Spectrophotometric Determination of Cobalt with 1-(2 - Pyridy lazo)-Znapht hol Separation from Interfering Ions GERALD GOLDSTEIN, D. L. MANNING, and OSCAR MENIS Analyfical Chemistry Division, Oak Ridge Nafional laboratory, Oak Ridge, Tenn. A, sensitive and selective spectrophotometric method for the determination of cobalt i s based on the reaction of cobalt(l1l) with 1-(2-pyridylazo)-2-naphthol (PAN) at a pH of 3 to 6. A chelate i s obtained which i s extractable with chloroform, giving a solution which exhibits absorbance maxima at 590 and 640 mp. Fewer interferences are encountered when the absorbance i s measured at 640 mp. Beer’s law i s obeyed over a range of 0.1 to 2.4 y of cobalt per milliliter, with a coefficient of variation of about 3%. The cobalt i s separated from thorium and most of the interfering elements such as iron, nickel, zinc, and cadmium b y an anion exchange method. Of the elements which accompany the cobalt through the anion exchange column, only copper interferes; others either do not react with PAN or can be masked by the addition of citrate prior to reacting the cobalt with PAN.
A
METHOD was developed for the de-
termination of cobalt in concentrations ranging from 0.1 to 10 p.p.m. in impure slurries of thorium oxide, for the purpose of indicating the rate of corrosion of pump components fabricated from alloys which contained cobalt. The slurries also contained corrosion products of various alloys, the constituents of which mere chiefly iron, nickel, chromium, manganese, titanium, zirconium, and molybdenum. To determine cobalt in these impure slurries, a more extensive study was carried out of the applicability of a colorimetric method for cobalt, first described by Cheng and Bray ( I ) , which is based on the absorbance of a 1-(2-pyridy1azo)2-naphthol-cobalt complex. They describe the reaction of this dye with many ions, and its application as an indicator for titrations with (ethylenedinitri1o)tetraacetic acid. A brief procedure for the colorimetric determination of zinc, copper, nickel. and cobalt is also indicated. In the present investigation, optimum conditions were established for the d e termination of microgram quantities of 192
ANALYTICAL CHEMISTRY
cobalt. Variables which were evaluated include the p H of the extraction, the choice of organic solvent to be used in the extraction, and the tolerance limits of diverse elements. Schemes were also developed for the elimination of some of the interferences. A procedure was established whereby this method could be used for the estimation of cobalt in slurries of thorium oxide and in steels. The precision of the method was also evaluated. REAGENTS
Standard cobalt solution, 1 mg. of cobalt per ml. Dissolve 4.037 grams of cobalt dichloride hexahydrate in water and dilute to 1 liter. Prepare less concentrated solutions by appropriate dilution of this stock solution. 1-(2-Pyridylazo)-2-naphthol, PAR, 0.1%. Dissolve 0.1 gram of PAN in 100 ml. of ethyl alcohol. Reagent grade PAN is available from the J. T. Baker Chemical Co. Potassium periodate, saturated aqueous solution. Ammonium acetate, 50 w./v. %. Dissolve 500 grams of ammonium acetate in water and dilute to 1 liter. Citric acid, 100 mg. per ml. Dissolve 10 grams of citric acid in water: then dilure to 100 ml. Hydrochloric acid, 10 M. Dilute 830 ml. of concentrated hvdrochloric acid to 1liter. Hydrochloric acid, 4M. Dilute 330 ml. of concentrated hvdrochloric acid to 1 liter. Anion exchange resin, Dowex 1-X10, 50-100 mesh. APPARATUS
Beckman spectrophotometer, Model DU. Beckman pH meter, Model H. Anion Exchange Column. Prepare a column, 5 cm. high X 1 cm. in diameter of Dowex l-XlO, 50-100 mesh, anion exchange resin. Place a glass wool plug on top of the column. Wash the column with about 50 ml. of 10M hydrochloric acid to condition the resin before each sample is placed on the column. After use, regenerate the column by washing it with about 50 ml. of water. Because some elements (zinc, molybdenum) , are irreversibly adsorbed,
a column should be discarded before its capacity for retention of these ions has been exceeded. RECOMMENDED PROCEDURE
