V O L U M E 25, N O . 12, D E C E M B E R 1 9 5 3 LITERATURE CITED (1) -4limarin, I. P., and Frid, B. I., Trudy Vsesoyuz. honferei&. ilnal. Khim.. 2.333 (1943). (2) Atkinson, R. H.,.Steigman, J., and Hiskey, C. F., Ax.4~.CHEX., 24,477 (1952). '3) Burstall, F. H., Swain, P., Williams, A. F., and Wood, G. 9., J. Chem. SOC.,1952,1497. '4) Eichholz, G. G., Can. Dept. Mznes und Tech. Surteys, Tr-93/51 (1951). 15) Freund, H., and Levitt, A. E., AXAL.CHEM., 23, 1813 (1951). (6) Goto, H., and Kakita, Y., Science Repts. Research Insfs., Tdholzu Univ.. 2.249 (1950). 7) Huffman, E. H.: Iddjngs, G. If.,and Lilly, R. C., J . Ani. Chem. Soc., 73,4474 (1951). 8 ) Karyakin, Y. V., and Telezhnikow,P. RI., Zhur. Praklod. Khim.. 19,435 (1946). (9) Krasilschikov, B. S., and Popova, N. M., d m l ~ s t ,73, 176 (1948). '10) Krasilschikov, B. S., and Popova, E.hI., Zavodskaya Lab., 11, 512 (1945). (11) Kraus, K. A , , and Moore, G. E., J . A m . Cheni. Soc., 73, 13 (1951) . (12) Ibid., p. 2900. (13) Krivoshlikov, N. F., Khim. Referat. Zhur., 5, 59 (1939). (14) Krivoshlikov, N. F., Trudy L K K h T I , 7, 103 (1939). (15) Krivoshlikov, N. F., and Platanov, h l . S.,Zhur. Priklod. K h i m . , 10, 184 (1937).
1807 (16) Lauw-Zecha, A. B. H., Lord. a. S.,J:.. and Hnme, D. N., AWAL.CHEM.,24, 1169 (1952;. (17) Leddicotte, G. W., and ZIoore, F. L...i. . 4 ~ Chrm. . SOC.,74, 1618 (1952). (18) Long, J. V. P., Analyst, 76, 644 (1951,. (19) Mercer, R. A , , and Willianis, -4. F., J. C ~ > O JSoc., ? . 1952, 3399. (20) Palilla, F. C., Adler, S . , and Hiskey, C. F., ANAL.CHEM.,25, 926 (1953). (21) Platanov, M. S.,and KriT-oshlikov, S . F., .4valyst, 73, 175 (1948). (22) Platanov, M. S., and KriT-oshlikov. 5 . F., Trudu, T7scsoyuz. Konferents. Anel. K h i m . , 2, 369 (1943!. (23) Platanov, hI. S., KriT-oshlikov. S. F., 2nd Narakaev, -4.il., Zhur. Obshsehei Khim., 6, I815 ;1936!. (24) Slavin, AI., Pinto, C. Jf,, and Pinto, 11.. "A Tantalita do Nordeste," Bol. 21, Rio de Jnrieiro. ZIinisterio da Agricultura, Departmento Sacional da Produpo Mineral, 1946. (25) Telep, G., and Bolts, D. F., ;Is.~L. CHEV..24, 163 (1952). (26) Thanheiser, G., M i t t . Knisr,-T7';lhaZ/~[-I~,~~. Eisenjorsch. Diisseldorf, 22, 255 11940). ( 2 7 ) Vinogravada, S . A , and Gushr?uk, E. I , ZntodsAoya Luh , 11, 233 (1945). (28) Wells, R. C.; U . S. Geo2. S!iri,ey. B i z : / ,858, 113-14 (1937). (29) Mrilliams,-4.F., J . Chem. Soc.. 1952, 3155. RECEIVED for review M a y 23, 1933. Accl-pied Beprembrr 4 1953. Publication authorized by the Director. L-.6. Geoiogicsi S i i x y y .
Ultraviolet Spectrophotometric Determination of Niobium in Hydrochloric Acid J. H. KANZELMEYER
AND
HARRY FREUND, Oregon State College, CorLurllis. Ore.
