Calomel electrode preparation

The preparation and properties of the calomel electrode have been reviewed 30 superior reproducibility is claimed for a calomel electrode prepared by shaking dry, finely divided (0.1-0.5 /un) calomel with pure mercury. The calomel spreads over the mercury to form a pearly skin, and some of this skin is introduced on to a mercury surface where it spreads immediately. So-... 

As with the saturated calomel electrode, the potential of the Ag/AgCl electrode is determined by the concentration of Cb used in its preparation. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

To set up a saturated calomel electrode, a saturated solution of potassium chloride is first prepared from pure potassium chloride and de-ionised water, and this is then shaken for some hours with analytical grade mercury(I) chloride so that the solution is also saturated with this substance. Pure mercury is placed in the electrode vessel to a depth of about 1 cm the platinum contact must be... 

Compact, ready-prepared calomel electrodes are available commercially and find wide application especially in conjunction with pH meters and ion-selective meters. A typical electrode is shown in Fig. 15.1(h). With time, the porous contact disc at the base of the electrode may become clogged, thus giving rise to a very high resistance. In some forms of the electrode the sintered disc may be removed and a new porous plate inserted, and in some modern electrodes an ion exchange membrane is incorporated in the lower part of the electrode which prevents any migration of mercury(I) ions to the sintered disc and thus... 

For some purposes, modifications of the calomel electrode may be preferred. Thus, if it is necessary to avoid the presence of potassium ions, the electrode may be prepared with sodium chloride solution replacing the potassium chloride. In some cases the presence of chloride ions may be inimical and a mercury(I) sulphate electrode may then be used this is prepared in similar manner to a calomel electrode using mercury)I) sulphate and potassium sulphate or sodium sulphate solution. 

Pipette 25 mL of solution B into a 100 mL beaker mounted on a magnetic stirrer and add an equal volume of TISAB from a pipette. Stir the solution to ensure thorough mixing, stop the stirrer, insert the fluoride ion-calomel electrode system and measure the e.m.f. The electrode rapidly comes to equilibrium, and a stable e.m.f. reading is obtained immediately. Wash down the electrodes and then insert into a second beaker containing a solution prepared from 25 mL each of standard solution C and TISAB read the e.m.f. Carry out further determinations using the standards D and E. 

Calibration curve in spectrophotometry, 674, 753, 755, 800 statistical tests for, 144 Calmagite 318 Calomel electrode 63 forms of, 551 potential of, 554 preparation of, 551 Capacitative cell 527 Capacitance as an analytical tool, 528 Carbohydrates D. of hydroxyl groups in, (ti)) 306... 

MoBr(NH)(dppe)2] Br. Compound BMo was prepared by treatment of the Mo(IV) dialkylhydrazido(2-) complex [MoBr NN (C5H10) (dppe)2]Br (compound AMo) with tert-butyllithium or electrochemically at —1.62 V vs. a saturated calomel electrode... 

Like the calomel electrode, the saturated KC1 version of this electrode is the most convenient to prepare. 

Charlie Focht of the Nebraska State Agriculture Laboratory refills a saturated calomel electrode with saturated potassium chloride while preparing to analyze animal feed samples for sodium chloride via a poten-tiometric titration. 

The redox properties of an electrode are determined by its potential measured relative to some reference electrode. Many different reference electrodes are used in the literature. In order to make cross comparisons easily, most of the electrode potential quoted for reactions have been converted to the scale based on the saturated calomel electrode as reference. Electrode materials and electrolyte solutions used by the original workers are quoted. In many cases, the electrodes could be fabricated from more modem materials without affecting the outcome of the reactions. In the not too distant past perchlorate salts were frequently used as electrolytes. This practise must be discouraged for preparative scale reactions because of the danger of an explosion when perchlorates and organic compounds are mixed. Alternative electrolytes are now readily available. 

Krasensky and Studnickova [221] have prepared quaternary ammonium amalgam via electroreduction of tetraethylam-monium tetrafluoroborate in the aqueous medium at the room temperature. At the applied voltage of —2.8 to —2.4 V versus sodium saturated calomel electrode (SSCE) hydrogen evolution occurred simultaneously. Composition of the black precipitate formed was found to be Et4N Hg, where x = 2.9 0.8. 

