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离子选择性电极简介

基本理论
离子选择性电极(ISEs)是以敏感膜为基础的电化学传感器,这层膜是使电极对特定离子有选择性响应的元件。
根据膜的材料不同可将离子选择性电极分为
4种:

 玻璃膜(如Na+pH
固态膜(如Pb2+
聚合体膜(如K+
气体渗透膜(如CO2

电极置于溶液内时膜上会形成一电势差。当样品内待测离子的浓度变化时,用离子选择性电极和一内置或外置参比电极一起使用能测出此电势差的变化。从原理上说,整个的设置可以作如下描述:

该测得的电势
E和样品内的离子活跃度ai之间的关系用Nernst数学方式可表述如下

斜率等于2.3 RT/ZiF,其中R为气体常量,8.314JK-1mol-1

    T为绝对温度K
    F
是法拉第等值,9.6487×104 Cmol-1
    Zi是所测离子的电荷

该斜率在
25 oC下为59.16mV/Zi。如所测离子是像钾离子那样的单价离子(Zi=+1),在浓度发生十进制变化时,25oC时可以观察到59.16mV的电势变化。类似的,像钙离子那样的二价离子(Zi=+2)在25oC下可观察到59.16mV/229.58mV的电势变化。

Eo是每一支离子选择性电极与特定参比结合时的恒定的电势差。ai是离子活度,但只在较稀释的溶液内和离子浓度相等。离子的活度取决于由离子内容决定的样品溶液中的离子强度。
所测离子的浓度在其高低端都会发生线性偏移,该偏移决定了测量的高低端的限度。离子电极可以精确测量到浓度为
10-6 M的样品溶液。高浓度情况下(通常高于0.1M)线性偏移是由于带相反电荷的离子的作用引起的。

校正液和离子强度调节剂/缓冲液
为确保标准液和样品液离子强度相同, 强烈建议使用离子强度调节剂(ISA),通常是样品液和ISA比例为50:1
另外,有些离子选择性电极只能用于一定范围
pH值溶液内。在离子强度调节剂内加缓冲液可以将标准液和样品液调节至要求的pH值。更为复杂一些的离子强度调节剂配方还会加入一些能和可能发生的干扰的离子进行螯合的添加剂。离子强度调节剂还含有如抗氧化剂之类的防腐剂。

为精确标定,强烈建议使用至少2份浓度已知的溶液(标准溶液)对电极进行校正。通常用存储的1000ppm待测离子溶液加去离子水稀释而成。样品溶液的浓度应在标准

液浓度的函盖范围之内,电极最好经常校正,至少一周一次。
可以通过在半对数曲线纸上测绘两份标准液的电势读数(标液
1 mV及标液2mV)来定离子选择性电极的响应曲线。从图线上来说,样品溶液内的离子浓度可通过测样品溶液电势并将此数值代入由两份标准液确定的反应曲线中来确定。从数学上可以用Nernst方程表示:

其中
Eo和斜率可从标准液浓度和mV读数中计算出。

离子选择性电极的响应会受到样品溶液内存在的其他离子的影响。对每一支特定的离子选择性电极来说,这些干扰离子的影响可以进行量化,如干扰离子的浓度已知,则可进行修正。其他情况下,可用其他方法对样品进行分析,如标准液添加法,在样品溶液比较复杂或含有大量干扰离子的情况下,该方法是非常精确的。

清洗
良好的离子选择性电极在一、二分钟内即可稳定下来。但如电极的膜被溶液内的油或微粒弄脏后其响应会变慢。玻璃电极的电极头可用酒精或温性洗涤剂清洗,如实验室清洗器皿用的洗涤剂。固态离子选择性电极可以用颗粒非常细洁的抛光条进行恢复。PVC的离子选择性电极用水漂洗即可。用去离子水彻底漂洗之后,应将电极放入用于校正的最低标准液内浸泡2到4个小时重新标定。如电极的斜率或响应时间不符合要求,固态电极可在标定之前浸泡时间要长一些,如一夜。

存储
玻璃的离子选择性电极可以存储在低浓度标准液内。固态、PVC和气体离子感应电极在测量间隙可存储在低浓度标准液内。如固态电极几个小时不用,PVC和气体离子选择性电极几天不用,则应该干燥保存。具体情况请参阅说明书。

离子选择性电极参考内容

什么是离子选择性电极,离子选择性电极是怎么工作的?

离子选择性电极测量溶液内特定离子的电势,该电势是参照一支电位恒定的参比电极测出的,两支电极之间的电势差取决于溶液内某一特定离子的活度。此活度和该特定离子的浓度有关,因而可以对该离子进行分析测量。

毫伏读数和浓度如何对应?

必须精确准备好浓度已知的标准溶液,然后用pH/mV表测这些溶液。记下每份溶液的mV读数并在半对数曲线纸上绘制出浓度-毫伏读数曲线。现在可以对未知溶液进行测量了,在曲线上对未知溶液的mV值进行定位并确定其相应的浓度。

几款不同类别的感应电极可供选择
根据制作电极的膜材料的性质可对离子选择性电极进行分类,就是这层膜决定了电极可以对某一特定离子进行选择。

1. 聚合体膜电极(有机离子交换及螯合剂)
    聚合体膜电极由各类离子交换材料加入如PVC、聚乙烯、聚氨酯或硅橡胶等惰性载体构成。膜形成后即封到PVC管的端口上,膜表面形成的电势和所测离
子的浓度相关。这一类型的电极包括钾离子、钙离子、氟硼酸根、硝酸根、高氯酸根离子选择性电极和水硬度测试电极。

2. 固态电极 
    固态电极膜内使用的是相对不可溶的无机盐。固态电极以同类和异类方式存在。这两种类型都由于离子交换过程而在膜上形成电势。此类电极包括银/硫离子、铅离子、铜离子、氰化物、硫氰酸根、氯离子和氟离子选择性电极。

3. 气体感应电极
    气体感应电极用来测量诸如氨气、二氧化碳、溶氧、氧化氮、二氧化硫和游离氯气等溶解气体。此类电极配有气体渗透膜和内部缓冲液,且其结构上不需要外参比电极。

4. 玻璃膜电极
    玻璃膜电极是在二氧化硅玻璃载体里添加各种化学品做成的。最常见的玻璃膜电极是pH电极。玻璃膜电极也可用于测量钠离子。


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