DIRECT OBSERVATIONS OF NONINTEGRAL HYDROUS FERRIC OXIDE SOLUBILITY PRODUCTS: K*S0=[FE3+][H+]-2.86

Abstract

Previous studies have indicated that hydrous ferric oxide precipitates, formed and aged at low temperatures, exhibit solubility products of the form K*S0 = [Fe3+][H+]-m, where m < 3 ([ ] # free ion concentration). Although previous observations have either been obtained over a very narrow pH range or have involved phase separations and solution chemical modeling, the present work involves direct potentiometric observations of log [Fe3+] and log [H+] over a wide range of Fe3+ concentrations and pH. Our observations obtained at 25°C in 0.7 molar NaClO4 provide substantial evidence for log [Fe3+] vs. pH relationships with slopes (m) smaller than three but larger than those obtained in previous works (m = 2.70 and m = 2.35). Over a four unit range in pH and a nearly 12 order of magnitude range in [Fe3+] the observed relationship between pH and log [Fe3+] in this work is log [Fe3+] = 4.28 +/- 0.05 - (2.86 +/- 0.009) pH. This solubility behavior has substantial implications for the behavior of FeIII in natural waters.