Potentiometric Determination of the Purity and Dissociation Constant of Potassium Hydrogen Phthalate Chem 28

POTENTIOMETRIC DETERMINATION OF THE PURITY AND DISSOCIATION CONSTANT OF POTASSIUM POTASSIUM HYDROGEN PHTHALATE PHTHA LATE

Submitted By: Frances Abegail G. Quezon Potentiometry is a method that measures the electric potential difference in a cell. [! "he ob#ecti$e of the e%periment &as to determine the purity and dissociation constant of  '(P. "he type of electrode used in the e%periment is a glass membrane electrode. A glass membrane electrode is an ion selecti$e electrode) or an electrode that is for speci*ic ions only.[+! "he titrant used in this e%periment is ,a-( &hile the analyte and standard used is '(P. "he reaction for this system is: ,a-(  '(P  ,a'P  (/- 01 For the standa standardi rdizat zation ion of ,a-() ,a-() norma normall acid2b acid2base ase titra titratio tion n &as &as done) done) since since potentiometric titration is time2consuming. A titr titrati ation on cur$e cur$e has + signi* signi*ica icant nt regio regions. ns. "hese "hese are are pre2e pre2e3u 3ui$ i$ale alence nce point) point) e3ui$alence e3ui$alence point) and post2e3ui$alence point. Pre2e3ui$alence Pre2e3ui$alence point is the region &hen the titrant &as #ust added. 4n acid2base reactions such as in the e%periment) this region is a slo&ly increasing cur$e cur$e because of the presence of a buffer. buffer. 53ui$alence 53ui$alence point is &hen the analyte and titrant &ere e3ual in the number of moles. "his is the steep part of the graph in a p( $s. 6 graph. "he post2e3ui$alence point is the last part of the graph. At this point) all of  the analyte are con$erted to another form. [+!

Figure . p( $s. 6 Based from the titration cur$e abo$e) at the pre2e3ui$alence pre2e3ui$alence point) there &as a slo& but steady increase in the p(. At the e3ui$alence point) p( changes drastically o$er a small $olume inter$al. At the post e3ui$alence point) the p( change returns to slo& and steady again. Belo& are the graphs for p( $s 67 and /p( $s 68.

Figure /. p( $s 67

Figure +. /p( $s 68 "he e3ui$alence point from plot  is a middle point on the steep cur$e. -n plot /) it  is the pea9) and on plot + it is the %2intercept of the graph. "he $olume of ,a-( at  e3ui$alence point for trial  is the same for all plots. -n the second trial) $olumes ha$e a difference of .; per plot. And on trial +) $olumes from plot  and plot / are the same but  plot + has a difference of .. Potentiometric titration does not ma9e use of any indicator) instead) it adds titrant  $olume in increments of m< or less &hile the p( is continuously monitored. A large #ump in p( indicated the e3ui$alence point. As the solution reaches its e3ui$alence point) the increments of titrant addition should be narro&ed do&n in order to ha$e better accuracy in determining &hen the p( #ump occurs) and conse3uently) better accuracy in determining the e3ui$alence point. =sage of a stirrer or a magnetic stir bar is important in order to mi% the reactants. "he bea9er cannot be s&irled #ust li9e in normal titrations because of the p( electrode dipped in the solution. "he electrode is fragile and might brea9 &ith manual stirring. Potentiometric titration allo&s the constant monitoring of p( changes) therefore the $olume at e3ui$alence point could be determined more accurately than in normal titration. "here is also no ris9 of o$ertitration that could affect your results. Also) the data obtained from this type of titration could be used to ma9e a titration cur$e for the system. (o&e$er) potentiometric titration is tedious and time2consuming. 4t is also dependent on the p( meter) and a faulty meter &ould cause the results to be incorrect. "here is also the added danger of the spin bar hitting the electrode of the p( meter as it spins inside the bea9er. 5rrors may arise from instrumental errors such as a faulty p( meter. "his is an indeterminate error and may cause inconsistencies in the results. After processing the data obtained from the e%periment) the a$erage percent purity of the sample &as determined to be +>.?@. 4t has an S of /ppt meaning that data &ere imprecise. A$erage p'a is ;.>. "his has a ;/@ error &ith the theoretical $alue of ;.; [/!. "he e%perimental 'a is C.?% 2D. p'a $alues ha$e an S of Cppt meaning that the data are not only inaccurate but also imprecise. "he e%periment and the laboratory techni3ues in$ol$ed &ere successfully e%ecuted) and the data) processed properly. Since the ob#ecti$e of  determining @purity and 'a is achie$ed) the e%periment could be considered successful. REFERENCES

[!(arris) . Quantitative Chemical Analysis ;th ed. E.(. Freeman and ompany: ,e& or9) /. [/!4nstitute of hemistry. Quantitative Inorganic Analysis Laboratory Manual  [+! S9oog) .) et al. Fundamentals of Analytical Chemistry  ?th ed. "homson .