existence of dimeric forms of these reagents in the nonpolar solvent. In progressing from the diethyl ester to esters with longer chain alkyl groups, there occurs a marked increase in the extractability. Inspeation of a S t u a r t Briegleb model of the zinc dialkylphosphorodithioate complex provides a possible explanation. When the linear portion of the alkyl chain exceeds two carbon atoms, normal rotation about the carbon-carbon bisnds provided a protective screen araund the central metal atom, restricting; access of solvent or solute molecules to the site. Branching in the alkyl group increases the extractability to a slight extent, paralleling the amall increases in acidit,y observed for the parent reagentci (8). The stability constant of the zinc complexes provides quantitative evidence for appreciable stability of the phosphorodithioate complex. Data are lacking for a direct comparison with dialkylphosphates. However, the extraction constant for the zinc di-(2-ethyl)hexylphosphorodithioate is 25 as compared with a value of 0.056 reported for
the formation of a 4membered ring with the central zinc ion with less strain and distortion of bond angles (1, 2 ) . LITERATURE CITED
v
Di-(Z-ethyl) hexyl
I
’k
d ,,L , COCil
L
001
AOUEOUS HYDR0GEN.iON
L
L
01
L
IO
CONCENTRATION, M
Figure 4. Distribution of zinc as a function of aqueous HCI concentration for four esters of phosphorodithioic acid, each 0.2M in CCld
zinc di-(2-ethyl)hexylphosphate, which is probably ZnR*(HR)Z in the organic phase (4,6), an increase in extractability by a factor of 450. Conceivably the larger radii of the sulfur atoms permits
(1) Chaney, E.G.,Fernando, Q., Freiser, H..J. Phus. Chcm. 63. 2055 (1959). (2) $ernando, Q., Freher, H., J.‘ Am. C h m . Sne. 80, 4928 (1958). T. H., Dean, J. A., ANAL. CHEM.34, 1312 (1962). ( 4 ) Kimura, K., Bull. Chem. Soc. Japan . 33, 1038 (1960). (5) Lamb, E., U. 8. Atomic Energy Comm. Rept. CF-60-6-132,p. 5 (1960). (6) Rydberg, J., Arkiv Kemi 8, 113 (1955). (7) Schweitzer, G K., Southeastern Regional Meeting, ACS, Gatliiburg, Tenn., 1962. (8) Zucal, R. H., Dean, J. A., Handley, T. H., ANAL.CHEM.35,988(1963).
RECEIVEDMarch 25, 1963. Accepted May 27,1963. Presented a t Southeastern Regional Meeting, ACS, Gatlinburg, Tenn. R. H. Zucal is indebted to the International Atomic Energy Agency for the award of a fellowship which made this work possib!e. ORNL is operated by Union Carbide Nuclear Co. for the U. S. Atomic Energy Commission.
2-Diethy ICI minoet ha nethio I Hy drochIoride as a S pect rophoto met ric Reagent f o r Rhodium SURESH C. SRIVASTAVA Deparfmenf of Chemislry, I ooisiana Sfate University in New Orleans, lakefront, New Orleans 22, la.
,Rhodium produces a yellow color on heating with an excess of 2-diethylaminoethanethiol hydrochloride in aqueous solution. Studies have been performed to investigate the optimum conditions for the spectrophotometric determination of trace amounts of rhodium using this reagent. The sensitivity of the color reaction is 0.0064 micron per square ‘centimeter. The region of maximum absorption of the complex lies at 330 mp when measured against a reagent b h k . About 30 minutes’ heating is required for the color to develop fully and the optimum pH range is 1.5 to 3.5. The system adheres to Beer’s law over a concentration range of 0.:3to 15.4 p.p.m. of rhodium, and the optimum range for the most accurate spectrophotometric measurements is 0.6 to 8.3 p.p.m. The average and maximum relative standard deviations, respectively, are 0.63 and 1.26%, as shown by the absorbance readings of samples containing 3.09 p.p.m. rhodium. The interferences due to other platinum metals have been studied.
T
a few analytical procedures described in the literature for the spectrophotometric determination of trace amounts of rhodium, but still there is a need for better and more sensitive reagents. Beamish and McBryde (2) reviewed the existing colorimetric methods for the determination of rhodium and concluded that the tin(I1) chloride method (1, 2, 6) is, by far, the most satisfactory one. The use of tin(I1) bromide reagent for the determination of rhodium has also been described (3, 4, 7). Jacobs (6) and Wilson and Jacobs (10) have studied N,N’-bis(3-dimethylaminopropyl) dithio oxamide and p-nitrosodimethylaniline, respectively, as reagents for the colorimetric determination of rhodium. Wagner and Yoe (9) have also described the spectrophotometric determination of rhodium with thiomalic acid and the simultaneous determination of rhodium and palladium. Most of the reagents that have been employed are not water soluble and also lack sensitivity. This paper describes the use of 2-diethylaminoethanethiol hydrochloride (DAT) HERE HAVE BEEN
aa a spectrophotometric reagent for the determination of trace amounts of rhodium. The reagent and the rhodium complex are soluble in aqueous media and the sensitivity of the reaction is fairly comparable with the procedures described in the literature. EXPERIMENTAL
Materials. STANDARDRHODIUM SOLUTION.A standard solution of rhodium nitrate was obtained from
