厦门大学分析化学：第02章 电位分析法.ppt

第二章 电位分析法 电位分析（Potentiometric Analysis）分为电位法(Potentiometry)和电位滴定（Potentiometric Titration）两种 电位法：测量某一电极的电位，通过能斯特公式直接获取待测离子浓度 根据滴定过程中的电位变化，依据所消耗的滴定剂的量进行计算 道南电位的产生在于其选择性或强制性引起界面两端离子浓度的差别，从而产生双电层结构，而有电位差 离子选择性电极 敏感膜选择性电极一般满足以下条件； 微溶性 导电性 可与待测离子或分子选择性响应 离子选择性电极 最早的pH电极出现在1906年，1943年出现Na+电极，上世纪五十年代建立了离子选择性电极的膜电位理论，出现商品化产品，1966年出现F-离子选择性电极 离子选择性电极主要包括（1）敏感膜，是最关键部分（2）内参溶液，含有与膜及内参电极响应的离子（3）内参电极，一般用Ag|AgCl BASIC THEORY OF ISE MEASUREMENTS. a) The pH Electrode This is a device for measuring the concentration of hydrogen ions and hence the degree of acidity of a solution - since pH is defined as the negative logarithm of the hydrogen ion concentration; The most essential component of a pH electrode is a special, sensitive glass membrane which permits the passage of hydrogen ions, but no other ionic species. When the electrode is immersed in a test solution containing hydrogen ions the external ions diffuse through the membrane until an equilibrium is reached between the external and internal concentrations. Thus there is a build up of charge on the inside of the membrane which is proportional to the number of hydrogen ions in the external solution. The potential difference developed across the membrane is in fact directly proportional to the Logarithm of the ionic concentration in the external solution. Thus, in order to determine the pH of an unknown solution, it is only necessary to measure the potential difference in two standard solutions of known pH, construct a straight line calibration graph by plotting millivolts versus pH (= - Log [H+]) then read off the unknown pH from the measured voltage. The relationship between the ionic concentration (activity) and the electrode potential is given by the Nernst equation: E = E0 + (2.303RT/ nF) x Log(A) Where E = the total potential (in mV) developed between the sensing and reference electrodes. E0 = is a constant which is characteristic of the particular ISE/reference pair. (It is the sum of all the liquid junction potentia