Mass transfer salt dissolution

The effect of agitation, as produced by a rotary stirrer, for example, on mass transfer rates has been investigated by Hixson and Baum 2-1 who measured the rate of dissolution of pure salts in water. The degree of agitation is expressed by means of a dimensionless group (Nd2p/fx) in which ... 

The theories proposed to explain the formation of passivation film are salt-film mechanism and acceptor mechanism [21]. In the salt-film mechanism, the assumption is that during the active dissolution regime, the concentration of metal ions (in this case, copper) in solution exceeds the solubility limit and this results in the precipitation of a salt film on the surface of copper. The formation of the salt film drives the reaction forward, where copper ions diffuse through the salt film into electrolyte solution and the removal rate is determined by the transport rate of ions away from the surface. As the salt-film thickness increases, the removal rate decreases. In the acceptor mechanism, it is assumed that the metal-ion products remain adsorbed onto the electrode surface until they are complexed by an acceptor species like water or anions. The rate-limiting step is therefore the mass transfer of the acceptor to the surface. Recent studies confirmed that water may act as an acceptor species for dissolving copper ions [22]. 

On the other hand, coordination complexes or organometallic compounds can be solubilized in ionic liquids, especially hydrophobic or anionic complexes [71, 78], It has been pointed out earlier that there are tricks to circumvent dissolution problems such as dissolving a metal salt and the IL in an organic solvent followed by solvent evaporation. Furthermore, the viscosity, which is much higher in ILs than in conventional solvents, will dramatically reduce mass transfer, which in turn will lead to a much slower metal salt dissolution [71]. 

The enhancement of mass transfer in the solid/liquid system is a frequent stirring operation. It should be remembered, that many salts must be dissolved in the liquid, to prepare a salt solution or to initiate a chemical reaction. In order that the dissolution process proceeds rapidly, the whole surface of the solid particles must be wetted as completely as possible by the liquid and the liquid flow should be turbulent, so that the boundary layer on the liquid side is small and the transfer of the dissolved material to the bulk of the liquid proceeds rapidly. 

In another study [232], which was also concerned with the dissolution of salts in liquids during simultaneous gassing, the following mass transfer relationships... 

The limiting current density is an important parameter for the analysis of mass transfer controlled electrochemical processes and represents the maximum possible reaction rate for a given bulk reactant concentration and fluid flow pattern. During anodic metal dissolution, a mass transfer limiting current does not exist because the surface concentration of the dissolving ion (e.g., Cu + when the anode is composed of copper metal) increases with increasing current density, eventually leading to salt precipitation that blocks the electrode surface. 

We will not be concerned here with diffusion per se instead we will concentrate on the issue of mass transfer between phases and how that is handled in the context of our analysis tools. The examples begin with an analysis of the dissolution of salt in water and move to more complex systems including the permeation of hydrogen through a palladium membrane. 

The details of the deposition and dissolution processes of the DAB-dend-(NHCOFc) in tetra n-butyl ammonium perchorate (TBAP)/CH2Cl2 solution were investigated using the electrochemical quartz crystal microbalance (EQCM) technique as well as admittance measurement of the quartz crystal resonator by Takada etal. [77]. It was found that the oxidized form of the dendrimers deposited onto the Pt electrode likely due to the low solubility of the salt composed of the oxidized dendrimer (ferricenium form) and C104 anions. On the other hand, the reduced form of the dendrimers easily redissolved except for the first monolayer, which appeared to be strongly adsorbed. Further, the mass-transfer process, during the redox reaction of the adsorbed dendrimers in an AN solution, was found to be of the anion exchange type. The resistance measurements of the quartz crystal resonator based on the admittance also supported the results obtained by EQCM. 

To ensure that the experimental procedure adopted results in the highest-possible colloidal nanoparticle concentration, the sequence of precursor addition was reversed. A second scheme which involved mixing the metal salt powder with microemulsions already containing the stoichiometric amount of NaOH solution was tested. Both schemes succeeded in forming stabilized colloidal nanoparticles however, higher uptake was obtained when scheme 1, solubUizing the metal salt before adding NaOH, was employed. The lower uptake associated with scheme 2 was attributed to the formation of a mass transfer barrier of the metal oxide/hydroxide at the surface of the salt powder, which prevented further dissolution. 

Anodic dissolution of vanadium metal at current densities below 1 A/cm leads to the formation of vana-dium(II) ions and the reaction kinetics are controlled by the diffusion of the reaction products formed. At higher current densities the anodic process is accompanied by various types of salt passivation. V(II) complexes can be reversibly oxidized to V(in) on a glassy carbon anode. This reaction is also controlled by mass transfer. 

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