Weigh a 3-gram sample of thorium oxide or a slurry of thorium oxide into a 50-ml. beaker. Dissolve the sample by heating it gently in 20 ml. of 1 to 1 nitric acid that contains a few drops of hydrofluoric acid. When the sample has dissolved, evaporate the solution to dryness. Add 10 ml. of hydrochloric acid and again evaporate to dryness. Repeat the addition of hydrochloric acid and evaporation to dryness. Dissolve the residue in 10 ml. of 1OJl hydrochloric acid. Heat the solution just to boiling. Pass the hot solution through the anion exchange column a t a flow rate of not more than 2 ml. per minute. Wash the column with 20 mi. of hot, 10M hydrochloric acid. Discard the effluent and washings. Elute the cobalt from the column with 30 ml. of 4M hydrochloric acid; then evaporate the eluate to dryness. Add 5 drops of concentrated hydrochloric acid to 15 ml. of water and dissolve the residue in this solution. Add 2 ml. of citric acid solution, 2 ml. of ammonium acetate solution, and 1 drop of a saturated solution of potassium periodate. The p H of the solution should now be about 4.5. Add 0.5 ml. of the PAN reagent solution. Transfer the solution to a 60-ml. separatory funnel and extract it with 5 ml. of chloroform, shaking for about 3 minutes. Allow about 15 minutes for the phases to separate. Drain the chloroform phase into a 1-cm. cell, after which measure the absorbance us. a reagent blank, prepared by the same procedure, a t a wave length of 640 mp, EXPERIMENTAL
Extractant. Tests were made to find a suitable organic solvent for the It was cobalt(II1)-PAN chelate. established that the chelate was rapidly and quantitatively extracted into chloroform with one equilibration and that the extraction was less rapid in carbon tetrachloride, amyl alcohol, or xylene. In some cases, with the latter solvents, a precipitate was formed a t the organic-aqueous interface. Under these conditions, the organic phase was USU-
ally turbid and it was not, therefore, suitable for use in making absorbance measurements without filtration. Absorbance Spectra. The absorption spectra of the chloroform extracts of the P A S chelates of cobalt, nickel, copper, and iron were measured 6s. a reagent blank; these are presented in Figure 1. For the cobalt(111) chelate, absorption peaks occur a t 590 and 640 mp. Although the absorbance a t 590 mp is slightly greater than a t 640 mp, the latter wave length was chosen to make absorbance measurements, because fewer elements interfere a t this wave length. Only a few metals, such as copper and iron, exhibit appreciable absorbance a t 640 mp. The decrease in sensitivity as compared to that attained by making the absorbance measurements a t 590 mp is slight. Concentration of PAN. The results of tests which were made t o determine the quantity of reagent necessary t o produce maximum absorbance reTealed that 0.2 ml. of a 0.1% solution of PAN is sufficient to produce maximum absorbance for test portions which contained up t o 12 y of cobalt. Sometimes other metals which form chelates with PAN interfere by consuming the reagent and causing the results for cobalt to be low, even though their chelates do not absorb light a t 640 mp. It is, therefore, recommended that 0.5 ml. of the chromogenic reagent be used rather than the minimum amount necessary for complexing the cobalt. Effect of pH. Because information n-as not available on the effect of pH on the formation and extractability of the cobalt(II1)-PAN chelate, the effect of this variable was evaluated. For this study, solutions were prepared that contained 5 y of cobalt(11). The cobalt was oxidized to the trivalent state with potassium periodate, after which the pH of the solutions was adjusted to different values by the addition of an acetate buffer solution. A 0.5-ml. portion of the PAN reagent n-as then added and the solutions were mtracted with 5 ml. of chloroform. The absorbance of the chloroform extracts was measured versus a blank which had been extracted a t the same p H as the sample solution. From the results, which are set forth in Figure 2, it was established that the amount of cobalt-PAN complex formed and extracted is constant over a pH range of 3 to 6. At higher and lower pH values, the absorbance of the complex decreases, This indicates that either the color reaction does not proceed to completion or the cobalt-PAN complex is not quantitatively extracted into the chloroform. As a consequence of these tests, a pH of about 4.5 was selected for use in the procedure. Adherence to Beer's Law. The ab-
Figure 1. Absorption spectra of metal-PAN chelates in chloroform Metal ion. 10 y Volume. 5 ml.
