Phosphoric acid metal dissolution

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Alkali metal alkoxides such as KOH, NaOH, and CH3ONa are the most effective catalysts in alkali-catalyst transesterification. When using KOH, NaOH, and CH3ONa alkali-catalyst for FAME conversion, the active catalytic species were the methoxide anion (CH 0 ), formed by the reaction between methanol and hydroxide ions of KOH and NaOH. In addition, the methoxide anion was formed by dissolution of sodium methoxide. Sodium methoxide causes the formation of several byproducts, mainly sodium salts, that have to be treated as waste and additionally require high-quality oil (16). However, KOH has an advantage because it can be converted into KOH by reaction with phosphoric acid, which can serve as a fertilizer. Since KOH is more economical than sodium methoxide, it is the preferred choice for large-scale FAME production process. 

When an alkaline oxide such as MgO is stirred in phosphoric acid, the pH of the solution rises slowly due to the neutralization of this acid. Initially, the phosphoric acid has pH 0, but initial dissolution of the oxide and reaction with phosphate anions precipitate phosphate salts. This neutralization of the acid raises the pH of the solution to >2. Even in this pH range, the acid dissolves sufficiently, and protons and H2PO4 anions are readily available to react with the ions produced by the dissolution of metal oxides. Subsequently, consolidation of the precipitate in the neutral solution leads to the formation of ceramics. 

For Co electrodissolution in phosphoric acid, Sazou and Pagit-sasi32 i33 carried out a systematic study of the dynamic behavior in the voltage/extemal resistance parameter plane. The skeleton bifurcation diagram they found is typical for an NDR oscillator that is, bistability between stationary states occurs at high values of ohmic resistance, whereas oscillations are observed at relatively low values of the external resistance. However, from a chemical point of view, Co dissolution seems to be among the most complicated metal electrodissolution reactions because quite a number of different oxide species are involved. Explanations of the dynamics hardly go further than a general statement that the instabihties are due to the formation of a passive film in combination with an IR drop. 

FIGURE 22.23 Thickness of passive oxide films on metallic iron in 0.15 kmol m 3 phosphoric acid solution and on metallic titanium in 0.1 kmol m-3 sulfuric acid solution as a function of electrode potential [30-32] L = film thickness, iFe = iron dissolution current, iTi — titanium dissolution current, and ia = anodic total current. 

The effect of water composition is not unique. Some water components can slow down the process, i.e., play the role of inhibitor, some others, on the contrary, accelerate it, playing the role of catalyst. For instance, alkali and alkali-earth metals usually slow down the process of alumosili-cate dissolution in acid solutions. Similar inhibitor role may also be played by other cations. For instance, AP+ noticeably obstruct dissolution of iron oxides. Inorganic ligands may accelerate and slow down minerals dissolution. A well-known example may be the effect of the phosphoric acid. 

Several known systems dissolve cellulose (126-129). These systems range from solutions in protonic acids (e.g., 78% phosphoric acid) to metallic complexes (e.g., cuprammonium). All known methods for dissolving cellulose can be fit into four main categories (128) cellulose acting as abase, cellulose acting as an acid, cellulose complexes, and cellulose derivatives. The cellulose derivatives are distinguished from those discussed previously in that dissolution occurs simultaneously with derivative formation and the derivative produced can easily be regenerated (129). 

This technique has the advantage that it reaches parts inaccessible to the other methods. Polishing action results from selective dissolution of the metal surface, and care must be taken to prevent etching and excessive removal of the metal. It is believed that the A1 surface is first oxidised to AI2O3 and this is then dissolved by the phosphoric acid. On rough surfaces, these reactions proceed more rapidly on the mountains than in the valleys , thus tending to smooth out the uneven surfaces. 

In addition to platinum dissolution discussed in Sect. 2, platinum area may also be lost by sintering. Two sintering mechanisms have been postulated for hot, concentrated phosphoric acid (1) the Smoluchowski collision model in which crystallites migrate, collide, and coalesce on the support and (2) Ostwald ripening, in which crystallites dissociate into metal atoms that diffuse to and associate with larger particles (Bett et al. 1976). 

Phosphoric acid solutions are used for pickling of metals, especially aluminium alloys. The dissolution rate can be controlled by adding chromates, which slow down the attack, depending on the acid concentration [23] (Table E.5.12). 

Applications Quantitative dry ashing (typically at 800 °C to 1200°C for at least 8h), followed by acid dissolution and subsequent measurement of metals in an aqueous solution, is often a difficult task, as such treatment frequently results in loss of analyte (e.g. in the cases of Cd, Zn and P because of their volatility). Nagourney and Madan [20] have compared the ashing/acid dissolution and direct organic solubilisation procedures for stabiliser analysis for the determination of phosphorous in tri-(2,4-di-t-butylphenyl)phosphite. Dry ashing is of limited value for polymer analysis. Crompton [21] has reported the analysis of Li, Na, V and Cu in polyolefins. Similarly, for the determination of A1 and V catalyst residues in polyalkenes and polyalkene copolymers, the sample was ignited and the ash dissolved in acids V5+ was determined photo-absorptiometrically and Al3+ by complexometric titration [22]. 

The process of metals electrodissolution in ionic liquids has been apvplied to develop several electropohshing procedures suitable for stainless steel, when a Type in eutectic has been involved containing ethylene glycol as the hydrogen bond donor (Abbott et al., 2004 Abbott et al., 2006) The anodic dissolution occurs in the same manner as in the case of the aqueous phosphoric and sulphuric acids electrolytes but the current efficiency was significantly improved up to about 90% and the ionic medium is much less toxic and corrosive. 

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