A Rapid and Selective Method for the Spectrophotometric Determination of Niobium Robert Villarreal and Spence A. Barker Argonne Nationtr/ Laboratory, Idaho Dicision,Idaho Falls, Idaho 83401
A rapid and selective method for the separation and determination of niobium in uranium-fissium alloy has been developed. The procedure is based on the extraction into toluene of niobium with N-benzoyl-N-phenylhydroxylamine (BPHA) from 9-12M HCI containing stannous chloride. The Nb-BPHA complex in toluene i s contacted with ammonium thiocyanate in 4M HCI and the color intensity of the Nb-BPHA-SCN complex in toluene is measured at 365 mp. Of 48 elements tested, only tantalum interferes at all levels. Milligram quantities of tartrate, oxalate, citrate, fluoride, and other common anions do not interfere; hydrogen peroxide interferes but is easily destroyed. The ionic molar absorptivity of the colored complex measured under the conditions of the procedure is 32,000. Beer’s law is obeyed from 0 to 50 pg Nb. At the 0.01% niobium level in uranium-fissium alloy, the relative precision of the method is + 2%. METHODS FOR THE DETERMINATION of niobium are generally long and tedious, and require preliminary separations for the removal of interfering ions. Masking reagents, added to improve selectivity, greatly reduce the sensitivity of many methods. Thiocyanate methods are perhaps the most commonly used for niobium, but are subject to several chemical interferences. Hydrogen peroxide and 8-quinolinol methods lack sensitivity and are also subject to chemical interferences. Chromogenic reagents such as 4-(2-thiazolylazo) resorcinol ( I ) , 4-(2-pyridylazo) resorcinol (2), bromopyrogallol red ( 3 ) , and various azo-dyes ( 4 ) were recently introduced for the determination of niobium. Although these reagents are very sensitive for niobium, they cannot be directly used on highly colored solutions or on solutions containing large amounts of uranium. N-Benzoyl-N-phenylhydroxylamine(BPHA) has been used for the gravimetric determination of niobium ( 5 , 6> and for the separation of niobium, tantalum, and titanium from other elements (7). Alimarin and Petrukhin ( 8 ) have investigated the BPHA extraction of niobium, titanium, tantalum, vanadium, and zirconium from sulfuric acid solutions. Lyle and Shendrikar (9) have studied the extraction of niobium, protactinium, and tantalum from hydrochloric acid solutions. Afghan er a/. ( I O ) used BPHA for the extraction and determination of titanium. Vita et ul. (11) reported a detailed study on the extraction of 52 elements with BPHA,
(1) V. Patrovsky, Talunta, 12, 971 (1965). (2) P. Pakalns, Anal. Chim. Acta, 41, 283 (1968). (3) R. Belcher, T. V. Ramakrishna, and T. S. West, Tulunta, 12, 681 (1965). (4) I. P. Alimarin and S. B. Savvin, ibid., 13, 689 (1966). (5) A. K. Majunidar and A. K. Mukherjee, Anal. Chim. Acta, 19, 23 (1958). (6) A. K. Majumdar and A. K. Mukherjee, ibid., 21, 245 (1959). ( 7 ) F. J. Langmyhr and T. Hongslo, ibid., 22, 301 (1960). (8) I. P. Alimarin and 0. M. Petrukhin, “Analytical Chemistry,” Elsevier, New York, N.Y., 1963, p 152. (9) S. J. Lyle and A. D. Shendrikar, Tulunta, 12, 573 (1965). (IO) B. K. Afghan, R. G. Marryatt and D. E. Ryan, Anal. Chim. Acta, 41, 131 (1968). (11) 0. A. Vita, W. A. Levier and E. Litteral, ibid., 42, 87 (1968).