\;.E
04
I
I
I
.E..'*.
c*
I Table 1. Effect of Certain Ions on Determination of Cobalt y
Quantity, XIg. 1
Ion Ti+4 ?\10+6 ~ 1 + 3
-1
Cr+3 Cr+6 Zr+4 W+6 Th+4 Sn+4 Sh+6 Ri+3 R h + 2
U +& y +I
sorbance of the cobalt(II1)-PAN chelate extracts adheres to Beer's law over a range of 0.5 to 12 y of cobalt in a volume of 5 ml. The optimum concentration range, established by the method of Ringbom (4), is 0.6 to 2 y per ml. Over that range, the coefficient of variation is about 3%. The molar absorbance index is approximately 20,000 for measurements made a t 640 mp, and about 25,000 at 590 mp, The color of the extract is stable for a t least 24 hours. The mole ratio of PAN to cobalt was found to be 2 t o 1 by the slope-ratio method (2). This ratio is in agreement with the work of Cheng and Bray ( 1 ) . Effect of Foreign Ions. The effect of impurities on the determination of cobalt was evaluated (Table I ) , Solutions were prepared that contained 3 y of cobalt plus the contaminant under test in a volume of about 15 ml. The test solutions were processed in accordance with the recommended procedure, except that all the steps prior to the addition of 2 ml. of citric acid n'ere omitted. Ions which interfere seriously are copper(II), when the copper-cobalt weight ratio exceeds 1 to 1, and nickel(II), when the nickelcobalt ratio exceeds 10 to 1. Substances which form colored PAN chelates and interfere when present in milligram quantities are iron(III), zinc(II), and cadmium(I1). Lesser quantities are readily masked by citrate. Qualitative tests by Cheng and Bray (1) indicate that many other cationic and anionic
Fpo4-3 0 3
Fe+3
0 1
Znf2
Cd+*
(i
Recovered 3.11 2.93 2.93 2.99 2.98 3.02 2 96 3 03 8 03 2 92 2.90 3.11 2 90 2 96 x 10 3 07 3 06 5 38b 3 15 4 O P
3 06 3 58* Sif2 3 65 Cobalt present 3.00, y . S o citrate added.
Difference 0.11 -0.07 -0,07 -0.01 -0 02 0.02 -0 04 0 03 0 03 -0 08 -0 10 0 11 -0 10 -0 04 0 10 0 07 0 06 2 38 0 15 1.07 0 06 0 58 0.65
substances such as arsenic, beryllium, ruthenium, and the alkaline earth elements, which were not tested, will not interfere. Separation of Cobalt by Anion Exchange. T o utilize PAN for the determination of microgram quantities of cobalt in thorium oxide, it u-as considered essential to devise a scheme which could be used not only to concentrate the cobalt, but also to separate it from the major portion of the thorium as well as from the interfering substances such as iron, nickel, zinc, and cadmium. This was accomplished by the application of an anion exchange separation in which elements are selectively adsorbed from concentrated hydrochloric acid solutions onto a strongbase, anion exchange resin such as Dowex l-Xl0. Kraus and Nelson (3) have investigated the separation of the majority of elements by this means. These findings with respect to the distribution of the elements of interest in the determination of cobalt are summarized in Figure 3. VOL 31, NO. 2, FEBRUARY 1959
193
A study was also made of the variables involved in the quantitative separation of cobalt in low concentrations from thorium and other elements by means of anion exchange resin. The effects of column dimensions, flow rate, and acid concentration of the sample solution, acid concentration of eluate, and temperature of the sample solution, wash solution, and eluate were evaluated. A short resin column, approximately 5 cm. in length, was found to be the best for the separation of large amounts of thorium, which is not adsorbed. The sample solution was made 1OM with respect to hydrochloric acid because cobalt is adsorbed most strongly from a solution of this acid concentration ( 3 ) . il 4M hydrochloric acid solution was chosen as the eluate. Although cobalt can be eluted more rapidly with more dilute hydrochloric acid solutions, the quantity of iron which is eluted is also increased. When the column separation was carried out at 25' C., appro.rimately 95% of the cobalt was recoTered. The results of a detailed study of the effect of temperature on column operation and the recovery of cobalt are shown in Figure 4. I n these experiments, 10 y of cobalt in 10 ml. of 1OM hydrochloric acid was placed on the column and, after the column had been washed with 20 ml. of 10M hydrochloric acid solution, the cobalt was eluted with 4M hydrochloric acid. The eluate was collected in IO-ml. increments and each increment mas analyzed for cobalt. Complete recovery of cobalt was attained only when the temperature of the sample solution was about 95' C. at the time it was placed on the column. After the sample solution had been placed in the column, the column was washed with 10M hydrochloric acid which was heated to approximately 90" C. The flow rate of the wash solution was maintained at 2 ml. per minute or less, because, at greater flow rates, some cobalt was eluted. Following the mashing step, the cobalt was removed from the resin with a 4-Vhydrochloric acid solution which was at room temperature.
