1680
ANALYTICAL CHEMISTRY
ican crudes are variable in composition. A rapid method of identification of the source of crudes from known authentics should be of value. A large number of tanker shipments of manifested crudes, imported through the Port of Baltimore over a period of about 3 years, were compared by means of the spot chromatogram method. The chromatograms from known crudes invariably gave identical radial fluorescence patterns. These crudes came from many sources, including Saudi Arabia, Kuwait, Iran, Mexico, Venezuela, Romania, Dutch East Indies, and Dutch West Indies. Residual oils manifested as fuel oils differed chromatographically from the crudes imported from the same areas. The residual oils were more highly colored or carbonized and showed more complex ring structures than the corresponding crudes. Semicrude distilled fractions, like Diesel fuel oils, lubricating oils, and paraffin oils, were chromatographed. The patterns were simpler than those formed by crudes, residual fuel oils, and mixed bunker oil types. The fluorescenre of the lubricating oils was even more pronounced than that of the crudes and fuel oils. In a series of lubricating oils from the same manufacturer, the fluorescence color varied with the viscosity of the oil.
Distilled oils showed distinctive patterns which lacked thc central spot present in most crudes and fuel oils investigated. For comparison other petroleum products-ie., paraffins, petrolatums, medicinal oils, kerosenes, cut-back asphalts, etc.-rerc chromatographed. Highly purified products, as indicated by their designation and water-white color, showed no appreciable fluorescence. Semirefined products-Le., crude paraffins, colored petrolatums, and Diesel fuels-shorred distinctive fluorescence depending on the degree of refinement. Coal tars, wood tars, and pitches showed characteristic patterns and fluorescence. ACKNON L E D G V E h T
The author wishes to acknowledge the assistance of It. S. Gibbs, principal chemist, Norfolk Xaval Shipyard, Captain G. E. McCabe, U. S. Coast Guard, Leonard Haddaway, chemist, C . S. Treasury Department; IT. p. Lovelace, Metallurgical Laboratory, Korfolk Naval Shipyard, who is responsible for the photographs of the spot chromatograms, and his other Rsiociates vcho have contributed to this method. RECEIJED .\larch 6,1951
Spectrophotometric Determination of Cobalt as Tetraphenylarsonium Cobaltothiocyanate HAROLD E. AFFSPRUNG', NEDRA A. BARNES2,.AND HERBERT A. POTRATZ Washington University, St. Louis, Mo.
In a search for a simple method for determining traces of cobalt in uranium materials, a procedure was devised which involves separation of cobalt from aqueous solution by chloroform extraction of tetraphenylarsonium cobaltothiocyanate. The cobalt content of the chloroform extract is determined spectrophotometrically. Experiments are here reported in which this procedure was studied with respect to its applicability to other materials, particu-
A"L
W B E R of pi ocedures for the colorimetric determination of cobalt as cobaltothiocyanate complex have been published. Young and Hall (20) surveyed the early literature on this subject, and recommend a method 11hich involves extraction of the complex into an isoamyl alcohol-ether mixture. Bayliss and Pickering (6)have determined the optimum conditions for this extraction. Putsche and Malooly ( I d ) propose a procedure in which the color of the cobalt complex is developed with acetone after removal of iron M-ith zinc oxide. Stengel (16) recommends a eimilar procedure. Kitson (10) also uses acetone for color development, but prevents iron interference by addition of chlorostannous acid. Cri (18)studied the conditions for photometric determination of cobalt by the thiocyanate method in ethyl alcohol-water mi\tures. He has sho-n that if sufficient thiocyanate is added the cobalt is practically quantitatively converted into a complev compound, the color intensity being the same when working with acetone or with ethyl alcohol. The procedure given here was suggested by Potratz and Rosen (11, 14). It is based on the observation that certain aryl onium ions react with cobaltous thiocyanate solutions to give bright blue, water-insoluble, chloroform-soluble compounds. Chloroform extraction of one of these compounds, tetraphenylarsonium 1 Present address, Chemistry Department. Ivf issourl Valle, College, Marshall, hfo. 2 Present address, Los Alamos, K R I