and the subsequent determination of niobium with PAR and zirconium with Arsenazo 111. The nuclear fuel used in Experimental Breeder Reactor I1 (EBR-11) is a uranium-fissium alloy consisting of 95 uranium and a 5 % mixture of molybdenum (2.5 %), ruthenium (1.9 rhodium (0.30z), palladium (0.19%), zirconium (O.lOz), and niobium (0.01 %). The routine determination of niobium in this fuel requires a method that is highly selective, sensitive, and preferably rapid. The outlined method has been successfully applied to the determination of niobium at the 0.01 % level in the analysis of uranium-fissium alloy. The selectivity of the procedure should make it widely applicable for the determination of niobium. In this highly selective method, niobium is extracted with BPHA into toluene from 9--12M hydrochloric acid in the presence of stannous chloride, which reduces metal ions. The N b B P H A complex in toluene is shaken with ammonium thiocyanate in 4M hydrochloric acid to form a colored complex in the toluene which is measured at 365 mp. An unsuccessful attempt was made to identify the exact composition of the Nb-BPHA-SCN complex. The ionic molar absorptivity for the colored complex measured under the conditions of the procedure is 32,000. The absorbance of the colored complex obeys Beer’s law from 0 to 50 pg N b in 10 ml of toluene.
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EXPERIMENTAL
Apparatus. Absorbance measurements were made with a Beckman Model B spectrophotometer in matched 1-cm silica cells against deionized water. Gamma counting of Nb-95 tracer was performed with an ND-180 512-channel analyzer. Conditions. All experiments and measurements were conducted at room temperature (25 “C). Reagents. 1% BPHA. Dissolve one gram of N-phenylbenzohydroxamic acid (Eastman Organic Chemicals, catalog No. 7297) in 100 ml of either ethanol or acetone. This solution is stable for at least one month if kept in the refrigerator. 40% SnC12. Dissolve 40 grams of stannous chloride by heating in 75 ml of 12M HC1. After cooling, make to 100 ml with 12M HC1. This solution should be made fresh every week. 25% N H S C N . Dissolve 250 grams of NH,SCN in deionized water and dilute to one liter. Extract the N H X N solution with 100 ml of hexone. Discard the hexone. Remove residual hexone from the solution by washing with 100 ml of chloroform. Discard the chloroform. Filter the washed solution through fast quantitative filter paper. Thioglycolic acid. Purified (Fisher A-319). Fluoboric acid. 48-50Z. Niobium solution (1 mgiml). Dissolve 1.0 gram of niobium metal by prolonged heating with concentrated sulfuric acid. Cool. Add 40 ml of 18N sulfuric acid, 50 ml of 10% tartaric acid solution and dilute to 1 1 with 6 M HC1. Chemicals used in interference studies were reagent grade. Dissolution of Uranium-Fissium Alloy. Place a 1-gram sample in a 125-11-11 Erlenmeyer flask containing 15 ml deionized water. Dissolve the sample by adding 20 ml of a 4:l HC1-HN03 mixture and heating on a hot plate. The dissolution should VOL. 41, NO. 4, APRIL 1969
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Figure 1. Niobium extracted
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1 ml of 19; BPHA in ethanol, 20 ml X xM HCI, 1 ml of 40:; SnCI,, 10 ml toluene
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proceed as rapidly as possible without spattering until the sample is dissolved. Any undissolved residue (primarily Ru) is separated by centrifugation in a 40-ml cone and dissolved in 2 ml of 6M NaOH and 5 ml of NaOCl heated in a hot water bath for 10 min. The alkaline solution is cooled, acidified with 12MHC1, added to the original solution, and this solution diluted to 50 ml volume with deionized water. Sample Preparation. Samples in which niobium may have polymerized or have precipitated should be strongly fumed with concentrated sulfuric acid. Also, sample aliquots larger than 3 ml should be fumed with 2 ml concentrated sulfuric acid to a volume less than 1 ml. The residual sulfuric acid is taken up with 20 ml of 12M HC1 and transferred directly to a separatory funnel for analysis. Analytical Procedure. For samples which do not require sulfuric acid