V O L U M E 2 5 , NO, 1 2 , D E C E M B E R 1 9 5 3 citrate buffer solution), and the reagent concentration was about 0.06 gram of phenanthroline per liter. The iron was reduc?d to the iron(I1) state by means of ascorbic acid. The lamp used was not the same as that used in the experiments in Table 11, hence the different 01 values. As the standards in experiments 4 through 7 were sufficiently close to the optimum values, the results \yere calculated. using Equation 20, whereas Equation 15 was used for experiments I through 3, and Equation 17 was used for experiment 8. The agreement is satisfactory. ACK\-OWLEDGMENT
The authors wish to thank Erkki 1-anninen for checking some of the analyses in this investigation.
1803 LITERATURE CITED
(1) Ayres, G. H., ANAL. CHEM.,21, 652 (1949). (2) Bastian, R., Ibid., 23, 580 (1951). (3) Bastian, R., Teberling, R., and Palilla, F., I b i d . , 22, 160 (1950). (4) Goddu, R. F. M , , and Hume, D. S . ,I b i d . , 22, 1314 (1950). (5) Hiskey, C. F., Ibid., 21, 1440 (1949). (6) Hiskey, C. F., Rabinowitr, J., and 1-oung, I., Ibid., 22, 1464 (1950). (7) Hiskey, C. F., and Young, I., I b i d . , 23, 1196 (1951). (8) Osborn, R. H., Elliott, J. H., and Martin, A. F., ISD. ENO. CHEM.,AXAL.ED., 15, 643 (1943). (9) Ringbom, A , , 2. anal. Chem., 115, 332 (1939). (10) Ringbom, A., and Sundman, F., I b i d . , 115, 402 (1939). (11) Walter, R. N., AXAL.CHEZI.,22, 1334 (1950). RECEIVED for review December 31,
1952.
Accepted July 30, 1953.
Ultraviolet Spectrophotometric Determination of Tantalum with Pyrogallol JOSEPH I. DINNIN, U . S . Geological Survey, Agricultural Research Center. Beltscille, Md. In a search for a more rapid method for the determination of tantalum in rocks and minerals, an intensive study w-as made of the tantalum-pyrogallol reaction recommended by Platanov and Krivoshlikov, and a better modified spectrophotometric procedure is given. The improved method consists in measuring the absorbancy of the tantalum-pyrogallol complex at 325 mp in 4N hydrochloric acid and a fixed concentration (0.0155.44) of ammonium oxalate. Beer’s law is follow-ed for the concentration range up to 40 y per ml. Sensitivity in terms of molar
K
NOWLEDGE of the analytical chemistry of tantalum and niobium has improved considerably since the recent publication of a critical analysis of the existing methods by Atkinson, Steigman, and Hiskey ( 2 ) . Since t h a t time, among other methods, chromatography on cellulose has been successfully applied to the separation and determination of the individual earth acids from complex mixtures and ores by Burstall et al. (S), PIfercer (19), and Williams (29); ion exchange separations using synthetic resins have been applied to selective mixtures by Huffman, Iddings, and Lilly (7) and Kraus and Moore (11, 12): a solvent extraction procedure by Leddicotte and Noore (17) has resulted in the separation of the earth acids from one another: neutron activation analysis has been applied by Eichholz ( 4 ) and I’ong (18) to the estimation of tantalum in the presence of several other elements. Although three sensitive colorimetric methods now exist for the determination of niobium (5, 16, 25), no satisfactory colorimetric method was available for tantalum until recently. Soon after the work described in this paper was completed a simultaneous spectrophotometric method for the determination of tantalum and niobium was reported by Palilla, Adler, and Hiskey (20). They found that by working with concentrated sulfuric acid it was possible to shift the spectra of the peroxy tantalates from the ultraviolet cutoff and thus make possible the determination of tantalum. Titanium and iron give serious interference in this determination, however. If they are not removed by the chloride volatilization technique ( 2 ) or other means, their effect would have to be circumvented by the usual series of calculations involving absorbancy measurements a t several wave lengths The use of the color-forming reaction betw ecn tantalum anti pyrogallol as the basis for A colorimetric determination ot tnnta-
absorbancy index is 4575. Most interferences are additive in character and readily correctable. Separations or major corrections are required in the presence of significant amounts of molybdenum, tungsten, antimony, and uranium. The method has been successfully applied to three ores previously analyzed by gravimetric techniques. The method affords greater speed, sensitivity, and reproducibility in the determination of tantalum in rocks and minerals. A more reliable technique for preparing standard solutions of tantalum has been developed.
