accomplished n ith ammonium persulfate. The investigation of interfering elements was restricted because the proposed procedure was designed for samples of relatively pure plutonium. Many elements remain in solution with plutonium(V1) when lanthanum is precipitated as the fluoride, but rare earths, cerium(IV), uranium(IV), americium(111) or (IV), thorium, and the alkaline earths will precipitate with lanthanum. For samples containing small amounts of thorium, it was convenient to determine this element in a separate sample ( 1 ) and correct the lanthanum results for thorium. The most successful separations of americium n-ere accomplished when ammonium persulfate n as em-
ployed in two successive oxidation-precipitation steps. There were some cases, however, where the unoxidized americium and plutonium accompanying the lanthanum amounted to as much as 2 or 3 y. These results emphasized the need for determining americium and plutonium in the final solution of lanthanum 8-quinolinolate by radioanalysis, if it is essential to correct the lanthanum determination for this low level of contamination. LITERATURE CITED
( 1 ) Bergstresser, K. S., Smith, 11. E.,
Los Alamos Scientific Laboratory, LA1839 (September 1954). (2) Hyde, E . K., in “The Actinide Elements,” G. T. Seaborg, J. J. Katz,
eds., 1st ed., Div. IV, Vol. 14-4,Chap 15, p. 576, National Nuclear Energy Series, RlcGraw-Hill, New York, 1954. (3) Popov, A. I., Knudson, G. E.,’ANAL. CHEX.26,892 (1954). (4) Rein, J. E., Los Alamos Scientific Laboratory, “A Modified Spectrochemical Method for Determination of Submicrogram Amounts of Certain Elements in Oxidized Plutonium Solutions,” in preparation. ( 5 ) Reinschreiber, J. E., Langhorst, Jr., A. L.. Elliott. $1. C.. Los illamos Scientific Laboratory, LA-1354 (February 1952). (6) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 509, Interscience, Kew York, 1950. RECEIVEDfor review January 21, 1958. Accepted May 12, 1958. Work performed under auspices of C . S. Atomic Energy Commission.
Cathodic Electrodeposition Methods for Cobalt DARNELL SALYER’ and THOMAS R. SWEET Mcfherson Chemical laboratory, The Ohio State Universify, Columbus I 0, Ohio
b
Radiotracer techniques and cobalt-
60 were used to study methods for the determination of cobalt by cathodic eledrodeposition. The quantity of cobalt left in the electrolytic solution was determined after electrodeposition by 10 of the most frequently used procedures for cobalt. Some cobali remained in solution in every case. The amount varied from 0.01 to 4.0 mg., with an average of 0.46 mg. of cobalt for 43 determinations. For the most part the cobalt left in solution is the result of fhe washing of the deposits and is not simply metal that was never deposited. A special washing procedure whereby the electrolytic solution was gradually replaced by distilled water until the current dropped to zero did not decrease the cobalt left in solution. The average positive error for 43 single determinations of cobalt was 0.74 mg. before, and 1.1 9 mg. after correcting for the cobalt left in solution. Correcting the data for the residual cobalt increased the errors.
T
most common difficulty encountered in the electroanalysis of cobalt is said to lie in the fact that special measures must be taken to deposit the last traces (8). The metal remaining in solution is generally determined gravimetrically (11) or colorimetrically (4, 6). It can be neglected for large cobalt samples (>0.25 gram), but beHE
1 Present Rome, Ga.
1632
address, Shorter ANALYTICAL CHEMISTRY
College,
comes increasingly important for smaller quantities. The aim of the present work was to examine some of the most frequently used methods for the electrolytic determination of cobalt. The use of radiocobalt and a solution counting technique provided a convenient method of determining the residual cobaltLe.. the cobalt left undeposited. REVIEW OF M E T H O D S
Satisfactory results for the electroanalysis of cobalt have been reported when deposition is made from ammoniacal or from certain slightly acidic solutions. The presence of various salts in the bath is said to be beneficial. Depolarizers are used to prevent anodic deposits of cobalt(II1) oxide. Acidic solutions that have been used include those containing formate or lactate, reported by Smith (f6),and the phosphate bath described by Perkin and coworkers (IS, 14). Electrolytic procedures have been described which utilize ammoniacal solutions of ammonium acetate, formate or lactate (15), oxalate (9), chloride ( I 7 ) ,bifluoride (S), sulfate (f9), bisulfite (I), thiocyanate (2), and borate (5). A review of most of the early work on the electroanalysis of cobalt has been given by Watts (18). A recent publication on the subject is by Hague, Maczkowske, and Bright (6). The conditions for cobalt deposition in some of the most frequently used methods are tabulated in Table I.