fuming, pipette an aliquot containing 2-20 pg of niobium into a 60-ml separatory funnel. Add 20 ml of 12M HCI, 1 ml of SnCL solution, and mix thoroughly. From this point on, the specified times should be followed fairly closely, as large deviations may lead to erratic results. Add 1 ml of 1 % BPHA, mix thoroughly, and allow to stand 1 min. Extract the Nb-BPHA complex into 10 ml of toluene by shaking for 1 min. Allow the phases to separate completely; drain off and discard the aqueous phase. Add 20 ml of 4M HC1 and 5 ml of 25% N H X N solution and shake for 1 min. As soon as the toluene phase is clear, transfer a portion or drain through cotton into a 1-cm cell and measure the color intensity at 365 mp. Color measurement must be made within 1 hour after adding the NHlSCN solution (see Color Development). Run blanks and standards with sample as described above. EFFECTS OF VARIABLES Nb-BPHA Extraction. Extraction of the Nb-BPHA complex into toluene is quantitative from 0-12M hydrochloric acid. Niobium carrier with niobium-95 tracer was extracted with BPHA into toluene from 0.1M to 12M HCl, the phases were separated by centrifugation, and aliquots of each phase were counted to determine the fraction of niobium extracted. The results of the tracer experiments are shown in Figure 1. The extraction of niobium-95 tracer with BPHA from 9-12M HC1 was complete within 1 min. Extraction of metal ions other than niobium is minimized at high acid concentrations and the possibility of hydrolysis or polymerization of niobium is reduced. Metal-thiocyanate complexes are insoluble in toluene but the Nb-BPHA-SCN complex is soluble in toluene. Selectivity of the procedure is greatly enhanced, because, under 612
ANALYTICAL CHEMISTRY
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360 380 4 0 0 WAVELENGTH (mp)
Figure 2. Absorption spectrum Curve 1. 33 p g Nb in 10 ml toluene Curve 2. 99 p g Nb in 10 ml toluene the conditions of the method, only toluene-extractable metalBPHA complexes can react with thiocyanate and remain in toluene. Effects of Other Ions. The effect of various ions was determined by adding known quantities of test ion to a niobium standard and blank and noting the effect on the final absorbance. Ten milligrams each of Li, B, Na, Mg, K, Ca, Fe, Y , Mo, Ru, Sn, Ba, La, mixed rare earths, Th, U, and 1 mg each of Al, Si, Ti, Cr, Mn, Ni, Cu, Zn, Co, Bi, Pt, Rh, Pd, In, W, and Re gave no interference in the method. Greater quantities of most of these test elements were not investigated for interference. Titanium and Pt in amounts greater than 1 mg and Zr in amounts greater than 20 pg gave positive interference. Small quantities of fluoride in the Ti and W test solutions may have contributed to the noninterference of these elements. Vanadium in amounts greater than 0.25 mg gave negative interference. Tantalum interfered at all levels. However, the interference of V and Zr up to 1 mg may be masked with 1 ml of 8 5 % phosphoric acid, but greater quantities of Zr will precipitate as zirconium phosphate. Ten milligrams of Zr may be masked with 0.1 ml of fluoboric acid. One milligram of Ag may be kept in solution by masking with thiourea. Ten milligrams of Bi gave positive interference but was masked by 0.5 ml of thioglycolic acid. Masking agents, if needed, should be added to the 9-12M HC1 before addition of BPHA. Reduction by SnC12 eliminates the interference of several cations, notably Co, Mo, and Fe, that interfere in most thiocyanate procedures. Up to 0.5 g of SnCl, added to 20 ml of 12M HC1 does not adversely affect the procedure. One hundred milligrams of citrate, oxalate, tartrate, thiourea, thioglycolic acid, phosphate, sulfate, nitrate, perchlorate, iodide, bromide, and 10 mg of fluoride gave no interference. Greater quantities of fluoride as H F leached impurities from the borosilicate separatory funnel and gave high results. At least 100 mg of fluoride as fluoboric acid does not interfere. Hydrogen peroxide, if present, interferes seriously but may be easily destroyed by heating the 9-12M HC1 to near boiling prior to addition of BPHA. Tantalum is the only ion found whose interference at all levels was not eliminated by complexing agents.