Of the metals which may accompany the cobalt through the separation procedure, traces of iron can be masked by citrate ( I ) , while uranium, titanium, vanadium, and zirconium do not interfere in the determination of cobalt. Citric acid up to 300 mg. has no effect on the formation of the cobalt-PAN chelate, and this quantity is enough t o mask up to 1 mg. of iron, zinc, and cadmium. Of the elements not removed by the anion exchange column or masked by citrate, only copper is a serious interference. Copper was not present in the materials for which the procedure was designed; hence it was not considered in the separation scheme. It may be removed in a separate step, such as by 194
ANALYTICAL CHEMISTRY
, Sample. Th+4, C O + ~Fe+3
X + 2 Cr+3 U+6,Zr+4,Al+3, !l?i +4, AIn +2, 'V +6, Mo +e, Zn +2, Cd t2
Dowes 1-XlO Column, lOhl HC1 (95' C. i
I
I--
I
Not adsorbed
Adsorbed
Washed with 1OM HC1 (90' C.) I Eluted with 4 h HC1 (25" C.) I
Edited
Co +2, Zr +4, V +6
(traces of Fe +3, Ti +4, U +6) Figure 3.
hdsoibed Fe+3, U+6, RIo+6, Zn + 2 1 Cd + 2
Separation of cobalt by anion exchange
electrolytic deposition, prior to separation of cobalt by the anion exchange procedure. Application. The proposed method was tested by applying it t o the determination of cobalt in synthetic slurries of thorium oxide, slurries submitted t o the laboratory for analysis, and three National Bureau of Standards standard samples. Test portions of synthetic slurries which contained a total of 2.00, 4.00, and 6.00 y of cobalt and 1.5 grams of thorium oxide, 3 mg. of iron(III), 200 y each of
.&
Figure 4. Effect of temperature on recovery of cobalt after anion exchange separation Table II. Test Results for Cobalt in Synthetic Slurries of Thorium Oxide Synthetic slurry. 1.5 grams of Tho,, 3 mg. of Fefa, 200 y each of Ni+2and Cr+6, 50 y each of Mn +*,A1 Zn +*, Zr +4J U +6,
Ti+',
y
Present Found 2 05 2 00 3 86 5 85
Differ- Co Recovery, c/G : ence 0 05 100 -0 14 -0 15
96 98
Table 111. Determination of Cobalt in Slurries of Thorium Oxide Weight sample, 2 . i to 3.2 grams
co-4dded, Slurry
y
A
3 00
B
3 00
C
3 00
D
Cobalt Found Total, y Slurry, ylgram 3 €11 0 23O 0 88 0 29 4 2 5 2 2 2
99 05 88 9G
67 93 2 07 2 82
0 0 0 0 0 0 0
64* 65 9Ta 98 9t 9s 94 0 95 10
Coefficient of variation, % (baaed on duplicate determinations) Corrected for standard addition of 3.00 y of cobalt. 5
Sample and wash solution a t 90' C.
B. Sample a t 90' C. C. D.
Ail solutions a t 25' C. All solutions at 90' C.
Mo+6, total in sample portion
Cobalt,
4.00 6.00
A.
nickel(I1) and chromium(VI), and 50 y each of manganese(II), aluminum(III), zinc(II), zirconium(IV), uranium(V1) , titanium(1V) , and molybdenum(V1) were analyzed by the recommended pro- , cedure (Table 11). The same procedure was used to estimate the cobalt content of four process slurries. Three of those were analyzed "as received" and also after the addition of a standard amount, 3.00 y, of cobalt. The fourth sample was analyzed in quadruplicate without the addition of cobalt (Table 111). National Bureau of Standards standard samples 126a and 161 were also analyzed by the recommended procedure, including the separation step. A 0.5-gram portion was dissolved in 15 ml. of aqua regia and diluted to 1 liter, after which suitable aliquots containing between 3 and 10 y of cobalt were withdrawn for analysis. A portion of NBS standard sample 153 was analyzed in a similar manner, except that no separation was made prior to the development of the color and the measurement of absorbance.
The results obtained in the analysis of XBS samples are set forth jn Table
Table IV.
117.
