photometric Determination of Thorium bdophosphsric Acid
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Bryan L. Madison and John C. Guyon Department of Chemistry, Uniuersity of Missouri, Columbia, Mo. 65201 A sensitive, spectrophotometric technique for the determination of thorium has been developed. The method is based upon the reduction product of a presumed thorium molybdophosphoric acid heteropoly complex. The effects of variables on the colorforming reactions are described, along with the optimum conditions for the formation of the blue hue. The method is sensitive for thorium in the range of 0.2 to 1000 pg and by proper choice of size of sample and cells the limits can be varied widely. The molar absorptivity was calculated to be 3.3 x lo4. The method is subject to several interferences; however, the thorium can be cleanly separated using a solvent extraction procedure. The system was applied to the determination of thorium in a monazite sand.
A NEW SENSITIVE spectrophotometric technique for determining micro amounts of thorium has been developed. Although a considerable number of color reactions employing organic chromogenic reagents are known for thorium, only a few of these are commonly used for spectrophotometric determinations. The most widely used reagent for the determination of small quantities of thorium is thoron ( I ) . Other reagents which have been utilized successfully are alizarin red S (2), naphthazarin (3))quinalizarin (4)) quercetin (5), carmine red (6)) carminic acid (9, Erichrome Black T (8), and parsonic acid (9). The most sensitive chemical procedure currently available appears to be the spectrophotometric method using morin (IO), which is capable of determining about 0.2 pg of thorium in a 50-ml volume using 5-cm cells. Advantage has been taken of the sensitivity of heteropoly acids to weak reducing agents in acidic solutions. The reduced heteropoly complex, in common with the other reagents for thorium, is a sensitive but not a selective reagent. The proposed method using molybdophosphoric acid is capable of determining about 0.2 pg of thorium. This sensitivity is comparable to that of the morin method, and considerably greater than that of the other spectrophotometric reagents. The method is more than twice as sensitive as the commonly used thoron technique. To overcome the disadvantage of being nonselective, a separation scheme was investigated which effectively separates all the interferences studied. (1) P. F. Thomason, M. A. Perry, and W. M. Byerly, ANAL. CHEM., 21, 1239 (1949). (2) D. V. N. Sarna and B. S. V . Raghava Rao, Anal. Chim. Acta., 13, 142 (1955). ( 3 ) T . Moeller and M. Tecotzkey, ANAL.CHEM., 27, 1056 (1955). (4) A. Purushottam, 2. Anal. Chem., 145,245 (1955). (5) D. L. Manning and G. Goldstein, ANAL.CHEM., 29,1426 (1957). (6) N. Eswaranarayana and B. S. V. Raghava Rao, 2. Anal. Chem., 146, 107 (1955). (7) R.H. Hall, U.S.At. Energy Comm. Rept. ACED-2437 (1948). (8) P. F. Lott, K. L. Cheng, and B. C. H. Kwan, ANAL.CHEM., 32, 1702 (1960). (9) W. Byerly, L. Niedrack, W. Davin, and H. Dyas, U. S. At. Energy Comm. Rept. CC-2670 (1945). (10) M. H. Fletcher and R. G. Milkey, ANAL.CHEM., 28, 1402 (1956).
