alkyl disulfides in the presence of alcohol cannot be achieved bv direct titrimetric methods Jvithout the removal of mercaptan and the reduced disulfide as silver mercaptides. In these mixtures the condensation of mercaptan Jvith acr\-lonitrile after the reduction of disulfide with zinc does not proceed even in presence of ethylenediaminetetraacetic acid, and tilus dialk\-l sulfide cannot be detprmined SatiSfaCtorilg by this method.
LITERATURE CITED
(1) Beesing, D. W., Tyler, R. P., Kurtz,
D. >I.. Harrison. S. A,. ANAL.CHEM.21. 1073 (1949).
(2) Earle, T.E., Zbid., 2 5 , i 6 9 (1953). (3) Haslam, J., Grossman, S., Squirrel, D. C. M., Loveday, S. F., Analyst 78,
92 (1953). (4) Hurd, C. D., Gershbein, L. L., J . Am. Chem. SOC. 69,2328 (1947). (5) Karchmer, J. H., - 4 ~ 4 CHEX ~ . 30, 80 (1958). (6) Karchmer, J. H., Ralker, -4. T., Zbid., 30, 85 (1958).
('7) Kimball, J. W., Kramer, R. L., Reid, E. E., J.Am. Chem. Soc.43,1199 (1921)
( 8 ) Kolthoff, I. M., Harris, W. E., IND.
E N G . CHEM.,!iNAL. ED. 18, 161 (1946). (9) Kolthoff, I. M., May, D. R., Morgan
P., Laitinen, H. A., O'Brien, A. S., Zbid., 18,442 (1946). (10) Siggia, S.,Edsberg, R. L., Zbid., 2 0 , 938 (1948). (11) Strafford, N., Cropper, F. R., Hamer, A., Analyst 75, 55 (1950).
RECEIVEDfor review June 25, 1958. Accepted December 3, 1958.
Spectrophotometric Determination of Traces of Nickel with 4-lso pro pyl-l ,2-cyclo hexanedioned10x1 me 1.
B. L. McDOWELL, A. S.
MEYER, Jr., R. E. FEATHERS, Jr., and J. C. WHITE
Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
b The use of xylene rather than chloroform to extract the nickel(ll)-4-isopropyl 1,2 cyclohexanedionedioxime chelate results in increased sensitivity in the spectrophotometric deterrnination of nickel. The extraction coefficient in xylene is more than five times as great as that in chloroform, and the low solubility of xylene in aqueous solution permits the use of much larger phase ratios (aqueous to organic). The system conforms to Beer's law for concentrations of nickel from 1 to 12 y of nickel per ml. of xylene. The precision of the method is 2y0. Iron, cobalt, and copper interfere seriously; however, their interference can b e minimized so that weight ratios of metal to nickel of 20, 2, and 8, respectively, can b e tolerated. Concentrations of nickel from 0.005 to 100 p.p.m. were determined in water, alkali metals, and several analytical reagents. Recoveries in excess of 95% were obtained when known amounts of nickel were extracted from aqueous solutions of relatively high ionic strength a t aqueous-toorganic volume ratios as high as 300 to 1.
-
R
-
for the determination of nickel have been adequately reviewed by Hooker and Banks (S), who noted that the vicinal dioximes are examples of a nearly ideal reagent for the determination of nickel. Of the several substituted cyclohexanedionedioximes recommended as reagents for the determination of nickel by gravimetric, spectrophotometric, and titrimetric procedures, these authors suggested 4-isopropyl-l,2-cyclohexanedionedioxime as the hest reagent for the spectrophotoEAGENTS
metric determination of nickel. They suggested that extraction of the nickel chelate from large volumes of aqueous solution into chloroform offers the most sensitive method yet reported for the determination of nickel. This report is concerned with further studies of 4-isopropyl-1,2-cyclohexanedionedioxime as a reagent for nickel. With xylene as an extractant for the nickel chelate, the reagent has been applied to the determination of 10 y of nickel in as much as 500 ml. of aqueous solution with a coefficient of variation of 2%. This order of sensitivity makes the method ideally suited for application to such samples as liquid metals, purified water, and reagent chemicals. Following the completion of this investigation, Blundy and Simpson ( I ) published a method applying 4-methyl1,2-cyclohexanedionedioxime to the determination of nickel with toluene as the solvent for the nickel chelate. APPARATUS
All absorbance measurements were made in 1.00-cm. Cores cells. The absorption spectra were recorded with a Gary automatic recording spectrophotometer, Model 14-11, and absorbance measurements were made with a Beckman Model DU spectrophotometer. REAGENTS
Prepare a standard nickel(I1) solution by dissolving 5 grams of nickel shot in hydrochloric acid and diluting the resulting solution to 500 ml. Determine the concentration of nickel gravimetrically ( 3 ) . The solution used for this study contained 9.97 mg. per ml. Make appropriate dilutions of this stock solution to yield standard solutions of lower nickel concentration.
