Mercury mercuration

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

Chemical Designations - Synonyms Albus Aminomercuric chloride Ammoniated mercury Mercuric chloride, ammoniated Mercury ammonium chloride Mercury (II) chloride ammonobasic Chemical Formula HgNHjCl. 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

Mercury/mercurous sulphate (Hg/HgSOj, SOJ ) Silver/silver chloride (Ag/AgCl, Cl )... 

Antimony/antimony oxide (Sb/Sb O, H ) Mercury/mercuric oxide (Hg/HgO, OH")... 

Mercury (Mercuric) Sulfocyanate (Mercuric thiocyanate, Mercuric rhodamide). Hg(SCN)2, mw 316.77, white powd mp (decompn), poisonous si sol in w (0.07g/100g at 25°), sol in ale and in NH3 NH4 salts. Can be prepd by pptn of Hg nitrate with Amm sulfocyanate and subsequent soln in a large quant of hot w, followed by crystn. Used in prepn of Pharaoh s Serpent and other fireworks (Ref 4)... 

Fig. 5.6 (Left) Comparison of band energy levels for different II-VI compounds. Note the high-energy levels of ZnSe. Representation is made here for electrodes in contact with 1 M HQO4. The reference is a saturated mercury-mercurous sulfate electrode, denoted as esm (0 V/esm = +0.65 V vs. SHE). (Right) Anodic and cathodic decomposition reactions for ZnSe at their respective potentials (fidp, Fdn) and water redox levels in the electrolytic medium of pH 0. (Adapted from [121])...

Similar designs are used for other REs on the basis of poorly soluble mercury compounds (1) the mercury-mercurous sulfate RE with H2SO4 or K2SO4 solutions saturated with Hg2S04, for which = 0.6151V and (2) the mercury-mercuric oxide RE, for measuring electrode potentials in alkaline solutions, with KOH solution saturated with HgO, for which = 0.098 V and E = 0.920 V. 

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

It is also important to understand how the potential gradient between an electrode and the bulk solution is established and controlled. Because the potential difference between the electrode and the bulk solution is not measurable, a second electrode must be employed. Although in general the potential difference between an electrode and solution cannot be determined, the potential difference between two electrodes in that solution can be determined. If the solution electrode potential difference of one of the electrodes is held constant by maintaining a rapid redox couple such as silver-silver chloride or mercury-mercurous chloride (calomel), then the potential... 

Mercury Mercuric iodide Mercuric nitrate Mercuric oxide Mercurous nitrate Mercuric sulphate Mercuric sulphide Mercurous sulphate... 

Property Elemental Mercury Mercurous Chloride Mercuric Chloride Methylmercury Chloride... 

Figure 4.4 The saturated calomel electrode. A platinum wire makes electrical contact with an electrode which is composed of a paste of metallic mercury, mercuric chloride (calomel) and potassium chloride. A saturated solution of potassium chloride completes the half-cell and provides electrical contact through a porous plug.

Basically, the calomel electrode consists of mercury, mercurous chloride (calomel), and chloride ion. The concentration of potassium chloride is 0.1 M in an aqueous-organic solvent (50 50) of the same nature as that contained in the solution to be investigated. The junction with the test solution is realized either with a capillary or a porous stone. When the capillary is used, a small hydrostatic pressure is maintained inside it in order to avoid any electrode contamination by the test solution. In the main part of our investigation, the porous stone junction was used. Moreover, the calomel electrode is thermostatted at 20°C, and temperature variations of this electrode giving appreciable emf variations involve uncertainty on the pon determination on the order of 0.2-0.3 poH unit/ 10°C. 

Next we discuss four types of reference electrodes hydrogen, calomel, silver-silver chloric, and mercury-mercurous sulfate electrodes. 

Mercury-Mercurous Sulfate Electrode. In this reference electrode the metal is mercury, the sparingly soluble compound is mercurous sulfate (Hg2S04), and the source of S04 anions is sulfuric acid or potassium sulfate. The electrode is made in the same way as a calomel electrode, and it is represented as... 

Raiko (Army) or Raisansuigin (Navy) Thunder Mercury . Mercuric Fulminate, Hg(ONC)2 mw 284.65, N 9.84% white to gray pdr, d 4.43, mp—decomp or detonates without melting Brisance by Sand Test—ca 55% TNT Expin Temp 210° in 5 secs Impact Sensitivity, BurMinesApp, 2-kg wt—5cm Power by Trauzl Test—51% TNT Rate of Deton 5000 at d 4.0 (AMCP 706-177, listed as Ref 8, p 201). Used straight by the Japanese in Instantaneous Fuzes, in Blasting Caps and in mixture called Bakufun (Ref 1, p 25 Ref 5, p 369)... 

Cinnabar and vermilion usually contain only small proportions of impurities from the prime materials vermilion may also contain impurities due to the method of preparation, namely, small quantities of metallic mercury, mercuric nitrate and free sulphur. These products are, however, often adulterated with ferric oxide, minium, chrome red, brickdust, gypsum, heavy spar, clay, ammonium chloride, dragon s blood, carmine and artificial organic dyes. Sometimes also arsenic and antimony sulphides are added to modify the colour. 

Gremoochaya Rtoof (Fulminate of Mercury, Mercuric Fulminate, MF). Prepd in the same manner as described in Vol 5 of the Encycl, pp F217-L to F220-R. It was used in initiating compns, usually contg K chlorate and Sb sulfide, but has been gradually replaced by Pb Azide and Pb Styphnate... 

MetaUInsoluble Salt/Ion Electrodes. Electrode potentials are usually reported relative to normal hydrogen electrode (NHE a(H+) = 1, p(H2) = 1), but they are actually measured with respect to a secondary reference electrode. Frequently used secondary reference electrodes are calomel, silver-silver chloride, and mercury-mercurous sulfate electrodes. These secondary reference electrodes consist of a metal M covered by a layer of its sparingly soluble salt MA immersed in a solution having the same anion Az as the sparingly soluble MA. The generalized reference electrode of this type may be represented as M MA AZ and may be considered to be composed of two interfaces one between the metal electrode M and the metal ions Mz+ in the salt MA... 

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