Anal. Chem. 1999, 71, 539-543
Determination of Oxide in Fluoride Salts Using an Yttria-Stabilized-Zirconia Oxygen Pump Sven E. Eklund, L. M. Toth,* J. Q. Chambers,* and Gleb Mamantov†
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
An improved method over a previous technique has been developed to determine the ppm oxide concentration of fluoride salts. The oxide is released as oxygen gas by the reaction of the test salt with potassium bromotetrafluoride at 450 °C. The molecular oxygen released is then passed through a zirconia oxygen pump which selectively removes the oxygen. The current response is recorded as a chronoamperogram, from which the ppm oxide content of the salt can be obtained. Oxygen recovery from an yttrium oxide standard was better than 99%. The precision of analysis of FLINAK was better than 13% for samples containing 110-170 ppm oxide. The LOD was 12 µg of oxygen, and the analytical range was 120 ppm to >20% for a 0.1-g sample. Molten fluoride salts have played an important role in many applications, including the electrowinning of aluminum from cryolite melts1 and as homogeneous fluid fuels in molten salt nuclear reactors.2 In addition, interest has been generated in the use of fluoride salts as baths for the deposition of refractory metals such as niobium,3 tantalum,4 and tungsten.5 An important consideration in many molten fluoride studies is the oxide content of the fluoride salt, which, depending on the concentration and other conditions, can cause desirable or undesirable effects. A reliable method for determining the oxide content of these fluoride salts at the ppm level was developed at Oak Ridge National Lab (ORNL) by Goldberg et al.6 They reacted the fluoride salts with potassium bromotetrafluoride (KBrF4) in a nickel cell at 450 °C for 2 h and then measured the oxygen produced manometrically. A mercury-filled Toepler pump was used to remove the product gases from the reaction cell and deliver them to the measurement apparatus. A major interference in these determinations was nitrogen gas released from the reaction due to absorbed nitrogen and nitride salts. The O2/N2 ratio had to be found by mass spectrometry or gas chromatography following the manometric measurement, or as shown by † Deceased, 3/11/95. (1) Grojtheim, K.; Krohn, C.; Malinovsky, M.; Matiasovsky, K.; Thonstad, J. Aluminum Electrolysis, 2nd ed.; Aluminum-Verlag: Dusseldorf, 1982. (2) Lane, J. A.; MacPherson, H. G.; Maslan, F. Fluid Fuel Reactors; AddisonWesley Publishing Co. Inc.: Reading, MA, 1958. (3) Matthiesen, F.; Christensen, E.; von Barner, J. H.; Bjerrum, N. J. J. Electrochem. Soc. 1996, 143, 1793-9. (4) Polyakova, L. P.; Polyakov, E. G.; Matthiesen, F.; Christensen, E.; Bjerrum, N. J. J. Electrochem. Soc. 1994, 141, 2982-8. (5) Senderoff, S.; Mellors, G. W. J. Electrochem. Soc. 1965, 112, 841-3. (6) Goldberg, Gerald; Meyer, A. S., Jr.; White, J. C. Anal. Chem. 1960, 32, 314-7.
10.1021/ac980830j CCC: $18.00 Published on Web 12/29/1998
© 1999 American Chemical Society
Dupraw and O’Neill,7 the oxygen could be removed by passing the O2/N2 mixture through a copper furnace followed by a manometric determination of the remaining nitrogen. The detection limit of this system was shown to be as low as a few hundred ppm. Other oxide determination techniques for fluoride salts have been developed with a wide variety of applications. The alumina content of cryolite was determined by carbothermic reduction using a LECO instrument.8 The alumina in a sample was reduced with carbon at high temperature, and the quantity of CO produced was measured. The standard deviation of free alumina analysis was found to be 0.11%. However, the LECO instrument does not perform well with many cations found in fluoride salts such as lithium and potassium. White9 reported that oxide added to a molten fluoride salt was directly proportional to the anodic peak current on a cyclic voltammogram, and recently, Polyakova et al.10 reported that the peak anodic current in the LiF-NaF-KF eutectic (FLINAK) at a glassy carbon electrode follows the equation
j550 ) 309.1cO2-
(1)
where j550 is the current density at 550 °C and cO2- is the oxide concentration from Na2O. Residual oxide was reported as low as 1.7 × 10-2 mol % (66 ppm). This technique is useful for in-situ oxide determinations but requires the salt to be in the molten state. Ito and co-workers11 used a Pt(O2)/stabilized zirconia electrode immersed in the LiF-KF eutectic and measured the residual oxide concentration of the salt as a function of potential down to a reported 4.0 × 10-4 mole fraction (about 150 ppm) range. This technique is useful for in situ oxide measurements as well but requires larger samples in the molten state. Bjerrum and co-workers12 showed that by reacting FLINAK with K2TaF7 and then comparing the tantalum fluoride and tantalum oxide vibrational (Raman and/or IR) peak ratios in the solid they could determine and oxide concentration as low as 0.2 mol % (about 800 ppm). (7) Dupraw, W. A.; O′Neill, H. J. Anal. Chem. 1959, 31, 1104-5. (8) Tarcy, G. P.; Rolseth, S.; Thonstad, J. Light Metals 1993; The Metallurgical Soc.: Warrendale, PA, 1993; pp 227-9. (9) White, S. H. Molten Salt Techniques; Plenum Press: New York, 1983; Vol. 1, p 50. (10) Bjerrum, N. J.; Berg, R. W.; Chistensen, E.; Kerridge, D. H.; von Barner, J. H. Anal. Chem. 1995, 67, 2129-35. (11) Ito, Y.; Yabe, H.; Nakai, T.; Ema, K.; Oishi, J. Electrochim Acta 1986, 31, 1579-84. (12) Polyakova, L. P.; Polyakov, E. G.; Bjerrum, N. J. Rus. J. Electrochem. 1997, 33, 1339-42.
