1、1Exp. Potentiometric Titration of Phosphoric AcidObjectivesEquipments and chemicals A pH meter with a compound electrode made of a glass and a calomel electrode. Magnetic stirrer with a Teflon- coated stirring bar. The titration of acid with NaOH is followed by measuring pH of the solution after eac

2、h addition of the titrant. The calculation of the ionization constants from the potentiometric titration curve is illustrated.uEquipmentsuChemicals Standard NaOH (0.1 mol/L); Phosphoric acid (approximately 0.1 mol/L); Potassium hydrogen phthalate solution (0.05mol/L), pH 4.00 2Principles1. Determina

3、tion of pKa H3PO4+ H2O = H3O+ + H2PO4- H2PO4-+ H2O = H3O+ + HPO42- HPO42-+ H2O = H3O+ + PO43- 43421 POHHPOHKapHpKHKPOHPOHVVWhenaaeqNaOH114342121 32242 POHHPOHKapHpKHKPOHPOHVVVWhenaaeqeqNaOH22422421)(21 32. Determination of end point The curve of potentimetric titration pH/V 2pH/V2 41.The standardiza

4、tion of the pH meterProcedures The glass electrode is activated by soaking it in deionized water for 24hr Wash the electrode thoroughly Calibrate the pH meter using a potassium hydrogen phthalate solution (0.05mol/L), pH 4.00 Wash the electrode thoroughly .2.The potentiometric titration of phosphori

5、c acid pipette a 10.00ml portion of 0.1mol/L phosphoric acid into a 100ml flask dilute with 20ml distilled water Place the electrode and a magnetic stirring bar into the beaker Add 0.1mol/L NaOH solution from burette and take readings after each addition. 5Noticesu1. After each addition of NaOH, wai

6、t a few seconds till the reading of pH is stable, and then record the data.u2. To add accurate volumes of NaOH ( 0.1 ml ) near the stoichiometric point, use a glass-stirring rod for help. u3. Choose a proper speed of stirring to avoid the splash of titrate.Calculations1. Calculate 2pH/V2 vs. millili

7、ters of NaOH at the two equivalence points and determine the V1 and V22. Plot pH vs. milliliters of NaOH. Determine pKa1 and pKa2 from the titration curve according to milliliters of NaOH at the two equivalence points.3. Determine the concentration of phosphoric acid by V1. 6三、確定電位滴定終點的方法：三、確定電位滴定終點

8、的方法：電位滴定數據電位滴定數據: :7三、確定滴定終點的方法三、確定滴定終點的方法1E V曲線法曲線法 滴定終點滴定終點：曲線上轉折點（斜率最大處）對應V 特點特點：應用方便； 但要求計量點處電位突躍明顯。 2E/V 曲線法（一階導數法）曲線法（一階導數法） 曲線曲線：具一個極大值的一級微商曲線 滴定終點滴定終點：尖峰處（E/V極大值）所對應 特點特點：在計量點處變化較大，因而滴定準確； 但數據處理及作圖麻煩。 32E/V2 V曲線法（二階導數法）曲線法（二階導數法） 曲線曲線：具二個極大值的二級微商曲線 滴定終點滴定終點：2E/V2由極大正值到極大負值與 縱坐標零線相交處對應的V。 特點特點：準確度高，可用內插法求滴定終點VPVPVP8三、確定電位滴定終點的方法：三、確定電位滴定終點的方法：電位滴定數據處理電位滴定數據處理: :E-V曲線 曲線VVE/2E/ V2-V曲線92E/V2-V曲線例如：例如：V1=11.30mL 2E/V2=5600 V2=11.35mL 2E/V2=-400mLmLxx35.11347.1156000560040030.1130.1135.11解得：注意：注意：利用上述三種作圖法確定滴定終點，主要是根據化學利