868 ciii.-l. The data on four perinanganates show prominent bands between 877 and 905 Cm*-l Ammonium 12molybdomanganate has a band at 858 cm.-l. vielv of the assumptions made here, any assignments must be considered purely speculative. The number of compounds investigated is small, and the extrapolation from a resonating ion such as phosphate (po4---) and tungstate (L1704--) to the binding in the heteropoly acids may be unwarranted. LITERATURE CITED
(1) Bellamy,
L. J., “The Infra-Red
Spectra of Complex Molecules,” p. 286, Methuen & Co., London, 1954. (2) Booth, H. S., “Inorganic Syntheses,” Vol. I, pp. 127-33, RlcGraw-Hill, New York, 1939. (3) Colthup, N. B., J . o p t . Soc. Amer. 40,397 (1950). (4) Copaux, H., Ann. chtm. phys. 17,217 (1908). (5) Corbridge, D. E. C., Lowe, E. J., J . Chem. Soc. 1954, 493. (6) Daniel, J . 7 Brackett, F. s., J . Opt. SOC. Amer. 43,960 (1953). ( 7 ) le^^, H. J , , ~ ~ J. s., d “Lfodern Aspects of rnorganic Chemistry,,’ pp. 180-95, Van Sostrand, New York, 1943. (8) Hunt, J. R.I., Wisherd,’M. P., Bonham, L. C., ANAL.CHEN.22, 1478 . (1950).
(9) Illingnrorth, J. W., Keggin, J. F., J . Chem. SOC.1935, 575. (10) Keggin, J. F., Proc. Roy. SOC. (London)A144,75 (1934). (11) Miller, F. A., Wilkins, C. H., ANAL. CHEM.24, 1253 (1952). (12) Pauling, L., “Nature of the Chemical Bond,” p. 186, Cornel1 University Press, Ithaca, 1945. (13) Schiedt, U., Reinwein, H., Z. Saturforsch. 7b, 270 (1952). (14) Sharpless, N. E., Gregory, D.,
Unpublished data.
(15) Signer, R., Gros, H., H e h . Chim. ~Acta 17,1076 ~ (1934). ~ ~ (16) jf7uj H., J . Bid. Chem. 43, 189 (1920).
RECEIVED for review February 11, 1957. Accepted July 16, 1957.
Na pht hylaminesuIfonic Acids A N e w Class of Organic Reagents for Spectrophotometric Determination of Trace Amounts of Osmium EDGAR L. STEELE and JOHN H. YOE Pratt Trace Analysis laboratory, Departmenf o f Chemistry, University o f Virginia, Charlottesville, Vu.
b Investigation of the naphthylaminesulfonic acids as analytical reagents for truce amounts of osmium has followed three main courses. The first deals with the applicability of the osmate(VI)-naphthylaminesulfonic acid complexes to the quantitative estimation of osmium. Several of the naphthylaminesulfonic acids used have sensitivities as high as 1 part in 20,000,000 and the complexes follow Beer’s law over a useful range. Some of the reagents are sensitive to 0.007 y per sq. cm. (Sandell’s nomenclature). The reactions are not specific. Osmium i s separated from all interfering substances by oxidation to the tetroxide(VIII) with nitric acid and distilling. The tetroxide i s reduced to the osmate by hydroxyl ions in the presence of the complex-forming naphthylaminesulfonic acid. Structural analysis of the complexes includes studies of reaction rates, mole ratios, stability, effect of temperature, pH, and presence of various ionic and molecular species. The nature, position, and number of amine and sulfonic acid groups were studied, to determine their effect on complex formation and stability. Various positions for the sulfonic acid groups on the naphthylene system are preferred over others, depending upon the position of the amine group.
