. (2) Standard Addition Method. This method is based on the fact that the wave height is a linear function of the concentration in the range of concentration employed. Here a known quantity of the standard is added to the unknown solution and a polarogram is taken before and after the addition of the standard containing the same composition of the supporting electrolyte. The magnitude of the heights of two waves and the increase in diffusion current is used to compute the concentration of the unknown as follows: For unknown solution, i1 = KCx, where i1 is the observed diffusion current (= wave height) of the unknown solution of volume Vcm and concentration Cx. After adding v cm3 of a standard solution of concentration Cs, to V cm3 of unknown, the diffusion current (= wave height) i2 is given by Ilkovic equation, 16-March-21
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- 12. Good results are obtained when the wave height is doubled by the addition of unknown amount of standard solution. The advantage of this method is that the unknown solution and known standard are measured under identical conditions at constant temperature. During quantitative analysis one must take into account the residual current. If two waves are close together, another supporting electrolyte can be used. For example, Cd2+ and Pb2+ cannot be distinguished in NaOH but can be differentiated in KCN. 16-March-21
- 13. 3) Pilot Ion Method OR Internal Standard method: This method also known as internal standard method is based on the fact that the relative wave heights or diffusion currents of two electroactive substances in the same supporting electrolyte supporting electrolyte are constant for the same concentrations and are independent are independent of the characteristics of the cápillary electrode and of the small of the small temperature differences. In this method the wave heights of the heights of the standard solution of the unknown (ld)x, and pilot or reference ion (Id)s, (whose half-wave potential should differ by at least 0.2 V from the unknown) are measured separately. Now the relative wave-heights with the unknown ion and the known amount of the pilot ion in the unknown solution are measured. The concentration of the unknown substance can be calculated from the following relation, 𝐶𝑥 = 𝑖𝑥(𝐼𝑑)𝑠 𝑖𝑠(𝐼𝑑)𝑥 × 𝐶𝑠 𝐶𝑠 and 𝐶𝑥 are the concentrations of the known and unknown substances respectively. Once the internal standard ratio (ld)s / (ld)x has been determined, a series of test solutions can be analysed with only a single standard solution. With the change in concentration of supporting electrolyte, the ratio should be determined again. The method has limited applications because a very few ions act as pilot or reference ion. 16-March-21