Prepare a 0.004M solution of the chromogenic reagent, 4-isopropyl-1,2cyclohexanedionedioxime [synthesized according to the procedure of Hooker and Banks (S)], by stirring 80 mg. of the reagent with 100 ml. of water for 24 hours and then filtering off the excess reagent. The 0.004M solution is a saturated aqueous solution of the diosime ( 6 ) . Xylene, hexane, chloroform, benzene, and toluene, all reagent grade. Ammonium acetate solution, approximately 10M. Sodium fluoride solution, 10 mg. of fluoride per ml. Sodium thiocyanate solution, 10% (W*/V.)*
Potassium (./.)
.
cyanide solution. 10%
Hydroxylamine hydrochloride solution, 10% (IV./T.). prepared fresh daily. Prepare a solution of Sulfi-Down (stabilized thioacetamide, distributed by A. Daigger and Co., Chicago, Ill.) by dissolving 2.5 grams of the solid reagent in 50 ml. of water and heating to 80" k 5" C. Cool immediately to prevent evolution of hydrogen sulfide. Store in a tightly stoppered flask and prepare fresh daily. PROCEDURE
Transfer a volume of solution which contains 10 to 150 y of nickel to a separatory funnel of appropriate size. Adjust to approximately p H 7 and add sufficient 1 O M ammonium acetate to make the final solution 1M with respect to acetate. For a final aqueous volume of 100 ml. or less, add 2 ml. of the saturated dioxime solution. For each additional 100 ml. of aqueous solution add another 2 nil. of diovime solution. After 30 minutes, add 10 ml. of xylene; a smaller volume may be used if the amount of nickel is very low. Shake the contents of the funnel VOL. 31,
NO. 5, MAY 1959
931
vigorously for 2 minutes; longer equilibration periods may be necessary for aqueous volumes in excess of 500 ml. After the phases have separated, discard the aqueous phase. Measure the absorbance of the xylene solution of the chelate a t 383 mp versus the xylene extract of a blank solution prepared in the same manner as the sample. On the basis of standards which contain 1 to 12 y per ml. of nickel in the organic phase, the coefficient of variation of this method is less than 2%.
1.0, 0.9
I
, ,
I
,.
L
I
I
I
I
I
I
I 1
x’;, CONDITIONS: C A R Y A U T O M A T I C RECORDING SPECTROPHOTOMETER. MODEL No. 14 M I cm. C E L L S N I C K E L CONCENTRATION, I O pq./ml.