1622
ANALYTICAL CHEMISTRY
T
increasing importance of osmium in catalysis (1, ?‘), biology (s), and metallurgy ( 2 ) justifies the search for a colorimetric method which is both simple and accurate. Among the reagents previously reported, only thiourea has met with some success. I n 1956 this laboratory reported 1naplithylamine-3,5,7-trisulfonicacid as a new reagent for the determination of osmium ( I O ) . This reagent is more sensitive than the thioureas that have been reported for this determination, and, the new reaction is carried out in a weakly acid medium. I t s chief disadvantages as reported ( I O ) were that osmium had to be in the Oxidation state of 6 and that 4 hours were required for maximum color development. A recently developed procedure eliminates the use of ordinary reducing agents and reduces the time for color development to 1 hour. Because of interference from a nuniber of substances, including the other platinum metals, osmium is separated by a conventional distillation technique ( 5 ) , but the osmium tetroxide is absorbed in 0.05N sodium hydroxide. HE
APPARATUS
-411 absorbance measurements IT-ere
made with a Beckman spectrophotometer, Model DU, using matched 1.000-cni. Corex cells. At wave lengths of 625 mp or below, the ultravioletsensitive phototube was used; above 625 mp, the red-sensitive phototube. The phototube circuit was kept a t maximum sensitivity and under these conditions the slit n-idth at 560 mp was about 0.040 mm., corresponding to a nominal band width of 2.3 mp. 4 Beckman p H meter, Model G, was used for all p H measurements. The meter was calibrated from time to time, using Fisher certified standard buffer of p H 4‘. Temperature bath, accurate to =to.5°
c.
REAGENTS
Standard Osmium Solution. -4 standard solution containing 100 p.p,m. of osmium was prepared by dissolving 0.0196 gram of potassium osmate (K20s04.2H20), obtained from the American Platinum Works, in distilled water and diluting to the mark in 100-ml. volumetric flasks. rlccording to Friend (4) this solution is stable, but for maximum stability i t is best stored in a refrigerator. Reagent Solutions. T h e naphthylaminesulfonic acids were purified by dissolving in a minimum amount of warm water and precipitated by adding
~
,
factors being equal-Le., temperature, time of development, concentration, and pH-the sensitivity of the individual reagent depends only on the number of sulfonic acid groups present and their position with respect t o the amine group. For general use, however, the practical sensitivity for the better reagents is of the order of magnitude of 0.05 y per sq. em. This corresponds to 1 part of osmium to 20,000,000 parts of solution. Interfering Ions. To determine interference by diverse ions, solutions were prepared containing 4 p.p.m. of 300 400 500 600 WAVE LENGTH Mp osmium, excess reagent, and varying concentrations of each ionic species to 1 -NAPHTHYLAMINE-4,6,8-TRISULFONlC ACID be tested. An increase or decrease of Figure 1 . Typical wave length-absorbance curve 0.005 absorbance unit was arbitrarily taken as an interference. The folloiving Reagent. 5 X lO-'ilf l-naphthylamine-4,6,S-trisulfonicacid ions in concentrations greater than 1:1 Complex. 2.08 X 10-6M l-naphthylamine-4.6,&trisulfonicacidinterfered: ruthenium(III), rhodiuniosmate complex palladium(II), iridium(IV), (111) * platinum(IV), aluminum(III), chroniium(III), vanadyl(II), iron(III), iron(11), cobalt(II), nickel(II), copper(II), isopi opyl alcohol. The recrystailizaperature, and position of the amine with zinc(II), lead(II), manganese(II), magtions were continued until a solution respect to the substituted sulfonic acid nesium(II), and silver(1). gave a straight line in the mole ratio groups. With a resonance favored reexperiment. No claim is made for These results indicate that osmium agent, which has little steric hindrancemaximum purity, yet it is felt that this must be separated from many metallic for example, l-naphthylamine-4,BJ8-trimethod yields a reasonably pure prodions before it can be determined by this sulfonic acid-the reaction is complete uct. ilfter one or more recrystallizamethod. 1 hour a t 35' C. and p H 2.5 to 3.0. rrithin tions, the reagent solutions were preSeparation. Osmium can be sepaOn the other hand, 2-naphthylamine-3,pared by dissolving the required rated from all interfering substances amounts in distilled water. 6,8-trisulfonic acid requires 24 hours to Buffer Solutions. Buffer solutions most easily by distillation of the tereact completely under the same conwere the Clark and Lubs type, pretroxide (ff), or by extraction of the teditions of p H and temperature. The pared by mixing 97 ml. of 0 . 2 s hydroorder of mixing the reagent with the troxide with carbon tetrachloride (11). chloric acid and 50 ml. of O.2N potassiuni chloride and diluting to 200 nil. Other Reagents. All other reagents n ere analytical grade, u s ~ d without rr 1 further purification. EXPERIMENTAL
