71 THE CLAISSE-CALCULATOR OF SAMPLES ACIDITY APPLICATIONS Fernand Claisse Fernand CLAISSE inc 2270, Léon Harmel, Suite 165 Québec, QC, G1N 4L2 ABSTRACT The CLAISSE-CALCULATOR is a new tool for XRF analysts who make fusion beads. It calculates the acidity of a sample and suggests the better Lithium borate flux composition for it. Its operation is briefly described. In addition, the Calculator has been used by the author to explain a) what causes the sample to have a preference for a given flux composition, and b) what causes molten glass sticking to crucibles and moulds. HISTORICAL In lithium borate fusion, acidity is the main factor that determines whether or not a fusion bead will be successfully done. In the old days of fire analysis of ores and rocks for the extraction of precious metals, the rule was: Basic samples need acidic flux, acidic samples need basic flux. No rule was used in the early days of fusion for XRF analysis [1] because only one flux composition was used, Sodium Tetraborate. On account of the very high dilution then used, one part sample in 100 parts flux, acidity was ignored. Later, when sodium became detectable by X-rays, Sodium Tetraborate was replaced by Lithium Tetraborate, and trouble began to appear. Various mixtures of chemicals started being tried. Only ten years ago, selection rules were proposed [2] for the determination of optimal flux composition, based on measured solubility of a number of oxides as a function of flux composition (Fig. 1). Fig. 1. Solubility of soe oxides as a function of flux composition [4] The rules were simple: Use Lithium Tetraborate for clearly basic samples such as Calcium oxide; Lithium Metaborate for clearly acidic samples such as Silicon dioxide; and a 50/50 mix of Lithium
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72 Tetraborate and Lithium Metaborate for oxides. Then, the problem that arose was: what to do with complex sample? Measured values of oxides acidity existed [3] but they were obtained from aqueous solutions, and some oxides dissolved in lithium borates did not seem to fit well with those of Smith. Oxides dissolved in a borate glass, in a certain way, should behave similarly to salts that are dissolved in water, but the values should be different. The solvents are different, and in the Smith case, one solvent (H 2 O) is considered as the origin of the acidity scale. Chemical acidity 15 10 5 Ba sic ox ide s 0 0 0,5 1 1,5 2 2,5 3-5 LiM Acidic LiT -10 oxides -15-20 Acidity Index (O/Me) Fig. 2. Comparison of the Smith (aqueous) and Claisse (fusion) acidity of oxides It was found necessary to find another way to define the acidity of oxides in fusion beads. Since the alkalinity of oxides seems to vary with the concentration of oxygen in their chemical formula, then the ratio number of oxygen atoms per number of metal atoms in the chemical formula of oxides, was used as the Acidity of the oxide [2,4]. That looked different from the Smith scale of acidity, but it was found later that both scales are related as shown when the values of one are plotted against those of the other as in Figure 2. COMPLEX OXIDES Let us start with a given example of complex oxide, Ilmenite. The chemical formula is FeTiO3, so that the A.I. is 3/2 or 1.5. That can also be written FeO + TiO2, in which case the A.I. of the two oxides are l and 2, and the average is (1+2)/2 = 1.5; we may conclude that a homogeneous compound and its components yield the same results, provided that they are expressed as molecules. Thus, the acidity of a complex sample may be calculated by expressing its composition in molecules, and taking the average of the acidity values of all its components. That is something analysts would normally not like to do frequently. The CLAISSE CALCULATOR To help analysts not do these long annoying calculations, a small software was developed [5], applicable to nearly all oxides of the periodic table in their maximum oxidation state, assuming that the oxides in fusion beads are in that state. For convenience, some oxides in other oxidation states were included. A single table only is needed to use the software (Fig. 3), and the procedure is simple : 1. Since acidity is independent of the size of the sample, its composition can be defined on any scale: percentage of each oxide, relative weights of oxides in grams, ounces, etc.
73 2. When each of these numbers is entered in the space next to the oxide, the description of the sample is done. 3. Pressing CALCULATE, the program makes all the calculations and yields the Acidity of the sample under ACIDITY NUMBER. 4. The objective of knowing the acidity is to chose the optional flux composition for a given sample. Based on the behaviour of pure oxides, five ranges of acidity were defined, and five flux compositions that should be the optimal flux for them were also defined. After the acidity value of the sample appears, the optimal flux composition also appears under OPTIMAL FLUX, expressed as percents of Li Tetraborate and Lithium Metaborate. 5. Since pure Lithium Metaborate always crystallizes except with SiO 2, Al 2 O 3, most sulfates and phosphates, when their concentrations are above about 25% in the fusion bead, the software recommends to switch to a flux composition of 35 Li Tetraborate / 65 Li Metaborate. Fig. 3. The Claisse-Calculator with data for a cement sample (from the Internet). RESEARCH APPLICATIONS 1. ACIDITY OF FUSION BEADS The Calculator can also be used to determine the acidity of fusion beads. It is only necessary to add the flux composition to that of the sample using the same units. The more units are the weights of Li 2 O and B 2 O 3 in the flux, and those of oxides in the bead. This was applied to a number of oxides, at the composition at which the solubility is maximal. That is the best flux-sample combination that represents the more stable condition for each particular bead. Some cases were estimated when the maximum solubility is outside the range of flux.
