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1、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,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
2、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,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,离子选择性电极构造示意图,BASIC THEORY OF ISE
3、MEASUREMENTS.,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
4、意图,Pore distributions of ormosils,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
5、 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 conce
6、ntrations. 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 extern
7、al 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 meas
8、ured 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 characteris
9、tic of the particular ISE/reference pair. (It is the sum of all the liquid junction potentials in the electrochemical cell, see later),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. Mo
10、st 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.,离子选择性电极类型,pH电极 阳离子玻璃电极 晶体电极 氟电极 硫、卤素电极 流动载体电极 气敏电极 酶电极 离子敏感场效应晶体管电极,Differences
11、 Between pH and Other Ion-Selective Electrodes,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
12、应管选择性电极,Application of ISE,Ion-selective electrodes are used in a wide variety of applications for determining the concentrations of various ions in aqueous solutions. The following is a list of some of the main areas in which ISEs have been used. Pollution Monitoring: CN, F, S, Cl, NO3 etc., in eff
13、luents, and natural waters. Agriculture: NO3, Cl, NH4, K, Ca, I, CN in soils, plant material, fertilisers and feedstuffs. Food Processing: NO3, NO2 in meat preservatives.,Salt content of meat, fish, dairy products, fruit juices, brewing solutions. F in drinking water and other drinks. Ca in dairy pr
14、oducts and beer. K in fruit juices and wine making. Corrosive effect of NO3 in canned foods. Detergent Manufacture: Ca, Ba, F for studying effects on water quality. Paper Manufacture: S and Cl in pulping and recovery-cycle liquors.,Explosives: F, Cl, NO3 in explosive materials and combustion product
15、s. Electroplating: F and Cl in etching baths; S in anodising baths. Biomedical Laboratories: Ca, K, Cl in body fluids (blood, plasma, serum, sweat). F in skeletal and dental studies.,Advantages.,1) When compared to many other analytical techniques, Ion-Selective Electrodes are relatively inexpensive
16、 and simple to use and have an extremely wide range of applications and wide concentration range. 2) The most recent plastic-bodied all-solid-state or gel-filled models are very robust and durable and ideal for use in either field or laboratory environments.,3) Under the most favourable conditions,
17、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).,4) They are particularly useful in applications where only an
18、order of magnitude concentration is required, or it is only necessary to know that a particular ion is below a certain concentration level. 5) They are invaluable for the continuous monitoring of changes in concentration: e.g. in potentiometric titrations or monitoring the uptake of nutrients, or th
19、e consumption of reagents.,6) They are particularly useful in biological/medical applications because they measure the activity of the ion directly, rather than the concentration. 7) In applications where interfering ions, pH levels, or high concentrations are a problem, then many manufacturers can
20、supply a library of specialised experimental methods and special reagents to overcome many of these difficulties.,8) With careful use, frequent calibration, and an awareness of the limitations, they can achieve accuracy and precision levels of 2 or 3% for some elements and thus compare favourably wi
21、th analytical techniques which require far more complex and expensive instrumentation. 9) ISEs are one of the few techniques which can measure both positive and negative ions. 10) ISEs can be used in aqueous solutions over a wide temperature range. Crystal membranes can operate in the range 0C to 80
22、C and plastic membranes from 0C to 50C.,测量仪器和参比电极,ISE的内阻很高达108，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1011,参比电极,参比电极满足三个条件：可逆性、重现性和稳定性 可逆性：衡量尺度是交换电流 重现性：温度和浓度变化时，能按照能斯特公式响应而没有滞后现象 稳定性：测量过程中保持不变和长期稳定,重要的参比电极,氢标准电极（SHE）：所有电极中重现
23、性最好的，人为规定标准氢电极的电位在任何温度下为零 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 ). 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
24、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,ISE定量分析的各种方法,直接比较法 温度和电极的响应斜率补偿和校正 标准曲线法 基体干扰，加入总离子调节缓冲剂 标准加入法（一次和连续标准加入法） 减少基体干扰的最佳方法，适用于组成复杂和份数不多的样品,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,The calculation of ioni
25、c concentration is more dependent on a precise value for the potential difference than is the pH value. it would take an error of more than 5 millivolts to cause a change of 0.1 pH units, but only a 1 millivolt error will cause a 4% error in the calculated concentration of a mono-valent ion and an 8
26、% 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 10-5 to 10-7 mol/Land very few can be used to determine concentrations below 1x10-7 mol/L. Thus, for low concentration samples, it
27、may be necessary to construct a calibration graph with several points in order to define the curve more precisely in the non-linear range.,It is more usual to plot a calibration graph using the ionic concentration with a logarithmic scale on the X-axis rather than the pX factor (analogous to pH) on
28、a linear axis. Some ISEs will only work effectively over a narrow pH range.,电位法测量的方法误差,相对误差 c/c 100 % = 3900 Z E 当电位测量误差达到1 mV时，一价离子的误差为3.9%，二价离子的相对误差为7.8%,电流滴定法,容量分析使用电化学分析指示终点的有电导、电位、电流和库仑滴定等方法 电流滴定使用一个指示电极和一个参比电极时，指示电极是极化电极，常用旋转铂微电极和滴汞电极，参比电极常用Ag/AgCl或甘汞电极 电流滴定有酸碱、沉淀络合和氧化还原滴定等类型,电流滴定装置,各种电流滴定的响应曲线,滴定反应与响应的类型,酸碱反应（玻璃电极） 氧化还原反应（铂电极） 沉淀反应（选用多种电极） 络合反应（使用个别反应电极或指示多种金属离子电极）,电 导 测 定,