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In this paper, an improved oxygen determination technique using the ORNL-developed procedure is described which employs the KBrF4 reaction scheme but which replaces the manometric measurement with an yttria-stabilized-zirconia (YSZ) oxygen pump. In this technique, the molecular oxygen produced in the reaction cell is instead swept through a stabilized zirconia electrochemical cell and “pumped” out with an applied potential across the zirconia membrane. The quantity “pumped” is recorded as a chronoamperogram from which the total coulombs of charge can be obtained and converted to ppm oxide. This technique eliminates the nitrogen background interference of the ORNL technique, the use of a mercury pump, and the necessity for a temperature calibration and also offers a possible lower detection limit. YSZ has been employed widely for various technologies including the automotive industry where it is used to control the air-to-fuel ratio in automobiles,13 and in gloveboxes to determine the ppm O2 present in the inert atmosphere. A YSZ tube used in these experiments measures the partial pressure of oxygen gas on the inside of the tube compared to a reference oxygen gas partial pressure (usually atmospheric) on the outside of the tube by creating a potential difference across the inside and outside of the YSZ tube. The potential difference is measured by making electrical contact to porous platinum coatings that have been applied to the inside and outside tube surfaces. If the reference gas oxygen partial pressure (P2) is known, then the unknown partial oxygen pressure (P1) can be calculated from the Nernst relation:13
EMF ) (RT/4F) ln(P1/P2)
(2)
where R is the gas constant, T the temperature in kelvin, and F the Faraday constant. Normally one would measure potential, but according to this equation the tube can also act as an oxygen “pump” if a dc voltage (V) is instead applied across the two platinum coatings. The YSZ tube can then selectively “pump” oxygen either into or out of the tube depending on the sign and magnitude of the potential applied. When a dc potential is applied across the tube, oxygen gas is reduced at the negative pole (cathode):
O2(g) + 4e- f 2O2-(s)
(3)
The YSZ tube then acts as a solid ionic conductor and oxide is oxidized to oxygen gas at the positive pole (anode):
2O2-(s) f O2(g) + 4e-
(4)
The current, I, has been shown13 to be proportional to the applied potential according to the equation,
I ) (V + EMF)/Ri
(5)
where Ri is the cell impedance. The current is also a function of the temperature (eq 2) of the YSZ tube, but the tube temperature (13) Logothetis, E. M. Ceram. Eng. Sci. Proc. 1987, 8, 1058-73.