The analytical methods available for the determination of niobiuni are not well suited for routine analysis because of critical or time-consuming operations. .4 hitherto unreported absorption peak at 281 mp serves as the basis for a spectrophotometric determination of niobium. In the absenceof large amounts of iron and certain other elements, the sample is obtained ina concentrated hydrochloric acid solution and the absorbance measured at the specified wave length. In the presence of interfering elements, themethod may be used following a preliminaqseparation. It is rapid, accurate, and suitable for routine analysis.
T
1j-O essentially different spectrophotometric methods for the cletermination of niobium have been reported in the literature: methods based upon the peroxy complex (3, 5 ) , and those whivh make use of the thiocyanate complex ( 2 , 4). The peroxy methods lack scnsitivit'y and suffer from the inconveniences inherent in the hydrogen peroxide-concentrated sulfuric acid reagent employed-i.e. , t'he corrosive nature of the wlutions and the annoyance of oxygen bubble formation. The 1ac.k of ~ensitivityof these methods is aggravated by the limited .wlulility of many materials in concentrated sulfuric acid, for rlii? often establishes a practical upper limit to the sample size. The procedures utilizing the thiocyanate complex, while posswsing adequate sensitivity, offer other disadvantages. The complex ic? unstable in aqueous media. Freund and Levitt ( 2 ) &ahilize the complex by the addition of a miscible organic solvent, :i(etone. The resulting solutions show increasing absorption n i t h time, making it necessary to specify accurately the time interval from the addition of the reagent to the measurement of absorbance. Lauvi-Zecha, Lord, and Hume ( 4 )extract t'he colored spec,ies with ether, a process further complicated by the necessity for accurate cont,rolof the acidity. Desesa and Rogers ( 1 ) report a method for the determination of iron using 6 M hydrochloric acid, and give the ahsorption curvep of other elements i i i thcx same solvent. Jt-ernctt (6)has
shown t h a t niobium hydrous oxide is appreciably soluble in highly concentrated hydrochloric acid. 4 hitherto unreported absorption peak oi a niobium species present in concentrated hydrochloric acid femes as the basis for a method for the determination of niobium. The interfering concentrations of certain ions are listed. APPARATUS AND SOLLTIONS
Apparatus. ,411 absorbance measurement's were made with a Beckman Model DU spectrophotometer. Matched I-cm. silica cells were employed; all solutions were measured against a hydrochloric acid solution of the same concentration as the sample. Hydrochloric Acid. The concentrated hydrochloric acid used (Baker and Adamson, ACS specification grade) was 11.8M. The concentration of hydrochloric acid solutions was determined by specific gravity. Chloride concentrations were determined by a potentiometric titration mit,h standardized silver nitrate solution, using a silver indicating electrode. Acid concentrations were determined by titration with carbonate-free standard sodium hydroxide solutions to the phenolphthalein end point. Standard Niobium Solution. =1 0 , s mg. per ml. standard niobium solution was prepared b>-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-
ANALYTICAL CHEMISTRY
1808 solved easily in concentrated hydrochloric acid, and the clear solution thus obtained was diluted to exactly 100.0 ml. with the same reagent.
'H I
EXPERIMENTAL
I11
2 3
1.32 57
4
.2*
5
,172
04
\
In order to investigate the effect of the hydrochloric acid concentration on the absorption of the niobium complex, samples containing 8 y of niobium per ml. were prepared by diluting the standard niobium solution with hydrochloric acid of different strengths. The absorption curves for these samples are shown in Figure 1.
I
M HCI I
0.01
240 260 WAVELENGTH
280
300
MI,
Figure 2. Effect of Variation of Hydrogen Ion Concentration on Ultraviolet Absorption of Niobium in Hydrochloric Acid 5 Y of niobium per ml.
WAVELENGTH M U Figure 1. Absorption Spectra of Niobium Solutions 1-cm. cells 8 y of niobium per ml.