J. Bishop and Co.
The stock solution was diluted suitably to give a final solution containing 1.00 mg. per ml. of rhodium. Rhodium content was determined gravimetrically by first precipitating rhodium as the sulfide then igniting to the oxide and finally reducing to the metal in presence of hydrogen. REAGENT SOLUTION. A stock solution containing 5 mg. per ml. was prepared by dissolving 2.6 grams of reagent grade Zdiethylaminoethanethiol hydrochloride (K and K Laboratories) in 500 ml. of distilled water. The solution was standardized by determining its sulfur content. It was stable for several days when stored in an inert atmosphere VOL. 35, NO. 9, AUGUST 1963
1165
I
t
0 20L
t
0 20
0 I0i
1166
ANALYTICAL CHEMISTRY
ri
nitrate on the system was studied. These ions were added in proper amounts to solutions containing 4.12 p.p.m. of rhodium. The amounts of foreign ions which produce a varihtion in absorbance by =k2% have been taken as the tolerance limits. Results are described in Table I.
0 50 O
ACKNOWLEDGMENT
03 0 L 0
I
I O
30
2 0
4.0
50
Thanks are due to hIary L. Good for her kind interest during the course of this study.
PH LITERATURE CITED
Figure 3.
Effect of pH on the stability of the complex;
330 mp Concn. of rhodium, 3.09 p.p.m. Concn. of DAT, 5.9 X 10-%4
reproducible color intensity.
Hence,
a 10-ml. total volume of the heating
solution is recommmded. Effect of Reagent Concentration on Solution Absorbance. A large molar excess of the reagent over rhodium (>1:80) is necessary to produce maximum color intensity. For example, a solution containing 3.09 p.p.m. of rhodium required a t least 10 mg. of the reagent for maximum color development. Adherence to Beer’s Law. A series of mixtures containing varying amounts of rhodium, 2 ml. of the buffer, 5 ml. of the reagent, and distilled water t o give a total volume of 10 ml. was prepared and heated on a water bath a t 90” C. for 30 minutes. The solutions were cooled to room temperature and diluted to 25 ml. with water. Absorbances of these solutions were recorded a t 330 mp against reagent hlanks and plotted against rhodium concentration. Results show that Beer’s law is obeyed by the system between 0.3 and 15.4 p.p.m. of rhodium. Effect of pH on the Stability of Color. The variation in absorbance of the complex with change in p H is shown in Figure 3. The color is stable between p H 1.5 and 3.5. A sodium acetate - hydrochloric acid buffer of p H 3.0 was adequate for the
system to give reproducible values of absorbance. Other buffer systems of p H 3.0 including potassium biphthalate-hydrochloric acid affected the nature of the complex. The volume of buffer solution had some effect also on the absorbance values. Two milliliters of the sodium acetate-hydrochloric acid buffer were adequate for buffering capacity and for the consistence of absorbance values. Sensitivity and Precision. The sensitivity of the reaction is 0.0064 micron per square centimeter as described according to the notation of Sandell (8). The practical sensitivity based on an absorbance of 0.010 unit is 0.064 micron per square centimeter. The optimum range for the most accurate spectrophotometric measurements is 0.6 to 8.3 p.p.m. of rhodium. Ten different solutions containing 3.09 p.p.m. of rhodium when treated according to the optimum conditions already described gave absorbance readings which show average and maximum relative standard deviations of 1 0 . 6 3 and Al.26%, respectively, Interferences Due to Diverse Ions. The effect of a number of diverse ions including iron(III), cobalt(II), nickel (11), ruthenium(III), palladium(II), osmium(IV), iridium(IV), platinum (IV), copper (II), silver (I), gold (111), chloride, bromide, iodide, sulfate, and
(1) Ayres, G. H., Tuffly, B. L., Forrester,
J. S., ANAL.CHEM.27,1742 (1955). ( 2 ) Beamish, F. E., McBryde, W. A. E., Anal. Chim. Acta 18, 551 (1958).
(3) Berman, S. S., Ironside, R., Can. J . Chem. 36, 1151 (1958). (4) Berman, S. S., McBryde, W. A. E., Analyst 81, 566 (1956). (5) W. D.. ANAL. CHEM.32. . ,514Jacobs. (1960). (6) Maynes, A. D., McBryde, W. A. E., Analyst 79,230 (1954). (7) Pantani, F., Piccardi, G., Anal. Chem. Acta 22, 231 (1960). (8) Sandell, E. B., “Colorimetrk, Deter-
mnation of Traces of Metals. Interscience, New York, 1959. (9) Wagner, V. I., Jr., Yoe, J. H., Tnlanta
2,239 (1959). (IO) Wilson, R. B., Jacobs, W. D., ANAL. CHEM.33, 1652 (1961).
RECEIVEDfor review March 22, 1963. Accepted May 27, 1963. Work supported by U. S. Atomic Energy Commission under Contract Number AT-(40-1)-2576.
Correction Infrared Spectra-Structure Correlation Study of Vanadium-Oxygen Compounds In this article by Leo D. Frederickson. Jr., and Donald M. Hausen [ANAL, CHEM. 35, 818 (1963)l on page 821 captions 6 and 10 are in correct order but the spectra are reversed.
VOL. 35. NO. 9 , AUGUST 1963
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