larly alloy steels and nickel-copper alloys. IXrections are given for analyzing these alloys without prior separation of copper, iron, nickel, chromium, and other constituents of the alloy solutions. The method is rapid and convenient. Results obtained in the determination of cobalt in National Bureau of Standards alloy steel samples 101c, 126a, 153, nickel-chromium alloy 161, and nickel-copper alloy 162 are reported. mbaltothioryanate, serves as the method for separating t i n ( ('s of cobalt from aqueous solutions. The cobalt content of the chloroform extract is determined spectrophotometrically. The procedure may be applied directly to the determination of robalt in cobalt-copper alloys and alloy steels. N o prior separation of iron, nickel, or copper is necessary. The conditions for eltraction are not critical and the resultant chloroform extracts :ire stable over a considerable peiiod of time. Khile the method a- described makes use of a spectrophotometer, measurpments could be made 15 ith a filter photometer or by visual color rompai iioii. APPARATUS AUD W i T E R I A L S
Absorption spectra were measured a i t h d Beckman AIodcl DU spectrophotometer using Corex cells of I-em. light path. A Beckman Model G p H meter was used for the p H measurements. Cobalt metal sponge (spectrographically standardized, 99 99 % pure), Jarrell-Ash Co. Chloroform, analytical reagent quality, containing approximately 0.5% ethyl alcohol as preservative. Tetraphenylarsonium chloride, Hach Chemical Co. Vanadium pentoxide, c.P., Eimer & Amend. All other chemicals were analytical reagent quality. Redistilled water from an all-glass (borosilicate glass) still was used for the preparation of the aqueous solutions. -4primary standard cobalt solution was pre ared by dissolving 11.91 mg. of Jarrell-Ash, 99,99% cobalt in d i k t e nitric arid and diluting to 100.0 ml. An additional stock solution was made by dissolving 4.945 grams of cobalt(I1) nitrate hexahydrate in water, adding 1 ml. of concentrated nitric acid, and diluting to 1 liter.
V O L U M E 23, NO. 11, N O V E M B E R 1 9 5 1 The final cobalt concentration of this stock solution as determined by spectrophotometric comparison with the standard solution by the procedure here described was 0.993 mg. per ml., which corresponds to 20.08y0 cobalt in the salt. The calculated is R20.25% ~O cobalt. Suhstandards \yere value for C O ( N O ~ ) ~ . ~ prepared by volumetric dilution of the stock solution.
1681 solutions is attributable to [Co(SCS),]-- ( 1 , S, 6-9, 15, 17). From the work of a number of investigators (6, 9) it would seem that cobalt(I1) in the "blue" type halide and thiocyanate complexes is in all cases tetracoordinated, a maximum of four halide or thiocyanate groups being present in the complex. Solvent molecules may appear in the inner shell of the complex ion but only when less than four halide atoms or thiocyanate groups are present. In conflict a-ith this view are recent observations by West and de I-ries (19). They present evidence for the presence of a hexathiocyanatocohaltate(I1) ion in both aqueous and alcohol\vater solutions. The formation of the blue color they attribute to selective attraction of nonaqueous molecules by this complex. C 4 LIBRATlOY
2ol IO
I
5"U *
I
1
1
5 5 L,
600
650
\FSVE LESCTH-IIILLIIllCRONS
Figiirc 1. Spectral Transmittancy Curve of Tetraphrnylarsonium Cobaltothiocy-anate in Chloroform 0.0198 m g . of cobalt per m l . Reckman 3lodel DE specrrophotornctcr 1-cm. light p a t h
,ABSOKI"rlOh SPECTRUM O F TETRAPAENYLARSONI~JRI (:OBA LTOTHIOCYANATE IN CHLOROFORM SOLUTION
A portion of the standard solution containing 0.496 nig. of colwlt was introduced into a small separatory funnel, and 5 nil. of ,50yo itqueous ammonium thiocyanate and 15 drops of 0.05 molar aqueous tetraphenylarsonium chloride solution were added. A light blue precipitate, a mixture of tetraphenylarsoniuni thiocyanate and tetraphenylarsonium cobaltothiocyanate, was obshed a t this point. The solution containing the precipitate, was then shaken with 8 nil. of chloroform and the chloroform layer was filtered off through a small paper (to remove suspended water droplets) into a 25-ml. volumetric flask. The extr.:rctioii was repeated two additional times using 5 drops of tetraphenylarsonium chloride and from 5 to 8 ml. of chloroform each tinir. The third time the precipitate was white (tetraphenglarsoiiiuiii thiocyanate) and the chloroform extract was colorleP The chloroform solution was diluted to the mark and the tran inittancy with reference to chloroform was determined at 5 mp intervals throughout the spectral range 500 to 700 mp. Measurrnirrit~ were made a t room temperature (approximatelv 25" C'.).
