电位滴定实验挂图-修改

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、确定电位滴定终点的方法：三、确定电位滴定终点

8、的方法：电位滴定数据电位滴定数据: :7三、确定滴定终点的方法三、确定滴定终点的方法1E V曲线法曲线法 滴定终点滴定终点：曲线上转折点（斜率最大处）对应V 特点特点：应用方便； 但要求计量点处电位突跃明显。 2E/V 曲线法（一阶导数法）曲线法（一阶导数法） 曲线曲线：具一个极大值的一级微商曲线 滴定终点滴定终点：尖峰处（E/V极大值）所对应 特点特点：在计量点处变化较大，因而滴定准确； 但数据处理及作图麻烦。 32E/V2 V曲线法（二阶导数法）曲线法（二阶导数法） 曲线曲线：具二个极大值的二级微商曲线 滴定终点滴定终点：2E/V2由极大正值到极大负值与 纵坐标零线相交处对应的V。 特点特点：准确度高，可用内插法求滴定终点VPVPVP8三、确定电位滴定终点的方法：三、确定电位滴定终点的方法：电位滴定数据处理电位滴定数据处理: :E-V曲线 曲线VVE/2E/ V2-V曲线92E/V2-V曲线例如：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：利用上述三种作图法确定滴定终点，主要是根据化学利