Ind. Eng. Chem. Res. 2006, 45, 4525-4529
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Determination of Total Fluoride Content in Electroslag Refining Fluxes Using a Fluoride Ion-Selective Electrode Jerry L. Yeager,†,‡ Michael D. Miller,‡ and Kandalam V. Ramanujachary*,‡ American Flux and Metal, 352 East Fleming Pike, Winslow, New Jersey 08095, and Department of Chemistry and Biochemistry, Rowan UniVersity, 201 Mullica Hill Road, Glassboro, New Jersey 08028
A method for a routine, direct analysis for fluoride in electroslag refining (ESR) fluxes, using potentiometry, is described that is faster, more economical, and less operator-dependent than previously known methods of fluoride analysis in these materials. Flux samples were prepared using a rapid sodium carbonate/borate fusion with subsequent digestion in dilute nitric acid prior to chelant addition and analysis by fluoride ion-selective electrode (FISE). Disodium ethylenediaminetetraacetate (EDTA), ethylenediamine, ammonium citrate, ammonium tartrate, and citric acid were used to chelate metal cations in solutions of flux containing calcium fluoride (fluorspar) as a major constituent. It was found that, for optimal recovery of fluoride, pH of the chelant/flux solution must be controlled between 8.0 and 9.0. High concentrations of calcium reduced the ability of the fluoride ion-selective electrode to detect fluoride. It is suspected that high concentrations of magnesium would also contribute to this reduced sensitivity, but such fluxes were not tested. The technique is well-suited for analysis of fluxes when the sample CaF2 content is e63% by mass. Analysis of several flux formulations showed that the new FISE method is more practical and produces results that are as accurate and precise as conventional methodology. 1. Introduction Fluxes are routinely used in metal refining to facilitate (1) fusion of minerals or metals, (2) removal of impurities in the melt, and (3) prevention of oxide formation.1-2 Electroslag refining flux (ESR flux), in particular, is a fused solid mixture of calcium fluoride, lime, magnesia, alumina, silica, and other metallic oxides used in the production of high-quality alloys and superalloys.2 ESR flux is important to the metals industry, where it is used in applications requiring metal strength and thermal resistance.2 Many types of ESR flux have been developed over the years in the industry, with specific mixtures of the constituent compounds being tailored to each application.2 For these fluxes to be effective in the process, the constituent mixture must adhere to specific compositions determined by the specific application(s) of the product(s) being produced.2 Thus, the need for rapid, accurate, and cost-effective analysis of these fluxes is of obvious importance. Verifying the chemical composition of fluxes is, often, a difficult and time-consuming process. The mixture of oxides, fluorides, and silicates, etc., in fluxes presents analytical challenges when quantitative analyses for major constituents and trace impurities are sought. Calcium fluoride (fluorspar) is one of the most important constituents of ESR flux; it is typically present in concentrations ranging from 30 to 70% (by mass) CaF2 with the 40 to 60% (by mass) CaF2 fluxes being the most common.2 However, in the presence of other calcium species and/or other metal oxides, such as Al2O3, MgO, and TiO2, the CaF2 content is difficult to quantify.2-4 A common and practical approach is to measure the concentration of fluoride in the flux and, then, calculate the stoichiometrically equivalent amount of calcium fluoride. The difficulty in this approach lies in the complex and variable composition of flux, which prevents the * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 856-256-5451. † American Flux and Metal. ‡ Rowan University.
