Kinetic Effects in On-line Precipitation and Coprecipitation

An important difference between the batch and the continuous mode of precipitation is the available reaction time. Different standing times are normally used in batch procedures to ensure the completeness of the precipitation reaction or/and the form of the precipitate to facilitate filtration and minimize contamination. Standing times of IS min to a few hours are typical, occasionally with elevated temperatures. Such procedures are obviously not feasible in continuous on-line precipitation systems where reaction times are typically in the range of a few seconds to a few tens of seconds. Quantitative recovery of analyte through precipitate collection is therefore not likely unless the precipitation (or coprecipitation) process is extremely fast. 

This should not be considered as a drawback when one realizes that FIA is a technique essentially performed under physically and chemically non-equilibrium conditions, and that reproducibility of the reaction process rather than its completeness is the key issue of the technique. With proper calibration, good results may be obtained under nonequilibrium conditions for the precipitation. This has been pfewen experimentally in the coprecipitation of cobalt and nickel with Fe(lI)-HMDTC [22]. Although the analyte collection efficiency was onl approximately 50%, good sensitivity and precision were achieved. with excellent agreement of analytical results with certified values of standard reference materials, covering a large variety of different sample matrices. 

The optimum conditions for the precipitation may also vary when a batch procedure is adapted to work in a continuous mode. Thus the optimum pH range for the coprecipitation of lead with Fe(II )-HMDTC (pH 2-3) was found to be much narrower in the FI procedure than in the original batch procedure (pH 1-7) [21]. This was also considered to be a kinetic effect due to insufficient time for equilibrium in the continuous mode, so that coprecipitation can only be complete under the most favorable conditions. This phenomenon could, however, be made use of to overcome certain interferences from coexisting elements by kinetic discrimination, and should not be simply regarded as a drawback of the continuous approach. In manual batch procedures one always waits for a reaction to complete, unfortunately too often allowing interfering side reactions to fully develop. The equilibrium is therefore often achieved with a significant loss in selectivity. 

After studying the interference effects of ten main potential interference ions in the determination of chloride by silver chloride precipitation, Valcarcel s group has shown that the FI approach exhibited 2-12 fold greater tolerance to the interfering ions when compared to the batch manual counterpart, all performed without precipitate dissolution [3,15]. These authors concluded that the improved selectivity of continuous precipitation 

The enhancement in selectivity of continuous precipitation processes is an important benefit which by far outweighs the incompleteness in precipitation. In fact, this should be an important field for future exploration. 

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