The spwt ral transniittancny curve, obtained with the Beckman Mntlel DC spectrophotometer, operated a t constant, sensitivit?, setting, is shown in Figure 1. A blank prepared by extracting ivith chloroform an aqueous solution n-hich contained all the reagc'iits but no added cobalt was run simultaneously. The blank showed no absorption in the region 500 to 700 mp. I3arries ( 4 ) has compared t,he absorption spectrum of an isoamyl alcohol-ether extract of ammonium cobaltothiocyanate with the spectra of chloroform extracts of triphenylsulfonium cob:iltothiucyanate, rn-xylyldiphenylsulfonium cobaltothiocyanate, trii)Iiem).lfielenonium cobaltothiocyanate, and tetraphenylarsoIlium cobaltothioeyanate. In the region 500 to 700 mp the spectra of all these solutions were found to be identical. m-Xylyldipheiiylsulfonium cobaltothiocyanate (11), precipitated from a11 aqueous cobaltothiocyanate solution, has been found to have the cornposition, [C8Ho(C6H5)S1&o(SC?;)4. These observations inciic,:rir th:it the blue color of the noriac~ucouscobaltothiocyanate
Solutions of known cobalt content were extracted by the foregoing procedure. Transmittancies of the chloroform extracts were measured at 620 me, and concentrations and extinction coefficient values were calculated on the assumption that extraction of cobalt was complete. The results are shown graphically in Figure 2, in which the logarithm of the cobalt concentration in micrograms per milliliter has been plotted against the per cent absorptance (100 - per cent transmittancy), according to the method of Ringbom ( 1 3 ) . The advantages of this type of plot have been dkcussed by Ayres ( 2 ) . The portion of the curve lying hetween 30 and 80% absorptancy is nearly a straight line, and this region, from a concentration of 5 to 25 micrograms of cobalt per milliliter, is the optimum range for measurements. Over this range, values of the specific extinction coefficient, k of the BeerLambert equation, log Io/Z = kcl, average 30.1. (The length of light path, I , is in centimeters; the concentration, c, is in gramq of c*obaltper liter.) The chloroform extracts of tetraphenylarsonium cobaltothiocyanate are stable. One such extract was allowed to stand in a locker for 60 days; there mas no change in transmittancy. The
100 I
I
2
4
8
6
10
C O N C E Y T R A T I O ~OF co
Figure
2.
40
20
60
. \IICROGRAMS/ML.
Standard Curve for Tetraphenylarsonium Cobaltothiocyanate in Chloroform
Beckman Model DU spectrophotometer a t 620 ms I-om. l i g h t p a t h
Table I.
Effect of pH on Extraction Cobalt Obtained, /
% Extracted 66 91 96 96 97 94 84
ANALYTICAL CHEMISTRY
1682 T a b l e 11. Effect of Diverse Ions Cobalt Present,
Foreign Substance Added
Amount
+ +
Error,
300 mg. C1 1 g. N H i F 300 mg. NO3 300 mg. SO4 50 mg. P O I 50 mg. S t 0 3 g. K I a0 mg. Cr 50 mg. M n 50 nig. Ni 50 mg. Fe
198 198 198 198 198 19s 198 198 I98 198 198
192 -6 195 -3 204 4-6 196 -2 194 -4 194 -4 0 198 198 0 195 -3 -1 197 Complete
50 nip. Fe [ 1 g. XH4F ,
198
195
-3
50 mg. Cu
198
340
+142
50 mg. RIo 50 mg. ZIo 500 mg.
