Mercury indicator electrodes

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

An interesting indirect end-point detection method is based on the work of Siggia et al [143], and Reilley and Schmid [119], who originally developed the technique for chelometric titrations of electro-inactive metals with a mercury indicator electrode. Assume that metal ion M is titrated by titrant Y (ionic charges are omitted for the sake of simplicity),... 

The reaction continues and current passes until all the iodide is used up. At this point some means of endpoint detection is needed. Two methods are commonly adopted. The first uses an amperometric circuit with a small imposed voltage that is insufficient to electrolyze any of the solutes. When the mercury ion concentration suddenly increases, the current will rise because of the increase in the concentration of the conducting species. The second method involves using a suitable indicator electrode. An indicator electrode may be a metal electrode in contact with its own ions or an inert electrode in contact with a redox couple in solution. The signal recorded is potentiometric (a cell voltage vs. a stable reference electrode). For mercury or silver we may use the elemental electrodes, because they are at positive standard reduction potentials to the hydrogen/hydrogen ion couple. 

The endpoint in the titration of the rare earths can also be determined potentiometrically using a mercury indicator electrode as reported by Reil et al. (1958). The determination of the hydrogen ions liberated as a result of complex formation has also been employed to,determine the amount of rare... 

Direct solution of the zinc electrode in the alkaline solution on open circuit is minimized by dissolving zinc oxide in the electrolyte and amalgamating the zinc in the electrode. Mercury levels used in zinc electrodes are usually in the range of 5 to 15% w/w. Great attention is also paid to the impurity levels in the zinc since minor cathodic inclusions in the electrode can drive the hydrogen generation reaction despite the precautions indicated. ... 

A number of the most common potentiometric electrode systems and their applications are summarized in Table I. One of the most important and extensively used indicator electrode systems is the glass-membrane electrode that is used to monitor hydronium-ion activity. Although developed in 1909, it did not become popular until reliable electrometer amplifiers were developed in the 1930s (modern pH meters use high-input-impedance digital voltmeters). Figure 1 gives a schematic representation of this electrode and indicates that the primary electrode system is a silver/silver-chloride (or mercury/mercurous-chloride) electrode in contact with a known and fixed concentration of hydrochloric acid (usually about 0.1 M). When... 

Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

For complexation titrations involving the use of EDTA, an indicator electrode can be set up by using a mercury electrode in the presence of mercury (II) EDT A complex (see Section 15.24). 

Discussion. The indicator electrode employed is a mercury-mercury(II)-EDTA complex electrode. A mercury electrode in contact with a solution containing metal ions M"+ (to be titrated) and a small added quantity of a mercury(II)-EDTA complex HgY2- (EDTA = Na2H2 Y) exhibits a potential corresponding to the half-cell ... 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

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