Anal. Chem. 1989, 67,2696-2699
2696
Determination of Phosphorus in Zircons by Inductively Coupled Plasma Atomic Emission Spectrometry J. C. Fariaas Departamento de Andisis Quimico, Instituto de Cerhmica y Vidrio, CSIC, 28500 Arganda del Rey, Madrid, Spain
A method for phosphorus determination In zlrcons by lnductlvely coupled plasma atomk emlsslon spectrometry has been developed. Sample decomposition Is achieved by means of Na,CO, ZnO sintering In a platinum crucible at 950 OC for 45 min and subsequent leaching wlth water of the attacked product. P Is separated from the matrix by means of yellow moiybdophosphoric acid complex formation and Isobutyl acetate extradlon. The analyses are performed in an aqueous phase after complex decomposition wlth (1 4) HNO, and evaporation of the organic extractant. The lowest quantitatively detennlnable concentration is 1.8 pg mL-' P,O,. The method was tested on zircon BCS-388 with a certified P,O, value of 0.12%. Short- and long-term precision and method precision reached relative standard deviatlons of 1.64%, 3.36%, and 2.50%, respectlvely. The accuracy of the method is excellent.
+
+
INTRODUCTION Zircon (ZrSiOJ is a raw material widely used in the manufacture of ceramics and glass. Its technological relevance derives fundamentally from its high refractoriness (dissociation temperature = 1676 f 7 "C). Among its many applications the following should be mentioned as particularly important: in the ceramic and glass industry it is employed in roofs for tunnel and blast furnaces; in metal foundry it finds application in continuous casting nozzles, in blast furnace bellies, and in refractory linings for casting ladles; in the field of new and advanced ceramic materials it is the fundamental raw material for the production of zirconia (ZrOz). The most important properties of zircon-based materials are the high thermal shock resistance and the high chemical resistance to alkali and slag attack. In addition and most essentially, zircon does not present any structural transformation below its dissociation temperature. The impurities that are generally found in zircon comprise major fractions of A1203,Fez03,Ti02, and HfOz and minor portions of MgO, MnO, NiO, NazO, K 2 0 , and P205. The presence of these impurities can have a negative impact on the aforementioned properties of these materials, degrading them considerably. Hence, accurate and reliable analytical impurity control is indispensable. For instance P205,which is found in zircons in concentrations in the order of 0.1 %, is a component that must be rigorously quantified. Nevertheless, in the literature on chemical analysis of zircons (1-9) no phosphorus determination method is mentioned. The general literature on silicate materials, however, contains frequent references to the determination of this element. The most widely used method is spectrophotometric analysis, either through yellow molybdovanadophosphoric acid complex formation (10) or by means of the blue molybdophosphoric acid complex obtained from the respective yellow molybdophosphoric acid complex by reduction with either ascorbic acid (12-17), ferrous ammonium sulfate (18,19) or tin(I1) oxalate (20). The interference of Si, forming colored complexes analogous to those of P, has been eliminated by (a) HF attack
(10-13,19), (b) pH control (18), (c) pH control and limiting free molybdate ion concentration (16), (d) pH control and addition of Fe(II1) in excess (20), and (e) tartaric acid addition (14). Kuroda et al. (21) applied flow injection analysis (FIA) in combination with blue molybdophosphoric complex spectrophotometric detection to P determination in silicate rocks. Riddle and Turek (22) described an indirect method for P determination in rocks through yellow molybdophosphoric acid complex formation, organic solvent extraction, complex decomposition, and determination of the molybdenum released in the aqueous phase by atomic absorption spectrometry. Direct current plasma (DCP) and inductively coupled plasma atomic emission spectrometry (ICP-AES) were likewise utilized for P determination in silicates, in spite of the fact that this element possesses only scarce emission lines (23) which, in addition, are little sensitive, so that P is one of the elements that presents the highest detection limits in ICP (24). Bankston et al. (25) used the 214.914-nm line for P determination through DCP after solubilizing the samples by LBOz fusion in a graphite crucible. Cook and Miles (26),Burman ( 2 3 ,and McLaren et al. (28) used the lines at 178.287,213.618, and 214.914 nm, respectively, for P determination by ICP, solubilizing the samples with H F attack in a platinum or PTFE dish. In all cases the lines used were free of spectral interferences. However, the detection limits of this element are not reported in any of the publications mentioned. In a previous work, FarSas and Valle (29) studied impurity determination (Al, Fe, Mg, Ti, Ni, Mn, and P) in zircons by ICP. The zircon samples were dissolved by fusion in a platinum crucible of 0.2 g of zircon with 2 g of a mixture of NaZCO3+ NazB407;the melt was solubilized in 10% HC1 and diluted to 200 mL. Although the first six elements could be accurately quantified by ICP, P resisted analysis, as P concentration was below the detection limit of any of its analytical lines. This was attributed to the considerable increase of the spectral background by the complex matrix resulting from the dissolution of the sample, together with the severe spectral interferences caused basically by Zr, rich in emission lines, as well as by other elements, such as Hf, Ti, and Cr. The aim of this research is to develop a convenient method for reliable and accurate P determination in zircons by ICPAES. The procedure comprises several steps, starting with a preconcentration of the element and separation of the matrix containing the solubilized P by means of Na2C03 + ZnO attack, yellow molybdophosphoric acid complex formation, extraction with an organic solvent, followed by decomposition to transfer P to an aqueous acid phase, where this element is measured by ICP.
EXPERIMENTAL SECTION Apparatus. A Jobin-Yvon Model JY-38 VHR sequential spectrometer with an ICP source was used. This is equipped with a 56-MHz radio frequency generator and a 1-m Czerny-Turner monochromator with a 3600 lines mm-' grating. The output, induced, and reflected powers are 2200,1600, and