The results given in Table I1 demonstrate that this method can be applied satisfactorily t o the estimation of microgram quantities of cobalt in slurries of thorium oxide which contain large quantities of thorium and lesser amounts of nickel, chromium, iron, and other impurities. The coefficient of variation is less than 5%. Even for cobalt in concentrations of 0.3 to 1 y per gram in contaminated slurries of thorium oxide, the coefficient of variation is no greater than 10% (Table 111). It is unnecessary to use a standard addition technique. No significant difference in test results (Table 111) is to be observed between results obtained with and without the use of this technique. This method is also suitable for the determination of cobalt in steels and other alloys as is evidenced by the results for the analysis of NBS standard samples given in Table IV. Apparently, no prior separation by means of an anion exchange resin is necessary if the ratio of interferences to cobalt is below the tolerance limits, or if the foreign ions can be masked with citrate or do not form che-
Cobalt in NBS Standard Samples
Sample SBS KO 153
TS Pe Co-&Io-W steel
Certified Value 8 45
161
Si-Cr casting alloy
0 4i
126a
High-nickel steel
0 30
lates with PAN. Even though the Separation step was omitted in the analysis of NBS sample 153, the results are in excellent agreement with the certified value for cobalt. I n all probability, this method is also applicable to the determination of cobalt in a variety of sample types which were not tested, particularly t o materials which contain, as their primary constituents, metals which are not adsorbed on a Dowex-1 resin, such as aluminum, the alkali, and alkaline-earth metals. ACKNOWLEDGMENT
The authors acknowledge the assistapce of H. P. House and hl. A. Marler in the preparation of this manuscript.
rl V
Found 8 43 8 49 0 48 0 50 0 31 0 30
Difference Difference, yo -0 02
n
04 0 01 o n3 0 01
0 1 4
2
0 00 LITERATURE CITED
(1) Cheng, K. L., Bray, R. H., XSAL. CHEJI.27, 782 (1955).
(2) Harvey, A4.E., Manning, D. L., J . Am. Chem. SOC.72, 6688 (1950). (3) Kraus, K. A., Selson, F., International Conference on Peaceful Uses of Atomic Energy, Paper 837 (U.S.A.), August 1955. (4) Ringbom, A , , 2. anal. Chem. 115, 322 (1939). RECEIVEDfor review August 4, 1958. Accepted October 9, 1958. Division of Analytical Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958. Fork carried out under Contract No. W-7405-eng-26 at Oak Ridge National Laboratory, operated by Union Carbide Nuclear Go., division of Union Cftrbide Corp., for the Atomic Energy Commission.
Ultraviolet Spectrophotometric Determination of Thorium with 2,4-Dichlorophenoxyacetic Acid SACHINDRA KUMAR DATTA1 Chemisfry Deparfrnenf, Darjeeling Government College, Darjeeling, India
b The thorium salt of 2,4-dichlorophenoxyacetic acid is soluble in an aqueous ammonium carbonate solution, The solution of this saltlike compound shows maximum absorbance a t a wave length of 230 rnp and follows Beer's law. The spectrophotometric determination of thorium is based on the measurement of the absorbance of this solution. The sensitivity of the method is 0.0 19 y of thorium per sq. cm., but the accuracy is poor in samples containing less than 2 mg. of thorium. The common metals do not interfere, but strong interference is exhibited b y iron, cerium(lV), and zirconium. The optimum results are obtained in a concentration range between 2 and 1 4 mg. of thorium per liter.
T
was determined gravimetrically by Datta and Banerjee (3) using 2,4-dichlorophenoxyacetic acid (2,4-D). This reagent proved useful for the HoRImf
separation of thorium from the rare earths, zirconium, titanium, iron (4),and uranium ( 5 ) . It was also employed for the recovery of thorium from industrial wastes, like used gas mantles and tungsten filaments ( 2 ) . In these methods, thorium is precipitated with 2,4-D as the triphenoxate and the precipitate is ignited and weighed as thoria. The thorium derivative of this reagent is highly soluble in an aqueous solution of ammonium carbonate, a soluble saltlike compound probably being formed in the reaction. The solution of this thorium salt in ammonium carbonate exhibits absorption in the ultraviolet region of the spectrum. The spectral curve indicates maximum absorbance a t a wave length of 230 mp. The absorbance is probably caused by the phenyl group of 2,4-D in the thorium salt, as the ammonium salt of 2,4-D produces a very similar spectrum, although its maximum absorbance occurred a t 285 to 286 mp. Thorium in ammonium carbonate solution shows
an entirely different spectrum. The present paper examines the possibility of using spectrophotometric study in the determination of small amounts of thorium. REAGENTS AND APPARATUS
Reagents. 2,4-Dichlorophenoxyacetic acid was prepared and purified as before ( 3 ) . A 1% solution in water was prepared by heating the constituents. A stock solution of thorium nitrate was prepared from thorium nitrate tetrahydrate (analytical reagent, hlerck) and standardized with oxalic acid. Ammonium carbonate solutions of various strengths were prepared and estimated with standard sulfuric acid. The solutions of other metal salts, all analytical reagent quality, were pre1 Present address, Department of Chemistry, Victoria College, Cooch Behar, West Bengal, India.
VOL. 31, NO. 2, FEBRUARY 1959
195