APPARATUS A N D REAGENTS
An Eberbach rotating electrode electroanalyzer was used for the depositions. Ordinary cylindrical platinum gauze cathodes were used. They were 2 inches long by 11/2 inches in diameter and had 4-inch solid wire platinum stems; the surface area of each was approximately 100 sq. em. The platinum anode stirrers mere 1 X 3/4 inch plates M ith stems. The apparatus for radiation measurement consisted of a Model LC1 liquid counter apparatus (Nuclear Instrument and Chemical Corp., Chicago) and a Potter predetermined decade scaler, Model 341. I n addition to the Nuclear D52 tube provided with the solution counter set, a Tracerlab TGC-6 G l I tube was also used. Cobalt solutions were prepared from spectrographically pure cobalt sponge of Johnson RIatthey and Co., Ltd., London. Stock solutions containing about 1 mg. of cobalt per ml. as the sulfate were made active by the addition of 0.5 to 1 me. of high purity cobalt60 per liter of solution. They were standardized by titrating with (ethylenedinitri1o)tetraacetic acid. High specific activity radiocobalt was obtained from the Oak Ridge n’ational Laboratory as cobalt(I1) chloride in hydrochloric acid solution. EXPERIMENTAL TECHNIQUES
Electrolytes. Using aliquots of t h e standard active cobalt solution (containing 50 t o 60 mg. of cobalt). electrolyte baths were prepared and the electrolyses were made as described in the method under investigation (Table I ) . T h e cathodes were removed, without interrupting the
current, b y slonly loivering t h e beaker t h a t contained t h e electrolyte. Simultaneously, the deposit was viashed with a spraj- of double-distilled water. The mixture of residual electrolyte and wash solution remaining in the electrolysis beaker was then transferred t o a 100ml. volumetric flask, diluted to the mark, and mixed well. T.T7hen the volume exceedc.d 100 ml., the solution was ex-aporated or an aliquot was taken. These solutions were counted directly in the liquid counter apparatus, and the total amount of cobalt left in solution was calculated from the activity. The cathodes n-ere dipped in e t h j l alcohol and ether, air dried, and placed in a n oven a t 90' C. for 5 minutes. The n-eight of the deposits was obtained by neighing the cathodes to the nearest 0 02 nig. before and after electrolysis. Cobalt deposits were stripped from the cathode by dipping it in cold concentrated nitric acid. Solution of the metal was rapid and complete when a few drops of hydrogen peroxide solution M ere added to prevent passivation. Liquid Counting. I n t h e activity range used for liquid counting, t h e ohserved activity was directly proportional t o the quantity of cobalt in solution. This was s h o n n b y counting a wries of solutions, each of nhich contained a known amount of active cobalt in 100 ml. T h e counts were corrected for background a n d t h e activity per mg. of cobalt n a s calculated. This quantity, hereafter called the solution specific activity, n a s determined for each of the standurdized active cobalt solutions and n-as used in tlie calculation of the amount of residual cobalt from the activities of the clcctrolyte-wash solutions. Because a high degree of accuracy was not necessary in the determination of small amounts of residual cobalt, the counting procedure was relatively siiiiple. It n-as possible to neglect coincidence corrections and to limit counting periods to 5 minutes per sample. Typical cwunting data are slionii in Tnhl~11. RESULTS AND DISCUSSION
The data of Table I11 show that some cobalt remained in solution in each of the 10 generally used electrolytic procedures. The amount varied from 0.01 to 4.0 mg. ~ i t han average of 0.46 mg. for 43 determinations. Slightly acidic electrolyte.. tend to rrtain more metal. If tlie only difficulty in the cathodic electrodepo.itioii method for cobalt were' the incomplete deposition of the metal on the electrode, then all the errnrs shown in column 4 of Table I11 would be negative and equal in magnitude to the measured quantity of r c d u a l cobalt. K i t h one exception (the acidic phosphate bath experiment 4) all results are positive, even n hen corrections are not made for residual cobalt. The average error for 43 determi-
Table I.
