The total solubility of amorphous FeIII‐hydroxide and α‐FeOOH in 3M NaClO4 has been determined using 59Fe as tracer. The solubility determination of the amorphous hydroxide extended over the pH‐range 3–14. At pH < 3.5 the results agree satisfactorily with the value calculated using the solubility‐product and the constants of hydrolysis. At pH 5–12 an average solubility of ∼ 10−7 M has been measured but the results show considerable fluctuations. Therefore a colloidal solubility has been assumed. In the alkaline range pH > 12 it could be shown that the equilibrium Fe(OH)3 + OH−⇆Fe(OH) 4− is established, the following pKS4 being found to be after 2 h ageing 4.50 ± 0.15 and after 96 h ageing 5.00 ± 0.15.
The state of iron(III)‐hydroxide in aqueous solutions of very low concentrations has been investigated using 59Fe as tracer. From very dilute solutions (10−5 M, 10−7 M and 10−9 M) brought to a pH between ∼ 5–12 practically all the iron hydroxide can be sedimented by ultracentrifugation. The particle size varies with the pH, being biggest at a pH ∼ 6.5 and showing a rather wide range of distribution at a higher pH. In the acid (pH 3–5) and the alkaline region (pH 12–14) the dissolved amount of FeIII‐hydroxide agrees satisfactorily with the values calculated using the solubility‐product and the constants of hydrolysis. In the precipitate two kinds of colloidal particles, spindel shaped α‐FeOOH and irregular aggregations of amorphous hydroxide particles, can be distinguished under the electron microscope.
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