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ANALYTICAL CHEMISTRY
No heteropoly acid technique utilizing the reduction product of a complex between thorium and molybdophosphoric acid has been developed. This paper reports the results of a study designed to develop a spectrophotometric method for thorium and to continue an investigation into heteropoly acid formation. EXPERIMENTAL
Apparatus. Absorbance measurements were made either on a Cary Model 12 recording spectrophotometer, or on a Beckman DU spectrophotometer each with matched 1-cm and 5-cm quartz cells against double-distilled water, Emission spectra were taken on a Jarrell-Ash 480-cm emission spectrograph with a Wadsworth mounting. Measurements of pH were made on a Beckman Zeromatic pH meter equipped with a Beckman No. 39183 combination electrode. Hypodermic syringes of 2-ml capacity were used. The extractors were 60-ml open top cylindrical extractors with inch stems. Stirring motors were variable speed, line-operated. Reagents. Thorium nitrate stock solution was prepared by dissolving 12.0 grams thorium nitrate tetrahydrate (Baker and Adamson) in water, adding 32 ml of concentrated nitric acid, and diluting to volume in a 500-ml volumetric flask. Standardization was carried out by evaporating 20-ml aliquots to dryness in a platinum dish and igniting the resulting T h o zto a constant weight. This solution contained approximately 10 rng of thorium per milliliter. Appropriate dilutions of this solution were made using 1-molar nitric acid to obtain standard thorium solutions. Thorium solutions are known to be stable in 1-molar nitric acid (11). Twelve :one molybdate-phosphate (0.007M) was prepared by mixing 600 ml of 0.1M NazMo04.2H20 (24.196 grams/ liter) with 100 ml of O.05M KH2POk (6.805 grams/liter). Eight :one molybdate-phosphate (0.054M) was prepared by dissolving 207.38 grams of NazMoOl.2Hz0 and 14.579 grams of KH2P04in water, filtering into a 2-liter volumetric flask and diluting to the mark. Sodium citrate, 2 5 z (w/v) was prepared by dissolving 125 grams of Na3C6H507.2Hz0 in water and diluting to the mark in a 500-ml volumetric flask. Chlorostannous acid, 0.5 (w/v> was prepared fresh daily by dissolving 0.595 gram of SnClz.2H20in 1 ml concentrated HC1 and diluting to the mark in a 100-ml volumetric flask. Procedure. PREPARATION OF CALIBRATION CURVE. Prepare a calibration curve by transferring 0-50 pg of thorium from appropriate standard solutions to 50-ml beakers containing 15 ml of 0.3M H2S04. To each beaker, add 3 ml of 8 :1 molybdate-phosphate mixture (0.0542M). Adjust the pH to approximately 3.0 using 2 0 z NaOH and start a stopwatch. After 10 minutes add, by a hypodermic syringe, 2 ml of 25% sodium citrate and exactly 10 seconds later, in the same way, 2 ml of 0.5 % chlorostannous acid. Transfer to a 50-ml volumetric flask and dilute to the mark. Measure the absorbance at 690 m p 30 minutes after reducing using 5-cm cells with distilled water as a reference. Construct an absorbance-concentration calibration curve. The sensitivity of the method may be decreased to 0.5-10 ppm by using 1-cm cells.
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(11) R. G. Milkey, ANAL.CHEM., 26,1800 (1954).
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.Figure 1. Effect of 12: 1 molybdate-phosphate volume
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Preliminary Studies. Evidence was sought first for a reaction between thorium, molybdate, and phosphate. To solutions containing 0.0, 1.0, and 2.0 mg of thorium, 1.5 ml of 12 :1 molybdate-phosphate mixture (0.007M) was added, the pH adjusted to 2.2 and the volume made up to 100 ml. The absorption spectra show that there is an interaction, although the change is quite small. An attempt to improve the sensitivity of the system by reduction was carried out. Thorium gave an excellent enhancement under reducing conditions. The reduced species gave a spectrum with a broad absorption between 600 and 800 mp, with a maximum at 690 mp. It was necessary to remove the interference of the excess molybdophosphoric acid. The intense yellow color that formed as a result of the thorium-molybdophosphoric acid interaction faded instantaneously and completely in moderateto-high concentrations of acid. All attempts to utilize acid to destroy excess molybdophosphoric acid failed because of the instability of the ternary species in acid medium. Dicarboxylic and a-hydroxycarboxylic acids are known to form complexes with molybdate. Aqueous solutions of oxalic, citric, and tartaric acids were tested for their ability to destroy by complexation the binary heteropoly species but not the ternary species. Oxalate gave complete destruction of both species and tartrate gave incomplete destruction of the binary species. Citrate was