Conditions for Electrodeposition of Cobalt
Composition of Electrolyte"
Conditionsb
2.2 g. KazC03,5 ml. concd.
4 amp., 30 min., 6 volts, 95" c. 2 to 3 amp., 25 min., 8 volts
lactic acid
20 ml. S H a O H ,3.5 ml. 94% HCOOH 2 ml. 5% H3P04, 25 ml. 10% SaHZPO4, 0.1 g. S H z O H . H,SO,, X H t O H to pH 4 5 ml. 1 to 1 H2S04,",OH
to neutral, then 40 ml.
2 amp., 20 min., i volts
0.5 to 2 amp., 30 min., 4.2 to 8 volts
2 amp., 1 hour
excess 25 ml. S H t O H , 10 ml. 2075 1.5 amp., 25 min., 6 volts H0.k 15 nil. SH40H,3 g. XH4CI, 2 to 5 amp., 45 min., 4 volts 0.1 g. SHZOH. HCI 50 ml. SHJOH, 5 g. XHdC1, 4 to 7 amp., 30 miri. 0.4 g. NaHS03 35 ml. ",OH, 20 g. (XH4 j20.5 to 0.8 amp., 1 hour, 4 SO,, 2.5 g. SHaHF2 volts 45 ml. KH40H,0.5 g. SHz.0.5 amp., 1 hour, 4 volts XH, HCI, 2 ml. XH, "2. HZO 5 Initial volume of earh solution, approximately 100 ml. Current values are in amperes per sq. em.; unlpss otherwise indicated, initial temperature was 20 to 25' C.
Table II.
Typical Counting Data
Counts per hlinute Expt. Yo. 1
9
Total activity. electrolyte
+ n-ash
Barkground
xet activity
2355 82 2607 76 .3ii 141 45i 2380
63 68 74 62 30 55 55 4i
2306 14 2519 14 54 1 86 402 2347
nations was 0.7 nig. before, and 1.2 mg. after correcting for residual cobalt. This represents errors of 1.4 and 2.2 %, respectively. The codeposition of foreign material is generally held responsible for the positive errors, although in analyses in which careful determinations of foreign matter were made, the corrections did not completely explain the results (6). It is possible that contaminants not tested for, such as oxide, hydroxide, or water. LT ere also present. The values of residual cobalt shown in Table I11 were larger than expected in view of negative results of qualitative tests that were performed on a few drops of the electrolyte before the cathodes were removed and the current ivas turned off. Sniall amounts of metal were apparently lost from the deposits during the removal and washing of the cathodes a t the termination of the electrolj-sis. Exprrinients described below have shown this to he the correct explanat ion.
Solution specific
Cotmlt Present,
activity
hlg.
2058 2058 2058 2058 20.38 1675 16'75 1675
1 12 0 01 1 24 0 01 0 27 0 05 0 21 1 40
~
COBALT LOSSES DURING REMOVAL AND WASHING OF DEPOSITS
To prove that losses of cobalt occur during the removal and washing of deposits, experiments N ere performed in which the amount of cobalt in solution before and after these steps was quantitatively determined by activity measurements. The electrolyte used was the ammoniacal chloride solution containing hydroxylamine hydrochloride as depolarizer (6). After an electrolysis had proceeded for 45 minutes, a 25-ml. portion of the residual electrolyte was removed with a pipet before the cathodes were removed or the current was turned off. This portion was placed in a 100-ml. volumetric flask, diluted to the mark, and set aside for counting, The cathodes mere then removed with washing in the usual way and the remaining 7 5 ml. of residual electrolyte plus wash water was also retained for counting. From the observed activities the cobalt losse~were calculated and are shown VOL. 30, NO. 10, OCTOBER 1958
1633
Table 111.