lum was proposed by Platanov, Krivoshlikov, and Marakaev (15, 23) in 1936. Several studies and numerom applications of the method have since been made (1, 6, 8-10, 13, 14, 21, 22, 26, $ 7 ) . The reported methods are lacking in sensitivity, however, and suffer from interferences caused by elements usually associated with the earth acids in rocks and minerals. Tedious separations are almost always necessary. In addition, most of the work is reported in Russian journals and is not readily accessible to English-speaking chemists except in the form of abstracts Thanheiser ( 2 6 ) studied the effect of sulfuric acid concentrations as high as I S on the absorbancies of the tantalum-, niobium- and titanium-pyrogallol complexes. The present investigation eutends the range of the hydrochloric acid concentrations studied to beyond 7 N . The improved absorbancj relationships found a t the higher acidities form, in part, the basis for the improved analytical procedure described later. Selection of Acid. Sulfuric acid was the medium used by others in previously reported work. All of the experimental lvork with hydrochloric acid described later 1m.s also performed using sulfuric acid for comparison. Sulfuric acid \vas found to result in greater interference from niobium and titanium. Nitric acid oxidized the pyrogallol to a j-ellow compound, which made measurements of the tantalum-pyrogallol complex impossible. Phosphoric acid depressed the absorbancy of the complex. Perchloric acid caused precipitation of salt? from solution. Hydrochloric acid was therefore found to be the best of the common mineral acids for use in this \I ork. GENERAL PROCEDURE
The following procedure for thr analysis of a typical tantalum-
ANALYTICAL CHEMISTRY
1804 bearing rock was adopted on the basis of the experimental n.ork later described: A 0.050- to 1.000-gram sample is fused with 10 grams of potassium bisulfate; 50 ml. of the ammonium oxalate extracting sohtion are then added to the cooled melt and the mixture is stirred continuously on a steam bath. The resulting solution is cooled and made up t o volume with water. An aliquot, estimated to contain approximately 1 mg. of tantalum dioxide, is then added to a 50-ml. volumetric flask containing the following reagents: 25 ml. of hydrochloric acid solution, 10 ml. of pyrogallol solution, and sufficient dilute ammonium oxalate solution to make the final ammonium oxalate concentration 0.125 gram per 50 ml. The flask is then made up to volume, mixed well, and a portion is used for the determination of the absorbancy a t 325 mp, compared with the absorbancy of a blank set at 100% transmittancy. STANDARDS, REAGEBTS, AND APPARATUS
Standard Tantalum Solution. The purest grade of tantalum pentoxide available, containing less than 0.10% niobium pentoxide and less than 0.01% titanium dioxide, was used for the preparation of this solution. It was ignited a t 1000° C. for 2 hours before use. An oxide obtained from another source contained more than 8% volatile matter and was quite hygroscopic even after ignition a t 1000" C. for 48 hours. A standard solution of the oxide, containing 0.20 mg. per ml., was prepared by the method outlined in the general procedure described above. Solutions thus prepared are stable for several months. Continuous stirring has been found to be of critical importance in the preparation of a true solution of tantalum. The literature on tantalum commonly available does not stress the need for continuous stirring during an ammonium oxalate leach of a bisulfate melt. I t s importance was indicated only after an intensive investigabion was undertaken to determine the source of variations which were encountered during an attempt to prepare standard solutions giving the same absorbancy, Kumerous methods for the preparation of standard tantalum solutions were tried without success. In the common technique involving fusion of the oxide with potassium bisulfate and extraction of the fused melt with ammonium oxalate solution, variations were made in the temperature and duration of fusion, mole ratio of flux to oxide, and temperature and duration of extraction. Dissolution of the oxide in a mixture of hydrofluoric and sulfuric acids and evaporation of the solution to fumes of sulfur dioxide and dilution with water was also tried. Variations were made in the duration of fuming, mole ratio of sulfuric acid to oxide; temperature and duration of heating the final diluted solution, and reagent solutions used for the final dilution. All of the variations listed above yielded solutions which gave absorbancies which could not be duplicated on retrial. In general, the absorbancy could be increased by maintaining the solution a t 90" C. for greater lengths of time. The absorbancies produced by such aging, however, were not