S pectro photometric Dete rmination of Ruthenium(111) Using 1 -Nitroso-2-NaphthoI-3,6-DisuIfonic Acid (Disodium Salt) C. SRIVASTAVA,
D. J. MILLER, S.
and M. L. GOOD
Department o f Chemistry, Louisiana State University in New Orleans, Lake Front, New Orleans, La. 70122
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-
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1 Nitroso - 2 naphthol'- 3,6 disulfonic acid forms a 2 : l colored complex with ruthenium(ll1) in aqueous solution. An investigation of its color reaction as a basis for the spectrophotometric determination of ruthenium has been carried out. The violet-blue complex exhibits a maximum absorbance a t 586 mb a t pH 4.5, when measured against a reagent blank. Beer's law is obeyed from 0.2 to 8.2 p.p.m. of ruthenium; the optimum range for accurate determinations being from 0.6 to 4.0 p.p.m. The average relative standard deviation, as determined from a series of measThe Sandell urements, is 1 to 2%. sensitivity of the reaction is 0.0047 pg./sq. cm. and the molar absorptivity of the complex is 22,200. The overall apparent stability constant (log K ) of the complex (as calculated by two different methods) has an average value of 9.7 =t 0.2.
I
a number of reagents have been described in the literature for the spectrophotometric determination of ruthenium (2-6, 8, 9, 11-1,& 17, 18). In most methods reported, a close control over the conditions for color formation is necessary. This, along M ith limited sensitivity and use of nonaqueous media in certain cases, limits the usefulness of many reagents. Thus, the search for better reagents having more sensitivity, greater solubility in aqueous media and a wider range of application becomes desirable. 1 - Nitroso - 2 - naphthol - 3,6 - disulfonic acid (disodium salt) (trivial name nitroso R salt; abbreviated hereafter as KRS) has found use as a spectrophotometric reagent for many metal ions including iron(III), cobalt (11), and palladium(I1) ( 7 , 16, $0). I n the present investigation, it formed a violet-blue colored complex with ruthenium(II1). The purpose of this study was to investigate the suitability of this reagent for the determination of ruthenium. Manning and Menis (11) have described the use of 2-nitroso-lnaphthol as a reagent for ruthenium. N RECENT YEARS,
Kitroso R salt behaves quite differently when complexed with ruthenium(II1) , and the reagent itself is completely soluble in aqueous media. The analytical method for ruthenium presented in this paper has several advantages over previously reported colorimetric methods because of its high sensitivity and larger tolerance for pH changes. EXPERIMENTAL
Apparatus. A Beckman DK-1 recording spectrophotometer was used t o obtain the absorption spectra of the various solutions. All other absorbance measurements were performed with a Beckman DU spectrophotometer. Matched ultraviolet silica cells of 1-cm. path-length were used in all cases. A Beckman Zeromatic p H meter equipped with glass and calomel electrodes was used for p H measurements. Materials. Standard Ruthenium Solution. A stock solution was prepared by dissolving analytical grade ruthenium chloride (Fisher Scientific Co.) in distilled water. The total ruthenium content was determined by the method described by Banks and O'Laughlin (3). Several 5-ml. aliquots of the stock solution were transferred into narrow porcelain boats, evaporated to dryness under an infrared lamp, then reduced in a stream of hydrogen a t about 600' C. Finally, after cooling to room temperature, ruthenium was weighed as the metal. To prepare a standard solution of Ru(III), a portion of the stock solution of ruthenium chloride [which contains a mixture of Ru(II1) and Ru(IV)] was reduced by heating with 0.1M KI in the presence of concentrated HCl until the brown solution turned clear straw-yellow. After cooling to room temperature, the p H was adjusted to approximately 4.5, using sodium bicarbonate. The solution was then diluted to give a final concentration of 5 X 1 0 - ~ ~ruthenium(II1). 1 Solutions of Ru(II1) thus prepared could be stored in an inert atmosphere for rather long periods of time without any appreciable conversion of RulIII) into Ru(1V). However, it is preferable to carry out the reduction of the stock solution to Ru(II1) each time i t is needed.