found to give complete and rapid destruction of the binary species with little effect on the ternary complex. A system was thus available which yielded a blue hue for thorium-containing solutions and a practically colorless solution for those containing no thorium. This system was further investigated. Effect of 12:1 Molybdate-Phosphate Concentration. This variable was investigated by adding various amounts of the 0.007M molybdate-phosphate mixture to 0.5 mg of thorium in a system otherwise identical to that described in the previous section. Under these conditions the absorbance (Figure 1) increased with increasing amounts of the 12:l molybdate-phosphate mixture up to a volume of 3.0 ml. Additional molybdate-phosphate had no further effect except to slightly increase the blank correction necessary in each case. Effect of pH. Like other heteropoly systems, the extent of complex formation, and hence the absorbance of the final solution, is a function of pH. To solutions containing 0.5 mg of thorium, 3.0 ml of 12 :1 molybdate-phosphate mixture (0.007M) was added, and the pH adjusted to various values. The reduction was carried out as before, and the volume adjusted to 100 ml. The ab-
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sorbances, but the resulting blue hue was unstable. Sulfuric acid was chosen because it gave a quite stable blue hue with excellent sensitivity. Effect of Time. To determine the time required for complex formation, a series of solutions, each containing 3.0 ml of 12 :1 molybdate-phosphate mixture (0.007M) and 0.5 mg of thorium, were prepared and the pH was adjusted to 3.0. Ths solutions were allowed to stand different periods of time and then reduced as described before. The absorbance of the reduction product was taken as a measure of completeness of complex formation. The absorbance increased with time and became constant after 10 minutes. Ten minutes was selected as the time required for formation of the complex. Effect of Temperature. Temperature change had little effect on the degree of rapidity of complex formation. When solutions were heated in boiling water, a considerable decrease in sensitivity was observed, because of the accelerated interaction between citrate and the ternary species. It was decided to perform subsequent experiments at room temperature. Effect of Sodium Citrate Concentration. Citrate ion was necessary in the determination of thorium to prevent reduction of excess molybdophosphoric acid. Citrate ion should not be added until sufficient time is allowed for the complex to form. Citrate ion interacts with the molybdate aggregate and forms a nonreducible complex. Solutions containing 3.0 ml of 12:l molybdate-phosphate mixture (0.007M) and 0.5 mg of thorium were adjusted to pH 3.0 and 10 minutes allotted for complex formation. Varying amounts of 5x sodium citrate were added via a hypodermic syringe and after 10 seconds, 2.0 ml of 0.5Z chlorostannous acid. The solutions were made up to a volume of 100 ml. The absorbance data (Figure 2) show that 2.0 ml is sufficient for removal of excess molybdophosphoric acid. Choice and Handling of Reductant. If given sufficient time, citrate ion decomposes the complex before reduction. To avoid partial complex decomposition, a reductant was required which would give instantaneous color development. Several reductants were studied. Hydrazine hydrochloride gave a weak coloration that formed slowly. A solution of VOL. 39, NO. 14, DECEMBER 1967
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Stability of Color. To study this variable, the blue hue was developed in a system identical with that of the previous section, and the solution was transferred rapidly to the spectrophotometer. The absorbance was measured at 690 mp OS. time. The absorbance data show that the reduction product fades rapidly during the first 15 minutes but remains stable for at least 70 minutes thereafter. A time of 30 minutes after reduction was selected for subsequent work. Conformity to Beer's Law. A straight-line calibration curve showed conformity to Beer's law for the concentration ranges studied. The useful range for a 1-cm cell is 0.05 to 1.0 mg of thorium per 50 ml. The useful range for a 5-em cell is 0.2 to 80 pg of thorium per 50 ml. On the basis of these data, the molar absorptivity calculated over the range studied is 33,000 and the mean sensitivity (Sandell method) is 0.0073 pg/cm2. Effects of Diverse Ions. The results of the erects of selected diverse ions are summarized in Table I. A 2 % error in the determination of thorium was considered tolerable. Ag+, Pb+2,and Ba+2form insoluble precipitates. Fe+2, Fe-3, Al+3, V03-, W04-2, Ni+2, Ti+*, and Zr+4 interfere seriously. The ions listed in Table I can be tolerated in the concentration range reported.