LEGEND:
0.6
-
0
CHCIJ
____
3
Figure 1 . Absorption spectra of nickel(Il)-4.isopropyl-l,2cyclo hexaned ionedioxime chelate in chloroform, benzene,
11
andxy‘ene
BENZENE
EXPERIMENTAL
Extractants for Nickel Chelate. Hooker and Banks (6) extracted nickel after precipitation as the Pisopropyl1,2-cyclohexanedionedioxime chelate from a neutral solution buffered with ammonium acetate. To effect quantitative extraction from large volumes of solution, they carried out repeated extractions with small volumes of chloroform, which were then combined and diluted to an appropriate volume. Other solvents were investigated in an attempt to accomplish quantitative extraction of the chelate into the organic phase with a single equilibration. The criteria on which the selection of the solvent was based were a large extraction coefficientof the chelate and a negligible mutual solubility with the aqueous phase. Preliminary’ screening tests of various solvents were carried out by extracting, with a single 10-ml. portion of the solvent, solutions which contained 50 y of nickel that had been precipitated with a solution of 4-isopropyl-l,2-cyclohexanedionedioxime (6). The extent of extraction was estimated either on the color of the chelate remaining in the aqueous phase or on the color of a second extract. Of the solvents tested, chloroform, benzene, and xylene appeared to be satisfactory (Table I). Tests were carried out to determine the distribution of nickel after extraction of the chelate into each of these solvents by precipitating 8.5 pmoles of nickel with 21 pmoles of the reagent [a 25% excess of that required to form the ;“\Ti(CsH1&,02)2 complex] ( 2 ) . The nickel chelate was then extracted into 10
0.4
0.3
r I
~
300
20
40
6C)
80
400
20
40
ml. of the solvent from 100 ml. of, aqueous solution buffered at p H 7 by the addition of 0.1 mole of ammonium acetate. The aqueous solution also contained 100 mg. of fluoride to complex any iron that might be present. After the phases were separated, the organic phase was discarded and the aqueous phase was centrifuged to remove the last traces of the organic phase. The concentration of nickel in the aqueous phase was determined by adding 2 ml. of a saturated solution of the dioxime in water, extracting the remaining chelate into 5 ml. of the appropriate solvent, and measuring the absorbance of these extracts at 383 mp. The concentration of nickel was obtained from a standard curve prepared by extracting known quantities of the nickel chelate into each of these solvents. The chelate conformed to Beer’s law for concentrations of nickel of 0.5 to 10 y per ml. for all three solvents. The extraction co-
Table 1. Comparison of Solvents as Extractants for Nickel Chelate Solvent Comments Chloroform Approximately 95y0 extracted rapidly to yield yellow soluCarbon tetrachloride tion Perchloroethylene Approximately 50% extracted slowly t o yield yellow solution Butyl acetate No apparent extraction; some chelate transferred as red n-Amyl alcohol precipitate No apparent extraction; some chelate transferred as red Paraffinic hydrocarbons precipitate Aromatic hydrocarbons Rapid, quantitative extraction of chelate t o yield yellow Benzene solution Extraction similar t o benzene, but less rapid Toluene Xylene Very rapid, quantitative extraction of chelate to yield yellow solution 932
ANALYTICAL CHEMISTRY
60
80
500
20
WAVELENGTH, mp.
efficients of nickel-Le., the ratio of the concentration of nickel in the organic phase to the concentration of nickel in the aqueous phase-for the three solvents were 210 for chloroform, 320 for benzene, and >lo00 for xylene. Xylene was selected as the most suitable solvent because it had the largest extraction coefficient. For extraction under conditions of high aqueous-to-organic volume ratios, xylene offers three additional advantages: low solubility in water (7), rapid rate of extraction of the chelate, and sharp phase separation. The absorption spectrum of the chelate in each of the three solvents is shown in Figure 1. For all three solvents the spectra are of the same general shape and exhibit absorption maxima a t 385 and a t approximately 335 mp. The position of the maximum a t the shorter wave length shifts somewhat with different solvents. Effect of pH of Aqueous Phase on Extraction. I n the presence of a small excess of reagent, the extraction coefficient reached a maximum value (approximately 1000) n-hen the p H of the solution was 7 or 8. The value of the coefficient decreased rapidly a t p H values lower than 6 and higher than 9, t o about 100 a t p H 5 and p H 10. The decrease in the coefficient a t high p H values may have been caused in part by the large amount of ammonium hydroxide required to adjust the pH. For the sake of convenience, p H 7 with 1M ammonium acetate as the buffer solution was chosen as the optimum acidity. Hooker and Banks recommend extraction from an acetate-buffered solution.