use of some traditional analytical methods to determine its fluorine content.3,4 Several wet methods for fluoride analysis exist, but these are generally time-consuming, expensive, and operator-dependent and require modifications to be useful for analysis of ESR flux.3,4 One of the oldest of these methods is the Berzelius method, which involves fusion of the flux sample, precipitation of constituent metals, and, finally, precipitation of fluoride as calcium fluoride, which must be washed, dried, and weighed.3 This method is reliable and accurate but involves toxic reagents, is time-consuming, and is operator-dependent. Another method involves the distillation from sulfuric acid or perchloric acid solution.3 This method is also time-consuming and is subject to interferences from aluminum and silica, both of which are present in ESR flux in high quantities.3 One method used by American Flux & Metal, called the Foote method (a variant pyrohydrolytic method developed by the, now defunct, Foote Mineral Co.) is an example of a method that has been used for analysis of fluoride content shown in the flowchart in Figure 1.3 This procedure is time-consuming, expensive, high maintenance, and subject to numerous operator errors. In addition, much of the apparatus for this method must be custom-made. Despite their widespread use in metals analyses, more conventional laboratory instrumental techniques, such as inductively coupled plasma-optical emission spectrometry (ICPOES) and atomic absorption-graphite furnace (AA-GF), have inherent limitations that prevent their application for detecting fluoride ions.4 Analysis of the solid flux for fluoride content has been conducted in the past with expensive instrumentation such as X-ray diffraction, X-ray fluorescence (XRF), electron microprobe analysis (EMPA), and/or glow discharge mass spectrometry (GD-MS).4 X-ray techniques are considered more suitable for this type of analysis, but numerous difficulties remain a concern when quantitative determinations are needed.5 Methods for potentiometric determination of fluoride in a variety of sample matrixes, such as drinking water and blood samples, have been extensively investigated utilizing a fluoride ion-selective electrode (FISE).6-9 Because an FISE responds
10.1021/ie060128a CCC: $33.50 © 2006 American Chemical Society Published on Web 05/23/2006
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Figure 1. Flow chart for pyrohydrolytic fluorine analysis (Foote method).
to F- ion activity in aqueous solution, cations in the solution matrixes that form metal complexes with fluoride ions (reducing the availability of free fluoride ions) are potential negative interferences to this FISE technique. In many common applications, such as drinking water analyses, these interferences are minimal, due to the low levels of cations present.4,7,9 However, in the case of ESR flux, interferences are cause for concern due to the abundance of interfering polyvalent cations such as Fe3+, Al3+, Mn4+, Ti4+, Mg2+, and Ca2+ in the flux matrix. Varying concentrations of these cations make it impossible to predict and mathematically correct for these interferences.10 The key to allowing the persistence of free F- in solution is to “tie up” interfering cations by adding ligands to the solution that have a greater affinity for the metal cations than does fluoride, releasing fluoride ions to be detected by the FISE. Aminopoly(carboxylic acids) such as ethylenediaminetetraacetic acid (EDTA), DPTA, CDTA, and nitrilotriacetic acid (NTA) are well-known chelating agents that form stable coordination spheres with metal cations.4,11-13 In addition, polycarboxylic acids, such as citric acid, tartaric acid, and salicylic acid, have been used as chelating agents.5,11,13,14 Based on availability, cost, solubility, and chelating effectiveness, a combination of chelating agents (EDTA, ammonium citrate, and ammonium tartrate) was used in this work to complex metal cations. An excess of these complexing agents masks the effect of cations on fluoride activity and drives complex formation equilibria toward coordination molecule formation.4,11,13,15,16 Table 1 lists the overall formation constants (log Kf) for many of the complexes formed by these chelants with metals commonly found in ESR flux. The reaction was carried out under conditions of relatively high pH (8-9) to maintain a high conditional formation constant of EDTA and drive complexformation reactions forward. The work presented in this research was undertaken to develop a rapid, low-cost, accurate, and reproducible method to analyze fluoride in fluxes using equipment that is readily available to analytical laboratories. Potentiometric techniques
Table 1. Overall Formation Constants (log Kf) for Metal Complexes When All Ligand Sites Are Deprotonateda metal ion
EDTA
aluminum calcium chromium(II) chromium(III) copper(II) iron(II) iron(III) lanthanum lead(II) magnesium manganese(II) nickel(II) titanium(IV) zirconium
16.11 11.0
ethylenediamine
citrate
tartrate
20.0 4.68
9.01
13.94 23 18.7 14.33 24.23 16.34 18.3 8.64 13.8 18.56 17.3 19.40
20.00 9.70
5.67 18.33
14.2 15.5 25.0 9.45 6.50 3.29 3.67 14.3
4.78 7.49 3.06 3.78 1.38
a The elements listed are metals expected to be present in ESR flux formulations. Source: Adapted with Permission from ref 4. Copyright 1995 McGraw-Hill.
were studied because they offer utility for a cost-effective, reliable, and timely fluoride analysis and most types of samples do not require extensive preparation.4 In addition, sample solutions with high turbidity are rarely problematic with potentiometric methods.4 The potentiometric method developed was validated with ESR flux samples and samples simulating the composition of ESR fluxes (i.e. control samples). In this paper, we present the results of our analytical approach in determining the total fluoride, reported as % CaF2 (by mass) (fluorspar), in ESR fluxes. 2. Experimental Section The experimental procedure for this method is presented in the flow chart given in Figure 2. Qualitative knowledge of the flux‘s composition should be known prior to FISE analysis. Ideally, aluminum, calcium, and magnesium contents should be known before performing this analysis. The reasons for this are further explained later in the Experimental Section.