Absorbance Curves. Absorbance curves of 28 naphthylaniinesulfonic acids were prepared from 5 X 10-4 M solutions. Figure 1 shows a typical curve along with an absorption curve for a 4 p.p.m. organo-osmate complex. I n every case, the reagents absorb only in the low wave length portion of the visible region and a t the maximum for the organo-osmate its absorption is negligible. Hence excess reagent does not appreciably increase the absorbance of the complex solution. ,411 complexes show a fairly sharp peak around 560 mp (maximum), Effect of pH. The organo-osmate complex shows an absorbance variation with change in pH, having a maximum a t p H 1.5. The purple color first formed upon addition of the reagent to a solution of osmate ions changes to blue a t p H 5 and finally to pale green a t p H 10 or higher. Color Development Time. The time required for maximum color t o develop depends upon the p H . tem-
Figure 2.
Distilling apparatus
osmate solution has no effect on the rate or extent of reaction; however, adding the buffer solution before the color has developed will substantially stop the reaction. Beer's Law. A11 the naphthylamine sulfonic acid-osmate complexes adhere to Beer's law over a n osmate concentration of 0.1 t o 6 p.p.m. Sensitivity. Using Sandell's expression for sensitivity (8) the naphthylamines have a range of sensitivities up to 0.0068 y per sq. cm. This corresponds roughly to 1 part of osmium to 140,000,000parts of solution. All other
As a spectrophotometric method for both osmium and ruthenium was t o be developed, the distillation was chosen because of the ease of separating ruthenium with the same apparatus. Figure 2 s h o w a diagram of the distillation apparatus. If small concentrations of osmium are to be distilledLe., 5 X lO-4M-only the first receiver is needed. If osmium and ruthenium are to be distilled together, it is usually best to distill both into the first receiver, then continue by distilling the osmium to the second after reducing the ruthenium with ferrous ion, and VOL. 29, NO. 1 1 , NOVEMBER 1957
1623
Table I.
No. 1 2 3 4 5 6
Distillation
of
Osmium
Osmium Osmium .4dded, Recovered,
Error,
Mg.
Mg.
Mg.
0.20 0.20 0.50 0.50 1.00 1 .oo
0.19 0.20 0.48 0.49 1.01 1.00
-0.01 0.00 -0.02 -0.01 +0.01 0.00
oxidizing osmium with 6 M nitric acid. Many oxidants have been used in these distillations; nitric acid is as convenient as any, though somewhat slower than some. A final choice for any given determination depends upon the nature of the sample. Care should be taken to remove any oxides of nitrogen, if ruthenium is to be distilled with the same apparatus. Table I shows the result of some osmium distillations. Nature of Complex in Solution. Three methods were used t o establish the empirical formula of the complex in solution: the mole ratio method of Yoe and Jones ( l a ) (Fig. 3), the continuous variations method of Job modified by Vosburgh and Cooper (9) (Fig. 4) and the slope ratio method proposed by Harvey and Manning (6). Results indicate a complex formed by two molecules of reagent reacting with one osmate ion. There is further evidence that with excess osmate a 1 to 1 complex is formed which rapidly changes to a 2 t o 1 complex on the addition of more reagent. The complex is stable in aqueous solution or ethyl alcohol. It is not extracted by any common organic solvent. Indeed, one method of concentrating the complex is to add petroleum ether to an ethyl alcohol solution; part of the ethyl alcohol dissolves, leaving the aqueous phase richer in the complex. The colored complex solution shows no change in absorbance value u p t o a month, but gradually decomposes over longer periods of time. Reaction Mechanism. T h e pH dependence of t h e rate of complex for-
Table 11.