It is clear that the hydrochloric acid concentration must be above 6 M before the analytically important absorption a t 281 mfi becomes useful. As the absorbance increases while the rate of change of absorbance decreases with increasing hydrochloric reagent acid concentration, the maximum concentration of conveniently obtainable should be used for maximum sensitivity and accuracy of the method. The effect of the independent variation of hydrogen ion and chloride ion is shown in Figures 2 and 3. The chloride ion concentration was maintained constant by dilution with 11.8M lithium chloride (Figure 2). The hydrogen ion concentration was kept nearly constant by dilution with 72% perchloric acid (Figure 3). In the latter case, the limited solubility of hydrogen chloride in perchloric acid caused the loss of some hydrochloric acid. The niobium complex concentration depends, in a complicated manner, on both the hydrogen ion and the chloride ion concentrations. These curves cannot be quantitatively interpreted in terms of present knowledge, but it is hoped that investigations now in progress will provide the additional information necessary t o assign structures to the complex species present. It is not the purpose of this paper t o elucidate structures, but rather to present a rapid method for the determination of niobium. The fact that the exact structure of the complex used is not *known does not impair its usefulness in analytical work. The significance of Figures 2 and 3 is that hydrogen chloride is required for the formation of the niobium complex under consideration; neither hydrogen ion nor chloride ion alone will suffice. An analytical curve was prepared by proper dilution of the standard niobium solution with concentrated hydrochloric acid. Beer's lan- was followed up to 10 y of niobium per ml. The absorbance of the latter solution was 1.05. No attempt was made to verify linearity of absorbance with concentration a t higher values. The reproducibility of the method was determined by measuring six runs of five samples each, prepared and measured over a period of one week (Table I). Five supposedly identical volumetric flasks n-ere used for all six runs. For each run, an aliquot of standard niobium solution was introduced, such that the final concentration of niobium would be 5 y per ml. when the flask was filled to the mark with concentrated hydrochloric acid. The data yere treated by the analysis of variance as a two-way
this
classification \Tith single replication. In this way the variance due to the differences among the flasks could be separated from the variance due to differences among runs, the latter being of interest as an estimate of the reproducibility of the method. The mean absorbance was 0.522, and the standard error of the runs was equal to 0.003. The obvious downward t,rend in the results is attributable to the fact that the hydrochloric acid used was kept in a ground-glass bottle from which hydrogen chloride could escape. Unless standards are to be carried along nith the samples, the hydrochloric acid used as a reagent for this method should be protected from loss of strength by storage in a pressuretight hottle. ___________
~.~~
.~ .__
Table I . Reproducibility Data Run hTo. 1 2 3 4 5
6
1
2 0.525 0.524 0.524 0.523
0,530 0.533 0.528 0.523 0.523 0,538
0.518
0,518
Flask No. 3 0.526 0.523 0,521 0,516 0,514 0,515
4 0.523 0.521 0.518 0,519 0,514 0.515
5 0.525 0.522 0.517 0.519 0.515
0.516
Interferences were studied by measuring the absorption curves of a large number of cations in concentrated hydrochloric acid. The weights of the elements which caused an absorbance equal to 5 % of the absorbance of 5 y of niobium per ml. were calculated
y
0.0'
CI'
/
y H*
I
2 40
280
260 WAVELENGTH
300
MU
Figure 3. Effect of Variation of Chloride Ion Concentration on Ultraviolet Absorption of Niobium in Hydrochloric Acid 5 y of niobium per ml.
