Precipitation of sparingly soluble alkaline-earth and lead salts; the effects of chelating additives on nucleation and growth rates

Abstract

The precipitation of some sparingly soluble calcium, strontium, and lead salts from aqueous solution was studied in the presence of chelating organic anions and low molecular-weight polyphosphates. Nucleation and growth rates were estimated from combined measurements on induction period (t) and crystal number (N). [t is defined as the time after mixing for the rapid surge in crystal growth.] At any metal salt concentration (C_s) nucleation rates decrease with increasing chelator concentration (C_x) according to an exponential relation and are reduced more than 10⁶ times at the ‘limiting chelator concentration’C_x=C_x* at t= 100,000 sec. C_x*:C_s ratios very with the stability of the corresponding metal cation–chelator complex from ca. unity for ethylenediaminetetra-acetate to >100 for acetate while C_x:C_s* <0·002 for tri- and tetra-phosphates. Growth rates decrease with increasing C_x according to an adsorption isotherm in C_x and are reduced ca. 100 times at C_x=C_x*. Induction periods increase with C_x according to an exponential relation.

For any metal cation, the effect of any chelator varies from salt to salt and with metal salt concentration. The very marked reduction in nucleation rates is explained by ‘chemisorption’ of chelator molecules on to the metal salt nuclei; some empirical quantitative relations based on this hypothesis are presented.