capable of being reliably duplicated. Moreover, some solutions were found to yield lower absorbancies when aged at elevated temperatures. Dissolution of a bisulfate fusion of the oxide with either tartaric or oxalic acid frequently resulted in a turbid solution which gave a very low absorbancy, even when continuous stirring was applied. The final procedure adapted-viz., fusion of the oxide with potassium bisulfate and dissolution of the fused melt with ammonium sulfate solution with continuous stirring-was found to allow considerable latitude in the permissible mole ratio of flux to oxide. Solutions giving the same absorbancy could be prepared consistently from mole ratios varying from 5000 to 1to less than 5 to 1. Duration of time of heating the extracting solution was also not critical. Stirring the solution for 30 minutes or until the fused cake was completely dissolved was adequate; stirring for several additional hours had no further effect. Aliquots of 11 different solutions prepared by stirring ammonium oxalate extractions gave absorbancies with a standard
deviation of 0.003 absorbancy unit. This was only slightly greater than the standard deviation of 0.002 obtained using 10 aliquots from the same solution; it was also only slightly greater than the deviations caused by slight contaminations on the absorption cells. The criterion of an adequate solution of tantalum oxide is usually implied to be a clear solution. A clear solution can be obtained, however, which gives only 1% or less of the expected absorbancy. Observation of the solutions for Tyndall effects showed a rough inverse correlation between the number of colloidal particles observed and the absorbancy of the solution. The cause of the colloidal particles can be ascribed to surface phenomena acting a t the interface of the melt and the extracting solution. Reagent Solutions. PYROGALLOL SOLUTION, 200 grams of pyrogallol, 100 ml. of concentrated hydrochloric acid, and 10 ml. of 2 M stannous chloride per liter. The presence of hydrochloric acid and stannous chloride increases the stability of the solution so that it can be kept for a t least a month without decomposition. The period of usefulness of a solution prepared without these two reagents is about 2 days. Stannous chloride also serves to reduce the ferric ion usually present in the samples being analyzed. HYDROCHLORIC ACID, 8 N . AMMONIUXOXALATEEXTRACTING SOLUTION, 50 grams per liter. DILUTEAMMONIUM OXALATE,POTASSIUM BISULFATESOLGTIOK, 12.5 grams of ammonium oxalate and 50 grams of potassium bisulfate per liter. Because of a gradual increase in absorbancy, a fresh solution was prepared every 2 weeks. Apparatus. The absorbancy measurements were made with a Beckman DU spectrophotometer equipped with ultraviolet attachments, and a set of matched 1.000-em. silica cells. Water followed by acetone proved adequate as wash solutions. All the data mere rechecked using a photomultiplier attachment with a 10,000-megohm resistor in the spectrophotometer circuit. This permitted much higher sensitivities to be obtained and smaller band widths to be used; the necessary slit wldths were reduced from approximately 1.5 mm. to less than 0.1 mm. EXPERIMENTAL WORK
Effect of Variables on Absorbancy. The effect of the different variables on the absorbancy of the tantalum-pyrogallol complex was measured a t 325 mM.
-
4
FINLL
HCI
5 C O Y C E h T R A T l O N IN)
6
I
8
Figure 1. Effect of Hydrochloric Acid on Absorbancy of Tantalum-, Niobium-, and TitaniumPyrogallol Complexes
HYDROCHLORIC ACID. The effect of varied concentrations of hydrochloric acid was studied in an effort to determine the optimum concentration to be used. Varying amounts of the hydrochloric acid solution were pipetted into a series of 50-ml. volumetric flasks; 10 ml. of the pyrogallol solution were then added. This was followed by the addition of 8 ml. of the dilute ammonium oxalate solution and 5 ml. of the tantalum standard solution (1.00 mg. of tantalum pentoxide). The solutions were made up to volume and mixed well. For each concentration of hydrochloric acid used, a blank
1805
V O L U M E 2 5 , NO. 1 2 , D E C E M B E R 1 9 5 3 containing no tantalum was prepared immediately afterwards in a similar manner. Measurements were made on each pair of solutions with the tantalum-free solution serving as the blank. Similar measurements were made on solutions containing 5 mg. each of titanium dioxide and niobium pentoxide. These data are plotted in Figure 1. *The absorbancy of the tantalum-pyrogallol complex was increased by increase in the hydrochloric acid concentration, whereas the absorbancy of the niobium complex remained quite low below 5-V hydrochloric acid. The absorbancy of the titanium complex was found to reach a minimum bet!?-een 2.5 and 4,V hydrochloric acid.
