A theoretical discussion in terms of molecular theory and the results of model experiments are employed to demonstrate that the conventional analysis of “anion fixation” data, through linear least squares regression of the points in a Langmuir plot, usually will not be sensitive enough to show the failure of the Langmuir equation whenever solubility product considerations are essential in the fixation reaction. The theoretical discussion points out that although the Langmuir equation is not restricted to two‐dimensional phenomena (i.e., adsorption), it cannot apply if the reacting anions must be present at some threshold concentration before fixation occurs. An analysis of model experiments on the reactions of OH‐ with Al‐ or Fe‐resin and of Cl‐ with Ag‐resin shows that secondary precipitation phenomena always result in a statistically significant, linear correlation of the variables in a Langmuir plot if the concentration of the reacting anions is much larger than the threshold value needed to initiate precipitation of the secondary solid phase. This condition generally can be expected to be met when anions such as o‐phosphate react with soils or soil constituents. Therefore, the Langmuir equation usually will appear to apply, on the basis of a statistical analysis, even when its applicability is impossible in principle. It follows that the Langmuir equation cannot be used statistically to determine whether adsorption or precipitation (formation of secondary minerals) is occurring during anion fixation reactions in soils and that the parameters in the Langmuir equation cannot be interpreted in terms of surface reactions without additional, independent evidence that only adsorption is involved in anion fixation.

Abstract--Four Na2S20,-reduced Na-vermiculites, each with some trioctahedral mica interstratified, were oxidized with H20 2 at pH 6.5 and again reduced with Na2S20 * in suspensions at pH 7.5-8.0. The layer charge (CEC + K +), measured at pH 6.50, did not change significantly when octahedral Fe was oxidized (7-92 mmole I00 g-1) or reduced (6-71 mmole 100 g-1). Electroneutrality was maintained within the octahedral sheet when Fe was oxidized or reduced. When Fe(II) was oxidized, electroneutrality was maintained by deprotonation of octahedral OH-groups,and by ejection of (dissolution of structural) octabedral metallic cations, The weathering increment was small since, of the total amount of Fe + Mg, less than 1"3 per cent was ejected from any of the four vermiculitic materials. When biotite was K-depleted, about 20 m-equiv of layer charge per 100g (300~ basis) was lost, while 51 mmole of Fe(II) per 100g was oxidized in the presence of Na2SzO 4 and 82 mmoles in its absence in the aqueous suspensions. Since sequential reduction-oxidation-reduction treatments of K-depleted biotite and mica-containing vermiculites did not cause signifcant changes in layer charge (r 2 = 0"04), the layer charge changes were concluded to be entirely independent of the oxidation or reduction of Fe in these minerals.

The formation of X‐ray amorphous analogs of variscite and Namontebrasite was demonstrated to occur when Al2O3 · nH2O, synthetic allophanes, and allophanic soils were reacted with sodium o‐phosphates, Na3‐AHAPO4, of varying acidity A. The formation of the two amorphous Al‐phosphates, Al(OH)2H2PO4 and AlOHNaPO4, respectively, was favored both by an increase in lability of Al (i.e., by large values of the hydration number n in Al2O3·nH2O or the presence of Si‐O‐Al bonds in the aluminosilicates) and by an increase in the acidity A of the added o‐phosphate solution.Evidence for the formation of amorphous Al‐phosphates by secondary precipitation rather than the formation of a surface o‐phosphate phase by adsorption was indicated by the slowness of the reaction between o‐phosphate and Al‐containing material, by the immediate and significant increase of Si in solution when aluminosilicates were reacted, and by the large amounts of P reacted (e.g., 82% of the P added as equimolar H3PO4 reacted with synthetic allophane). The presence of amorphous Al(OH)2H2PO4 in the products was demonstrated by showing that a solid phase, Al‐P compound was formed in all of the reactions studied that had the same pH stability limites (2.5 ≤ pH ≤ 9) and solubility product pKso (28.1 ± 0.1) as were observed in a previous investigation for amorphous Al(OH)2H2PO4 formed directly from the reaction of Al‐hydroxy‐chlorides and o‐phosphate. The presence of amorphous AlOHNaPO4 in the products was inferred from an analysis of their formation curves (P in a solid phase vs. pH) and from measurements of the amount of Na bound with Al and P, corrected for adsorption of Na, as a function of pH. The formation of the amorphous, basic, Na‐containing AlOHNaPO4 begins at pH ≃ 6 and ends with the appearance of Al(OH)4‐ at pH ≃ 10.