Reagent Solution. 1.8868 Grams of 1-nitroso-2-naphthol 3 : 6-disulfonic acid (disodium salt) (Eastman Organic Chemicals) were dissolved in 500 ml. of distilled water to give a 0.01M stock solution. The reagent solution is stable indefinitely. Buffer Solution. A sodium acetatehydrochloric acid buffer of pH 4.5 was prepared by mixing proper amounts of 2 M solutions of hydrochloric acid and sodium acetate and diluting to 500 ml. with distilled water. Two milliliters of this solution, when diluted to 25 ml., gave a fairly stabilized p H for the determinations. More concentrated solutions of the buffer may be employed in case of samples requiring greater buffering capacity. Diverse Ions. Stock solutions of the various ions were prepared from analytical grade reagents. Recommended Procedure. T o a suitable aliquot of the sample solution, add approximately 2 ml. of concentrated HCl. Heat on a hot plate and when the solution first begins to boil, add from 6 to 10 drops of 0.1M KI. Boil the solution for 5 t o 10 minutes and cool to room temperature in a n ice bath. Adjust the p H of the solution to 4.5, using sodium bicarbonate and dilute to a standard volume. Transfer 2 ml. of this solution into a 25-ml. volumetric flask and add 2 ml. of the HC1-XaOAc buffer of pH 4.5, 1 ml. of 0.01.V reagent, and 6 ml. of water. Heat the mixture on a boiling water bath for 1 hour and cool to room temperature in an ice bath. Dilute to 25 ml. with distilled water and measure the absorbance a t 586 mp against a distilled water blank. Determine the ruthenium content from a standard calibration curve. RESULTS A N D DISCUSSION
Nature of the Ru(II1)-Nitroso R Salt Color Reaction. With nitroso
R salt, Ru(II1) forms a violet-blue complex which has a n absorption maximum a t 586 mp. Figure 1 shows the absorption spectra of the complex us. reagent blank and the reagent 2's. water blank. The absorption maximum of NRS lies a t 374 mp, and it has no appreciable absorption above 500 mp. The reaction of Ru(II1) with nitroso R salt is slow a t room temperature, but color development IS hastened by VOL. 37, NO. 6, M A Y 1965
739
Table
hlethod employed Anderson and coworkers Mole ratio Slope ratio
I.
Composition and Stability Constant of Ru(III)-NRS Complex
Ionic strength, microns
Temperature, O C.
PH
25 25 25
4.5 f0.1 4.5 f0.1 4.5 f0.1
X,,
0.3
mp
Composition (Ru+3: NRS)
Stability constan t, log K
Free energy of formation AG (Kcal.)
1:2 1:2 1:2
9.6 f0.2 9.8 f0.2
-13.1 f 0 . 3 -13.4 f 0 . 3
586 586 586
...
...
1.0
-
...
, . .
I
0 301
400 490 WAVELENGTH,
580
610
Mr
Figure 1 . Absorbance curves of ruthenium complex and nitroso R salt A. E.
Reagent alone, 4 4.04 p.p.m. Ru(lll)
heating. The absorbance first increases with increasing time of heating, and then becomes constant after 50 minutes. A heating time of 1 hour and a total volume of 10 to 12 ml. of the solution to be heated have been found optimum for color development. At least an eightfold excess of the reagent over ruthenium is necessary for full color development, as shown in Figure 2. The absorbance remains constant a t higher reagent to metal ratios. The effect of p H on the stability of the complex was determined by preparing a series of mixtures as described earlier, but with pH values varying from 1 to 12. The wavelength of maximum absorbance and the absorbance a t the wavelength maximum remain unchanged (within experimental error) between p H 3 and 7 and thus rigid control of the p H during color formation is not necessary. A sodium Table
O3
ti
X lo-%
II.
acetate-hydrochloric acid buffer of pH approximately 4.5 gives satisfactory pH control and does not interfere in the determination in any way. A plot of absorbances of solutions measured at 586 mp against the concentration of ruthenium shows that Beer's Law is obeyed over the concentration range 0.2 to 8.6 p.p.m. of ruthenium.