Results of Cobalt Determinations by Several Methods
ExDt. Nb.
Taken
1
59.8
Cobalt, Mg. Weight of deposit 59 7 60 7 59 9 60 2
2
59.8 49.0
3
59.8 49.0
4
59.8
A0 61 61 48
6 8 5 7
61 61 60 50
3 0 6 0
59 56 47 47
6 8 9 6
Error
-0.1 +0.9 +o. 1 $0.4 Av. $0.33 $0.8 +2.0 +1.7 -0.3 Bv. +1.10 +1.5 +1.2 +0.8 +1.0 Av. $ 1 . 1 3 -0.2 -3.0 -1.1 -1.4 Av. -1.43 +1.7 +2.6 +1.3 +0.6 Av. +1.55 t0.8 $1.8 $0.9 +1.3 .4v. 1.20 +0.6 +1.7 +2.7 $0.8 $0.6 Av. $1.28 -0.2 $0.2 +2.1 $0.9 Av. $0.75 -0.7
Residual co, Mg. 1.12 0.01 1.24 0.01 0.60 0.35 0.05 0.01 0.68 0.28 0.01 0.07 0.06 0.07 0.05 0.65 4.03 1 64 1.66 2.00 0.18 0.24 0.14 0.68 0.30 0.27 0.05 0.16 0.07 0.14 0.04 0.21 0.43 0.15 0.04 0.18 1.315 0.66 0.40 0.97 0.83 0 27 0.05 0.24 1.40 0.49 0.02 0.05 0.04 0.17 0.02 0.02 0.05
S e t Error, Mg. +l.O +0.9 +1.3
$0.4 +0.9 $1.2 $2.1 $1.7 $0.4 +1.6 $1.5 $1.3
;0.9 +1.1 +1.2
$0.5 t l . O 49.0 t0.5 +0.3 $0.6 5 59.8 61 5 +1.9 62 4 1-2.8 61 1 +1.4 49.0 40 6 +1.3 t1.9 6 59.8 60 6 $1.1 61 G $1.9 60 7 +1.1 49.0 50 3 $1.4 +1.4 7 59.8 60 4 +0.6 49.0 50 7 t1.9 51 7 +3.1 49 8 +1.0 49 6 +0.6 +1.4 8 59.8 59 6 +1.1 60 0 +0.9 61 9 +2.5 49.0 49 9 +1.9 +1.6 9 59.8 59 1 b -0.4 49.0 49 4 $0.4 +0.5 49 4 +0.4 +0.6 -0.5 48 5 +0.9 Av. - 0 , 10 $0.4 10 59.8 +2.1 61 9 $2.1 61 3 +1.6 +1.5 49.0 50 0 +1.0 +1.0 $1.2 +1.0 50 0 50 0 $1.0 +1.0 49 9 +0.9 +0.5 Av. $1.25 $1.3 Values include some cobalt which was in a sulfur-sulfide layer that came off in wash. Values include correction for platinum migration. Corrections were 0.1, 0.4, 0.2, and
+
0.0 mg., respectively.
Table
IV.
Cobalt Losses during Washing of Deposits
.o.o
17 9 2 6
0.000 0 052 0 008
0 0 1 0
0 000
0 005
249,5 218 3 142 5 213 Od 188 0 159 I
0.240 0 209 0 138
0,228 0.261) 0.227
Expressed in terms of 100 ml. Standard active cobalt solution with solution specific activity of 1386 c.p.m. per mg. Active cobalt solution with solution specific activity of 934 c.p.m. per mg. Activity of entire 100 ml. of electrolyte bath plus wash.