Montmorillonite, vermiculite, and a sulfonic resin were treated with different concentrations of Al(OH)BCl3‐B with the basicity B ranging from 0 to 2.2. The Al exchanged by vermiculite and the resin was always the nonbasic Al3+, independent of B and the amounts of Al(OH)BCl3‐B added. The Al exchanged by montmorillonite was Al3+ when exchanged from Al(OH)BCa0.5B‐montmorillonite containing 32 mmoles Al/100 meq CEC and became basic when large amounts of Al(OH)BCl3‐B had been added before the exchange reaction. In contrast to vermiculite and the resin, the montmorillonite adsorbed preferentially relatively large OH‐Al polymers and, therefore, accumulated much more Al in the interlayers than vermiculite and resin which excluded large OH‐Al polymers because of their smaller d spacings and/or high charge densities. When more OH‐Al accumulated in the interlayers of montmorillonite, the Al was adsorbed less strongly, and relatively large amounts of basic Al became exchangeable. It was shown also that an Al3+‐saturated vermiculite is much more unstable than an Al3+‐montmorillonite. These results are employed to discuss the widely‐observed formation of gibbsite when smectite reacts with basic Al. Finally, the exchange acidity from an Al‐saturated exchanger was shown to consist of both Al3+ and H+. The protons come from (i) the hydrolysis of exchanged Al3+ which, in turn, can be readsorbed as OH‐Al; (ii) from the hydrolysis of adsorbed Al; and (iii) by the exchange of H+ from permanent charges. The latter portion mechanism provides an amount of exchangeable acidity that, if it exists, is relatively small.

Bildung von o‐Phosphaten. I. Bestimmung von Aluminium‐o‐phosphaten in Gegenwart von Natriumionen In der vorliegenden Arbeit sollten zunächst die sich bildenden Al‐Phosphate aus Reaktionen zwischen Aluminium und o‐Phosphat ermittelt werden, die besonders bei Umfällungs‐ und Fällungsreaktionen von Interesse sein können. Dazu wurden H3PO4‐AlCl3‐Mischlösungen unterschiedlicher Molverhältnisse mit NaOH titriert und die Reaktionsabläufe potentiometrisch verfolgt (Abb. 1). Die jeweils titrierten H3PO4/AlCl3‐Molverhältnisse wurden gegen die NaOH/AlCl3‐Molverhältnisse an den korrespondierenden Wendepunkten in ein x‐y‐Diagramm aufgetragen (Abb. 2). Es entstanden Geraden, deren mathematische Auswertung u. a. drei Al‐Phosphate erbrachte: das dem Variszit analoge Al(OH)2H2PO4, das dem Wavellit analoge Al3(OH)8H5(PO4)2 sowie ein dem Na‐Montebrasit analoges Al(OH) NaPO4. Die Existenzbereiche der drei Al‐Phosphate wie der Nebenprodukte sind in Abb. 3 in Abhängigkeit vom pH und vom vorgelegten H3PO4/AlCl3‐Molverhältnis dargestellt. Dabei hat das Al3(OH)8H5(PO4)2 den relativ geringsten Existenzbereich (zwischen Gerade LIa und Gerade LIIIa). Der Existenzbereich von Al(OH)2H2PO4 ist weit größer, als in der Literatur angenommen wird, und beginnt unterhalb der Geraden LIa, d. h. bei pH < 3, und endet etwa mit der Geraden LIVb, d.h. bei pH ˜ 8. May. Das Na‐haltige, basische Al(OH)NaPO4 beginnt etwas unterhalb der Geraden LIIIb (bei pH ˜ 5.0) u. endet mit der Bildung von Aluminat mit der Geraden LV (bei pH ˜ 10). Beide Al‐Phosphate, Al(OH)2H2PO4 und Al(OH)NaPO4, sind in Anwesenheit von Al(OH)3 existent, was bei einer Gesamtanalyse sowie der Reaktion zwischen o‐Phosphaten und Al‐Hydroxiden berücksichtigt werden müßte.