198
66
-132
198
194
;
Fe(?;Oajn -t XHaF Cu(iv01)z
Cobalt Found,
Y
~~~
__
interferenre . .. ... ......
Cu(NO3jt
KI
NarSIOa (NH4)aM07Oz4
+
(~H4)ahloiOzr NHiF
+
VmOs VtOa
+
VzOa
+ 2 nil. 1M HCl
2 ml. 1M HCl 2 ml. 1 M HC1 SH4F
Fe(NHr)z(SOdn NHIF
+
+
50 mg. VrOs 50 nig. VZOS' 1 5 g. XHdFJ 50 mg. VzOs I 350 mg. Fe(ivH4jz-1 (Sod?6H20
1 5g.
"4I.
-4
198
210
+12
198
190
-8
238
236
-2
j
same sample then stood in diffuse sunlight for 15 days nith no observable change. .4 sin ilar sample developed a green color after standing in direct sunlight for 25 days. Itc~peatedobservations indicate no rhange in \\-C(%;.P if t h e solutioii is hc,pt out of direct sunlight. EFFECT O F HYDHOGEh IOY COhCENTRATION
The effect of hydrogel1 ion concentration upon the efficiency of e\traction was investigated by the following procedure: Two milliliters of robalt stoch solution were added to 10 ml. of 50Yu ammonium thiocyanate; the solution was diluted and the p H was then adjusted with hydrochloric acid or ammonium hydroxide. The solution was finally diluted to 50 ml. and a portion was removed for the p H measurement. A second portion of the solution, 10 ml. in volume, was shaken x i t h 15 drops of tetraphenylarsonium chloride solution and 10 ml. of chloroform. Five milliliters of this extract were diluted to 25 ml. with chloroform to prepare the solution for spectrophotometric analysis. The results, shown in Table I, indicate that the hydrogen ion concentration is not critical. I n the p H range 1.9 to 6.8 over 90% of the cobalt is removed in a single extraction. INTERFERENCES
There are two ways in which interferences in the determination of cobalt by the thiocyanate-tetraphenylarsonium-chloroformextraction procedure are most frequently encountered. 1. The interfering ion reacts with SCN- and tetraphenylarsonium ion to give a colored, chloroform-soluble com ound. 2. The interfering ion oxidizes SCN- to colored, ckloroformsoluble substances of undetermined composition.
Ferric and cupric ions may be cited as examples of interferences of the first type. These ions form colored complexes with thiocyanate that extract (as tetraphenylarsonium compounds) into chloroform, thus interfering with the subsequent photometric analysis. Pervanadate and certain other ions will oxidize thiocyanate. At least one of the osidation products of the thiocyanate ion is colored and, under the conditions required for the analysis, extracts into chloroform, thereby interfering with the transmittancy measurements. The commoner interfering substances are considered below. Additional information concerning interferences is given in an earlier publication (11). Iron. Iron(I1) does not interfere. Iron(III), however,
forms the familiar blood-red thiocyanate complex or complexes which extract into chloroform and obscure the color of the cobalt complex. The interference caused by iron(II1) may be completely removed by addition of excess ammonium fluoride. The fluoride ion reacts to give hexafluoferrate( 111)and other colorless fluoride complexes which do not interfere with the transmittancy measurements. In the determination of traces of cobalt in steels, about 1 gram of ammonium fluoride was found to be sufficient for each 0.2 gram of the steel sample. The amount of ammonium fluoride needed varies with the sample, so the amount necessary in any particular case must be found by experiment. I n any case sufficient ammonium fluoride must be added to remove completely the red color of the iron(II1) thiocyanate complex. Copper. The copper(I1) ion interferes by reacting to give a colored, extractable thiocyanate complex. This interference is most easily avoided by reducing copper(I1) to the plus one oxidation state with iodide ion. Traces of cobalt in the presence of large amounts of copper may be successfully extracted if the copp O I ) ion is first reduced in this manner. I n the presence of more than 0.1% of copper, solid potassium iodide is added until the precipitate of copper(1) iodide dissolves to form the