The calomel electrode is a commonly used reference electrode.By mercury andMercurous chloridestaypotassium chlorideIt is formed by contacting saturated solution in aqueous solution.There are three commonly used calomel electrodes:Potassium chloride solutionThe saturated calomel electrode is the saturated solution, the equivalent calomel electrode is the potassium chloride solution concentration of 1mol/L, and the calomel electrode is the potassium chloride solution concentration of 0.1mol/L.At 298.15K, the electrode potential of equivalent calomel electrode is 0.2801V.The preparation and storage of calomel electrode are very convenient, and the electrode potential is very stable, so it is widely used.[1]
The calomel electrode is composed of mercury, calomel andPotassium chloride solutionReference electrode.Also known as "calomel reference electrode" or "calomel half battery".At a certain temperature, when the concentration of Cl in KCl solution is fixed, the potential of calomel electrode is a fixed value, so it can be used as a reference electrode.Common calomel electrodes include saturated calomel electrode with saturated potassium chloride solution as internal reference solution and standard (equivalent) calomel electrode with 1 mole of KCl (potassium chloride) solution as internal reference solution.The calomel electrode is composed of metal mercury, calomel (HgtwoCltwo)[5]And chloride (usually KCl) solution.Its structure is shown in the figure below.It belongs toType II electrode, which is related to anion (Cl)Nernst (W. H. Nernst, 1864-1941, physical chemist).The structure is as follows:[2]
Since the potential of a single electrode cannot be measured, it is usually borrowedStandard hydrogen electrodeAs a reference electrode, the relative electromotive force of the electrode is measured.However, although the standard hydrogen electrode is stable, it is troublesome to operate, so the calomel electrode with good reproducibility and relatively stable is often used as the reference electrode.The calomel electrode is an electrode composed of mercury, calomel and KCl solutions of different concentrations. Its electrode potential can be accurately measured by forming a battery with a standard hydrogen electrode, so it is also called a secondary standard electrode.
There are generally four types of electrodes: metal metal ion electrode, gas ion electrode, metal metal insoluble salt electrode andRedox electrode。Calomel electrode belongs to metal metal insoluble salt electrode in electrode classification.
The potential value of the calomel electrode is unstable when the temperature is above 70 ℃, and the electrode has a life of only 9 hours when the temperature is above 100 ℃. Therefore, the calomel electrode should be used below 70 ℃, and the silver silver chloride electrode should be used when the temperature is above 70 ℃.
The electrode reaction and electrode symbol of calomel electrode are as follows:
Reference electrode is also called "reference electrode".Since the electrode potential of a single electrode cannot be measured directly, when measuring the electrode potential of an electrode, another electrode with known electrode potential must be added to make it a battery.The electrode potential of the measured electrode can be marked only after the electromotive force of the battery is measured.The electrode with known electrode potential is called "reference electrode".
The electrode potential of the primary reference electrode is generally taken as zero.The electrode potential measured with the standard hydrogen electrode (SHE) as the primary reference electrode is called "hydrogen standard electrode potential" orStandard electrode potential。The commonly used secondary reference electrodes are calomel electrode andSilver chloride electrode, their hydrogen standard electrode potentials have been accurately measured, so after the electrode to be measured is paired with them to form a battery, it is very convenient to measureBattery electromotive forceCalculate its hydrogen standard electrode potential.The mercury electrode is another primary reference electrode, which stipulates that the charge on the electrode is zero relative to the electrode potential when the solution is zero, and the measured electrode potential is called "absolute potential". This electrode is rarely used.[4]
