电化学分析--电位分析法

电位滴定过程中E 值的明显变化
离子选择性电极 Ion-Selective Electrode
The equipment required for direct potentiometric measurement includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device. Fig.1 shows a schematic diagram of an electrochemical cell for potentiometric measurement.
The composition of the membrane is designed to yield a potential that is primarily due to the ion of interest. The trick is to find a membrane that will selectively bind the analyte ions, leaving co-ions behind. Membrane materials that process different ion-recognition properties have thus been developed to impart high selectivity.
（一）电位分析法原理
1. 直接电位法基本原理 将金属浸入该金属离子的水溶液中，则 在金属和水溶液之间产生电极电势 ：( MnFra bibliotekn/M )


(M
n
RT ln aM n /M ) nF
n
aM 为M n+ 的活度 a M 式中， ,当浓 c 度很小时，活度系数 M 1 ，则：
Information on the composition of a sample is obtained through the potential appearing between two electrodes. Potentiometry is a classical analytical technique with roots before the turn of this century. However, the rapid development of new selective electrodes and more sensitive and stable electronic components over the past 25 years has expanded tremendously the range of analytical applications of potentiometric measurements.
第二章

电位分析法
离子选择性电极和膜电位
Ion Selective Electrode and Membrane Potential
离子选择性电极的性能参数 Parameters of Ion Selective Electrode 直接电位法

Direct Potentiometry

电位滴定法
EK （2.303RT / zi F） log ai
Equation above has been written on the assumption that the electrode responds only to the ion of interest, i.
In practice, no electrode responds exclusively to the ion specified, the actual response of the electrode in a binary mixture of the primary and interfering ions (i and j, respectively) is given by the Nikolskii-Eisenman equation .
By the way, the ISEs sense the activity rather than the concentration of ions in solution. The term “activity” is used to denote the effective concentration of the ion. The difference between concentration and activity arises because of ionic interactions.
Thermodynamic arguments tell us that the potential produced across the membrane corresponds to the free energy difference (Δ G) associated with the gradient of activity of the analyte ions in the outer and inner solutions. Such a potential arises whenever the membrane separates two solutions of different ion activities.
EK （2.303RT / zi F） log （ai kij a
zi / z j j
)
EK （2.303RT / zi F） log ai
Compare to the two equations above,we may……
where kij is the selectivity coefficient, a quantitative measure of the electrode’s ability to discriminate against the interfering ion (For example, a measure of the relative affinity of ions i and j toward the ionselective membrane, if an electrode is 50 times more responsive to i than to j, kij has a value of 0.02, a kij of 1.0 corresponds to a similar response for both ions).
The speed at which this field has developed is a measure of the degree to which potentiometric measurements meet the need of the analytical chemist for rapid, low-cost, and accurate analysis. In this chapter, the principles of direct potentiometric measurements, based on ion-selective electrodes, will be described.
The resulting potential of the ion-selective electrode that reflects the unequal distribution of the analyte ions across the boundary is generally monitored relative to the potential of a reference electrode.(why?) Since the potential of the reference electrode is fixed, the measured cell potential reflects the potential of the ISE, and can thus be related to the activity of the target ion.
Emem and Ej are the potentials generated between the internal-filling /the referencebridge solutions, respectively, and the external sample solution.
( M n / M )
RT ln aM n nF
参比 为参比电极电位。 式中，
因为当温度一定时， 参比 和 ( M
都是常 数，故根据测得的电动势E，可求得离子的 活度 aM n 。
n
/M )
2．电位滴定法基本原理
当采用电位滴定法测定 M n+ 时， 在化学计量点附近（滴定突跃内）时， 一滴滴定剂可使指示电极的电位发生 突变，这将导致测得的电动势E也发生 明显的变化。通过测量不同体积滴定 剂时E值的变化，绘制相应的滴定曲线 （V-E），根据滴定曲线来确定化学计 量点时滴定剂消耗的体积，从而计算 出待测组分的浓度或含量 。
Ion-Selective Electrodes are mainly membrane-based devices, consisting of permselective ion-conducting materials, which separate the sample from the inside of the electrode. On the inside is a filling solution containing the ion of interest at a constant activity. The membrane is usually nonporous, water insoluble, and mechanically stable.