粉状白钨酸的制备

钨酸的颜色有黄有白,并有粉状与胶状的区别。其中黄色胶状物是黄色粉状与白色胶状的混合物。一般制得的白色粉状钨酸,又或多或少地杂有胶状物,不易定其组成。所以长时期来,只把黄色粉状钨酸作为钨酸的代表,并作为制备金属钨及其他钨化合物的基本原料。黄色粉状钨酸和白色钨酸的生成机理不同,但二者在生成时每易交织在一起。黄色粉状钨酸是均相沉淀的产物,即是含钨络阴离子的分解产物。

THE PREPARATION OF TUNGSTIC ACID IN WHITE AND POWDERLY FORM

It has been shown in previous articles^[1,2], that yellow powderly tungstic acid is not merely a metathetical product between a tungstate salt with an acid. Its formation involves a series of consecutive reactions, in which complex tungsten ion is an intermediate form. Improper regulation of conditions^[3] will invariably result in admixture of impure white tungstic acid,revealed in its slow dissolution in cold ammonium hydroxide solution, different from the ready solution of the yellow powdery form.When sodium tungstate solution is acidified by dropwise addition of any acid, white tungstic acid will appear due to local reaction. It is curdy in form, and is incapable of being isolated on account of its succeeding reaction with existing tungtates in solution. M. L. Freedman^[4] at 1959 was however able to isolate white tungstic acid (C-Phase) in virtually powder form. But Na was still found present in small amount, which indicated the occulation of certain gelatinous form. The yield was also low, only 30%.On recognizing the definite presence of white powderly tungstic acid as a distinct species, and after considering the possible mechanism for its formation, we were enabled to design a procedure for the preparation of pure white powderly tungstic acid in rather high yield. This involves using low acidity, room temperature, an acid with low ability to form complex ion of tungsten, prevention of colloid formation as well as its peptization, and most important of all, the "reversed way of acidification", i.,e., addition of sodium tungstate solution to a dilute acid solution.Filtration and washing were rapid; the filtrate was clear. After washing with pure water, final hashing with ethyl alcol or acetone enabled the removal of adsorbed water, so as to prevent the gradual discoloration of the white powder to faint yellow. The yield after drying at room temperature was 95%.