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Figure 2. Flow chart for direct potentiometric fluoride analysis (FISE method).
To analyze ESR flux via potentiometric techniques, the flux must first be placed into aqueous solution. As stated previously, ESR flux is nearly insoluble in mineral acids. A flux fusion using crystalline sodium tetraborate decahydrate and anhydrous sodium carbonate, in 2:5:7 mass ratios (1 g of sample to 2.5 g of Na2B4O7‚10H2O to 3.5 g of Na2CO3), was used to prepare solid matrixes that were digestible by dilute nitric acid.4,17 The resulting solution was diluted with deionized (DI) water to make a solution with a dilution factor of 1 g of ESR flux in 200 mL of solution. All water used for dilutions was 18 MΩ DI H2O. This procedure was repeated for the control standard, and all other flux samples were analyzed. The control standard contains a mixture of ACS grade reagents from Alfa Aesar certified at >99.99% purity (metals basis) and was prepared by weighing 0.5982 g of 99.99% purity (by mass) CaF2, 0.1003 g of 99.76% (after subtracting water content) purity (by mass) CaO, 0.0999 g of 99.998% purity (by mass) MgO, 0.1002 g of 99.999% purity (by mass) Al2O3, 0.0497 g of 99.995% purity (by mass) SiO2, and 0.0498 g of 99.995% purity (by mass) TiO2 into a platinum 30 mL crucible and fusing the mixture as previously mentioned. The calculated mass of the fluoride in the control standard is 0.2911 g in the sample, yielding a concentration of 0.0766 M F- in 200 mL of prepared solution. To prepare the chelated solution, 4 mL of ESR flux solution, 20 mL of 2 M ammonium citrate dibasic, 10 mL of 2 M ammonium tartrate dibasic, 10 mL of 2 M citric acid, 4 mL of ethylenediamine, 25 mL of 50% (m/v) disodium-EDTA solution, and 5 mL of 2 M sodium chloride were added to a 100 mL volumetric flask, in this specific order, with DI water making up the balance. This order of the additions was important because precipitates formed when the order was altered. The final theoretical fluoride concentration of the control standard is 3.064 × 10-3 M when prepared in this manner. All reagents used were ACS reagent grade. The pH of these chelated solutions was held between pH 8 and 9, because chelation of metal cations by EDTA is dependent on the pH of the solution.4,11,17 A pH of 9 is the high limit for the electrode set by the manufacturer (Accumet). For fluxes that contained relatively high aluminum content (>30% Al2O3 by mass) such as F30 and F40 (Table 2), the pH of flux solutions was adjusted closer to pH 8.0 by lowering the amount of ethylenediamine added. This was done because the high stability of the aluminum-citrate and aluminum-EDTA
Table 2. Comparison of Analytical Results for ESR Flux by FISE and Foote Methods fluxa
Nb
Footemeanc
FISEmeand
RSDFISEe (%)
F30 F40 F48 F56 F58 F60 control F61 F63 F65 F70f F90f
5 8 4 6 4 4 6 4 4 4 1 1
30.7 40.0 47.9 56.4 58.6 59.2 59.3 60.4 62.5 64.3 69.8 89.5
31.1 40.6 48.9 56.1 58.8 60.7 58.9 60.3 61.7 54.4 62.8 68.5
2.8 2.4 1.9 2.0 3.9 3.2 1.8 0.6 2.8 1.9
a Flux formulations with the designation “F” followed by the expected mean %CaF2 content. b Number of measurements performed for each flux designation. c The mean %CaF2 of the results obtained for each flux designation by the Foote method. d The mean %CaF2 of the results obtained for each flux designation by the FISE method. e Relative standard deviation (%RSD) for FISE results. f Only one replicate was run for F70 and F90.