iodocuprate(1) ion, which is colorless. Ferric ion if present is reduced siinultaneously. I n the analysis of steel samples containing less than 0.1% of copper the interference by copper(I1) thiocyanate complps was so slight that it could not be detected. The large excess of fluoride added to complex the iron(II1) possibly aided in preventing copper interference. In the reduction of copper(I1) with potassium iodide the free iodine formed reacts with the excess of iodide to form the highly colored triiodide ion which may be extracted into chloroform as tetraphenylarsonium triiodide. To prevent this interference the triiodide ion is reduced with thiosulfate. If copper is present in large amount, a large excess of iodide will be necessary to keep it in solution. Because iodide ion in acidic solutions is oxidized to iodine by air, difficulty may be encountered if the acidity of the solution is not controlled. I n analyzing material of high copper content the p H of the solution should be 4 or greater a t the time of the extraction. For efficient removal of tetraphenylarsonium cobaltothiocyanate from solutions containing an excess of iodide ion, a large excess of ammonium thiocyanate was found to be necessary. Ten milliliters of 50% ammonium thiocyanat,e, added to about 25 ml. of t,he prepared solution, gave the necessary excess for efficient extraction. Molybdenum. Molybdenum(V) interferes by forming a deep red extractable thiocyanate complex. Molybdenum(V1) also interferes, but the cause of the interference is uncertain. A substance is formed which is extracted with tetraphenylarsonium ion and chloroform, and which gives a yellow color to the extract. The addition of ammonium fluoride was found to mask the molybdenum interferences completely. Vanadium. Pervanadate ion oxidizes thiocyanate to a yellow substance which is extracted into chloroform. The addition of a very large excess of ammonium fluoride prevents this. The large excess of ammonium fluoride required, however, decreases the efficiency of the cobalt extraction (see Table 11). A better method of eliminating vanadium interference is reduction of plus five vanadium to the plus four osidation state by addition of iron(I1). Ammonium fluoride is then added to prevent iron( 111)interference. I n the presence of trace amounts of vanadium in steels the reduction was found to be unnccrssary. The ammonium fluoride required to remove the interference of iron(II1) was also sufficient to prevent vanadium(V) interference. Nitric Acid. Sitric acid if present in high concentration oxidizes thiocyanate to give a chloroform-soluble, pink substance as one of the products. It is recommended that solutions obtained from dissolving alloy samples in nitric acid be fumed with sulfuric acid t o remove most of the volatile oxidizing materials. Solids. Large amounts of solid material tend t o emulsify the
V O L U M E 2 3 , NO. 11, N O V E M B E R 1 9 5 1
1683
chloroform, making the extraction difficult. Chromium, for example, if present in large amounts may partially precipitate as chromium(II1) fluoride and cause trouble. The extraction may usually be carried out successfully, however, if more water is added to increase the volume of the aqueous phase. Residues of molybdic or tungstic acid, which appeared during solution of some steel samples, were allowed to settle in the volumetric flask when the sample solutions were made up and were ignored in \ubsequent operations. In making the interference studies, the results of which are shown in Table 11, the procedure was as follows:
h weighed quantity of the substance or substances to be investigated was dissolved in 25 ml. of a cobalt nitrate solution containing 198 micrograms of cobalt. The solution was then extracted with thiocyanate tetraphenylarsonium chloride and chloroform. The extract was diluted with chloroform to 25 ml. and the transmittancy was measured at 620 mp.
Table 111. Test of Method with Bureau of Standards Samoles Sample and X.B.S. Certificate Values
2.
