Dissolution of Microparticles

Small particles of calcite (calcium carbonate, CaCOs) dissolve in acidic solution by means of the following mechanism  

The following rate law has been measured [R.G. Compton etal. Freshwater Biology 22 (1989) 285] for the reaction of protons at the calcite surface  

Explain the form of the rate equation and suggest an explanation for the observation that while large ( 100/rm) particles of calcite dissolve with a rate controlled by the diffusion of protons to the calcite surface, for much smaller particles the surface controlled reaction quantifed by Eq. 5.3 is found to apply. 

The rate equation shows that the flux, Jq 2+, of dissolving Ca ions depends on the concentration of protons, [H ]o, adjacent to the particle surface (hence the subscript o on the concentration), in a first-order manner. Given the units of flux are molcm s and those of concentration are molcm , the units of the first-order rate constant ki are cm s .  

Whether the dissolution reaction will be controlled by diffusion or by the surface reaction kinetics depends on the relative rates of the two processes. If we assume the CaCOa particles to be approximately spherical then (using Eq. 5.2) the diffusion-controlled flux of protons to the surface can be calculated from 

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Diebold SM, Dressman JB. Dissolution of microparticles—a hydrodynamically based contribution to an unresolved pharmaceutical issue. Pharm Res. In preparation. 

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