Acidic solutions, adsorption

Vysotskii ZZ, Strazhesko DN (1973) Isoelectric state of disperse silicas and ion exchange in acid solutions, adsorption and adsorbents, 1st edn. WUey, New York, pp 55-71... 

Nishihara C and Nozoye H 1995 influence of underpotentiai deposition of copper with submonolayer coverage on hydrogen adsorption at the stepped surfaces Pt(955), Pt(322) and Pt(544) in sulfuric acid solution J. Electroanal. Chem. 396 139-42... 

The citric acid solution is deionised at this stage to remove trace amounts of residual calcium, iron, other cationic impurities, and to improve crystallisation. In some processes, trace-impurity removal and decolorization are accompHshed with the aid of adsorptive carbon. 

In modern practice, inhibitors are rarely used in the form of single compounds — particularly in near-neutral solutions. It is much more usual for formulations made up from two, three or more inhibitors to be employed. Three factors are responsible for this approach. Firstly, because individual inhibitors are effective with only a limited number of metals the protection of multi-metal systems requires the presence of more than one inhibitor. (Toxicity and pollution considerations frequently prevent the use of chromates as universal inhibitors.) Secondly, because of the separate advantages possessed by inhibitors of the anodic and cathodic types it is sometimes of benefit to use a formulation composed of examples from each type. This procedure often results in improved protection above that given by either type alone and makes it possible to use lower inhibitor concentrations. The third factor relates to the use of halide ions to improve the action of organic inhibitors in acid solutions. The halides are not, strictly speaking, acting as inhibitors in this sense, and their function is to assist in the adsorption of the inhibitor on to the metal surface. The second and third of these methods are often referred to as synergised treatments. 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

Blocking of reaction sites The interaction of adsorbed inhibitors with surface metal atoms may prevent these metal atoms from participating in either the anodic or cathodic reactions of corrosion. This simple blocking effect decreases the number of surface metal atoms at which these reactions can occur, and hence the rates of these reactions, in proportion to the extent of adsorption. The mechanisms of the reactions are not affected and the Tafel slopes of the polarisation curves remain unchanged. Behaviour of this type has been observed for iron in sulphuric acid solutions containing 2,6-dimethyl quinoline, /3-naphthoquinoline , or aliphatic sulphides . 

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

For the titration of chlorides, fluorescein may be used. This indicator is a very weak acid (Ka = ca lx 10-8) hence even a small amount of other acids reduces the already minute ionisation, thus rendering the detection of the end point (which depends essentially upon the adsorption of the free anion) either impossible or difficult to observe. The optimum pH range is between 7 and 10. Dichlorofluorescein is a stronger acid and may be utilised in slightly acid solutions of pH greater than 4.4 this indicator has the further advantage that it is applicable in more dilute solutions. 

Similar remarks apply to the determination of bromides the Mohr titration can be used, and the most suitable adsorption indicator is eosin which can be used in dilute solutions and even in the presence of 0.1 M nitric acid, but in general, acetic (ethanoic) acid solutions are preferred. Fluorescein may be used but is subject to the same limitations as experienced with chlorides [Section 10.77(b)], With eosin indicator, the silver bromide flocculates approximately 1 per cent before the equivalence point and the local development of a red colour becomes more and more pronounced with the addition of silver nitrate solution at the end point the precipitate assumes a magenta colour. 

This very interesting work is the first demonstration of NEMCA for a hydrogenation reaction in aqueous solutions. It is also the first demonstration of NEMCA in an acidic solution, as all previous electrochemical promotion studies in aqueous media were restricted to alkaline solutions. This work also underlines the importance of the supporting electrolyte and of the strength of adsorption of the anions in aqueous electrochemical promotion studies. 

Adsorption of surface-active substances is attended by changes in EDL structure and in the value of the / -potential. Hence, the effects described in Section 14.2 will arise in addition. When surface-active cations [NR] are added to an acidic solution, the / -potential of the mercury electrode will move in the positive direction and cathodic hydrogen evolution at the mercury, according to Eq. (14.16), will slow down (Fig. 14.6, curve 2). When I ions are added, the reaction rate, to the contrary, will increase (curve 3), owing to the negative shift of / -potential. These effects disappear at potentiafs where the ions above become desorbed (at vafues of pofarization of less than 0.6 V in the case of [NR]4 and at values of polarization of over 0.9 V in the case of I ). 

Dederichs, F., Friedrich, K F. and Daum, W. (2000) Sum-frequency vibrational spectroscopy of CO adsorption on Pt(l 11) and Pt(llO) electrode surfaces in perchloric acid solution effects of thin-layer electrolytes in spectro-electrochemistry. J. Phys. Chem. B, 104, 6626-6632. 

Wang JX, Markovic NM, Adzic RR. 2004. Kinetic analysis of oxygen reduction on Pt(ll 1) in acid solutions Intrinsic kinetic parameters and anion adsorption effects. J Phys Chem B 108 ... 

Iwasita T, Vielstich W. 1988. New in-situ IR results on adsorption and oxidation of methanol on platinum in acidic solution. J Electroanal Chem 250 451-456. 

Figure 7.13 Variation of the CO stripping charge formed from formic acid dissociative adsorption as a function of adatom coverage for a Pt(lll) electrode modified with Bi and Se, as indicated, in 0.5 M H2SO4 solution.

Climent V, Coles BA, Compton RG. 2002b. Coulostatic potential transients induced by laser heating of a Pt(lll) single-crystal electrode in aqueous acid solutions. Rate of hydrogen adsorption and potential of maximum entropy. J Phys Chem B 106 5988-5996. 

Eeliu JM, Orts JM, Gomez R, Aldaz A, Claviher J. 1994. New information on the unusual adsorption states of Pt(l 11) in sulphuric acid solutions from potentiostatic adsorbate replacement by CO. J Electroanal Chem 372 265-268. 

Schmidt TJ, Grgur BN, Behm RJ, Markovic NM, Ross PN. 2000b. Bi adsorption on Pt(l 11) in perchloric acid solution A rotating ring-disk electrode and XPS smdy. Phys Chem Chem Phys 2 4379-4386. 

Szabo S, Nagy F. 1978. Investigations of bismuth adsorption via the ionization of hydrogen adsorbed on platinized platinum in hydrochloric acid solutions. J Electroanal Chem 87 261-265. 

---