Solubility of Reagent in Xylene. The extraction coefficient of the chelating agent was determined a t p H 7 and 5 . The amount of reagent remaining in the aqueous phase was determined by measuring the absorbance of xylene extracts obtained after the addition of excess nickel. The absorption spectra of these extracts coincided with those of extracts from solutions that contained excess reagent. When 15 mg. of reagent was initially present in the aqueous phase, the coefficient m s 0.46 a t p H 7 as compared to 8.3 a t pH 5 . Therefore, a t pH 7 and a t aqueous-to-organic phase ratios of 10 to 1, practically all the chelating agent remains in the aqueous phase. Extraction Efficiency at Extreme Phase Ratios. I n order to obtain a more precise estimation of the extraction coefficient so that a limiting phase ratio for quantitative extraction could be estimated, equilibration measurements were carried out with increased quantities of nickel and reduced concentrations of excess reagent. Solutions which contained 17 pmoles of nickel as the chelate in 10 ml. of xylene (prepared by direct dissolution of the dried chelate) were equilibrated with 100 ml. of aqueous solutions to which 0 to 6.9 pmoles of reagent had been added. Following equilibration, the concentration of nickel in the aqueous phase was determined as previously described. The formation and extraction of the chelate can be represented by the equation Ni++.
+ 2RH2, e
+
Yi(RH~i20 2H+, (1) where RHZ = 4-isopropyl-l,2-cyclohexanedionedioxime. Under the conditions of these experiments the reactants are present in relatively low concentrations. Therefore, all activity coefficients should remain constant, and the concentration equilibrium constant may be expressed as:
in which the concentration of the undissociated reagent, RH,,, has been expressed in terms of the excess reagent before extraction, R,, by algebraic solution of conventional equilibrium expressions in which kl and k2 are the first and second ionization constants of the reagent, D is the distribution ratio of the reagent between the organic and aqueous phases, and P is the phase ratio. Since these equilibrations were carried out under conditions of constant phase ratio and
II.
Effect of Excess Reagent on Equilibrium Constant of Nickel(ll)-4lsopropyl-l,2-CyclohexanedionedioximeChelate in Xylene (Nickel added, 17.1 pmoles; aqueous-to-organicphase ratio, 10; pH of aqueous phase, 7; Table
1M ammonium acetate) Equilibrium Ni Concn. in Constant, K', Excess Reagent Aq. Phase after (Liter per Mole)2 Equilibration, Added, ?moles x 10-1s per Liter pmoles per Liter 1.3 0 3.21 530 2.4 1000 3.0 1.71 1.6 3.0 820 2.08 5.2" 11.8 0.22 43000 0.35. 59 >10000 0.14 a Concentration of nickel in aqueous phase based on absorbance measurements less than
0.010.
k, acidity, the value of the terms, [H+la' klk2 D [~+1:, and p , in Equation 2 are con-
stant; therefore, a practical concentration equilibrium constant, k', which is a function of the hydrogen ion concentration and the phase ratio, may be defined as follows:
of the concentration range for precise photometric measurements (150 y), the concentration of excess reagent, R,, is about 3 X lO-5M. From Equation 4, the extraction coefficient, E:, is calculated t o be about 25,000. If 50 y or less of nickel is present, a larger excess of reagent is present, and the extraction coefficient, E:, exceeds 100,000
K'(P, [H+]) = INTERFERENCES
Values of K ' , calculated on the assumption that no undissociated chelate remains in the aqueous phase after extraction, are given in Table 11. In view of the low concentrations of nickel remaining in the aqueous phase and the inherent limitations of phase separation, the agreement between the values of the equilibrium constant is satisfactory. The value of the equilibrium constant, approximately 3 X 10'3, should not be materially different a t the concentration of reagent specified in the analytical procedure. From Equation 3 it is obvious that the extraction coefficient,E:, is given by the expression
E(R,, [H+],P )
=
K'{&
+ 2[Ni++],)2 (4)