I
10 CONCENTRATION
b
a t 0.2 mg. per 50 ml., and varying the ammonium oxalate concentration. These data, shown in Figure 3, show the pronounced effect of ammonium oxalate on the absorbancy of the complex. The ammonium oxalate concentration in all the work performed was fixed a t the arbitrary concentration of 0.125 gram per 50 ml. The relatively high concentration was chosen so as to enable greater amounts of tantalum solution to be used when desired; i t also permitted taking larger aliquots in the analysis of samples loiv in tantalum pentoxide concentration. Ammonium oxalate solution, measured against rrater in the reference cell, was also found to have an appreciable absorbancy in hydrochloric acid solution; this again necessitated the strict regulation of its concentration. The dilute ammonium oxalate solution previously described was prepared for use in bringing solutions up to the fixed ammoniuni oxalate concentration. POTASSIUX BISULFATE. Similarly, the effect of varying concentrations of potassium bisulfate was studied by holding the concentrations of all other reagents constant. It was found to have no measurable effect on the absorbancy of the tantalumpyrogallol complex. It did, however, affect the absorbancy of the blank slightly; 0.5 gram per 50 ml. caused an increase in absorbancy of the blank of 0.01 absorbancy unit. For this reason, potassium bisulfate w s also added to the dilute ammonium oxalate solution and its concentration was controlled along with that of ammonium oxalate.
4 :
OF
20 PYROGA-LOL (9150 ml)
Effect of Pyrogallol on Absorbancyof Tantalum-Pyrogallol Complex
Figure 2.
0000
0025
0050 AMMONIUM O X A L A T E
3375
(9,s;
0 IC0
i ,25
Figure 3. Effect of Ammonium Oxalate on -4bsorbancy of Tantalum-Pyrogallol Complex
A concentration of 4h’ hydrochloric acid was selected as the optimum compromise because it allowed a minimum interference from the two troublesome elements, titanium and niobium, while permitting a moderately high absorbancy of the tantalum complex. PYROGILLOL. similar study was made of the effect of pyrogallol concentration on the absorbancy of the complex. The procedure used was identical with the hydrochloric acid measurements escept that the amount of pyrogallol added to each set of flasks was varied and the hydrochloric acid concentration was held constant a t 4N. The variations of absorbancy with pyrogallol concentration are shon-n in Figure 2. The absorbancy of the tantalum-pyrogallol complex q-as found to increase with pyrogallol concentration. -1limit was set on the permissible amount of pyrogallol, however, because of the high absorbancy it produced in the blank reference solution and the correspondingly larger slit widths required. The amount of pyrogallol was fixed a t 10 ml. of the pyrogallol solution ( 2 grams) in subsequent work. . ~ X M O X I U M OXALATE. The effect of ammonium oxalate was studied in a similar manner by holding the hydrochloric acid concentration fixed a t 4N, the pyrogallol concentration constant at 2 grams per 50 ml., the tantalum pentoxide concentration fixed a \
oco!
I , 300
3i5
35c
375
4CO
WAVCLENGTH I m p 1
Figure 4.
Absorption Curves of Tantalum-, Niobium-, and Titanium-Pyrogallol Complexes
Absorption and Standardization Curves. The absorption curve for the tantalum-pyrogallol complex was measured a t the concentration level of 1.00 mg. of tantalum pentoxide per 50 ml. Curves for the niobium and titanium complexes were measured a t five times this concentration in order to moderate the effects of any slight contamination of the absorption cells. The color development procedure outlined previously was followed and the absorbancies were measured against the absorbancy of a reference blank containing the same constituents with the exception of tantalum. The DU spectrophotometer and ultraviolet attachments were used for measurements below 350 mp. For convenience, the Model B was used for measurements between 350 and 400 mp. The data are shown in Figure 4. The maximum absorbancy of the tantalum-pyrogallol complex was found a t 315 mp; 325 mp \?-as chosen as the operating wave length so that it would fall within the wave length range offered by the Model B spectrophotometer. The standardization curve was found to follow Beer’s law up to the concentration of 2.00 mg. of tantalum pentoxide per 50 ml.