Per Cent Found ~______~ 1
Ni steel 126A Co 0.30, Xi 35.89, C 0.056, M n 0.414, Si 0 . 3 0 0.194, Cu0.092, Cr0.054 C o , Mo, W steel 153 Co8.45. C 0 . 8 6 4 , M n 0.219, P 0.025, S 8 . 4 5 0.008, Si 0.187, Cu 0.099, Xi 0.107, Cr 4.14,V2.04,hlo8.38,W1.58 Ni. Cr. alloy 161 Co 0.47, C 0.34, RIn 1.29. P 0.012, S 0 . 4 8 0.005, Si 1.56, Cu 0 . 0 4 , SI 64.3, C r 16.9, V0.03, hI00.005, Fe 1.5.0 SI,Cu alloy 162 Co 0.54. Xi 66.38, Cu 28.93. .\In 2.34, 0 53 Si 0.67, Fe 0.34, C r 0.24. AI 0.23, Ti 0 20, c o . 1 1 , s o . 0 0 2
3.
4.
3.
2
3
4
0.30
0.29
0 30
8.42
8.42
8.44
0.48
0.48
...
0 52
0 62
0 32
DETERMINATION O F COBALT
Procedure in Absence of Copper and Vanadium. Introduce a suitable aliquot of the solution to be analyzed into a small separatory funnel. Add water if necessary to increase the volume to about 25 ml. Add 2 to 5 ml. of 50% ammonium thiocyanate solution. Add solid ammonium fluoride until the red color of the ferrithiocyanate complex is completely removed and then add 200 to 300 mg. in excess. Add 15 drops of 0.05 N aqueous tetraphenylarsonium chloride solution and shake the resulting precipitate and solution with 8 to 10 ml. of chloroform. Tranvfer the chloroform layer to a 25-ml. volumetric flask. Repeat the extraction two times, adding each time 5 drops of tetraphenylarsonium chloride and 5 ml. of chloroform. Dilute t,he m1nt)iried extracts to volume with chloroform and measure the tranamittancy against a chloroform extract of the reagents or :igainst pure chloroform if the reagent blank has been shown to kw riegligible. Procedure in Presence of Copper. Take a suitable aliquot of the solution in a small beaker, neutralize, and then make just, widic with hydrochloric acid. Add solid potassium iodide until t,he precipitate of copper( I) iodide redissolves, and then add 500 mg. in excess. Add 10Co sodium thiosulfate solution until ihe color of the triiodide ion is completely removed. Add 3 to 5 drops in excess. Add 500 mg. of solid ammonium fluoride. Transfer the solution t,o a small separatory funnel. Add 10 ml. of 50% ammonium thiocyanate and 15 drops of 0.05 M tetraphenylarsonium chloride solution. Proceed with the extraction as outlined above. Procedure in Presence of Vanadium. Introduce a suitable aliquot of the solution to he analyzed into a small separatory funnel. Add solid ferrous ammonium sulfate until all the ranadium has been reduced to the plus four oxidation state. iidd solid ammonium fluoride unt,il the solution has a greenishyellow color; do not stop a t the blue color of t,he vanadyl ion. Add thiocyanate, tetraphen\‘larsorliulll chloride, and chloroform xnd proceed as above.
ACCURACY
The accuracy of the procedures has been r.stablished by the analysis of five Bureau of Standards alloy samples (Table 111). LITERATURE CITED
(1) Aflsprung, H. E., M.*L thesis, Washington L niversity, 1950. (2) Xyres, G. H., .\SAL. CHEM., 21, 652 (1949). (3) Rabko, A . K., and Drako. 0. F., J . Geri. Chem. ( U . S . S . R . ) .19, 1809 (1949). (4) Barnes, K.A , , h1.S. thesis. Washington University, 194i. (5) Bayliss, N. S., and Pickering, R. W.,IND.Eso. CHEM..A s . 4 ~ . ED.,18, 446 (1946). (6) Bobtelsky, hl.. and Spiegler. K. S.. ,J. Cheni. SOC..1949, 143. ii) Dwver. F. P.. Gibson. S . .I..and Kyholm. R. S.,J . Pro? Rou. ,