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is kept constant to eliminate this uncertainty in the current response. The YSZ tube surface area and platinum coating quality also limit the maximum current by affecting Ri. EXPERIMENTAL SECTION Apparatus. The system was composed of a reaction cell, a circulating system, and a YSZ oxygen-pumping apparatus. This is shown in Figure 1. The reaction cell was machined from 1-in. inconel bar and was isolated from the circulating system by two Hoke inconel diaphragm valves. The valves were silver-soldered to 1/4-in. Monel tubing which was in turn welded to the cell body. The cell was capped with a brass top to prevent galling of the threads. An insert machined from nickel fit between the cell and cap and reached down to the top of the inlet/outlet holes of the welded tubing. A nickel divider was also welded to the insert to promote better sweep of the gases through the cell. A Teflon gasket was used to seal between the insert and cell. The circulating system was composed of 1/4-in. Monel tubing connected with Monel Swagelock unions. All valves used were either straight or tee pattern Hoke inconel diaphragm valves or Whitey rising plug valves with Teflon seats and Monel stems. The tee pattern valves were used to eliminate eddy pockets from sidearms while the rising plug valves prevented restricted gas flow. A miniature fan (486 CPU cooler) was placed in line with the tubing to circulate the gases at a rate of approximately 100 mL/ min. It was found that this provided the best flow for the gases in the system for oxygen pumping. The circulating fan housing (Figure 2) was machined from aluminum and designed to promote even gas flow. Part of the circulating line was coiled and immersed in LN2 to trap the volatile bromo and fluoro compounds produced in the reaction cell. These compounds degrade the platinum coating and cause the background current to drift. Helium was used as the carrier gas so that it would not condense in the LN2 trap. The reaction cell and LN2 trap were isolated from the main circulation loop in order that the initial background O2 present in the system could be circulated through the YSZ tube and pumped out. This established an O2 background baseline from which the coulombs of charge from the O2 produced in the reaction cell could be separated. The oxygen-pumping apparatus consisted of a platinum-coated YSZ tube placed in line with the circulating system. The YSZ tube was attached to an EG&G PAR 273 potentiostat/galvanostat which was used to apply a dc potential across the inside and outside of the tube and record the current response as a function of time (chronoamperogram). The area under the resultant curve was integrated to obtain the total number of coulombs of charge passed. The YSZ tubes were either purchased complete with platinum coating from Thermox (Amatek), or uncoated tubes were purchased (Zircoa, stabilized with 8% yttria) and then coated with a platinum ink (Engelhard) in-house which in turn was thermally decomposed at successively higher temperatures up to 800 °C to produce the porous platinum coating. The relative quality of the tubes was tested by applying a predetermined potential across the tube that pumped oxygen into the system, while the system was under a vacuum, and observing the peak current. Using this test method, the in-house coated tubes performed as well as or better than the tubes purchased from Thermox. The reference
Figure 1. Oxide determination apparatus. Table 1. Oxide Determinations for FLINAK, Yttrium Oxide, and the Background sample
weight mC (g) pumpeda
backgroundb
FLINAK Figure 2. Circulating fan.
in all cases was atmospheric oxygen. The tubes were sealed in the system with a Cajon fitting and thermostated at 800 °C. Alumina tubing was inserted inside the YSZ tube to sweep the gas over the inner surface. The YSZ tube assembly was isolated electrically from the rest of the system with Teflon bushings to reduce background noise on the chronoamperograms. Reagents. The eutectic LiF-NaF-KF (46.5, 11.5, 42.0 mol %), FLINAK, was prepared from the individual salts (Fluka, >99%) by premelting under vacuum in a graphite tube. The FLINAK was then treated with an HF (Matheson, 99.99%) sparge at 550 °C for 24 h to reduce the oxide content to the ppm level before being separated into test samples. Yttrium oxide (A. D. Mackay, Inc., 99.99%) was dried under vacuum at 350 °C overnight to remove condensed moisture. KBrF4 was made in situ by adding 12 mL of BrF3 (Ozark-Mahoning) to 10 g of KF and preheating to 450 °C. Procedure. FLINAK samples ranging from 100 to 225 mg (see Table 1) were used to test the system for reproducibility. All the samples were taken from the same batch of FLINAK with the
Y2O3c
0.1363 0.2033 0.2250 0.1574 0.1151 0.1264 0.0116 0.0103 0.0104 0.0110
598 597 651 712 695 703 642 277 316 378 283 155 230 294 × 102 263 × 102 266 × 102 280 × 102
µg of O2 49.6 49.5 54.0 59.0 57.6 58.3 53.2 23.0 26.2 31.3 23.5 12.9 19.1 24.4 × 102 21.8 × 102 22.1 × 102 23.2 × 102
ppm
std dev 4.0 µg
168 18 ppm 129 139 149 112 151 210 × 103 1 × 103 ppm 212 × 103 212 × 103 211 × 103
a For FLINAK and Y O , the average background has been sub2 3 tracted. b The limit of detection is 12 µg of oxide. c The relative error for an expected 213 × 103 ppm was -0.8%.
assumption that they all contained the same ppm oxide. Y2O3 samples were used to test the system for accuracy. In practice, the samples were added by opening the reaction vessel cap under helium pressure and quickly adding the sample from a glass vial to minimize addition of moisture. The entire reaction vessel was then heated to 120-130 °C under vacuum until the pressure dropped to less than 2 µm of mercury, thus removing condensable gases. At this point, the valve to the vacuum line was closed off and the heat at the bottom of the cell was increased to 450 °C for Analytical Chemistry, Vol. 71, No. 3, February 1, 1999
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2 h while the cap was maintained at 120 °C to prevent condensation of BrF3 (from the decomposition of KBrF4) so that it would recombine as the reactant. The reaction between the KBrF4 and a representative oxide (Y2O3) is believed to be
2Y2O3 + 3KBrF4 f 4YF3 + 3KBr + 3O2(g)
(6)
with traces of other products such as Br2 and BrF. Upon cooling to room temperature, the cell was opened to the circulating path where the product gases were swept through the system by the circulating fan. The gas passed through the LN2 trap first where the volatile fluorides and bromides were trapped and then through the YSZ tube where the oxygen was pumped out and the quantity recorded as a chronoamperogram. The millicoulombs of charge passed can be converted to ppm oxide by Faraday’s law:
(mC) (0.08291)/sample wt ) ppm oxide
Figure 3. Typical O2 pumping chronoamperogram.
