the equation Ell2 = -0.152-1.25 log (SCN) for the relationship between E112 (volts US. S.C.E.) of mercuric ion and thiocyanate concentration. The value of Ellz for mercury is a function of total mercury concentration as well as the concentration of thiocyanate, and unfortunately, Korshunov and Shchennikova did not specify the mercury concentration. As a result, exact comparison is difficult. Assuming a reasonable concentration of total mercury on the order of 10-4 to 10-3~v, their potentials are in the range expected on the basis of comparison with those obtained here. The value of the “theoretical half-wave potential” for reduction of mercury(I1) to mercury used by those authors was 0.387 volt us. S.C.E., which corresponds to the value of E: = 0.477 volt us. S.C.E. employed here. Using an equation similar to Equation 6, the value 0.387 was determined by Korshunov and Shchennikova from knowledge of the half-wave potential of mercury in iodide media and the stability constant of [Hg14]-2,taken as 2 X 1030. As the constant for the iodide complex is similar to those most often quoted (W), one can only surmise that some error, experimental or otherwise, was made in their roundabout procedure.
tained in bromide media leads one t o suspect that both may be in error due t o failure to take into account properly all the possible equilibria in these complicated systems. The ratio of K 3 / K zcorresponds to the stepwise formation constant, k3, and the value found here is 7.53 X lo2, a factor of 16 greater than that reported by Gallais and Mounier (4). Furthermore, the ratio of K4/K3 = k4 = 98 is approximately a factor of 20 greater than that reported by these authors. No immediate explanation is apparent. The value of K4 = 8.7 X lo2(’,when compared with those reported earlier, differs least from the value 1.7 X loz1 reported by Toropova ( l a ) , where the differences in ionic strength employed could account for the discrepancy in the two values. Thus the value obtained polarographically in this investigation would lend support to the value reported by Toropova from elwtromotive force measurements. The question arises as t o why the earlier polarographic result (7) for K a is only 1/40 that reported herein. Because of the general agreement of the value of K 4 reported here and the value of Toropova, it would appear that the earlier polarographic value is in error. The earlier authors presented
ACKNOWLEDGMENT
The authors are grateful to the Office of Ordnance Research, U. S. Army, for financial support under Contract No. D.4-04-200-ORD-567. LITERATURE CITED
(1) Bjerrum,
J., Schwarxenbach, G., Sillen, L. G., “Stability Constants of Metal-Ion Complexes. Part 11. Inorganic Ligands,” Spec. Publ. 7, p. 42, Chemical Society, London, 1958. (2) Ibid., p. 121. (3) DeFord, D. D., Hume, D. N., J . Am. Chem. SOC.73, 5321 (1951). ( 4 ) Gallais, F., RIounier, J., Compt. rend. 223,790 (1946). 15) Grossmann. H., 2. anora. Chein. 43.
RECEIVEDfor review July 6, 1959. -4ccepted Pl‘ovember 6, 1959.
-
0
r
e
.
Some Factors Affecting Flame Photometric tmission of Rubidium in an Oxygen-Acetylene Flame T. E. SHELLENBERGER, R. E. PYKE,’
D.
B. PARRISH, and W. G. SCHRENK
Kansas Agricultural Experiment Station, Manhattan, Kan.
b The flame photometric characteristics of rubidium were studied in the presence of relatively large quantities of potassium and other cations. Intensity of the rubidium line a t 780 mp was recorded using a Beckman Model DU flame photometer equipped with a photomultiplier and a strip-chart recorder. Excitation source was an oxygen-acetylene burner. When potassium was present in relatively large quantities, the apparent emission of rubidium was increased by overlapping of emission from the 770-mp potassium line and enhancement of rubidium emission in the flame. The overlapping potassium emission was corrected by background subtraction. Effect of potassium emission on rubidium may b e controlled by adding excess potassium to the test solution. Flame variations during excitation may b e minimized by including lithium (671 mp) as an internal control. 210
ANALYTICAL CHEMISTRY
G
LESDEKIXG,
Parrish, and Schrenk
(1) have discussed difficulties in
determining rubidium by chemical and spectrographic methods. These n-orkers reported a flame spectrographic method for rubidium, using a large Littrow spectrograph equipped Kith a natural gas-oxygen flame excitation source and special plate mask. They found that a Beckman flame photometer used without a recorder apparently did not permit sufficient resolution for the determination of rubidium in the presence of relatively large quantities of potassium. Pro, Nelson, and Mathers (S), however, determined rubidium and cesium added to whiskey using a Beckman DU flame photometer with simulated standards to circumvent potassium and sodium interferences. A Beckman DU flame photometer equipped with a multiplier phototube and strip-chart recorder was used in
this study to investigate factors affecting rubidium emission. EXPERIMENTAL PROCEDURE
Reagents and Standards. Stock solutions as chlorides were made t o known concentrations with analytical grade reagents, from which aliquots were taken and diluted t o give the ion concentrations used in the various trials. Analytical solutions were made t o a final concentration of 20y0 isopropyl alcohol by volume ( 2 ) . Apparatus. A Beckman spectrophotometer Model DU, equipped with a flame photometer attachment, Model 9200; oxygen-acetylene burner; photomultiplier unit, Model 92300; spectral energy recording adapter; and Brown Electronik recorder, with 0- t o 10-mv. scale and 1/2-second pen response was used. 1 Present address, Texas Woman’s Univereity, Denton, Tex.
