Al-P system The present paper dealts with the interaction between H_3PO_4 and AlCl_3 solutions varying their concentration and proportion at different pH values. The experimental conditions and procedures except using AlCl_3 as an Al source are quite similar to those of Fe-P system in previous paper. Results obtained this time were as follows : (1) Fixation curve of P in Al-P system was substantially in agreement with that of Fe-P system in its shape. However, fixation of P took place at higher pH range, from about 3 to 10,compared to range of the Fe-P system. (2) Optimum pH range of P-fixation was affected by P/Al ratio of the system; and the lower the ratio, viz., the higher percentage of Al was, the higher the pH of range was. (Fig. 1). (3) Complete fixation of P was observed only in the series where P/Al ratio was less than unity. Therefore, when P/Al ratio of the system was above unity, viz., when added P in the system was present equivalently or excessively to Al, P was not so much fixed as 100 per cent; concequently some portion of P remained in a soluble form. For instance, the percentages of fixed P in each series, P/Al ratio of which being 2,1 and below 0.5,were 50,80 and 100,respectively (Tab.1). (4) The pH range of complete fixation (retention) of Al coincided with that of the maximum fixation of P, but it was shifted to higher pH values with decrease of P/Al ratio of the system (Tab. 2 and Fig. 2). Comparison of Fe-P system with Al-P system. (1) Being added in the both systems P was fixed by Fe or Al at all pH zones except alkaline and strong acidic reactions; and its maximum fixation took place at several pH value according to conditions namely, the pH value varied with P/Al or P/Fe ratio of the system; the lower the ratio was the more the pH was shifted to higher value. (2) In the both systems the percentage of the P-fixation decreased with increase of P/Fe or P/Al ratio of the system, and no difference was between the both systems in their decreasing manners. (3) Fixation of total P was completed only in the series, when P/Al or P/Fe ratio was less than one equivalent (Tab. 3). (4) However, the pH zone which caused the maximum fixation of P was of higher value in the Al-P system than in the Fe-P system (Tab. 3). (5) SWENSON has recently noted that Fe- or Al-compounds of P formed in the pH range of 2 to 7 should be basic Fe- or Al-phosphate formulating Fe(H_2O)_3(OH)_2・H_2PO_4 or Al(H_2O)_2・H_2PO_4. From this point of view, the conclusions were obtained with precipitated products in the following way. (Tab. 4). Only in the series, P/Fe or P/Al ratio of which being 2,all the preciptates were produced in such a form as the above mentioned. With further lowering of the ratio below unity, basic Fe- or Al-phosphate decreased in the precipitates and hydroxides of iron and aluminum increased. Though all of retained P in the series whose P/Fe or P/Al ratio was 2 was in the form of basic Fe- or Al-phoshate, the proportion of the phosphate present in the precipitates was only one half of added P; while remaining half portion was in a soluble state for depletion of Fe or Al. On the other hand, when the above described ratio was below unity, all of added P was fixed as basic Fe- or Al-phosphate. (6) The retention curve of Fe was nearly identical with that of Al, aside from the following two points pointed out as a discrepancy : a) The solubility of Fe which was raised once at a weak alkaline reaction started to be again suppressed with further advance of pH value, and finally all of Fe flocculated. Contrarily, in the Al-P system, such a phenomenon has not been found as to Al, because of the fact that Al enters into solution depending upon the formation of aluminate at an alkaline reaction. In short, the point of this temporal solubility reduction of Fe at a weak alkaline reaction in the Fe-P system was correspondent to a point of inter section of the solubility curve of

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