Spectrophotometric Determination of Rhodium with Nitroso R. Formula for the Rh(lI1)-Nitroso R Complex Orville W. Rollins and Morris M. Oldham Contribution from the Department oj. Chemistry, U . S . Nasal Academy, Annapolis, Md. 21402
A CRITICAL EVALUATION of the known colorimetric methods for the determination of rhodium has been reported by Beamish ( I ) . Subsequent to this review article, Nath and Agarwal ( 2 ) reported a spectrophotometric method for determination of this platinum metal employing nitroso R salt. We were unable to obtain satisfactory results in analyzing for rhodium by the aforementioned method. We have evaluated the method which was proposed by Nath and Agarwal and, after considerable modification of it, have developed a sensitive and useful colorimetric method for determination of rhodium using nitroso R salt. EXPERIMENTAL
;
(1965). 146
:
L
"
0. I
0 380
420
460
500
540
580
620
Wavelength, nm
Apparatus. The absorption spectra shown in Figure 1 were obtained with a Carey Model 15 Spectrophotometer. All other absorbance measurements were made with a Beckman Model DU-2 Spectrophotometer. A Sargent pH meter equipped with a combination electrode was used to measure pH. Reagents. A standard rhodium(II1) sulfate solution was obtained from Matthey Bishop, Inc., Malvern, Pa. It was prepared from sponge rhodium metal (99+% pure) and was certified to contain 69.04 i 0.06 grams of the metal per liter of solution. The concentration of this solution was verified gravimetrically (to 2 ppt) by electrolytically depositing all of the Rh(II1) from an aliquot onto a platinum gauze cathode. All solutions were prepared by appropriate dilutions of this standard rhodium sulfate solution. The nitroso R salt which was used was obtained from K & K Laboratories, Inc. with a certified purity of 99% minimum. This was verified by potentiometric titration with standard NaOH solution. In a similar titration using a sample of Fisher Certified nitroso R salt, 80 of the calculated acidic hydrogen ions were titrated. The following solutions were used to obtain the desired pH values: 33 sodium acetate, 6 M acetic acid, 6 M nitric acid, and dilute sodium hydroxide. Deionized water was used throughout this study. Absorbance Spectra. The absorbance spectra for solutions of the red Rh(II1)-nitroso R complex and nitroso R itself are shown through a part of the visible region in Figure 1 . The solution containing the complex was 6.039 X 10-5M with rhodium. Both solutions contained 5 ml of a nitroso R solution (0.500 g/100 ml), they were buffered at pH 5.5, then boiled for 70 minutes and cooled to room temperature before diluting to a final volume of 100 ml. The absorbance spectrum of the Rh(II1)-nitroso R complex has not been reported heretofore. Effect of pH. In order to determine the effect of p H on formation of the Rh(II1)-nitroso R complex, solutions containing a fixed amount of Rh(II1) and an excess of nitroso R were buffered at different pH values, boiled for 70 minutes, cooled, diluted to the same volume, and then their absorbance was measured. The results obtained at different wavelengths are shown in Figure 2. (1) F. E. Beamish, Tulanta, 12, 789 (1965). (2) S . Nath and R. P. Agarwal, Chirn. Anal. (Paris), 47
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(9, 257
Figure 1. Absorption spectra of solutions at pH = 5.5 Upper curve. Rh(II1)-nitroso R complex Lower curve. Nitroso R blank
O.I 0 2.0
3.0
4.0
5.0
6.0
7.0
0
PH
Figure 2. Effect of pH on formation of Rh(111)-nitroso R complex at wavelengths indicated