Stoichiometry
of
Components.
Three different methods: the methcd of continuous variations (IO),the mole ratio method ( $ I ) , and the slope ratio method (9) were employed to determine the composition of the ruthenium (111)-nitroso R salt complex under the conditions of study described earlier. The method of continuous variations was used with both equimolar and nonequimolar solutions of the reactants as shown in Figures 3 and 4.
Concn. Foreign ion Fe( 111) Co(11) Ni(I1) Rh(II1) Os(VII1) Ir(1V) Pt(1V) Cu(I1) A& 1)
Added as Fe( NO3)3 9H20 Co(NO3)2.6H?O Ni( N 0 3 ) 26H20 .
RhC13
os04
(NH&IrCle HrPtCla .6Hz0 Cu(NOi)n.3HzO Agr\iOa
rel. error, p.p.m. 3.8 4.3 26.9 5.4 5.2 0.2 4.0 26.9 15.4 22.1 53.9
ANALYTICAL CHEMISTRY
P B
0.8
Direction
Per cent relative to Ru
0.4
-
94.1 106.4 645,8 133.6 128.7 4.9 99.0 665.8 381.2 547 0 1134.1
0.2
of change
-
++ +--
HAuC14 Au(II1) PdC12 Pd(I1) 103 ... Bromide KBr 740.0 ... KI Iodide 103 ... Sulfate KnSOc 103 ... KNOI Nitrate KC1 103 ... Chloride Final concentration NRS, 4 X l O - 4 M . a Final concentration Ru(III), 4.04 p.p.m.
740
1 .o
a 0.6 g
Interferences Due to Diverse Ions"
for f 3 %
A series of mixtures was prepared for the mole ratio method, containing a constant amount of the metal ion, but with increasing ratios of the metal to the reagent (and vice versa). When the absorbances of the mixtures are measured and plotted against the concentration ratios, the curves rise from the origin as a straight line and break sharply to a constant absorbance a t the molar ratio of the components in the complex. In the slope ratio method the stoichiometry was arrived at by comparing the slopes of the two straight line plots
0
0.2 0.4 [Ru'~l/[Rut31
0.6
+ [NRS]
0.8
1.0
Figure 3. Determination of composition by continuous variations method using equimolar solutions. p H = 4.5; p = 0.3; X = 586 mp A.
E. C.
Ru(lll) concentration, 5.0 X 1 O-'M NRS concentration, 5.0 X 1 O-'M Ru(IIl) concentration, 2.5 X 1 O-'M NRS concentration, 2.5 X 1 Ob4M Ru(lll) concentrotion, 1 .O X 1 O-'M NRS concentration, 1 .O X IO-%
1.4 -
1.6
2.0
0
4.0
6.0
8.0
10.0
Ru‘a, rnl.
Figure 4. Determination of composition by continuous variations method using nonequimolar solutions. p H = 4.5; p = 0.3; X = 5 8 6 m p A. 6.