WAVE
LENGTH (mu)
Figure 1. Resolution of emission responses of equal weight concentrations of potassium and rubidium (0.10 mg. per mi.) Beckrnan Model DU with a Brown strip-chart recorder
Operation of Apparatus. Oxygen and acetylene pressures \\-ere those recommended by the manufacturer for the individual burners used. Data mere collected a t several slit widths; however, 0.03 mni. \\as used for most of this n-ork. Larger slit widths reduced the resolution of the adjacent potassium and rubidium lines. Smaller slit widths gave better resolution of the lines but reduced sensitivity. Slit width selection thus became a compromise between resolution and sensitivity. The emission responses of rubidium and potassium were obtained by setting the recording adapter to scan the spectrum automatically from 800 to 760 mp. The lithium emission was determined by manually moving the wave length selector and then scanning the lithium line a t 671 mp. All curves were ohtained with a scan time setting of 30 minutes (4//5 of a revolution of the wave length drum). Emission responses were measured from the peak of the curve to the base line directly below. RESULTS AND DISCUSSION
Emission Spectra and Interference Studies. Figure 1 shows an emission spectrum obtained from a solution containing equal weights (0.10 mg. per ml.) of potassium and rubidium ions. Resolution was adequate for obtaining the 766- and 770-nip lines of potassium as well as the 780- and 795mp lines of rubidium. The 780-mp line is the most sensitive rubidium line and is free from possible interference of calcium at 420 mp. However, the proximity of the potassium doublet a t 766 and 770 mp has a pronounced effect on the intensity of rubidium a t 780 mp. Figure 2 illustrates the progressive enhancement and background interference of potassium on rubidium. Overlapping interference resulting in back-
ground enhancement can be corrected by measuring the distance, h, from the top of the rubidium peak to a point directly below on a line representing background emission of potassium. As indicated by measurements of h (Figure 3), progressive levels of potassium enhance rubidium emission a t 780 mp in addition to causing general background interference. This background-corrected emission, how ever. reaches a relatively constant value a t high levels of potassium. Rubidium emission a t 795 mp behaves in a similar manner (data not included); the height of this peak was more than doubled as the potassium level increased from 0 t o 40 mg. per ml. Because rubidium emission a t 780 and 795 mp is due to a resonance line of the rubidium atom, these data, although limited, suggest that the presence of potassium increases the relative concentration of rubidium atoms in the flame, thereby causing the increased emission.
a.
c
b.
I
d I
K C O N C . !Mp./MI)
Figure 2. Effect of increasing levels of potassium on emission of rubidium (0.10 mg. per mi.) at 780 mp Mg. K per MI. a. 0
b. 5 c.
10
d.
30
These data indicate that background correction is essential in any successful determination of rubidium and that cation interaction in the flame also must be considered. The addition of large quantities of potassium, as a buffering ion, such that increments of potassium have relatively little effect on the background-corrected emission of rubidium is similar to the so-called “method of excess” commonly used in spectrographic procedures. The use of excess potassium and of lithium as an internal standard for controls in the determination of rubidium is discussed later. Sodium, calcium, and magnesium were tested for possible interference with rubidium emission. I n agreement with other workers (1, s),sodium (0 to 10 mg. per ml.) was found to enhance the recorded emission of 0.10 mg. per ml. of rubidium. Calcium (0 to 10 mg. per ml.) did not affect rubidium emission. Glendening, Par-
Table 1. Effect of Increasing M a g nesium Concentrations on Emission of Rubidium a t 780 Mp
Concentration, hIg
hIg/Rb Ratio
n
n
Mg./Ml.