Color Development. The greatest color and stability for the Nb-BPHA-SCN complex was achieved by shaking the NbBPHA complex in toluene with an acid solution of 0.65M NH4SCN. The optimum acid concentration for color development with N H S C N was 3-4M HCl. Higher acid concentrations greatly increased the blank while lower acid concentrations gave much less color. The Nb-BPHA-SCN color formed rapidly with a 1-min shaking time and was stable for 1-2 hr if kept in the dark. Exposure to indirect sunlight in the laboratory increased the absorbance of the solution; however, the blank increased at the same rate and the net absorbance remained constant for 1-2 hr. Exposure of the colored complex in toluene to direct sunlight quickly caused formation of a yellowish turbidity. Production of turbidity by laboratory fluorescent light was very much slower than by sunlight. One milliliter of thioglycolic acid added to the 4M HCl before addition of NHISCN stabilized the colored complex for 1-2 hr in indirect sunlight and fluorescent light of the laboratory. Absorption Spectra. The absorption spectra of both the Nb-BPHA-SCN complex and the Nb-BPHA complex in toluene are shown in Figure 2. The peak wavelength for both complexes is in the near ultraviolet region from 360 to 365 mp. The wavelength of 365 mp was selected to provide maximum net absorbance readings together with an acceptably low blank. Sensitivity. The absorptivity of the extracted Nb-BPHA-
SCN complex measured under the conditions of the procedure at 365 mp was calculated from a Beer’s law plot to be 344 l/g-cm. The corresponding molar absorptivity (ionic molar absorptivity) is 32,000 and the sensitivity index for the reaction is 0.0029 pg Nb/cmz. The colored species obeys Beer’s law over the range studied from 1 to 50 pg niobium in 10 ml toluene. The Nb-BPHA complex in toluene may be used for the determination of niobium, but the molar absorptivity for the Nb-BPHA complex in toluene is only 10,000 at 365 mp, which is similar to 8-quinolinol but is much more selective. The Nb-BPHA complex in chloroform is the basis of a method described by Shigematsu et al. (12). Precision. Analyses of 8 aliquots taken from a uraniumfissium solution gave 7.89 + 0.12 pg Nb/ml. At the 95z confidence level, this represents a relative precision of + 2 z . ACKNOWLEDGMENT
The authors express their appreciation to John Young for preliminary work on the procedure and to Earl Ebersole for his technical assistance and help in preparing the report.
RECEIVED for review November 4, 1968. Accepted December 31, 1968. (12) T. Shigematsu, Y . Nishikawa, and S. Goda, Bull. Inst. Chem. Res., Kyoto University, 43, 347 (1965).
Design of Membrane-Covered Polarographic Gas Detectors Daniel P. Lucero Electro-Analytical Transducer Corporation, Fullerton, Gal$ 92633 Membrane-covered polarographic gas detectors are designed by employing the relationships between their design constraints, specifications, and performance parameters. The application of the detector determines the functional dependence between specificity, sensitivity, time rate of response, and wearout time. Specificity is established by the electrochemical characteristics of the detector cell. Sensitivity is determined by the membrane material and its thickness. Time rate of response characteristics are determined by the properties of the electrolyte reservoir and the membrane and the internal geometrical design of the detector. The detector can wearout by several modes which depend upon the application. Wearout by depletion of the electrolyte solvent medium is the most common. Temperature response characteristics can be established by signal temperature compensation and active thermal control. Total gas pressure, humidity, and shock and vibration also affect the detector performance by altering its parametric relationships.
MEMBRANE-Covered polarographic gas detectors are electrochemical devices which measure the partial pressure of a particular molecular species present in gas mixtures and/or dissolved in liquids. They are commonly comprised of a sealed cartridge containing the essential elements of an electrolytic cell which is exposed to the gas molecules of the external environment through a semipermeable membrane. Their optimal operating mode is in the diffusion limited condition
-Le., the mode where the electrode reaction consumes the electroactive molecular species at a greater rate than it can migrate through the membrane or diffusion barrier. Thus, the operating and performance characteristics of these detectors are primarily established by the mass transport properties of the membrane and its dimensional and geometrical configuration (1). Every aspect of the design is affected by the membrane characteristics. In only a few cases is the membrane of secondary importance in which consideration of its parameters may be relegated to the latter design stages. During the early phase of the design, it is mandatory that the design constraints and performance specifications be totally considered in order to characterize fully the detector elements and their parametric interdependence and to delineate and project design tradeoffs which will be required. The basic configuration of a detector electrolytic cell is illustrated in Figure 1. All the essential elements are shown with the critical dimensional parameters of the cell. The configuration of the cell can be drastically changed to reflect the optimum design for a specific application. However, the general configuration represented by Figure 1 serves more effectively to relate the fundamental principles and techniques in the design of membrane-covered polarographic gas detectors. (1) D. P. Lucero, ANAL.CHEM., 40, 707 (1968). VOL. 41,NO. 4, APRIL 1969
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