Effect of Heating. Solutions containing a fixed amount of Rh(II1) and an excess of nitroso R were buffered at pH 5.5 and boiled for periods of 10, 30,40, 50,60,70, and 80 minutes to determine the effect of heating on formation of the complex. Maximum absorbance was obtained (at 520 nm) after boiling for 60 minutes while approximately 75% of the color intensity was developed after boiling for 30 minutes. There was no appreciable formation of the colored complex at room temperature. Determination of Formula for Rh(II1)-Nitroso R Complex. The mole ratio method (3) was used to determine the formula for the Rh(II1)-nitroso R complex. The final concentration of Rh(II1) was 6.039 X lO-5M in each solution, they were buffered at pH 5.5, boiled for 70 minutes, cooled to room temperature, diluted to the same volume, and the absorbance was measured at 500 nm and 520 nm in 1-cm quartz cells with (3) J. H. Yoe and A. L. Jones, IND.ENG.CHEM., ANAL.ED., 16, 111 (1944).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
:::I
I Table I. Effect of Foreign Ions
0.7
0.6
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1
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'
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1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.010.0 11.0 Moles Nitroso R per mole R h ( E I )
J
Figure 3. Determination of formula for the Rh(II1)-nitroso R complex in solution a t pH = 5.5
water as the reference. The results were the same at both wavelengths, and those obtained at 520 nm are displayed graphically as Figure 3, where it is clearly shown that the ratio of ligand to metal in this complex is 3 to 1. Isolation of Sodium Salt of Rh(II1)-Nitroso R Complex. Thirteen milliliters of a rhodium sulfate solution (0.389M, pH = 0.5) was pipetted into approximately 50 ml of a solution containing 7.34 grams of nitroso R (molar ratio of ligand to metal, 3.8 to 1.0), the pH was adjusted to 5.5 with sodium hydroxide and the resulting solution was boiled for 1 hour. This solution was then taken down to a volume of approximately 35 ml on a steam bath, and allowed to stand at room temperature whereupon red colored crystals of the product separated. These were recovered by suction filtration, washed with water followed by acetone, and air-dried. The product, Na6(Rh[CloH4N02(S03)2]3) . N a 2 S 0 411.5 . HzO, weighed 3.3 grams. Analysis of Na6 { Rh[CIOH4N02(S03)2]3 ] .N a 2 S 0 411.5 . H20. The sodium analysis was performed by flame photometry using a NIL Flame Photometer obtained from the National Instrument Laboratories, Inc. Rhodium was determined by the method being reported in this paper. Carbon, nitrogen, and sulfur were determined by Aldridge Associates & Co., Inc., Washington D. C. The dehydrations were carried out in an oven at 230 "C. A n d Calcd. for Na6( Rh[CloH4N02 (S03)2]3).Na2S04. 11.5 H 2 0 : Na, 11.64; Rh, 6.50; C, 22.8; N, 2.66; S, 14.2; HzO, 13.1. Found: Na, 11S4, Rh, 6.45; C,23.2; N , 3.12; S, 13.2; H20, 12.9. Conformity to Beer's Law. Aliquots of exactly 2, 3, 4, 5, 6, and 7 ml were taken from a rhodium sulfate solution, 1.0064 x lOU3Mwith the metal, 5.0 ml of a nitroso R solution (0.500 gram per 100 ml) was added, and the solutions were buffered at pH 5.5 with NaAc and HAC. After boiling for 70 minutes, the solutions were cooled to room temperature, diluted to 100 ml, and their absorbance was measured against a blank of nitroso R which was prepared from 5 ml of that solution and which was treated in the same manner. Conformity to Beer's law is exhibited at wavelengths of 490 nm, 500 nm, and 520 nm through the concentration range 2.00 x IO-jM to 10.0 X 10-6M--i.e., 2.06 through 10.4 ppm of rhodium. The molar absorptivity for this complex at these wavelengths is 1.312 X l o 4 , 1.175 X l o 4 , and 8.350 x 103, respectively. Effect of Foreign Ions. In order to determine the effect of various ions on this spectrophotometric method, solutions containing Rh(II1) and the ion in question were treated exactly as described above. The final concentration of Rh(II1) in all solutions was 4.026 X 10d5M,the absorbance was measured at 520 nm against a nitroso R blank, also treated as described above. Boiling a solution of nitroso R at pH 5.5 for 70 minutes causes no change in its absorption spectrum in the region being studied (see Figure 1).
