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Forensic Chemistry - Ion Selective Electrode. The objective of experiment is to determine the chloride and nitrate (toxic anions) in samples from the calibration curve

Extracts from this document...

Introduction

SINGAPORE POLYTECHNIC SCHOOL OF CHEMICAL LIFE SCIENCES Diploma of Applied Chemistry with Pharmaceutical Science Experiment No. 3 Metal Toxicity Module Code (CP 4098) Desmond Seah (P1006812) Year of Study: Year 2 DACP/FT Lecturer Jeffrey Whey AY 11/12 ________________ Content Page Synopsis Page 1. Introduction 1-2 2. Theory 2-12 3. Procedure 12-13 4. Results and calculation 13-14 5. Discussion 14-24 6. Conclusion 24 References List of Illustration S/N Figures and Tables Page Number 1 Types of electrode with diagram 7 2 Internal structure of Ion selective electrode 8 3 Possible interferences with reference solutions stated 9 4 Calibration curve of Ion selective electrode 12 Synopsis The objective of experiment is to determine the chloride and nitrate (toxic anions) in samples from the calibration curve. Ion-selective electrodes respond selectivity towards one (or several species) in the presence of others and are used in a wide variety of applications from agricultural, pollution monitoring to dairy products. The potential difference E for an electrode reaction can be given by Nernst equation: [ E = E0 ? RT/nF ln ( 1/ aMn+) ] The ionic activity and concentration are practically identical but in solutions containing many ions, activity and concentration may differ. It is possible to 'fix' the solution so that activity and concentration are equal. This can be done by adding a constant concentration of an inert electrolyte (T.I.S.A.B) to the solutions under test. In order to produce the calibration curve, we have prepared the stock solution for both chloride and nitrate and diluted them accordingly for 3 other concentrations and plotted the potential reading against the concentration in log paper. Hence, we have obtained the concentration of the samples by reading from the calibration curve. Afterwards, we have conducted a t-test and it shows that the unknown correspond to sample D. We have also discussed on some corrective actions that ought to be taken and establishing standard addition method to improve the accuracy of the analysis. ...read more.

Middle

Nevertheless, it must be noted that if the samples to be measured are likely to have a total ionic strength of less than about 0.01M for monovalent ions (0.001M for divalent ions) then the activity effect should be insignificant and it may not be necessary to add ISAB. However, it must be noted that ISAB may be useful when using a double junction reference electrode with a non-equi-transferrent outer filling solution, in order to compensate for drift in the liquid junction potential, and in general most ISE systems give a stable reading more quickly in high ionic strength solutions. The slope of the calibration graph is the mV response per decade of concentration change. This is typically around 54 mV/decade for monovalent ions and 27 for divalent ions and will have a negative value for negative ions - i.e. a higher concentration means more negative ions in solution and therefore a lower voltage. a) Linear Range. The linear range of the electrode is defined as that part of the calibration curve through which a linear regression would demonstrate that the data points do not deviate from linearity by more than 2 mV. For many electrodes this range can extend from about 0.1 Molar down to 10-6 or even 10-7 Molar. b) Total Measuring Range. The total measuring range includes the linear part of the graph as shown below together with a lower curved portion where the response to varying concentration becomes progressively less as the concentration reduces. Samples can be measured in this lower range but it must be noted that more closely spaced calibration points are required in order to define the curve accurately and the percentage error per mV on the calculated concentration will be progressively higher as the slope reduces. c) Limit of Detection. For monovalent ions, the IUPAC definition is: that concentration at which the measured potential differs from that predicted by the linear regression by more than 18 mV. ...read more.

Conclusion

or worn electrodes which may not be completely linear over their whole range, as long as the slope is stable and reproducible over the limited range of the samples. The main disadvantage of these methods is that it is necessary to mix together accurately measured volumes of standard and sample, and they involve more complex calculations than simple direct potentiometry. This makes them a more skilled and time-consuming procedure and they have traditionally been less popular with most ISE users. It must be noted, however, that these analyses can now be made much more easily and quickly using software to calculate a suitable volume and concentration for the standard, measure the electrode potentials, and calculate the results - and by using automatic syringe pipettes to dispense the measured volumes. Further slight disadvantages are that this calculations are only valid within the linear range of the electrode and hence these methods cannot be used for low-concentration samples, near to the detection limit, and the approximate concentration of the sample must be known before commencing the analysis in order to choose an appropriate standard concentration and suitable volumes for the two solutions. Nevertheless, if the sample concentration is completely unknown, it can easily be determined by making a quick Direct Potentiometry measurement first. This can be done using an old calibration graph - but the most accurate results can only be obtained if the electrodes are calibrated using two standards which span the expected range of the samples immediately prior to analysis. 7. Conclusion Conclusively, we can state that the calibration of the ISE was successful despite having low slope value and experiencing severe drifting effects. In the discussion section I have suggested several corrective actions that should be taken to resolve the low slope value and drifting effects. Furthermore, I have established a t-test and determine the source of the unknown at 99% confidence. The results of the t-test for nitrate and chloride have unanimously concluded that the unknown corresponded only to sample D. 8. ...read more.

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