complexes, as shown in Table 1, does not require as high of a pH under the solution conditions to drive aluminum-complex formation.11 In general, this will be the case for the majority of metals having high stability constants with both EDTA and citrate (the primary chelants); however, Cu2+, Fe2+,3+, and Ni3+ (Table 1) were not present in appreciable quantities in the tested fluxes. Only Al3+ followed this exception in the samples tested. A pH closer to 9.0 in these sample solutions was unfavorable and skewed results high by several percent. In contrast, for samples having high calcium fluoride content (g60% (by mass) CaF2), the pH was adjusted as close to pH 9.0 as possible by adding additional ethylenediamine (due to the lower stability of calcium-chelant complexes) to increase the masking effects of the chelants on metal cation interferences.11 In this case, higher OH- content helped to achieve better masking of calcium-fluoride interactions, yielding good results.11 These adjustments were made to minimize the possible interference of excess OH- ions on electrode performance.17 In addition, the disodium-EDTA solution (obtained from VWR International) contained additional sodium hydroxide that helped buffer pH. The ammonium salts also served to buffer the solution, minimizing the effect that variations in flux matrixes
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Figure 3. Typical FISE calibration curve obtained. Figure 4. Comparison of CaF2 results obtained by Foote and FISE methods. Error bars are (1 standard deviation of an individual measurement. The straight line represents perfect agreement between the two methods.
have on the pH of the chelated solutions, and the sodium chloride adjusts the solutions’ ionic strength.4,17 ACS reagent grade 0.1000 M NaF was used as the source of fluoride for the calibration standards since Na+ cations do not form stable complexes with F- ions in aqueous solution.4,9 Five calibration standards were prepared with concentrations ranging from 5.00 × 10-4 M F- to 4.00 × 10-3 M F- to encompass a broad range of expected fluoride content. The chelating agents used for the ESR solutions were also used for these standards, so as to match the matrixes between samples and standards. The amount of ethylenediamine used was 3 mL for each standard; slightly less than that used for ESR flux samples because CaF2 is not present in these calibration standards. In addition, 4 mL of fused flux blank solution containing 3.5 g ((0.01 g) of anhydrous sodium carbonate, 2.5 g ((0.01 g) of crystalline sodium tetraborate decahydrate, 0.5001 g of CaO, 0.0505 g of MgO, 0.4002 g of Al2O3, 0.0199 g of SiO2, and 0.0307 g of TiO2 (reagents previously described), fused and brought to 200 mL volume with deionized H2O, was added to each standard to better match the matrix to that of the chelated ESR flux solutions. Finally, detection of fluoride activity in the standards is not as dependent on pH as in ESR flux solutions, since the concentration of metal cations in ACS grade NaF is negligible. The method used is a direct-read method for potentiometric analysis utilizing an Accumet Excel25 potentiometer/pH benchtop meter (Dual Channel) and an Accumet BNC LaF3 glassbody fluoride ion-selective combination electrode. All samples and standards stayed at room temperature (22 °C) during analyses, which was monitored via ATC probe. The pH meter was calibrated via a three-point calibration using pH 4, pH 7, and pH 10 certified pH buffers. For fluoride ion analysis, the instrument was calibrated using the five fluoride calibration standards previously discussed. Samples and standards were each allowed to equilibrate for 3-5 min according to standard potentiometric procedure.4,17 Assuming the raw materials other than CaF2 used in the manufacture of the flux contain only trace levels of fluoride, observed fluoride ion concentration can be directly correlated to total CaF2 content.
