Kinetic Effects in Precipitate Dissolution

The dissolution speed of the precipitate is usually not regarded as an important factor in batch procedures, as long as the dissolution could be completed in a few minutes or even somewhat longer time. For extremely slow dissolutions, the rate of dissolution may be accelerated by increasing the temperature. For on-line precipitate dissolutions, the speed of dissolution must be much faster, especially if the objective is to achieve sensitivity enhancements. Elevated temperatures are of limited use in on-line dissolution systems as this may produce undesirable gas bubbles in the system conduits and deteriorate precision. 

In on-line precipitation-dissolution systems with flame AAS detection, the volume of solvent used to dissolve the collected precipitate should be as small as possible in order to increase the concentration of the analyte in the eluent. On the other hand, the flow-rate of the dissolution solvent should be reasonably close to the free uptake rate of the nebulizer, so as not to starve the nebulizer excessively. Failing to do so will result in a decrease in sensitivity and possibly deterioration in precision due to the formation of air bubbles in the solvent. Strong solvents have to be used in order to satisfy these requirements. However, the form of the precipitate is also important for achieving fast dissolution. Gelatinous and curdy precipitates have larger surface areas than crystalline precipitates, and thus are more readily dissolved using the same solvent. 

An example of the importance of the choice of dissolution solvent is illustrated by the dissolution of the precipitate from the reaction of cobalt with 1-nitroso-2-naphthoI [12]. Variations in peak widths and heights were observed owing to the different kinetic properties of the solvents, and ethanol was chosen for its faster dissolution and cheaper cost. 

---