Experiment 8 Potentiometric Determination of Ionization Constant of a Weak AcidBase.doc

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Experiment No. 8: Potentiometric Determination of Ionization Constant of a Weak Acid/BaseI.AbstractThe measurement of hydronium concentration in a solution using a pH meter exploits the principles ofpotentiometry in which a glass electrode is used as the indicator electrode and calomel electrode as reference.In titrations, the accurate measurements of pH is crucial for plotting the titration curves because there is anample amount of information that can be derived from the curve; one of which is the dissociation constant of theanalyte. At half equivalence point, the negative logarithm of the hydronium concentration is equal to the negativelogarithm of the analyte’s dissociation constant. Knowing this constant would be one step towards knowing theidentity of the unknown.The results in this experiment showed that the average pKa value of the unknown acidis 2.607.II. Keywordspotentiometry, ionization constant, glass membrane electrode, half-equivalence point, pH meterIII. IntroductionPotentiometric methods are analyticalmethods that are based on the potential ofelectrochemical cells.These techniques haveseveral different uses such as determining theend point during titration.Using ion-selectivemembrane electrodes, the potential isdetermined to measure the ion concentration ofsolutions.Potentiometric measurements areactually made every day.Manufacturers obtainthe pH of consumer products; industrial runoff ismonitored to determine the pH andconcentration pollutants and oceanographersmeasure the carbon dioxide levels in the water.Potentiometry is also used to determineequilibrium constants such as Ksp, Kaand Kb.The Kaand Kbare called ionizationconstants.These values are indications of acidand base strength respectively.A large valuefor the ionization constant would mean astronger acid/base.The ionization constant isunique to a certain acid/base.Determining theionization constant through potentiometricmethods would help in determining theunknown compound.There are two ways to determine theionization constant of a compound.Both waysmake use of a pH meter.The first is plotting thevolume of the titrant versus the pH.Theequivalence point is obtained and the pH of thehalf-equivalence point is determined.This pHvalue is equal to the ionization constant.Thesecond method involves a “half volume”method.A specific electrode used in pH meters isa glass membrane electrode.It consists of asilicate lattice and is very sensitive and specificto H+ions.It is also responsive to othermonovalent cations.IV. ExperimentalUsing litmus paper, the unknown wasclassified whether it was an acid or a base.A0.1 M titrant was prepared for the titration of theunknown.

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