V O L U M E 25, NO. 1 2 , D E C E M B E R 1 9 5 3 Table 11. Weight Ratios of Elements Causing 5% Error Ratio (M/xb)
Element
Element
Ratio (M/Xb)
1809 stannous chloride, but measurements must be made almost immediately, since the iron is rapidly reoxidized by air. Titanium may be determined independently by the peroxide method and its contribution to the absorbance a t 281 mp subtracted from the total absorbance. The remainder is due t o niobium. Comparatively simple means of separation are available to reduce the concentrations of the other ions below the interfering ratio. The one exception is found in tantalum. The permissible weight ratio of tantalum to niobium was increased to 10 by the addition of sufficient hydrofluoric acid to make the solution about 0.01M in fluoride. Sulfate does not interfere up to 0.1M concentration, introduced as potassium bisulfate. Sulfuric acid tends to decrease the solubility of the hydrogen chloride. SUMMARY
Figure 4. Absorption Spectra of Interfering Cations i n Concentrated Hydrochloric Acid
The chloride complex which occurs in concentrated hydrochloric acid forms the basis for a simple, accurate, and rapid spectrophotometric method for the determination of niobium, in which absorbance is measured a t 281 mp. The method may be applied to any material that may be obtained in a hydrochloric acid solution. Interfering ions are vanadium(V), chromium(III), lead(II), iron(III), copper(II), molybdenum, and titanium(1V). The interferences from moderate amounts of iron(II1) and copper(I1) may be eliminated by reduction to the lower valence with tin(I1). Tantalum up to ten times the amount of niobium measured can be tolerated.
5 Y of metal per ml.
LITERATURE CITED
O.OC
240
” 260
280
WAVELENGTH
300
up
from the absorbance of several different concentrations of each ion a t 281 mp. These calculated weight ratios, given in Table 11, are presented as guides to the relative errors caused by the presence of the various ions. The absorption curves of several interfering elements are shown in Figure 4. The interference due to the presence of copper may be eliminated by addition of a small measured excess of stannous rhloride dissolved in concentrated hydrochloric acid to both the sample and blank solutions. The interference of iron can be reduced temporarily by the addition of a five- to tenfold excess of
(1) Desesa, AT. A., and Rogers, L. B., Anal. Chim. Acta. 6, 534-41 (1952). (2) Freund, H., and Levitt, A, ANAL,CHEM.,23, 1813-16 (1951). (3) Geld, I , and Carroll, R., Zbid., 21, 1098-1101 (1949). (4) Lauw-Zecha, A. B. H., Lord, S. S., and Hume, D. N., Ibid., 24, 1169-73 (1952). (5) Telep, G., and Boltz, D. F., Zbid., 24, 163-5 (1952). (6) Wernet, J., Z . anorg. u. allgem. Chem., 267, 213-37 (1952). RECEIVEDfor review June 24, 1953. Accepted September 28, 1953. Presented in part before the Division of Analytical Chemistry a t the 123rd LOBh g e l e s , Calif. Published Meeting of the AMERICAX CxiEhircaL SOCIETY, with the approval of the Oregon State College Monographs Publications Committeeas Research Paper No. 232, Department of Chemistry, School of Science.
Spectrophotometric Determination of Arsenic and Tungsten as Mixed Heteropoly Acids DELORA K. GULLSTROM WITH M. G. -MIELLON Department of Chemistry, Purdue University, Lafayette, Znd.
I
T HSS been known for years that a yellow color is formed on
adding an excess of molybdate and vanadate solutions to one containing orthophosphate. The colored component is assumed to be molybdovanadophosphoric acid, a mixed heteropoly complex. Misson (4, 6) first proposed this reaction as a colorimetric method for the determination of phosphorus in steel. The color reaction and its analytical application Fere investigated spectrophotometrically by Kitson and Mellon ( 2 ) . Wright and Mellon (11) studied the analogous mixed complex, tungstovanadophosphoric acid. I t s use for the determination of vanadium was proposed by Sandell (7). It seems reasonable that other mixed acids might have useful analytical applications. More specifically, since the addition of vanadate enhances the color of molybdophosphoric acid, it was of
interest to study such a possibility with the colorless molybdoarsenic acid. At the same time tungstovanadophosphoric acid was restudied with the aim of using this complex for the determination of tungsten.
EXPERIMENTAL WORK Apparatus and Solutions. Absorbance measurements were made in 1-cm. cells with a Beckman DU spectrophotometer. Incidental reagents were prepared as follows: 5% sodium molybdate dihydrate, NanMoOc.2H20, in water; 1 % sodium metavanadate, NaV03, in dilute sodium hydroxide, subsequently neutralized with hydrochloric acid; 1.7N hydrochloric acid (1 t o 6). A standard solution of arsenic was prepared by dissolving, with heating, arsenic pentoxide in water and diluting to a concentra-