1634
ANALYTICAL CHEMISTRY
in Table II-, experiments 1, 2, and 3. These results indicate that the major loss is not due to cobalt which R-as never deposited, but due to cobalt which redissolved when the deposits were washed. Lingane (IO) has attributed similar findings for lead and cadmium to air oxidation during washing. MODIFIED METHOD FOR REMOVAL AND WASHING OF DEPOSITS
An attempt iTas made to reduce or eliminate the amount of cobalt that is redissolved during the washing of the deposit by slowly replacing the electrolyte bath 514th distilled water before the cathode was removed or the current was turned off. With the ammoniacal chloride electrolyte, electrolyses n-ere conducted in an electrolysis vessel with a n outlet tube a t the bottom. After the 45minute electrolysis period, a 25-ml. portion of residual electrolyte was removed for activity measurements. The remaining electrolyte was slon-ly drained off while Tvater was added continuously to maintain the volume. This process was continued until the current dropped to zero, and required the addition of approximately 600 ml. of water during a period of about 40 minutes. The mixture of electrolyte and m-ater was collected, evaporated to 100 nil., and its activity was measured. Deposits n-ere dried and weighed in the usual manner, and showed positive errors as in the previous study. Typical results of this study arc those of experiments 4, 5, and 6, Table IT’. Blthough the liquid level was maintained and the deposits were not exposed to the air during the removal of electrolyte, cobalt losses occurred as the electrolyte was replaced by water. They are of about the same order of magnitude as when the usual washing method is employed. FORMATION OF SURFACE COMPOUNDS
Young (19) stated that the electrolysis of cobalt should not be prolonged after depositing all the cobalt or the results will be high. Nicol (la)observed that deposition from very dilute solution gives oxides and hydroxy compounds; he stated that solutions having a concentration of cobalt less than 0.005.V are subject to detectable oxide and hydroxide formation. His ivork dealt with water solutions of cobalt salts and not ammoniacal solutions. Nicol prepared several basic cobalt salts by ordinary chemical methods and obtained the thermogravimetric ignition curves for them. The ignition curve most similar to that of the electrodeposited cobalt compounds 11 as given by CoS04.~ C O ( O HHzO. ) ~ . The electrolytically prepared compound n as
more soluble in water and salt solutions than the corresponding basic salt. This information leads t o the following hypothesis: During the electroanalysis, cobalt is deposited as the metal until the cobalt concentration in the plating bath drops t o about 0.0051V. ilt this point COSOS3CO(OH)2 H,O and/or related compounds are deposited. .It the end of the deposition, part of the cobalt surface compounds are washed off or dissolved by the wash water, but enough remains on the deposit to give positive errors. The absolute errors due to such a process n ould be essentially independent of the total amount of cobalt if the initial concentration is above 0.00LV. With a sufficiently large sample the error ~oouldbe negligible. This seems to be true n-hen heavy cobalt deposits are made (6). I s the reaction and the washing of the deposits xvould not be expected to be reproducible, it is not surprising that there is a large variation in the positive errors. The formation of surface compounds is to be distinguished from the occIusioii of foreign material to which reference is generally made. Experiments mere made in which the depositions were purposely stopped before electrolysis was complete. The amount of metal that remained undeposited was determined with the liquid counter. Coincidence corrections were applied and suitable counting periods were used to provide the necessary accuracy. Deposits were removed, washed, dried, and neighed as before. The results are shown in Table V. The deposits tlint n-eigh 35 mg. or
Table V. Test for Formation of Surface Compounds a t Low Concentrations
(49.60 mg. of cobalt taken) Co Remaining Deposit, in Soln., Sum, Error, hfg. Mg. hfg. Mg. 50 40 0 09 50 49 $0 89 50 79 0 08 50 87 $1 27 35 75 14 35 50 10 +O 50 41 00 9 64 50 64 +1 04 38 88 10 95 49 83 +O 23 40 45 10 28 50 73 + I 13 30 36 20 65 51 01 + I 41 46 37 4 88 51 25 +1 65
less, after correction for the cobalt in the liquid, all show positive errors and are not significantly smaller than those obtained when nearly complete deposition is made. Hence, it appears either that the surface compound explanation as given above is incorrect, a t least insofar as the assumption that surface compounds form only n-hen the concentrations of the cobalt are very low; or that the formation of surface compounds a t low concentration causes so small a part of the positive error that its effect was not observed in this test. Another possibility is that as a result of concentration polarization near the cathode, surface compounds still form a t the cathode, even though the cobalt concentration is greater than 0.005N in the bulk of the solution. LITERATURE CITED
(1) Brophy, D . H., IND.ENG. CHEM., A S A L . ED.3, 363 (1931).