Foreign ion
Concentration,
Ru(II1) Os(VII1) Pt(1V) Ir(II1) Y(II1) Au(II1) Ga(II1) Cd(I1) Mn(I1) Zn(I1) Mg(W In(II1) Zr(IV) Cr(II1) M004'Al(II1) Ce(1V) Ce(I I I)
3.1 18.7
PPm
1.9
0.6 16.5 2.2
6.0 14.0 6.5 6.5 4.9
17.5 9.1 6.0 2.0 27.0 5.8 15.6
Significant interference" COC12 1.2 C~(N03)z 1.2 Ce(SO4In 11.5 Fe(II1) alum 1. O w04'NazW04 1.7 Ni(I1) NiS04 1.2 Au(II1) AuC13 11.o Pd(I1) Pd(N03)z 1.o Ir(II1) IrC13 2.9 a The criterion for significant interference was an absorbance reading which varied more than 2.5 % from the expected value. Co(I1) Cu(I1) Ce(1V) Fe(111)
Although the difference in absorbance between the Rh(II1)nitroso R complex and a blank is greater at 490 nm than it is at 520 nm, the absorbance readings were taken at 520 nm in order to take advantage of the very low absorbance of the blank (approximately 0.010 unit) as shown in Figure 1. The results of this study are summarized in Table I. DISCUSSION This spectrophotometric method for the determination of rhodium differs considerably from that proposed by Nath and Agarwal (2). First, we find that the Rh(II1)-nitroso R complex is red and not blue as reported by those authors. Second, we find that the complex reaches its maximum absorbance in a narrow pH range (as shown in Figure 2) and is not stable and constant through the pH region 2.0 to 8.0 as reported. Third, we find that solutions containing rhodium(II1) and nitroso R develop practically no color at room temperature and must be boiled for at least 1 hour to develop full color. Nath and Agarwal ( 2 ) have proposed that this platinum metal may be determined colorimetrically with nitroso R without heating the solutions, and they state that the color intensity of the complex is not modified from that developed at 25 "C by heating to 100 "C. This information was presented in table form where the colorimeter reading was shown to be the same for a solution heated to 25, 35, 40, 50, 60, 80, and 100 "C. Finally, those authors report that Beer's law is obeyed in the concentration interval of 1.03 to 41.2 ppm of rhodium, whereas we find that solutions of the complex show negative deviations from Beer's law above 10 ppm of rhodium. The fact that solutions of Rh(II1) containing nitroso R must be boiled for 1 hour in order for the reaction to go to
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
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completion points to the extreme inertness of the hydrated Rh(II1) ion. In fact, if these solutions were merely heated to 100 “C, and not boiled, there would be very little color development. This may be the reason for the fact that Nath and Agarwal did not observe the effect of continued boiling upon formation of the Rh(II1)-nitroso R complex. Nath and Agarwal used a Klett-Summerson colorimeter with filter number 54 (which transmits in the 520- to 580-nm region), so their study and ours were performed in essentially the same spectral region. Their source of rhodium(II1) was its chloride (obtained from JM) which was assayed gravimetrically according to Scott (4). Our solution studies show that the Rh(II1)-nitroso R complex has a ligand to metal ratio of 3 t o 1. This is depicted in Figure 3 where it is also evident that the complex dissociates appreciably at room temperature. The formula for the complex is also substantiated through the isolation and analysis of its sodium derivative. In a potentiometric titration of this (4) W. W. Scott, “Standard Methods of Chemical Analysis,” D. Van Nostrand Co., New York, N. Y., 1964, Vol. I, p 746.
derivative, we found no evidence for acidic hydrogen atoms which shows that the nitroso R is coordinating through the oxygen atoms of its naphthol group, a fact that is also supported by the sodium analysis. All of this evidence suggests that the nitroso R is functioning as a bidentate ligand in accord with the not unexpected coordination number of six for Rh(II1). The amount of sodium sulfate which crystallized with the Rh(II1) complex amounts to approximately a 10% “contamination” by weight in the final product. The Rh(II1)-nitroso R complex is so stable that solutions of it show no change in absorbance on standing for 24 hours. ACKNOWLEDGMENT We thank Dr. Curtis D. Herron of Matthey Bishop, Inc. for the standard rhodium sulfate solution which was used in this research.
