Mercury Salts adsorption

Warburg (Wted. Ann. XLi. 1, 1890) observed that the surface tension of the interface between mercury and dilute acid docixiascs as the amount of the corresponding mercury salt present in the solution increases. He therefore concluded that the salt is positively adsorbed in accordance with Gibbs adsorption equation. The adsorption by mercury of its salts from aqueous solution has been directly observed by McLewis jPA /s. Ghem. Lxxvil. 129,... 

It remains to be determined to what extent the dye adsorption technique is applicable to other substrates. No evidence was obtained for Pseudocyanine adsorption to Mn02, Fe2Os or to pure silver surfaces, although this dye can be bound to mica, lead halides, and mercury salts with formation of a /-band (61). Not only cyanines but other dye classes can yield surface spectra which may be similarly analyzed. This is specifically the case with the phthalein and azine dyes which were recommended by Fajans and by Kolthoff as adsorption indicators in potentio-metric titrations (15, 30). The techniques described are also convenient for determining rates and heats of adsorption and surface concentrations of dyes they have already found application in studies of luminescence (18) and electrophoresis (68) of silver halides as a function of dye coverage. 

Fig. 32.6. (a) Optimisation of the thin mercury film in terms of f mm using d.c. adsorptive stripping voltammetry of 5.0 x 10-8M aminopterin in pH 5 acetate buffer. Mercury salt solution = 1.0 x 10 3M in 0.5M HC1 tm — 60s Eacc — 0.0V, tacc. = 60s film was stripped anodically at lOOmV/s. (b) Optimisation of the thin mercury film in terms of film using d.c. adsorptive stripping voltammetry of aminopterin. Emm — -0.8 V. Rest of conditions as in (a), (c) Optimisation of the thin mercury film in terms of film versus a.c. stripping voltammetry of aminopterin 5 x 10 9M. Rest of the conditions as above, (d) Two anodic stripping peaks of the thin mercury film under optimum deposition conditions tmm = 90s Emm = -0.8V scan rate = lOOmV/s. 

In some cases, like reduction of azulene or for anodic waves, corresponding to mercury salt formations with various ligands, two or even three consecutive adsorption waves can be observed at gradually increased concentration. Two or three adsorbed layers can be formed, which can differ in chemical composition, in number and structure of adsorbed layers, or in orientation of compounds in such layers. 

Barbital, Phenobarbital, Pentothal. Barbital can be determined in a borate buffer of pH 9.3 by means of an anodic wave that corresponds to mercury-salt formation. Since the wave height is governed by adsorption at higher concentrations, it is necessary to keep the concentration of barbital below 1 x 10 Af. 

On the other hand, determination of IV in the presence of I requires the use of one of its mercury salt cathodic stripping peaks (IVic or IVac). Folic acid adsorptive stripping behaviour is not affected by the accumulation potential ( 0.300 V) necesary for this purpose. Fig.3 (B) shows the possibility to carry out such a determination in 1 1 mixtures where the cathodic stripping peaks of IV are unaffected by the presence of I, and they show a linear response to external additions of IV. The second cathodic peak (IVsc) yields a better linearity and keeps growing even in a four fold presence of I, whereas the first one (IVic)is suppressed. 

Similarly, the effects of molecular weight, as well as other specific structural influences, were observed for other polarographic phenomena in which adsorbability plays an important role adsorption currents were observed especially for substances of higher molecular weight containing hydrophobic groups, e.g. for those with more condensed aromatic rings. In some instances free radicals are adsorbed and a chemisorption can be assumed. Such adsorption probably also operates in the adsorption of mercury compounds. Another factor which can play a role in the adsorbability of mercury salts is the polymerization of those salts. 

Xanthates may be determined in flotation liquids< ) by utilizing a stock solution consisting of 0-05 N sodium hydroxide, 0-1 N potassimn chloride and 0-001 M eosin. The presence of eosin counteracts the adsorption and extends the useful concentration range of 0-05 mM to 2-5 mM solutions. The anodic waves of the xanthates, corresponding to a mercury salt formation are recorded at —0-3 to —0-4 V. 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

SWV has been applied to study electrode reactions of miscellaneous species capable to form insoluble salts with the mercury electrode such as iodide [141,142], dimethoate pesticide [143], sulphide [133,144], arsenic [145,146], cysteine [134, 147,148], glutathione [149], ferron (7-iodo-8-hydroxyquinolin-5-sulphonic acid) [150], 6-propyl-2-thiouracil (PTU) [136], 5-fluorouracil (FU) [151], 5-azauracil (AU) [138], 2-thiouracil (TU) [138], xanthine and xanthosine [152], and seleninm (IV) [153]. Verification of the theory has been performed by experiments at a mercury electrode with sulphide ions [133] and TU [138] for the simple first-order reaction, cystine [134] and AU [138] for the second-order reaction, FU for the first-order reaction with adsorption of the ligand [151], and PTU for the second-order reaction with adsorption of the ligand [137]. Figure 2.90 shows typical cathodic stripping voltammograms of TU and PTU on a mercuiy electrode. The order of the... 

