页数：105页文件大小：20.46 MB来源： 仪器分析及实验ppt课件
本课件围绕电位分析法展开，从电位分析法概述讲到扩散电位与道南电位，并延伸至离子选择性电极与敏感膜，说明相关原理、仪器结构与定量方法要点，逐页图示与文字版对照编排。
Chapter ThreePotentiometric Analysis第三章电位分析法精品课程仪器分析(含实验)
扩散电位示意图相互接触但浓度不同的溶液，如HCl，由于浓度差异产生扩散，同时由于离子迁移速率的差异，导致溶液界面的电荷分布不均，产生电位梯度出现电位差。相同电荷的溶液与离子间，存在静电排斥，使扩散达到平衡，溶液界面有稳定的界面电位，即液接电位。液接电位不仅出现在液-液界，也出现在固-液界面扩散电位
道南电位示意图膜电位
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
构造示意图
BASIC THEORY OF ISEMEASUREMENTS.Ion-Selective Electrodes are part of a group of relatively simple and inexpensive analytical tools which are commonly referred to as Sensors. The pH electrode is the most well known and simplest member of this group and can be used to illustrate the basic principles of ISEs.
玻璃的微观孔径结构示意图
Pore distributions of ormosilsPore ranges(nm)Rel.Vol, %S2-1S2-2S2-3S2-4100.0 -10.030.2414.4118.9111.1810.0 -5.011.145.906.5912.165.0 –2.032.2434.9031.4231.782.0 –1.513.5516.6018.4223.441.5 –1.011.5524.3322.7117.201.0 –0.903.381.902.650.9 –0.80.550.4701.600.8 –0.00.75000
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.
For practical use in measuring pH, it is not normally necessary for the operator to construct a calibration graph and interpolate the results for unknown samples. Most pH electrodes are connected directly to a special pH meter which performs the calibration automatically. This determines the slope mathematically and calculates the unknown pH value for immediate display on the meter.
In contrast to the pH membrane, other ion-selective membranes are not entirely ion-specific and can permit the passage of some of the other ions which may be present in the test solution, thus causing the problem of ionic interference.
液体膜电极
气敏电极
酶电极
场效应管选择性电极
3) Under the most favourable conditions, when measuring ions in relatively dilute aqueous solutions and where interfering ions are not a problem, they can be used very rapidly and easily (e.g. simply dipping in lakes or rivers, dangling from a bridge or dragging behind a boat).
5) They are invaluable for the continuous monitoring of changes in concentration: e.g. in potentiometric titrations or monitoring the uptake of nutrients, or the consumption of reagents.
7) In applications where interfering ions, pH levels, or high concentrations are a problem, then many manufacturers can supply a library of specialised experimental methods and special reagents to overcome many of these difficulties.
测量仪器和参比电极
ISE的内阻很高达108Ω，如果检出电流为10-9A，则造成的电压降将为0.1 V，相当于2个pH单位；为减少高内阻测量引起的误差，使用高输入阻抗的电子毫伏计；R电极；测量误差%=×100%；R电极+R仪表；因此要使误差小于0.1%，R仪表应大于1011Ω
重要的参
重要的参比电极1.氢标准电极（SHE）：所有电极中重现性最好的，人为规定标准氢电极的电位在任何温度下为零2.Ag-AgCl电极：银丝或细银棒表面覆盖AgCl而成。除SHE电极外，Ag-AgCl电极重现性最好，温度系数小，可在80℃ 以上使用，标准电位+0.2223 V (25℃ ).3.甘汞电极：最常用的参比电极，标准电位+0.2444 V ，电极电位相当稳定。
离子选择性电极的性能和参数
不同检测下限的计算方式
电位选择性系数越小，选择性越高
电极本身性质、待测离子性质、共存离子的性质、测定条件（温度、搅拌）
pA (pH)值的实用定义定量基础是能斯特公式，电位响应与离子活度关系为：+ 为阳离子，-为阴离子响应，K包括内参比电极电位、膜内相间和不对称电位通常必须和标准溶液比较获得结果ISE的定量方法介绍
活度和浓度为保证测定样品与标准溶液的离子强度的相近，一般测定中加入离子强度调节剂
一次标准加入法原理溶液的电池电动势加入标准溶液Vs后的电池电动势
电池电动势的变化为一次标准加入法的精确计算公式：
Detail content，See page 288 in the text
it would take an error of more than 5 millivoltsto cause a change of 0.1 pHunits, but only a 1 millivolterror will cause a 4% errorin the calculated concentration of a mono-valent ion and an 8%error for a di-valent ion
Most ISEs have a much lower linear range and higher detection limit than the pH electrode. Many show a curved calibration line in the region 10-5to 10-7mol/Land very few can be used to determine concentrations below 1x10-7mol/L. Thus, for low concentration samples, it may be necessary to construct a calibration graphwith several points in order to define the curve more precisely in the non-linear range.
Some ISEs will only work effectively over a narrow pH range.
电流滴定装置
各种电流滴定的响应曲线
电导测定
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