RECEIVED for review July 27, 1970. Accepted October 26, 1970. We are grateful to the United States Naval Academy Research Council for financial support for this work.
Determination of Polymeric Isocyanate in the Presence of Reactive Halides Phillip M. Beazley Marathon Oil Company, Denver Research Center, Littleton, Colo. THEISOCYANATE CONTENT of polymers derived from reactive halide intermediates ( I ) can not be determined by established amine titration procedures (2-4) because the residual halides react with the amines used. Kinetic studies showed that dicyclohexylamine reacts slowly with reactive halides and quickly with isocyanate. A titration technique using this amine was then developed. Excess dicyclohexylamine is reacted with the isocyanate in dimethylformamide, D M F , for a prescribed time at room temperature. The excess amine is then titrated with HCl in isopropanol using bromcresol green indicator. Because the total amount of amine taken is known, the amine consumed by the sample is calculated by difference and is equivalent to the isocyanate content of the sample.
EXPERIMENTAL Reagents. STANDARD 0.1N HCI. Dilute 9 ml of concentrated HCl to 1 liter with reagent grade isopropanol. Standardize this solution with tris-hydroxymethyl aminomethane using methyl red indicator. DICYCLOHEXYLAMINE. Dilute 50 ml of dicyclohexylamine, Eastman White Label 4627, t o 1 liter with dry D M F ( 5 ) . (1) P. A. Argabright, V. L. Sinkey, and B. L. Phillips, U. S . Patent
3,458,448 (1969).
(2) Sidney Siggia, “Quantitative Organic Analysis via Functional Groups,” 3rd ed., John Wiley and Sons, New York, N. Y., 1963, p 558. (3) R. Venkataraghavan and C . N. R. Rao, Chemist-Analyst, 51, 49 (1962). (4) Joe A. Vinson, ANAL.CHEM., 41, 1661 (1969). ( 5 ) E. Lieber, C. N. R. Rao, and T. S . Chao, ibid., 29,932 (1957). 148
e
BROMCRESOL GREEN INDICATOR. Slurry 100 mg bromcresol green, Matheson, Coleman and Bell, Inc., with 1.5 ml of 1 N N a O H and dilute t o 100 ml with distilled water after all the indicator has dissolved in the hydroxide. Procedure. Dissolve the sample containing about 1 mequiv of isocyanate in 5 ml of dry dimethylformamide. Pipet 10 ml of amine solution into the sample and after 2 minutes, add 40 ml of isopropanol and 8 drops of indicator. Titrate the unreacted amine with 0.1NHCI until the indicator remains yellow for at least 15 seconds. Run a blank to determine the total amount of amine taken and calculate the equivalent isocyanate content of the sample by subtracting the amount of unreacted amine from the total amount of amine taken. RESULTS AND DISCUSSION The reaction rates of benzyl bromide and o-tolylisocyanate with piperidine, dibutylamine, dicyclohexylamine, and diisopropylamine were compared. Equal portions of benzyl bromide in D M F were added to equal amounts of amine in D M F at room temperature. After 5 minutes, the reactions were quenched and the unreacted amines were titrated with HC1. The following results were obtained. % Benzyl bromine Amine reacted Piperidine 96 84 Dibutylamine Diisopropylamine 10 Dicyclohexylamine 6 In the same way, the reactions of o-tolylisocyanate with piperidine and dicyclohexylamine were compared.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