As an analogous example, the behavior of sulfonium salts can be mentioned. At mercury electrodes, sulfonium salts bearing trialkyl (Colichman and Love 1953) or triaryl (Matsuo 1958) fragments can be reduced, with the formation of sulfur-centered radicals. These radicals are adsorbed on the mercury surface. After this, carboradicals are eliminated. The carboradicals capture one more electron and transform into carbanions. This is the final stage of reduction. The mercury surface cooperates with both the successive one-electron steps (Scheme 2.23 Luettringhaus and Machatzke 1964). This scheme is important for the problem of hidden adsorption, but it cannot be generalized in terms of stepwise versus concerted mechanism of dissociative electron transfer. As shown, the reduction of some sulfonium salts does follow the stepwise mechanism, but others are reduced according to the concerted mechanism (Andrieux et al. 1994). 

Whether the diminution in mercurous salt concentration of a solution in contact with an expanding mercury surface be regarded as due to the adsorption of the salt on the newly formed surface as imagined by Gibbs and Warburg or as due to the transference of mercurous ions from the aqueous to the metallic phase as postulated by Lippmann and Nemst, we have seen that the number of grm. equivalents removed from the solution per unit increase in 1... 

Fig. 7.45 Experimental corrected (<2dscvc/A ) — E curves (symbols) and (7cv /v) — E ones (idashed lines) corresponding to the disodium salt of the 2,6-Anthraquinonedisulfonic acid (AQDS) 1.0 pM + HC104 0.5 M adsorpted on a mercury electrode. The sweep rate in both techniques is v = 1.0 V s 1. The values of AE (in mV) of the DSCVC curves are 1 (dark gray diamonds), 3 (white circles), 5 (gray squares), and 10 (black triangles). The initial potential in both DSCVC and CV technique was E iai = — 150mV. rs = 0.0316cm, and T= 296 K. A vertical dotted line indicates the value of the formal potential E = 0.062 V vs. Reference. Reproduced from [4] with permission...

The experimental verification of the theoretical behavior of DSCVC responses is presented in [4] for two systems which have been adsorpted on a mercury electrode forming stable sub-monolayers the disodium salt of the 2,6-Anthraquinonedisulfonic acid (AQDS) 1.0 pM + HCIO4 0.5 M, which behaves as reversible (Fig. 7.45), and the 4-PhenylazoPhenol 5 pM + 0.5 M KNO3 (pH = 8.0), which behaves as quasi-reversible (Fig. 7.46). 

Diagram (a) shows the results obtained with several inorganic salts, the adsorption of whose anions at the mercury surface increases in the order Cl < Br < CNS < I < S". The uppermost curve, for potassium hydroxide, is nearly symmetrical. The effects of the adsorption of the anions are a considerable depression of tension on the ascending part of the curve, a small or moderate depression at the maximum, and a shift of the applied voltage required to reach the maximum, to more negative values. The descending part of the curve is not appreciably altered a short distance past the maximum. 

The method of atomic adsorption analysis has been proposed for quantitative determination of mercury in the correspondent salt of 5-nitrotetrazole [1182], Chromatography is widely used for analysis and separation of nitroazoles. For example, thin-layer chromatography was used for separation of nitropyrazoles [1431, 1432], nitroimidazoles [1133, 1309, 1431], nitrobenzoxazole derivatives [1433], and 5-nitro-2,l,3-benzoselenadiazole [1434],... 

The majority of aliphatic ketones give the secondary alcohol on reduction at electrodes of carbon, mercury, lead, or platinum. The usual choice of electrolyte has been dilute sulfuric acid, acetate buffer, or a neutral salt solution, which will become alkaline during the course of reaction that consumes protons. Relatively few studies have been recorded of the isomer ratio obtained by reduction of open chain ketones with a prochiral center adjacent to the carbonyl function [32,33]. Results are collected in Table 2, and one aromatic carbonyl compound is included here for convenience. In general, the erythro-alcohol is favored and in an excess over that present in the equilibrium mixture [32,33]. These results are explained in terms of adsorption of intermediates at the electrode surface. For many of the examples in Table 2, the total yield of alcohol is low and this result is not generally typical of aliphatic carbonyl compounds, as can be seen from Table 3. 

Nature of This Study. The following represents preliminary results of an initial study on the adsorptive behavior of two organometal-llcs. Numerous organometalllcs comprise the active ingredient of various pesticides. Examples include phenylmercurlc salts, diphenyl mercury, triphenyl, tributyl, and tricyclohexyltin salts, as well as organoarsenic compounds (Table 1). Because of the toxicity of these substances their behavior and associations in natural water systems is a matter of concern. 

It will be useful to emphasize the practical aspects of the problem which are twofold the solution side and the metal side. On the solution side at the interphase, a level of impurities which does not interfere with dl measurements over the time scale of a mercury-drop lifetime, which is 4 s, could completely hinder observations of significant current-potential curves [i( )] or meaningful differential capacity-potential curves [C(E)] at a solid metal electrode which will stay 2, 3, or 4 h in the same solution. Not only must the water, salts, and glassware be kept clean, but also the gas used to remove oxygen and the tubing for the gas. Of course, conditions are less drastic for studies of strong adsorption than in the case of no adsorption also bacteria develop less in acid solutions than in neutral ones (which cannot be kept uncontaminated more than one or two days). This aspect will not be discussed in this chapter. 

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