Glass Electrode - Glass Electrode

A typical electrode for measuring pH, using a thin glass membrane of a specific composition. A sufficiently thin glass membrane exhibits the property of selectively allowing hydrogen ions to pass through. When two liquids with different hydrogen ion concentrations are separated by a thin glass membrane, a membrane potential appears that corresponds to the ratio of the pH of the two liquids. Therefore, if the pH of one liquid is known, it is possible to determine the pH of the other liquid. An example of the structure of a battery used in actual pH measurement is as follows:

There is a linear relationship between the potential obtained through the glass membrane and the pH of solution II, but at pH levels below 1 and above 10, deviations from this straight line occur, which are called the acid error and alkaline error, respectively. Furthermore, normal glass membranes are subject to a phenomenon known as asymmetric potential; that is, even if exactly the same solutions are used in I and II in the above battery, the equilibrium voltage of the battery will show a value that is slightly different from zero. Furthermore, there is some liquid junction potential difference between solutions II and III in the battery. Therefore, in order to correct for the asymmetric potential and liquid junction potential difference, the glass electrode system is practically calibrated by using a standard solution with a defined pH for solution II in the battery.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

特定組成のガラス薄膜を使ったpH測定用の代表的な電極．十分に薄いガラス膜は，水素イオンを選択的に通過させる性質を示す．水素イオン濃度の異なる2液をガラスの薄膜で隔てると，2液の pH の比に応じた膜電位が現れる．したがって，一方の液の pH がわかっていると他方の液の pH を求めることができる．実際のpH測定に用いられる電池の構成は，たとえば次のようなものである．

ガラス膜を通して得られる電位と溶液Ⅱの pH との間に直線関係があるが，pH 1以下および pH 10以上では，この直線からの偏移が起こり，それぞれ酸誤差およびアルカリ誤差といわれる．また，通常のガラス膜には不斉電位とよばれる現象，すなわち，上記の電池においてⅠとⅡにまったく同じ溶液を用いても電池の平衡電圧は，わずかではあるがゼロと異なる値を示すという現象がある．そのうえ，電池の溶液ⅡとⅢの間には多少なりとも液間電位差が存在する．したがって，不斉電位や液間電位差を補正するため，実用的には，電池の溶液Ⅱに pH の定義された標準溶液を用いてガラス電極系を較正する方法が用いられる．

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