mation suggests that the reaction between the naphthylaminesulfonic acids and the osmate involves the amine group; 1- or 2-naphthylamine gives a similar reaction and was just as sensitive as most of the reagents used. Various other substituted naphthylamines and naphthylamine sulfonic acids were investigated. Substitution on the amine nitrogen, such as N-phenyll-amin0-3~6-disulfonic acid, failed to give a colored complex. The faint color obtained with N-methyl-l-amino3,6-disulfonic acid is thought to be due to an impurity of 1-naphthylamine-3,6disulfonic acid. Further evidence for the amine-osmate mechanism is the change of p H during the reaction. The pH of the solution decreased during complex formation, lvhich could be explained by a proton coming from the amine group as it donated a pair of electrons to the OSmate for octahedral formation. One mole of osmate should liberate two moles of hydrogen ion, assuming complete reaction. This was found to be the case within the limits of measurements. Another point in favor of the amineosmate mechanism is the observed steric effect. I n every case where sulfonic
”2
3-5-7 3-5-7 3-6-8 3-6-8 4-8 4-8 5 5
1624
ANALYTICAL CHEMISTRY
>A
.2
/ 1
2
3
4
5
X Figure 4. Method of continuous variations
Reagent.
l-Xaphthylamine-4,6,8-trisul-
fonic acid
acid groups were adjacent to the amine group the reaction was extremely slow, requiring at least 24 hours for completion. A resonance effect was also noted in the complex formation. For the 1naphthylamines, sulfonic acid groups substituted in the 2,4,5 or 7 positions gave increased absorbance. For the 2naphthylamines, sulfonic acid groups substituted in the 1,3,6, or 8 positions gave increased absorbance. If the steric effect from adjacent groups is neglected, the best reagents for osmium are the naphthylamines with the largest number of sulfonic acid groups substituted in the favored positions. Table I1 s h o w the results of some of the reagents used.
( 1 ) Borisov, P. P., Stepanov, S. S., Scz. Repts. Moscow State Univ. 1936, No. 6, 347. (2) Carter, F. E., Materials and Methods 28, No. 5 , 5 5 (1948). (3) Collier, W.A., Krauss, F., 2. Krebsforsch. 34, 526 (1931).
(4) Friend, J. X., “Textbook of Inorganic Chemistry,” Vol. IX, Part I, Charles Griffen, London, 1925. (5) Fritzmann, E., 2. anorg. allgem. Chem. 163,165 (1927). ( 6 ) Harvey, .4. E., Manning, D. L., J . Am. Chem. SOC.72,4488 (1950). (7) Sa, *4.,Rev. centro estud. farm. y bioquim. 27, 19 (1937). (8) Sandell, E. B., “Colorimetric Deter;; mination of Traces of Metals, 2nd ed., Interscience, New York,
Steric and Resonance Effect
SO@H Position
6
LITERATURE CITED
( 4 p.p.m. osmium solution)
Position
.e
1q.m
Absorbance Max. At 4 hr. 0,555 0.490 0.160 0.280 0.444 0.197 0.360 0.260
0.555 0.579 0.160 0.535 0.444 0.197 0.360 0.260
(9) Vosburgh, W , C., Cooper, G. R . , J . Am. Chenz. SOC.63, 437 (1941). (10) Wingfield, H. C., Yoe, J. H., Anal. Ch;’m. Acta 14, 416 (1956). (11) FF‘ortenburg, H. V., A n n . 440, 97 (1924). (12) Yoe, J. H., Jones, A. L., IND.ENG. CHEM.,ANAL.ED. 16, 111 (1944).
RECEIVEDfor review April 25, 1957. Accepted July 15, 1957. Division. of Analytical Chemistry, 131st Meetlng, ACS, Miami, Fla., April 1957.