For each sample, Figure 4 graphs the average result obtained by each method, showing error bars of (1 standard deviation where multiple runs were made. FISE results were read directly from the potentiometer in units of molarity and converted into % CaF2(total) (by mass). As shown in Table 2 and Figure 4, excellent agreement was obtained between the two methods for samples containing 63% (by mass), indicating a point of diminishing returns when compensating for higher calcium content in fluxes using pH adjustment. Standard deviations of the analytical results by the FISE and Foote methods were not significantly different for any of the 10 flux samples measured in replicate, when statistically tested using an F-test. Relative standard deviations of individual measurements were roughly 2-3% for both methods, indicating good precision across all concentrations for both methods (Table 2). When CaF2 content was e63% (by mass), the difference between the mean CaF2 values obtained from the Foote and FISE methods (Table 2) were acceptable and differed by a maximum of 1.5%. When statistically tested with the student’s t-test, means of four to eight replicate measurements, each by the FISE and Foote methods, were not significantly different for any of the 10 flux samples measured in replicate when the mean result of the Foote method was 90% less than X-ray methods. The reagents are easily and inexpensively obtained and have low toxicity, and the consumable parts for the instrumentation are rugged and inexpensive. Even a laboratory on a strict budget with minimal staff can find utility in this method. In specific situations, this method can exhibit limitations. First, analysis of unknown samples can be problematic, since adjustments must be made, as discussed previously, depending on the sample content. At a minimum, roughly qualitative knowledge of sample composition is necessary. In addition, when the total sample fluorspar content exceeded 63% (by mass), the interference of calcium on the fluoride activity became significant (Figure 5). As previously discussed, this is a direct result of the relatively lower stability of chelant-calcium complexes as listed in Table 1. Further dilution of samples did not overcome this problem. Magnesium would also be expected to be a significant interference due to its lower stability when complexed (Table 1), but none of the fluxes tested were of greater magnesium content than 10% (by mass). It is evident from the results of this work that the rapid fusion with subsequent chelation and fluoride ion determination described in this procedure is suitable for analysis for the total fluoride content in many fluxes. Future work, seeking alternative combinations of chelating agents to improve the method’s utility
We would like to thank Rod Werner and Joachim Rudoler for supporting the financial needs of this research. We would also like to thank Rowan University of New Jersey for providing faculty support and supplemental resources and McGraw-Hill for providing permission for adaptation of Table 1. Finally, we would like to thank the following people for their guidance: Roberto N. Feudale, Robert J. Newland, James A. Fraley, and Jerry L. Fields. Literature Cited (1) Condensed Chemical Dictionary, 14th ed.; Lewis, Sr., Richard, J., Eds.; John Wiley & Sons: New York, 2001; pp 508-509. (2) Hoyle, G. Electroslag Processes; Applied Science: New York, 1983. (3) Jeffery, P. G. International Series of Monographs in Analytical Chemistry. In Chemical Methods of Rock Analysis, 1st ed.; Belcher, R., Freiser, H., Eds.; Pergamon Press: Elmsford, NY, 1970; Vol. 36, pp 227237. (4) Analytical Chemistry Handbook; Dean, J. A., Nalven, G. F., Miller, V. L., Eds.; McGraw-Hill: New York, 1995. (5) Ottolini, L.; Camara, F.; Bigi, S.; Am. Mineral. 2000, 85, 89-102. (6) McQuaker, N. R.; Gurney, M. Anal. Chem. 1977, 49, 53-56. (7) Capka, V.; Bowers, C. P.; Narvesen, J. N.; Rossi, R. F. Talanta 2004, 64, 869-878. (8) Kuznetsova, I. V.; Khmelev, S. S. J. Anal. Chem. 2004, 59, 770774. (9) Kartal, S. S.; Sahin, U. Turk. J. Chem. 2004, 28, 203-211. (10) Nambiar, C. H. R.; Narayana, B. Chem. Anal. 1999, 44. (11) Conway, M.; Smallwood, H.; Jones, L.; Leenhouts, R.; Williamson, G. Chem. Eng. 1999, 86-90. (12) Shriver, D.; Atkins, P. Inorganic Chemistry, 3rd ed.; W. H. Freeman: New York, 1999; pp 211-250. (13) De Moor, R. J. G.; Mertens, L. C.; Verbeeck, R. M. H. Dent. Mater. 2005, 21, 318-323. (14) Jones, M. R.; Yu, H.; Delehanty, J. B.; Blake, D. A. Bioconjugate Chem. 2002, 13, 408-415. (15) Kraemer, S. M.; Chiu, V. Q.; Hering, J. G. EnViron. Sci. Technol. 1998, 32, 2876-2882. (16) Nowack, B. EnViron. Sci. Technol. 2002, 36, 4009-4016. (17) Shugar, G. J.; Dean, J. A. In Chemist’s Ready Reference Handbook; Crawford, H. B., Fogarty, D. E., Eds.; McGraw-Hill: New York, 1990.
ReceiVed for reView January 31, 2006 ReVised manuscript receiVed April 3, 2006 Accepted April 18, 2006 IE060128A
