电位滴定实验挂图-修改.ppt

Exp. Potentiometric Titration of Phosphoric Acid 三、确定滴定终点的方法 1．E ~V曲线法 滴定终点：曲线上转折点（斜率最大处）对应V 特点：应用方便； 但要求计量点处电位突跃明显。 2．⊿E/⊿V~ ?曲线法（一阶导数法） 曲线：具一个极大值的一级微商曲线 滴定终点：尖峰处（⊿E/⊿V极大值）所对应? 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。 3．⊿2E/⊿V2~ V曲线法（二阶导数法） 曲线：具二个极大值的二级微商曲线 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：准确度高，可用内插法求滴定终点 * Objectives Equipments 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 each addition of the titrant. The calculation of the ionization constants from the potentiometric titration curve is illustrated. Equipments Chemicals Standard NaOH (0.1 mol/L); Phosphoric acid (approximately 0.1 mol/L); Potassium hydrogen phthalate solution (0.05mol/L), pH 4.00 Principles 1. Determination of pKa H3PO4+ H2O = H3O+ + H2PO4- H2PO4-+ H2O = H3O+ + HPO42- HPO42-+ H2O = H3O+ + PO43- 2. Determination of end point The curve of potentimetric titration △pH/△V △2pH/△V2 1.The standardization of the pH meter Procedures 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 phosphoric 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. Notices 1. After each addition of NaOH, wait a few seconds till the reading of pH is stable, and then record the data. 2. To add accurate volumes o