Table 1. Effect of Diverse Ions (0.5 mg ThjlOO ml solution) Amount permitted, p.p.m. 10
100 100 100 12 100
12 2 20 8
100 100 100
100 40 100 60 100
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l-amino-2-naphthol-4-sulfonic acid in sodium sulfite behaved similarly. Ferrous ammonium sulfate reacted more rapidly and gave higher enhancements. Chlorostannous acid gave considerably faster and greater enhancements and was chosen as the reductant in the system. The time between addition of the citrate and addition of the reductant was studied. The absorbance data showed that 10 seconds was necessary for citrate to remove excess molybdophosphoric acid. Time periods greater than 10 seconds resulted in partial complex decomposition. For this reason the reductant was added rapidly, using a hypodermic syringe, and exactly 10 seconds after the addition of sodium citrate. Effect of Chlorostannous Acid Concentration. Varying amounts of reducing agent were added via a hypodermic syringe to solutions prepared as in the previous section. The absorbance increased with increasing amounts of chlorostannous acid up to 2.0 ml of the 0.5% solution. Higher concentrations of reducing agent gave increasing blank absorbances and 2.0 ml of 0.5% chlorostannous acid was chosen as the most satisfactory volume. Effect of Volume on Complex Formation. Varying amounts of water were added to 3.0 ml of the 12:l molybdate-phosphate (0.007M) mixture and 0.5 mg thorium. The pH was adjusted to 2.2 and after standing three minutes, 2.0 ml of 5 sodium citrate was added. Exactly 10 seconds later, 2.0 ml of 0.5% chlorostannous acid was added. The solutions were transferred to 100-ml volumetric flasks, diluted to the mark, and the absorbance was measured 15 minutes after reducing. The data showed a substantial decrease in absorbance when the volume exceeded 5 ml. To obtain completeness of reaction between thorium and molybdophosphoric acid, 3.0 ml of a molybdate-phosphate mixture (0.054M) was selected for future work. Effect of Molybdate-Phosphate Ratio. This variable was investigated by adding 3.0 ml of 0.054Mmolybdate-phosphate solution to 0.5 mg of thorium in a system otherwise identical to that previously described. The absorbance increased up to a ratio af 3 :1 with no further change at higher ratios. A blank of minimum absorbance was obtained using the 8 :1 ratio, and this ratio was incorporated in further work. Optimum reagent concentration remained the same for any molybdate-phosphate ratio greater than 3 :1.
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ANALYTKAL CHEMISTRY
APPLICATION OF METHOD
An 8 :1 molybdate-phosphate mixture (0.054M) was used in preference to the 12:l mixture (0.007M) because of the general inconvenience of maintaining the volume at less than 5 ml for complex formation. To prevent reduction of excess molybdophosphoric acid, a 25 % sodium citrate solution was used in the reduction step. The method was evaluated using a m o n a d e sand obtained from the New Brunswick Laboratory, Atomic Energy Commission. The ore was decomposed using the sodium peroxide fusion method of Carron, Skinner, and Stevens (12). Separation of the thorium prior to the colorimetric determination was accomplished using the solvent extraction procedure of Ross and White (13) with tri-n-octylphosphine oxide (TOPO). A 0.2939-gram sample of the monazite ore was fused with 4 grams of sodium peroxide in a porcelain crucible over the full heat of a Meker burner for 1.5 minutes. The melt was cooled, the crucible transferred to a 250-ml beaker, and the melt leached with 100 ml H20. After the reaction ceased, 25 ml of 1-1 HNO, was added and the solution evaporated to approximately 10 ml. The solution was filtered through rapid flow filter paper into a 100-ml volumetric flask and diluted to the mark, A 25-ml aliquot was taken and the extraction procedure of Ross and White used, The spectrophotometric determination was carried out on the 0.3M H2S04 strip solution without further treatment as described in the preparation of the calibration curve, The average of three determinations showed the ore to be 0.051 in thorium. The value from the New Brunswick laboratory showed the ore to be 8.05 thorium. The relative error was 2 . 0 z .
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RECEIVED for review July 14, 1967. Accepted September 8, 1967. Work supported by the Director of Chemical Sciences, Air Force Office of Scientific Research under Grant AFAFOSR-205-67. Presented at Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 21, 1966. (12) M. K. Carron, D. L. Skinner, and R. E. Stevens, ANAL.CHEM., 27, 1058 (1955). (13) W. J. Ross and J. C. White, Ibid.,31, 1847 (1959). ~