1806 The absorbancy indes under the optimum conditions used was found to be 0.0215 ml. per microgram em. Slight but significant, variations from this value were sometimes caused by variatiorls in the concentrations of hydrochloric acid or pyrogallol. For accurate work it is reconmetided that the same reagent solutions and pipets be used for the determination of the absorbancy index and the analysis of unknon-11 solutions. The molar absorimicy index. with concentration expressed as gram atoms of tantalum per liter, is 4Ti5. Factors Affecting Color Stability. TIMEOF STANDING.A reference blank solution and a solution containing 1.0 mg. of tantalum pentoside in addition to the recommended concentrations of reagents were permitted t o stand a t room temperature for 7 hours. Samples for absorbancy measurements were taken at regular intervals. The absorbancy of the reference blank measured against water was found to increase 0.015 absorbancy unit in the full 7 hours, S o measurable difference was noted in 1 hour. The absorbancy of the tantalum-pyrogallol complex measured against' a reference blank of similar age y a s found to decrease by 0.010 absorbancy unit,. No measurable change was noted, however: after only 1 hour. Measurements could thus be made immediatell- or up to 1 hour from the time of mixing the solutions without a significant change in absorbancy. The temperature coefficient for this reaction in the range 20" t,o 35' C. was found to he negligible. The order of addition of reagents is immaterial, IXTERFEREXCES. In the study of the effects of diverse ions on the absorbancy of the tant,aluni-pyrogallol complex those elements were considered which have been reported to be constituents of the many tantalum-lwaring rocks and minerals. Each of the elements, usually taken as 0.100 gram of the oxide, was treated as outlined in the general procedure, Two series of measurements were made. I n the first series the absorbancy of the reagent solution containing the diverse ion toget,her with 1 mg. of tantalum pentoside was measured. Coniparison between this measurement, and the absorbancy of a reagent solution containing 1 mg. of tantalum pentoxide alone showed the effect oi the ioii on the absorbancy of the tantalunipyrogallol complex. Coniparisori of the difference between these two measurements atid 3 second measurement showing the absorbancy of a solution of the ion alone, with no tantalum present, indicated whether the etTect was additive or otherwise. The data, expressed 8s apparent milligrams of tantalum pentoxide, are presented in Table I. W t h i n detectable limits, in practically all cases the effect of the various ions was additive in character and readily correctable. The elements t,hat were fouud to require only moderate corrections for their presence in significant amounts were titanium, niobium, zirconium, chromium. vanadium, bismuth, and copper. The elements necewitating large correct.ions or separations n-ere uranium, niolybdenuni: t.ungsten. and antimony. These elements, however, are rarely piwent in tantalum-bearing minerals in troublesome concentrations: if encountered, they should be separated from the earth ncids. Iron did not interfere in the presence of stannous rhioride. In addit,ion to the ions nirntionecl above, tartrate Tas found to depress the absorbancy of the coniplex moderately; fluoride bleached the color vvhen present even in trace concentrations, Oxidizing agents oxidized the pyrogallol to black opaque solutions. Traces of platinum gave a very high absorbancy, probably because of the stannous chloride present; fused silica crucibles were used for the fusions for this reason. With the exception of fluoride, however. none of the interferences in this group are generally present in the rocks arid minerals encountered, and their presence can usually be avoided. APPLICATION OF METHOD TO RIINERALS AND ROCKS
The method as described in the general procedure \vas applied to the analysis of th!ee niiwrals. The results of the analysis of
ANALYTICAL CHEMISTRY Table I. Effect of Diverse Ions
Expressed as Nb206 Ti02 ZrOp
0.12 0.07 0.04
NazO
0.80 1.4 2.5 >5.0 >5.0 >5.0 >5.0
-fusing 0.0715 gram of niobium pentoxide ( A . D. Mac&\- Co.) with potassium bisulfate. The melt was dissolved in sulfuric acid. and the niobium precipitated as t,he hydrous oxide with ammonia. The gelatinous precipitate thus obtained was centrifuged. n . a ~ h r dwith distilled water, and centrifuged again. Thi- Fre~hl!. p:~vi!Iii::+ed hydrous oxide die-
