Abstract
The rate of precipitation of nearly monodisperse cubic PbS sols can be controlled by the addition of the chelating agent EDTA. This reaction has been studied turbidimetrically in 0.1M NaCl, and a rate law describing this precipitation process has been evaluated at room temperature in the pH range 8.0–8.5. The rate law has the form: dm PbS /dt = K ∗ obs A( PbEDTA 2−) 3.3 ( HS −) 3.4/ ( HEDTA 3−) 1.8 , where m PbS is the mass concentration of PbS in g/ml, A is the surface area of the sol in cm 2/ml, and K ∗ obs is the rate constant, equal to 3.58×10 4 when time is expressed in seconds. Temperature changes over the range 25–50°C had no discernible effect on the rate of precipitation, indicating that the activation energy of the reaction was near zero. A variety of other strong Pb(II) chelating agents were also tested for precipitation-inhibiting properties. Experimental evidence strongly suggests that high coordination number ligands are required to inhibit precipitation, not only to supress the nucleation reaction involving free Pb(II) and HS −, but to limit growth on existing particles as well.
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