Rb n- . i_ n_
Mm.5 36 5
37.0 34.0 32.5 looil 30.5 180/1 25.0 Height measured according t o h,
0.10
0.10 0.10 0.10 0.10
i.0 4.0 7.0 10.0 18.0
Height of Rb Peak,
10/1 40/1 70/1
Figure 4.
rish, and Schrenk ( I ) reported that magnesium had no effect on the rubidium emission and Pro, Nelson, and Mathers (S), using 5 p.p.m. of rubidium, found that 1 to 10 p.p.m. of magnesium had no effect. Table I shoas that magnesium, from a Mg/Rb ratio of 60/1 to 18011, had a depressing effect on rubidium emission. Lithium a s Internal Standard. Potassium is present in relatively large quantities in many samples, particularly plant tissues; therefore, it seemed desirable to investigate means of overcoming the effect of this ion on the emission of rubidium in flame photometry. I n addition to the insertion of a special mask to control scatter radiation of potassium, Glendening, Parrish, and Schrenk ( 1 ) used lithium as an internal standard and reported satisfactory results. Data (Table 11) show similar responses of lithium and rubidium to changes in potassium levels and, hence, the validity of using lithium emission a t 671 m p as an internal standard. Li/Rb intensity ratios were calculated from the heights (hl and h2) of the lithium ana rubidium emission peaks (Figure 4). The concentration of lithium (0.001 mg. per ml.) chosen provided a line of sufficient intensity to give a reasonable response on the strip-chart recorder; it had no measurable effect on the flame response of the elements studied in either water or water-alcohol s o h tions.
K
CONC
(Mg/MI)
Figure 3. Effect of progressive levels of potassium on backgroundcorrected emission of 0.10 mg. of rubidium per ml. VOL. 32, NO. 2, FEBRUARY 1960
21 1
Table II.
Response of Lithium and Rubidium Emission and Li/Rb Ratio to Changes in Potassium Concentration
Ion Concn., Mg.lM1.a Rb K
Sample
Av. Height of Peak,b RIrn."
Li
Rb
Li/Rb Ratio 2.16 2.05 2.05 2.01 2.10 1.98 1.88 1.90 1.46 1.39 1.40 1.47
71.1 20 153.5 0.10 77.8 159.2 30 0.10 82.6 169.1 40 0.10 84.5 170.2 0.10 50 156.7 74.4 0.10 20 170.4 86.0 0.10 30 175.2 92.8 7 40 0.10 86.3 163.5 0.10 50 8 103.5 0.16 20 150.8 9 118.2 0.16 164.8 30 10 122.7 172.5 0.16 40 11 121.2 50 178.2 0.16 12 All solutions contained 0.001 mg. Li/ml. as internal standard. h measured as in Figure 4. Averages from trials 1to 4 shown in Table 111. Samples 1 to 4 run on one day; samples 5 to 12, several days later. Id 2 3 4 5 6
5
b c d
Table 111. Ion Concn., Mg./Ml.a Rb K
Sample
Figure 4. Emission spectra of 0.10 mg. of rubidium and 0.001 mg. of lithium per ml. in presence of excess potassium (30 mg. per ml.)
Replication of Li/Rb Intensity Ratios
Trial 1
2
3
1
_- -1V.
Li/Rb Intensity Ratiob
2n 2 ox 2 17 2.23 2.15 2 16 ~. 30 2.0s 2.13 1.91 2.07 3.05 40 2.09 2.16 2.00 1.95 2.05 50 2.13 1.98 2.04 1.91 2.01 2.10 20 2.02 2.13 2.16 1.98 A 30 1.96 2.01 1.98 1.88 7 0.i0 40 1.84 1.92 1.89 1.90 8 0.10 50 1 90 1.90 ... I 46 9 0.16 20 1.43 1.53 1.46 1 39 10 0.16 30 1.37 1.40 1.42 1.40 1.37 1.42 11 0.16 40 1.42 1.47 1.34 50 1.52 1.54 12 0.16 4 All solutions contained 0.001 mg. Li/ml. as internal standard. b Li/Rb ratios calculated from heights of response of both lines measured as in Figure 4. c Samples 1 to 4 run on one day; samples 5 to 12 several days later.
n- . i_ n_
IC
2 3 4 5
0.10 0.10 0.10 0.10 o in
Table IV.
