| 摘要 |
<p>The calcium tungstate is prepd. by pptn. from aqs. salt solns, in which a first aqs. soln. contg. a tungstate and a second aqs. soln. contg. a calcium salt are added with stirring to a third aqs. soln. contg. either tungstate ions or calcium ions. The concn. of the first and second solns. is at most 0.5 mol/l, and the rate of their addition and the concn. of the third soln. is such that during the entire mixing process, a slight excess of tungstate or calcium ions is present. The mixing is effected slowly with vigorous stirring so as to prevent any local excess and the temp. of the resulting mixture is increased, after pptn. of the calcium tunstate, so as to facilitate crystallisation. The concn. of the first and second solns. is 0.2-0.5 mol/l., the excess of tungstate or calcium ions in the reaction mixture is below 0.05 mol/l, the concn. of the third soln. is 1/10 that of both the other solns., and the temp. is not allowed to exceed 50 degrees C during the reaction.</p> |