(2) Csokan, P., Z. anal. Chem. 119, 418 (1940). (3) Fine, M. M., U.S. Bur. Mines, Rept. Invest. 3370,59 (1938). (4) Fischer, A., Sleicher, A,, “Electroanalytische Schnellmethoden,” pp. 235, 237, Ferdinand Enke, Stuttgart, 1926. (5) Guzman, J., Rial, M., Anales SOC. espaii. f i s . y quim. 34, 636 (1936). ( 6 ) Hague, J. L., Maczkowske, E. E., Bright, H. A., J. Research Natl. Bur. Standards 53, 353 (1954). (7) Jilek, A,, VieSfal, J., Collection Czechoslov. Chem. Communs. 7, 512 (1935). (8) Kolthoff, I. M., Sandell, E. B,., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 410, Macmillan, New York, 1952. (9) Kpvalenko, P. N., Zhur. Priklad. Khzm. 24,951 (1951). (10) Lingane, J. J., “Electroanalytiral Chemistry,” p. 289, Interscience, New York, 1953. (11) Lundell, G. E. F., Hoffman, J. I., J.Znd. Eng. Chem. 13, 540 (1921). (12) Nicol, A., Ann. chim. 2,670 (1947). (13) Perkin, F. M., Hughes, W. E., Chem. News 101. 52 (1910). (14) Perkin, F. i f . , Prebble, W. C., Trans. Faraday SOC.1, 103 (1905). (15) Smith, E. F., “Electroanalysis,” 6th ed., pp. 137, 146, Blakiston’s, Philadelphia, 1918. (16) Taggirt, W. T., J . Am. Chem. SOC. 25, 1039 (1903). (17) Wagenmann, K., Metall. u. Erz 18, 447 (1921). (18) Watts, 0. P., Trans. Am. Electrochem. SOC.23,99 (1913). (19) Youpq: R. S., “Industrial Inorganic Analysis, p. 73, Wiley, New York. 1954. ~
RECEIVEDfor review March 4, 1958. Accepted May 12, 1958. Taken in part from a dissertation presented to thr Graduate School of The Ohio State University by Darnel1 Salyer in partial fulfillment of the requirement for the doctor of philosophy degree.
Polarograph with Automatic Corrector for the Electrode Potentia I SYOTARO OKA’ instruments Division, Shimadzu Seisakusho, Ltd., Kyoto, Japan
,A new polarographic instrument automatically corrects the potential of the indicator electrode in accordance with the applied voltage. The difference between the electrode potential and the applied potential is set on the auxiliary potential applier, which is controlled servomechanically. This instrument continuously compensates for any potential drops arising from internal and external resistances in the cell circuit. Electrolysis can b e carried out using the indicator electrode against the mercury pool anode or
other electrodes. As the voltage axis of the resulting polarogram is always represented against the reference electrode, the polarizable micrometallic electrode can also b e used as the anode. Reproducibility and accuracy are within the limits of normal polarographic analysis.
T
HE accurate determination of halfwive potentials is essential for many applications of polarographic analysis, but the values determined with the auto-
matic recording polarograph in common use are sometimes in error. Much work in organic polarography has been conducted in solutions containing little or no water. The electrical resistances of these cell solutions are very high, resulting in distortion of the current-voltage curves. This problem mas investigated by IlkoviE ( 1 ) and Jackson and Elving ( g ) , and automatic compensators for 1 Present address, Instrument Division, Shimadzu Manufacturing Co., Kyoto, Japan.
VOL. 30, NO. 10, OCTOBER 1958
1635