A cell was prepared by dipping a Cu wire and a saturated calomel electrode into 0.10 M CuS04 solution. The Cu wire was attached to the positive terminal of a potentiometer and the calomel electrode was attached to the negative terminal. 

E. A titration of 50.0 mL of unknown Fe2+ with 0.100 M Ce4+ at 25°C monitored with Pt and calomel electrodes gave data in the following table.6 Prepare a Gran plot and decide which data lie on a straight line. Find the x-intercept of this line, which is the equivalence volume. Calculate the molarity of Fe2+ in the unknown. 

Reference electrodes are divided Into two groups. One comprises the saturated calomel electrode, its variants (such as the "lithium S.C.E.", Hg/Hg2Cl2(s.), LiCl( s)> and others of the same ilk), and the normal hydrogen electrode. These are almost Invariably prepared with water, so that their use with a non-aqueous solution entails a liquid-junction potential between the non-aqueous solution of the compound being studied and the aqueous solution In the reference electrode. Some workers have sought to circumvent this by preparing similar electrodes In the same solvents or solvent mixtures that contain the compounds they study when this has been done, the symbol "(o)" (for "organic") follows the abbreviation that would denote the ordinary aqueous form of the reference electrode. 

For reductions, hanging mercury drop electrodes or mercuryfilm electrodes are frequently used owing to their microscopic smoothness and because of the large overpotential for hydrogen evolution characteristic for this electrode material. Mercury film electrodes are conveniently prepared by the electrochemical deposition of mercury on a platinum electrode from an acidic solution of an Hg2+ salt, e.g. the nitrate. However, the oxidation of mercury metal to mercury salts or organomercurials at a low potential, 0.3-0.4 V versus the saturated calomel electrode (SCE), prevents the use of these electrodes for oxidations. 

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

Although extensive data are available for the calomel electrode, the silver-silver chloride electrode appears to be superior to it because of its ease of preparation and use, lower sensitivity to the presence of oxygen, and small temperature hysteresis. 

Because special preparative procedures are not necessary (except for the preparation of mercurous sulfate electrolytically), this reference electrode is recommended for use in place of the silver chloride or calomel electrodes when chloride ion must be rigorously excluded. 

The mercurosulphate electrode is most suitable for work with the sulphate solutions. It is prepared analogously as the calomel electrode, with one exception that the paste is composed of mercurous sulphate, ground together with mercury and an alkali metal sulphate solution of a definite concentration. In the electrode the equilibrium is attained, according to the equation... 

Mercuric and mercurous salts — Salts of Hg(II) and Hg(I), respectively. Soluble mercuric and mercurous salts such as acetates and nitrates are used for the deposition of mercury films on conducting substrates (see -> anodic stripping voltammetry). Insoluble salts, e.g., chloride and sulfate of Hg(I) in chloride and sulfate medium, respectively, can be used to prepare reference electrodes (see -> calomel electrode). The formation of insoluble salts of mercury on - mercury electrodes determines, among others, the positive limit of their voltam-metric potential window. 

There are two other common versions of this half-cell the normal and tenth normal csAo-mel electrodes, in which the KCl concentration is either 1.0 or 0.1 N. The saturated electrode is the easiest to prepare and the most convenient to use but has the largest temperature coefficient. The half-cell potential for each of the calomel electrodes has a different value relative to the standard hydrogen electrode these emf valnes are given in Table 1. Calomel electrodes can be easily prepared in the laboratory and are also available commercially. Two typical calomel electrode designs are shown in Fig. 7. 

Fig. 9.33 Absorption spectra of dried thin films of electrochemically polymerised PAni on a Pt electrode (a) yellow-green partially reduced, (b) and (c) green and (d) blue partially oxidised films prepared in water/HCl at 0.72 V and oxidised at -0.2, 0.4 and 0.8 V relative to a saturated calomel electrode, respectively. Reproduced with permission of the Institute of Physics from Monkman et al., (1987).

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