ETERMINATION OF PURITY AND ACID DISSOCIATION CONSTANT DETERMINATION OF POTASSIUM HYDROGEN PHTHALATE BY POTENTIOMETRIC TITRATION D.C T AÑA DEPARTMENT OF C HEMICAL HEMICAL E NGINEERING NGINEERING , C OLLEGE OLLEGE OF E NGINEERING NGINEERING NIVERSITY OF THE PHILIPPINES ITY HILIPPINE S U NIVERSITY , DILIMAN , QUEZON C ITY , P HILIPPINE DATE PERFORMED: NOVEMBER 20, 2018 INSTRUCTOR’S NAME: GALLEPOSO, E.C.
1. What are the significant regions in the titration curve? Relate the characteristics of each region to the pH results obtained. Three significant regions of the titration curve are the pre-equivalence region, post-equivalence region, and the equivalence point. As the experiment involves a neutralization titration, the graph is a curve that begins from low to high pH. The preequivalence region pertains to when the analyte is still in excess against the titrant and is from the beginning of the titration to the equivalence point. At this point, the curve has an upward concavity and this is due to the buffer formed by the acid and the base as it resists large changes in the pH. At halfequivalence point, the amount of titrant added is half the amount of analyte thus, pH of the solution is equal to the pKa of the analyte. The equivalence point occurs when the analyte and titrant added are mixed in stoichiometric amounts. This point is illustrated as the point of inflection of the curve wherein a large change in pH is observed. A large change in pH is due to the anion of the analyte being hydrolyzed. At the post-equivalence region, the amount of titrant is added in excess compared to the analyte. The resulting amount of titrant determines the pH of the solution and causes an increase in pH1,2.
2. Why is continuous stirring important in potentiometric titration? Continuous stirring was imposed throughout the whole experiment as it enabled even distribution of titrant and analyte throughout the solution, ensuring that it was completely homogenous. Apart from being efficient, this was done to prevent analyzing only a certain area of the solution wherein there would be a larger amount of analyte or titrant as this would cause errors in reading3.
3. Why should the increments of addition be narrowed down as the titration approaches the equivalence point? Increments of addition were narrowed down as one approached the equivalence point as the pH of the solution increased drastically with respect to the amount of titrant added. The increments of the titrant added to the solution were narrowed down to allow the plotting of the sharp region of the titration curve and to effectively determine the equivalence point better.
4. Why is potentiometry a suitable method in determining the pK a of weak acids? Potentiometry is a method which involves the measurement of pH of a solution
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after a certain volume of titrant is added. As a buffer system is incorporated within the preequivalence point, the Henderson-Hasselbach equation is relevant to use. Using this equation, pKa of the weak acid can be computed with the pH of the solution, concentration of the weak acid, and the conjugate base. Apart from this, the pKa of the weak acid is equal to the pH at the half equivalence point. To determine the half equivalence point, the equivalence point must first be determined graphically and could then be computed 1,4.
5. What are the possible sources of errors in the experiment? Explain how these errors affect on the calculated parameters. A possible source of error could be attributed to mis-calibration of the pH meter due to inaccurate standard buffer solutions which would then result to displaying wrong pH readings. Alkaline error would result if the glass electrode would respond to both the hydrogen and alkali metal ions at pH readings greater than 9. The pH value would be lower than the actual value of the solution. Apart from parallax errors in titration, variation in junction potential could also be present due to the differences between the standard and sample solutions 2,4.
REFERENCES [1] Skoog, D.A.; West, D.M.; Holler, F.J.; Crouch, S.R. Fundamentals of Analytical Chemistry, 9th ed.; Brooks/Cole: USA, 2014. [2] Harris, D. C. Quantitative Chemical Analysis, 8th ed.; W. H. Freeman and Company: New York, 2010.
[4] Institute of Chemistry, University of the Philippines Diliman. Analytical Chemistry Laboratory Manual ; 2016; pp 8-12. [5] Purdue University. Analytical Crib. 2012. https://www.chem.purdue.edu/docs/ex ams/2012/October_20_2012_crib.pdf (accessed November 22, 2016).
