Colorimetric study on the redox number of P2O5–PbO–Fe2O3 glasses

Semi-conducting property of lead iron phosphate glasses is assumed to be due to the hopping process. Redox number of glass is usually determined by Mössbauer spectroscopy. However, the technique is limited by low sensitivity in lower concentration of iron ions. Neither is suitable for the traditional method since Fe2+ in the glass is apt to oxidize and results in the uncertainty of the concentration. In this paper, a colorimetric method was performed to study the redox number of the glasses. Sample solutions have been prepared by using a vanadate as an oxidizing inhibitor for the equilibrium of Fe2+/Fe3+ pairs. The V5+/V4+ couple, which is strongly acid-dependent, oxidizes the Fe2+ to Fe3+ in the highly acidic solution, and it is regenerated when the solution was adjusted to pH 5 from V4+ preservative. The measurement of the electric conductivity of the glasses has indicated the fact that, other than the concentration of the electric conducting carriers, redox number of the glass plays a significant role for the semi-conducting phosphate glasses. The method was validated by analyzing a mixture of ammonium iron (II) sulfate and/or ammonium iron (III) sulfate solutions and has been proved to be simple, accurate and reliable.