Mononuclear cyano- and hydroxo-complexes of iron(III)
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A detailed investigation of the iron(III)-cyanide and iron(III)-hydroxide systems has been made in NaClO4 media at 25 °C, using combined UV-vis spectrophotometric and pH-potentiometric titrations. For the Fe(III)/OH- system, use of low total Fe(III) concentrations (≤10 μM) and a wide pH range (0 ≤ pH ≤ 12.7) enabled detection of six mononuclear complexes, corresponding to the following equilibria: Fe 3+(aq) + rH2O ⇔ Fe(OH)r(3-r)+(aq) + rH+(aq), where r = 1-6 with stability constants (log *β1r) of -2.66, -7.0, -12.5, -20.7, -30.8, and -43.4, respectively, at I = 1 M (NaClO4). It was also found to be possible to measure, for the first time, stability constants for most of the following equilibria: Fe3+(aq) + qCN-(aq) ⇔ Fe(CN)q(3-q)+(aq), despite a plethora of complicating factors. Values of log β1q = 8.5, 15.8, 23.1, and 38.8 were obtained at I = 1.0 M (NaClO4) for q = 1-3 and 6, respectively. No reliable evidence could be obtained for the intermediate (q = 4 or 5) complexes. Similar results were obtained for both systems at I = 0.5 M(NaClO4). Spectra for the individual mononuclear complexes detected for Fe(III) with OH- and CN- are reported. Attempted measurements on the Fe(II)/CN- system were unsuccessful, but values of log β 16(Fe(CN)64-) = 31.8 and log β 15(Fe(CN)53-) ≈ 24 were estimated from well established electrode potential and other data.
|Publication Type:||Journal Article|
|Murdoch Affiliation:||School of Chemical and Mathematical Science|
|Publisher:||American Chemical Society|
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