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reagents in the potentiometric determination of gold include either ferrous or stannous .... where E~2 and E°u are the standard potentials of I-/I2 and. Au(III)/Au(I) ...
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Fresenius J Anal Chem (1993) 345:441 -444

© Springer-Verlag1993

Potentiometric titration of gold in ores with potassium iodide L. C. Robles, C. Garcia-Olalla, and A. J. Aller

Department of Biochemistry and Molecular Biology, University of L6on, E-24071 L6on, Spain Received March 30, 1992, revised July 13, 1992

Summary. A direct potentiometric titration method for the determination of gold in ores and alloys is described. It is based on the reduction of Au(IIl) with iodide ions yielding accurate and reproducible results. Detection limit and sensitivity were 0.06 and 0.032 mg/1 Au, respectively. The linear response range was between 0.1 and 120 mg/1 Au. The method can be used for the routine assay of gold in different kind of samples.

Introduction

Gold determination in alloys, minerals and soils is usually carried out by spectrometric methods [1-5]. All these methods, although in some instances very sensitive, are quite complicated and very expensive. On the other hand, gold determination by potentiometry has enjoyed wide acceptance among analytical chemists, due to its simplicity and rapidity. Results obtained by potentiometric methods are also reliable and usually comparable to those obtained by other chemical methods. Most of the titrimetric procedures for gold determination require a precipitation of the metal, and in all these methods filtration of precipitated gold is usually necessary in order to avoid interferences. There is available a wide variety of reducing agents, such as Fe(II), Ti(III), Sn(III), Cr(III), Cu(I), hydroquin0ne, arsenite, ascorbic acid, hydrazine, SO2, and others [6-11]. However, the most commonly used reagents in the potentiometric determination of gold include either ferrous or stannous ions. Iodide is an exception to the general type of reductant involved in gold titrations, because of the lot of papers published about this reagent since its first application [12]. In all the iodometric methods for the determination of gold, the end points are determined by potentiometric techniques using back-titrations against the liberated iodine with usually a standard sodium thiosulphate or mercury (II) solution [13,14]. These methods show, however, the common drawbacks of indirect determinations. The iodometric methods were able to obtain accurate results on samples of high concentrations of gold, but for small levels of this metal these methods have usually been found inapplicable.

Correspondence to: A. J. Aller

Besides the optimum amounts of gold, other additional problems related to the back-titration iodometric methods include confusing and contradictory results concerning the influence of parameters such as acidity, iodide concentration, the reaction stoichiometry, etc. Thus, in most acceptable iodide procedures, the nitric-hydrochloric acid solution of the gold caused a strong interference, probably due to either the precipitation of some metal chlorides or the modification of the redox potential of the system, resulting in the need of controlling the pH [11, 13, 14] of the solutions. One of the most important problems in the determination of gold from a chloride solution is the formation of a precipitate of AuI, as a result of the following reactions [13], AuC14- + 2 I-
AuC12- + I2 + 2 C1-

AuClf+I-

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