74 Oxide Li Flux Solubility. A.I. A.I. % Meta g oxide/ Oxide Bead 6g flux ZnO 0 1,5 1 1,15 BaO 0 1,0 1 1,16 PbO 0 0,5 1 1,14 CaO 0 2,0 1 1,14 Sb2O3 50 1,5 1,5 1,13 Fe2O3 50 1,5 1,5 1,15 FeO.TiO2 65 1,3 1,5 1,12 ZnSO4 50 2,0 1,8 1,20 CaSO4 100 2,0 1,8 1,16 TiO2 50 0,7 2 1,15 V2O5 70 1,3 2,2 1,17 Average 1,15 Table 1. Acidity Index of fusion beads of some pure oxides at their maximal solubility (A.I. means Acidity Index) composition. The results are shown in Table 1. An interesting observation is that all fusion beads have essentially the same value of acidity at their solubility peak. At flux compositions on each side of the maximal solubility, the solubility is lower because the flux composition is less optimal. Fusion beads apparently reach a neutral, more stable condition when the acidity index reaches 1.15. That value is not far from that of 1.17 for pure Lithium Tetraborate. Indeed, that flux is barely acidic; the term acidic is used for practical reasons to mean the lesser basic. 2. GLASS STICKING TO PLATINUM Sticking occurs with transition metals oxides only [4], and seems to be due to the simultaneous existence of all possible oxides of the dissolved metal element in the fusion Weight Acidity Index Oxide* g LiT 50/50 Fe2O3 1,0 1,19 1,18 FeO 0,9 1,16 1,11 Fe 0,7 1,10 0,81 Weight of oxide for constant amount of metal in 6g of flux Table 2. Acidity index of two Fe oxides and Fe, dissolved in two different fluxes bead. These oxides are the result of a chemical equilibrium that depends on the surrounding conditions. That is a well known concept in aqueous solutions. In borate fusion, it is estimated that the responsible oxide for sticking is the metal itself, the one with the lowest oxidation state, because free metal atoms can plate the crucible and induce sticking. The Calculator helps understand this phenomenon for the case of Fe oxides (Table 2). In the 50/50 flux, for the same number of Fe atoms in the fusion beads, and different oxidation states, the acidity index of Fe 2 O 3 is the closest to the ideal value of 1.15 (Table 1), so that this oxide is the more stable of the three (Table 2) and the one with a higher solubility in the bead. FeO should be present also but in a smaller proportion. The probability to have free Fe atoms is very low compared to Fe 2 O 3, so that sticking is not likely to occur. In Lithium Tetraborate flux, the situation is different. FeO is the more probable oxide present in the bead, Fe 2 O 3 should be in lower proportion, and Fe atoms have a greater probability to exist. The life time of the latter is certainly short, but the atoms that are next to the platinum surface may continuously alloy with platinum and cause sticking. That is what is observed in the crucible and in the mould. Weight Acidity Index Oxide* g LiT 50/50 CuO 1,0 1,16 1,06 Cu2O 0,9 1,13 0,93 Cu 0,8 1,10 0,81 Weight of oxide for constant amount of metal in 6g of flux Table 3. Acidity index of two Cu oxides and Cu, dissolved in two different fluxes Copper oxides are in a similar situation except that the Calculator predicts that the difference of probability of dissolution is closer between CuO and Cu in the 50/50 M/T flux (Table 3), so that sticking is probable in both fluxes. That is what is observed, and larger concentration of Lithium Metaborate in.
75 the flux should improve the situation, although the solubility decreases with the Metaborate content. CONCLUSION The CLAISSE CALCULATOR is a simple but efficient tool for the analyst and for research. In most analytical work, it might be needed only occasionally, for example when samples are rather similar. It is actually accessible on the FCinc. website [5] REFERENCES [1] Claisse F. (1957) Accurate XRF Analysis Without Internal Standard. Norelco Reporter III, 3, 3-7, 17-19 [2] Smith D.W. (1947) An Acidity Scale for Binary Oxides. J. Chem. Educ. 64, 480 481. [3] Claisse F. (1997) Selection of Fluxes for Fusion of X-Ray Samples : Optimum Lithium Tetraborate Metaborate Ratio for Specific Sample Types. Pittsburg Conference [4] Claisse F., Blanchette J. (2004) Physics and Chemistry of Borate Fusion 135 pp, ed. Fernand Claisse inc. [5] Claisse F. (2006) www.fernandclaisse.com