Flame Photometric Recovery of Rubidium from Solutions with Wide Variations in Alkali Mineral Concentrations
Sample Composition, Mg./Ml. Rb added K Na Li K/Rb Rb found
1 0.100 40.0 10 0 10.0 0,001 400/1 0.105
2 0,100 35.0 3.0 4.5 0,001 350/1 0.109
3 0.050 17.5 1.5 2.3 0,001 350/1 0.053
4 0.050 35.0 3.0 4.5 0,001 700/ 1 0.053
5 0.050 50.0 3.0 4.5 0,001 1000/1 0.055
Table V. Comparison of Flame Photometric and Flame Spectrographic Method for Determination of Rubidium in Solutions of Known Mineral composition
Sample Composition, Mg./Ml. Rb K Na Mg
Li K/Rb
1 0.050 17.5 1.5 2.3 0,001 350/1
2 0.050 35.0 3.0 4.5 0.001 700/1
3 0.100 40.0 10.0 10.0 0.001 400/1
4 0.100 35.0 3.0 4.5 0,001 350/1
R b Recovered, Mg./Ml. Photometric Spectrographic 212
0.051 0.054
ANALYTICAL CHEMISTRY
0.052 0.064
0.099 0.093
0.106 0.129
Replication of Li/Rb ratios on successive determination is shown in Table 111. The over-all average of Li/Rb ratios of samples 1 to 4 is 2.08; samples 5 to 8, 1.97; samples 9 to 12, 1.43. The standard deviation for all trials on samples 1 to 4 is 0.073; samples 5 t o 8, 0.095, indicating good precision for these determinations. Precision in samples 9 to 12 is better (S. D. = 0.062), probably because of the higher rubidium concentration. Lithium, therefore, is useful in minimizing minor fluctuations in rubidium intensity when the potassium concentration in the test solution is high. Analysis of Known Solutions. A series of samples of known rubidium concentrations was prepared t o investigate t h e practicability of this flame photometric technique a s a n analytical procedure. Each sample vas made t o represent various mineral compositions t h a t would be encountered in analysis of a typical plant tissue, and, in addition, contained lithium for the internal standard, Calcium was omitted from these solutions because it apparently does not affect rubidium emission. Standard curves were made from solutions that contained 0 to 0.16 mg. of rubidium, 30 mg. of potassium, and 0.001 mg. of lithium per ml. Rubidium emission was obtained as described above. Data (Table IV) indicate that this technique is capable of giving good results from a single calibration curve on samples with a wide variation in mineral content. A second series of samples was prepared and analyzed. When compared to results obtained using the flame spectrograhic method of Glendening, Pari-ish, and Schrenk (1) (Table V), results by the flame photo-
metric technique were in closer agreement with the quantities added than those by the spectrographic method. Thus, this detection method appears applicable to a practical analytical method for rubidium. However, any practical flame photometric method for the determination of rubidium should include an internal standard (such as lithium) and excess potassium on samples that contain variable potassium lwels. In this work, the standard curves were obtained from calibration solutions containing excess potassium only. Over a \Tide range of high potassium levels, Li R b ratios remain constant (Tables I1 and 111); in addition, it appears that thc excess potassium present acts as a predominant ion and minimizes
variations caused by other ions. Consequently, the use of calibration standards containing only one high level of potassium seems justified, provided the samples also contain a large quantity of potassium (above 20 mg. of potassium per ml.). Although this flame photometric study is based on solutions of known mineral composition, the data suggest application to determinations that require resolution of potassium and rubidium spectral emission lines. Results are immediately available from the strip chart, eliminating the usual time losses of exposure, development, and plate reading required for spectrographic procedures. The flame photometric technique for the determination of rubidium in thc presence of large
concentrations of potassium is easier and appears more accurate than flame spectrographic methods. LITERATURE CITED
(1) Glendening, B. L., Parrish, D. B., Schrenk, W. G., ANAL. CHEM. 27, 1554 (19%). \----,.
(2) Kingsley, G. R., Schaffert, R. R., Science 116, 359 (1952). (3) Pro, M. J., Nelson, R. A., blathers, A4.P.. J. Assoc. Ofic. - Aqr. Chemzsts 39,
506 (1956).
