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Date performed: Sept. 15, 2010
POTENTIOMETRIC DETERMINATION OF THE PURITY AND DISSOCIATION CONSTANT OF POTASSIUM HYDROGEN PHTHATLATE
COLLEGE OF HOME ECONOMICS, BS. Food technology
UNIVERSITY OF THE PHILIPPINES, Diliman, Quezon city
Date received: Sept. 22, 2010
RESULTS AND DISCUSSION
Potentiometry is the use of electrodes to measure voltages that provide chemical information. Potentiometric titrations are generally used to characterize a newly synthesized orhanic acid. The equivalent weight and ionization constant or constants are obtained from the plots of:
1.pH vs volume of the titrant
2..ΔpH / ΔV against average volume of the titrant V’ and
3.Δ2pH / ΔV2 against average of the average volume of the titrant V’’.
In this experiment, the standard used is KHP (potassium Hydrogen phthalate) and it is titrated with NaOH and by the help of a potentiometer, the pH during the different significant regions are determined. The first region is the pre-equivalence point. It is where the dominant species present is the standard used (KHP) which is being converted into HP- and because of too little NaOH added, the increase in the pH is just small. The second significant region is the equivalence point. It is when the amount of titrant added is stoichiometrically equal to the amount of analyte present in the sample: the smallest amount of titrant that is sufficient to fully neutralize or react with the analyte. During this point, the pH has increased rapidly. It is clearly seen (see data sheet) that from pH 6.70 in jumps to pH 10. 43. This is because HP- is a weak acid and the titrant is a strong base thus making the pH at the equivalence point basic (due to the hydrolysis of phthalate ion). And the third and last significant region is the post – equivalence point. It is when the titrant is now the dominating species. In this region, the pH is now basic due to the basic characteristic of the titrant used.
A. Standardization of Sodium...
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