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Experimental Methods   Det 3
 

Experimental Methods Det 3

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    Experimental Methods   Det 3 Experimental Methods Det 3 Document Transcript

    • Experimental Methods DAY 1 An approximately 0.1M solution of NaOH was prepared by weighing out 2g of NaOH and dissolving it in 500ml of water. The solution was transferred to a storage bottle. ∗ Potentiometric Titration The pH electrode was calibrated as detailed in the pH meter instruction manual using buffers of pH 4, 7 and 10. The acid mixture – approximately 0.05M HCl and 0.1M CH3COOH – provided was pipetted (25ml) and placed in a clean, dry 150ml beaker. The solution was stirred with a stirring rod and then the pH electrode lowered into the solution. Three (3) drops of screened methylorange was added and the pH and initial burette reading recorded. From the burette, approximately 1ml of NaOH was added and the burette reading recorded. The mixture was stirred and the pH of the solution recorded when the pH meter stabilised. The titration was continued in this way - adding approximately 1ml aliquots of NaOH, noting the burette readings and pH values after each addition – until just passing the first equivalence point by about 2ml. Three (3) drops of phenolphthalein indicator was added and the titration continued to about 5ml beyond the second equivalence point. The titration data was plotted on a pH vs. titrant volume graph and the end points determined. ∗ Standardisation of NaOH Three (3) 0.3g portions of dried AR potassium hydrogen phthalate was weighed out and placed in three 250ml conical flasks. These were dissolved in 50ml of distilled water and three (3) drops of phenolphthalein indicator added then the whole was titrated with NaOH to a light pink colour. DAY 2 The pH electrode was recalibrated as done in the previous weeks and the pH found for buffers of pH 4, 7 and 10. ∗ Potentiometric titration The apparatus was set up as before and the same amount of components added to the solution prior to the equivalence point of the titration was added. Upon approaching the 3
    • first equivalence point or at pH 2.0, the addition of NaOH was switched to a drop-wise addition. The pH and burette reading was recorded after each drop. This was done up to about 2ml beyond the equivalence point or pH 3.8. The addition of NaOH was then returned to 1ml aliquots until approaching the second equivalence point at pH 5.8. The drop-wise addition was done up to a pH of 12.22 and following this five additional 1ml portions were added. ∗ Standardisation of NaOH The standardisation of NaOH was carried out in a similar manner as in the previous week and the necessary calculations done. 4