Leaching local concentrations

One simple way to increase the solubility of organic substrates in aqueous solution is the addition of co-solvents to an aqueous two-phase system. This increases the reaction rate of the Rh/TPPTS catalyst, but at the same time the selectivity drops [7]. Furthermore, it can result in troublesome phase separation and leaching of the co-solvent into the product phase [7]. Instead of the addition of co-solvents triphenylphosphine can be added to the two-phase Rh/TPPTS system. The local concentration of the active rhodimn complex at the interface will be increased, resulting in a rate enhancement in the hydroformylation of 1-octene by a factor of 10-50 [8]. The active species at the interface contains both TPPTS and triphenylphosphine hgands. Test reactions performed in methanol confirmed that this increase in reaction rate was due to promotion of interfacial catalysis. However, after recycling the catalyst Rh/TPPTS was found in the aqueous phase whereas the PPhs was in the organic phase and in the subsequent reaction with freshly added organic phase the activity of the catalyst is similar to that of the biphasic reaction in the absence ofPPhs. 

Several hundred tonnes of solder containing cadmium are used in the UK every year and since this is a relatively volatile metal (B.Pt. 767 ), soldering operations inevitably lead to high local concentrations of cadmium in the atmosphere. There are also hazards associated with the use of cooking vessels glazed with cadmium-based enamels, for appreciable amounts of cadmium can be leached out during cooking. 

Lead levels ranging between 10 and 30 pg/L can be found in drinking water from households, schools, and office buildings as a result of plumbing corrosion and subsequent leaching of lead. The combination of corrosive water and lead pipes or lead-soldered joints in either the distribution system or individual houses can create localized zones of high lead concentrations that exceed 500 pg/L (EPA 1989f). 

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