RECEIVED for review July 6, 1959. Accepted November 4, 1959. Contribution 586, Department of Chemistry, Kansas Agricultural Experiment Station, Kansas State University, Manhattan, Kan. Work supported in part by a grant from the U. S. Public Health Service.
Spectrophotometric Determination of Cobalt and Nickel with Oxamidoxime GEORGE A. PEARSE, Jr.,l and RONALD T. PFLAUM Department of Chemisfry, Sfafe Universify of Iowa, Iowa Cify, Iowa
b A method i s presented for the determination of cobalt and nickel in a mixture of diverse ions. Oxamidoxime is used as the reagent for the simultaneous precipitation of nickel and the complexation of cobalt as a soluble colored species. Nickel is quantitatively precipitated as Ni(C2HbN40212. 2Hz0 at pH 8 to 9.5. The formation of the yellow cobalt oxamidoxime complex is quantitative under identical conditions. The method for cobalt and nickel is rapid and accurate within spectrophotometric limits. Results on selected samples and the effects of diverse ions are discussed.
T
determination of cobalt and nickel has long presented a problem to the analytical chemist. As these two transition metals readily form colored coordination compounds with many organic ligands, the colorimetric method of analysis has been widely used. However, the reagents employed have not been specific for either metal ion, and the methods developed have suffered from interferences due to the presence of other metal ions (3, 4, 6, 9). A study of the reactivities of amidoximes with transition metal ions disclosed useful color reactions resulting HE
Present address, E. I. du Pont de Semoura & Co., Inc., Seaford, Del.
from the coordination of cobalt(I1) ion with oxamidoxime ( 7 ) . Precipitation of nickel(I1) ion with oxamidoxime, HON: (NH2)C-C(KHz):NOH, has been reported ( 1 ) and vias observed in this study. As a consequence, the cobalt(11) and nickel(I1) oxamidoxime systems were investigated thoroughly and a spectrophotometric method for the two metal ions was developed.
weighed amount of reagent in deionized water . Standard solutions of cobalt(I1) and nickel(I1) ions were prepared from the corresponding perchlorate salts (G. Frederick Smith Chemical Co.). Cobalt solutions were standardized gravimetrically using 3,5-dimethyIpyrazole as the cobalt precipitant (8); nickel solutions, by gravimetric determination of the dimcthylglyoxime precipitate. ,411 other reagents were prepared from reagent grade chemicals.
APPARATUS AND REAGENTS
All spectrophotometric measurements were made a t 25" C. with a Gary Model 11 recording spectrophotometer, using 1-em. matched silica cells. A Beckman Model G pH meter was used for all p H measurements. Oxamidoxime was prepared by the reaction of 0.5 mole of dithio-oxamide (rubeanic acid) with 1 mole of hydroxylamine (6). Hydroxglammonium chloride, neutralized with an equivalent amount of sodium carbonate, was added slowly, with stirring, to a hot solution of the dithio-oxamide in methanol. After refluxing for 0.5 hour on a steam bath, the solution was concentrated to one half its volume and cooled. The resulting crystals were recrystallized from water with the use of decolorizing charcoal. A 75% yield of pure white compound, with melting point of 2023" C. (literature value 202" C.), was obtained. A 0.1M stock solution of oxamidoxime, stable for more than six weeks, wa3 prepared by the dissolution of a
RECOMMENDED PROCEDURE
Dissolve the sample containing cobalt and nickel by appropriate means using a minimum of solvent. Evaporate t o 1-nil. volume and add 15 nil. of 12M hydrochloric acid. Place the solution on a 20 em. X 0.75 sq. em. column of Dowex 1-X8, 50 to 100 mesh, resin. Elute the column with 15 to 20 ml. of 4M hydrochloric acid. Adjust the acidity of the sample solution to pH 6 and the volume to about 50 ml. Add 10 ml. of 0.1M reagent solution for each 5 mg. of combined cobalt and nickel ion and sufficient solid sodium acetate to raise the pH to 8 to 9.5. Collect the nickel oxamidoxime precipitate (which mag' contain insoluble hydrous oxides of diverse heavy metal ions) into a medium-porosity sinteredglass filter and wash with two 5-ml. portions of O.1M sodium acetate solution. Combine the filtrate and the wash solution, adjust to volume with 0.1M sodium acetate, and measure the absorbance a t 350 mu. Calculate the VOL. 32, NO. 2, FEBRUARY 1960
213