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Figure 5. Slope ratio method for the composition of the complex a t 5 8 6 mp; pH = 4.5
p l.P --
Final concentration of tlxed cornponent (in excess) 4.0 X 10-4M A. Ru(lll) varying B. NRS varying
-
u
0.6 0.4
t
Ru(lll1 concentrotion, 2.5 X I O-‘M NRS concentration, 5.0 X 1 O-‘M Ru(lll) concentration, 5.0 X lO-‘M NRS concentration, 2.5 X 1 O-‘M
obtained by varying one component in the presence of a large excess of the other, as shown in Figure 5. The results of all t,hese methods establish the composition of the comples to be 1 : 2 [ R u ( I I I ) : K R S ] . Stability Constant of Complex. The overall apparent stability constant of the 1 : 2 complex was calculated by two different methods as described previously ( I 9). Table 1 shows the results. Sensitivity and Precision. As expressed according t o the notation of Sandrll ( 2 5 ) ,the sensitivity of the reaction is 0.0047 pg./sq. cm. The optimum concentration range for accurate measurcments as determined by a plot of per cent transmittance us. log concentration is between 0.6 and 4.0 p.p.m. (Figure 6). The average relative standard deviation, as determined from a spries of measurements made according to the optimum conditions, ranges between i1-2%. Effect of Diverse Ions. Results of a study of the interferences due to diverse ions are given in Table 11. The tolerance of the system to uranium was determined separately. Table 111 gives the results of this study. The high tolerance of the method for
-
0 P.5 5.0 1.5 CONCN. OF VARIABLE COMPONENT
Figure 6. Ringbom plot for ruthenium-NRS complex a t 5 8 6 mC1 NRS concentration, 1
X 10 -3M
0
Determination of Ru(lll) in Presence of U(VI)“
Uranium added
~ o , ~ z , Absorbance at 586 mp
p,p,m,
E a t e r blank Sample blank 0.890 4 0.890 0 : 890 10 0.900 0.900 100 0.890 0.890 500 0.895 0.890 a Final concentration of Ru(III), 4.04 p.p.m. 0
10 M
I
- 1
f
E
g
4o
560
P g
80
0.1
uranium is significant since ruthenium is commonly found associated with large excesses of uranium in some natural samples and in fission materials. The tolerance of the method for Ni (11), Cu(II), Au(III), and Pd(I1) is also very high, though other ions like Fe(III), Co(II), Ir(IV), Os(VIII), and Pt(1V) interfere with the determinations. However, this presents no severe problems since ruthenium can be separated from the sample mistures prior to the determination either by distillation of the volatile tetroxide from sulfuric acid solutions, or by other well known methods ( 1 ) . LITERATURE CITED
Table 111.
x 10 - 5
(1) Avtokratova, T. D., “Analytical Chemistry of Ruthenium,” pp. 136-57, Israel Program for Scientific Transla-
tions Ltd., Jerusalem, 1963. (2) Bandyukova, V. A., Bzull. Izobreteni 13, 50 (1961). (3) Banks, C. V., O’Laughlin, J. W., ANAL.CHEM.29, 1412 (1957). (4) Beamish, F. E., LIcBryde, W. A. E., Anal. Chim. Acta 9, 349 (1953). (5) Belew, W. L., Wilson, G. R., Corbin, L. T., AKAL.CHEM.33, 886 (1961). (6) Brandstets, J., Crestal, J., Collectzon Czech. Chem. Commun. 26, 392 (1961). (7) Griffing, hl., hlellon, 11. G., IND ENG. CHEM., ANAL. ED. 19, 1014 (1947).
0.5
1.0
3.0 RU(III) P.P.M.
7.0
(8) Hara, T., Sandell, E., Anal. Chim. Acta 23, 65 (1960). (9) Harvey, A. E., JIanning, D. L., J . A m . Chem. SOC.72, 4488 (1950). (10) Job, P., Compt. Rend. 180, 928 (1928), Ann. Chim. (10) 9, 113 (1928). (11) Manning, D. L., Menis, O., ANAL.
CHEM.34. 96 i 1962). (12) Oka, Y . >Kato, T., iyippon Kagaku Zasshi 84 (3), 254 (1962). (13) Pilipenko, A. T., Sereda. I. P.. Zh. Anal. Khim. 16, 73 (1961). ’ (141 Pooa. G.. Cuirea. I. C.., Lazard. ~~-~ C.. Rev. Chim.,’Acta Rep. Populaire Rok: maine 7, 1161 (1962). (15) Sandell, E. B., “Colorimetric De~
‘
termination of Traces of Metals,” Interscience, New York, 1959. (16) Sangal, S. P., Dey, S. K., 2. Anal. Chem. 202. 348 11964). (17) Shlenskaya,