(7)
where the sample weight is in grams. This equation assumes 100% current efficiency for the oxygen-pumping process. Blank runs were performed in the same manner including opening the reaction cell cap and simulating adding a sample. RESULTS AND DISCUSSION Oxide Determination. The background reproducibility was between 50 and 60 µg of oxygen, similar to the 30-50 µg reported in the ORNL technique, and gave a limit of detection of 12 µg of oxygen (Table 1). The accuracy was tested by analyzing samples of yttrium oxide. As can be seen in Table 1, more than 99% of the oxide was recovered from the this salt. The precision of analysis was tested (Table 1) using FLINAK samples from the same batch, which had an oxide content near the detection limit of 12 µg. The analysis gave a coefficient of variation better than 13% and indicated the FLINAK samples contained residual oxide in the 110-170 ppm range after the HF purification. With these data, the analytical range for a 0.1-g sample would be 120 ppm to >20%. Since this technique relies on the absolute quantity of oxide, larger sample sizes would, in principle, lower the detectable ppm. For a 1-g sample, the detection limit would be 12 ppm. Sample sizes larger than 1 g are not recommended for this system though since this may require longer reaction times and would deplete the reactant more quickly. If a lower detection limit is required, then a modified version of this system may be designed for that purpose. Chronoamperograms. A typical chronoamperogram generated by the oxygen pumping can be seen in Figure 3. The peaks were generally sharp with a noticeable amount of tailing. As long as the baseline was established before pumping out the oxygen produced in the reaction vessel, the baseline background could be eliminated from the peak by drawing a horizontal line across the base and calculating only the coulombs of charge above that line. The blank run coulombs could then be subtracted from the sample run coulombs to get the final charge. The circulating system was designed to produce sharper chronoamperogram peaks by increasing the current and thus reducing the pumping time. This was achieved by minimizing the system volume to maximize the oxygen concentration. Often with larger quantities 542 Analytical Chemistry, Vol. 71, No. 3, February 1, 1999
Figure 4. Overloaded O2 pumping.
of oxygen in the system, the zirconia tube would be “overloaded” on the first pumping pass and a second current maximum could be seen on the chronoamperogram (Figure 4). “Overloading” was interpreted as being due to a slug flow of O2 that was not completely pumped out on the first pass through the YSZ tube. Then on passing a second time, another peak appeared for the remainder of the slug. The time for this second peak is consistent with the pumping speed of the system. A third peak is slightly visible as well. This did not significantly affect the accuracy or precision of the determination since the oxygen continued to circulate and eventually was pumped out. The chronoamperogram did have increased tailing and thus increased pumping time. Typical pumping times ranged from 1000 to 3000 s. Increased YSZ tube surface area, smaller internal circulating volumes, and faster circulating rates reduced this time significantly. Increasing the applied potential did not necessarily decrease the pumping time though, and the system was found to work best with an applied potential of 0.25 V. Potentials greater than 0.3 V caused the background current to drift, and potentials greater than 1.0 V tended to decompose the zirconia. Reaction Cell Seal. The Teflon ring insert used to seal the reaction cell lasted for more than 40 sample determinations as opposed to 6 reported by the ORNL method. Since a batch of KBrF4 will provide 20-25 determinations, the Teflon insert was replaced when a new batch was prepared. The source of the background variation is not known with certainty but is assumed to arise from either addition of moisture during the addition of the sample or minute leakage of the Teflon
seal during the sample run, since the cell is under vacuum during the reaction period. Steps to reduce the background and the variation are being taken so that a lower detection limit can be achieved and thus improve the performance of the technique. CONCLUSIONS An oxide determination apparatus has been developed for use as a simple, inexpensive tool for routine, ppm oxide analysis of fluoride salts such as the fuel and flush salts from the molten salt reactors. This technique can be used for solid samples in the 10250-mg range, depending on the oxide content, and can be assembled in most part from materials found in fluoride salt laboratories. The detection limit could be lowered significantly
by reducing the standard deviation of the background. Further work on these improvements will be continued in this laboratory. ACKNOWLEDGMENT This work was supported by a contract with Lockheed Martin Energy Systems, Inc. and AFOSR for a fellowship to S.E.E. Charmaine Mamantov and Haiming Xiao are thanked for many helpful discussions.
Received for review July 28, 1998. Accepted November 11, 1998. AC980830J
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