Under the usual conditions of analytical measurements, the concentration of uncomplexed nickel is negligible and the value of the extraction coefficient is equal approximately to K'(RJ2. As it has been determined that the reagent remains substantially in the aqueous phase when extractions are carried out a t pH 7 and at a phase ratio of 10, the value of is negligible and K' and E: are therefore independent of the phase ratio a t ratios of 10 or greater. However, it is necessary to increase the amount of reagent when larger aqueous volumes are used in order to maintain the same concentration of reagent in the aqueous phase. In the recommended procedure, a t least 2 ml. of the saturated solution of the reagent is specified. If the quantity of nickel corresponds to the upper limit
The effect of 100-mg. quantities of several cations and anions on the determination of 15 y of nickel by the extraction of the nickel chelate into chloroform was investigated by Hooker and Banks. They reported serious interference only from cobalt(II), copper(II), iron(II), iron(III), and ruthenium(II1). Except for ruthenium, which is easily removed by volatilization (6),suitable masking agents or extraction procedures were proposed for the elimination of these interferences. They recommended extracting the iron(II1) as the cupferrate, complexing the cobalt as the hexacyanocobaltate(II1) ion, and complexing the copper as the dithiocyanocuprate(1) ion. Of the interferences reported by Hooker and Banks, iron, cobalt, and copper were of primary importance in this investigation. When the chelate is extracted into xylene, the interference of these substances is more serious than that reported by Hooker and Banks, probably due to increased effectiveness of the solvent. Cobalt and copper yield xylene extracts with absorbances proportional to the concentration of the interfering substances. The interference of iron is not proportional to its concentration because it is partially hydrolyzed in the aqueous phase. Because separations were considered to be impractical for large volumes of sample solution, the use of masking agents to eliminate the interference of iron was investigated, Of the complexing agents investigated-fluoride, citrate, tartrate, and o-phenanthrolinefluoride was the only reagent which, when added to the aqueous solution, would reduce the interference of iron without reducing the effectiveness of the VOL. 31, NO. 5, M A Y 1959
933
Table 111.
Effect of Interfering Ions (Nickel, 50 y; aqueous-to-organic phase ratio, 10; reagent concentration, 0.00008S~)
Interfering Ion Co(II1) Cu(I1) FeUI) FeiIII) . ,
Tolerance Limit without Masking Agent, y 2 7 , error 5% error 4 10 2 4 2 5 2 5
Masking Agent HiOz, KCN Sulfide
...
Fluoride 0 . 1 gram 0 . 2 gram 0.6 gram
Tolerance Limit with Masking Agent, y 2Yc error 5% error 100
400
800
200 400
1000
2000
tered by addition of fluoride; however, since procedures for the elimination of the interference of cobalt and copper reduce the precision of the method from 27, to ti%, the use of these procedures should be restricted to samples containing these contaminants. Because of the large volumes of solution which may be analyzed by this procedure, extreme care must be exercised to prevent contamination by copper. Maximum tolerance limits of the interferences are reported in Table 111.
Table IV.
Determination of Nickel in Diverse Samples (-411 samples extracted with 10 ml. of xylene) Vol. of
1Iaterial Tap water Demineralized \Tater 100 y Ni Reagent grade HCl
Sample Weight, Grams
+
A B
500
Aqueous Phase, MI. 600
1000
1200
300 900
loo=
1ooa
Nickel Found Y P.p.m. 4.1 0.008
APPLICATIONS
Recovery of -4dded Kickel -!
99 6.0 5 2
% 99
0.020
0.005
Sodium-potassium alloy 3 1 200 8 3 2.7 ,4 B 2 4 200 9.7 4.0 Lithium metal -4 11.2 600 1.6 0.14 B 61.0 2800 6 . 22 0.098 Sodium chloride 100 350 6.5 0.065 Potassium chloride 50 Y S i 100 500 2.1 0.021 49 98 ~Lithium chlorideb 50 y Ni 375 3000 48 96 a Evaporated to very small volumes to remove free acid. b Lithium chloride solution extracted to remove any nickel prior to nickel addition.
+
+
nickel extraction. The tolerance limits for iron in the presence of various concentrations of fluoride are reported in Table 111. As much as 100 y of cobalt produced no measurable interference when converted to the hexacyanocobaltate(II1) ion according to the procedure of Feigl and Kapulitzas (4), in which the cobalt is oxidized with peroxide in the presence of cyanide, and the excess cyanide is destroyed with formaldehyde to decompose the nickel-cyanide complex. While copper can be eliminated as the dithiocyanocuprate(1) ion, it was impossible to form this complex and the hexafluoroferrate(II1) complex in the same solution. As no satisfactory reagent ITas found for masking copper(I1) in the aqueous phase, the copper and nickel chelates were extracted into the xylene, and the interference of copper was eliminated by washing the xylene extract with 5 nil. of a slightly acidic solution of thioacetamide. The copper, along with some of the nickel, was extracted into the aqueous thioacetamide solution. The nickel was returned to the xylene phase by adding 1 ml. of reagent solution and 5 ml. of 1031 ammonium acetate and re-equilibrating the solutions. The copper remained in tho aqueous phase as suspended copper sulfide. 934
ANALYTICAL CHEMISTRY
Because this procedure introduces a significant increase in the absorbance of the extracts, samples must be compared to reference extracts that have been prepared in exactly the same manner. As much as 800 y of copper can be removed from the organic phase by a single wash with a solution of thioacetamide. However, an amount of chelating agent sufficient to complex the copper and nickel completely and to provide excess reagent must be added to the solution, because the copper competes with the nickel for the chelating agent. Similarly, iron, which cannot be removed from the extracts by washing with a neutral fluoride solution, can be complexed with fluoride after removal from the xylene by extraction with a n acidic aqueous solution. This technique must be used to remove the interference of iron from extracts of sample solutions that contain constituents which are not compatible with large amounts of fluoride ions-e.g., lithium or calcium. I n general, since iron is a common contaminant and may be present in concentrations sufficient to compete with nickel for the dioxime, it is usually desirable to complex the iron(II1) by direct addition of fluoride to the solution of the sample prior to the extraction of the copper and nickel chelates. The precision of the method is not al-
The results of analyses of diverse samples, as n-ell as recoveries of added nickel, are shonn in Table IV. For all samples escept those which contain lithium, fluoride was added to the aqueous phase to complex any iron present, following which the organic extract was washed with an acidified solution of hydrogen sulfide to remove any copper. The organic extracts from the lithium samples were m-ashed n-ith an acidified fluoride solution after separation of the phases, because addition of fluoride ion to the aqueous phase would precipitate lithium fluoride. The recovery of knon-n amounts of nickel is satisfactory, particularly in the analysis of lithium chloride. The success of this estraction at the unfavorable phase ratio of 300 to 1lends considerable credence to the validity of the results shown, despite the fact that the amount of nickel available in some of the samples was less than the recommended minimum. The nickel concentrations in the samples shown in Table IT- are reasonable values for materials of these types. LITERATURE CITED
(1) Blundy, P. D., Simpson, 11. P., “Determination of Nickel by a Solvent Extraction Method,” Atomic Energy Research Establ. Gt. Brit. CE/M-215 (1958).
(2) Bradv, 0. L., Muers, M.M.,J . Chem. Soc 125, 1599(1930). (3) Diehl, H., “Application of the Dioximes to Analytical Chemistrv,” p. 30, G. F. Smith Chemical Co., Columbus, Ohio, 1940. (4) Feigl, F., Kapulitzas, H. J., 2. anal. Chem. 82, 417 (1930). (5) Gilchrist, Raleigh, Wichers, Edward, J . Am. Chem. SOC. 57, 2565 (1935). (6) Hooker. D. F., Banks, C. V., “Preparation, Properties, and Applications of Some Substituted Alicyclic zic-Dioximes,” -4mes Laboratory, Iowa State College, 1%-597 (March 1955). (7) Seidell, A.;, “Solubilities of Organic ComDounds, vel. 11, p. 607, Van Sost‘rand, New York, 1941. RECEIVEDfor review June 19, 1958. Accepted December 1, 1958. Division of Analytical Chemistry, 134th Meeting, .4CS, Chicago, Ill., September 1958. Work carried out under contract No. IT7405-eng-26 at Oak Ridge National Laboratory, operated hy Union Carbide Suclear Co., a division of Union Carbide Corp., for the Atomic Energy Commission.
