Determination of Small Amounts of Cobalt in Steels and Nickel Alloys By the Isotope Dilution-Anodic Deposition Method DARNELL SALYER and THOMAS R. SWEET McPherson Chemical laboratory, The Ohio State Universify, Columbus J 0, Ohio
b A method involving cobaltinitrite separation, isotope dilution, and anodic electrodeposition can be used for the determination of small amounts of cobalt in steels and nickel alloys. The time of standing of the cobaltinitrite precipitate is 30 minutes. Several check electrodeposits may b e obtained from a single cobalt solution. The observed absolute standard deviation varied from 0.00570 for a sample known to contain 0.1470 cobalt to 0.02570 for a sample known to contain 0.47% cobalt. The maximum observed absolute error of the mean was
of nickel and cobalt by alkalies in the presence of an oxidizing agent (6). Either separation is followed by a separation of cobalt from nickel b y the dimeth ylgl yoxime, the 1-nitroso-2-naphthol, or the potassium cobaltinitrite method. Ion exchange separations have also been successful ( 2 ) . The final 11eighing form is usually the metal, cobaltous sulfate, or cobaltous cobaltic oxide. The blue cobaltothiocyanate complex that is formed 11-hen cobalt reacts with ammonium thiocyanate can be extracted with amyl alcohol and ether and the cobalt determined colorimetrically. The routine procedure is given by Young (ZO), m-ho also gives directions for eliminating the interference of iron and vanadium. Young, Pinkney, and Dick ( 1 1 ) have described the nitrosoR salt photometric method for cobalt in metallurgical products. The method depends on the fact that the cobalt complex of this reagent resists attack by nitric acid, while complexes of most common elements are destroyed. Ac-
0.0270.
T
DILuTIoK-anodic electrodeposition method (8) can be used advantageously for the determination of small amounts of cobalt in steels and nickel alloys. Gravimetric methods for cobalt in alloys and steels involve a separation based either on the estraction or precipitation of iron or on the precipitation HE ISOTOPE
Table I.
Sample NBS 162
Material Ni-Cu alloy, 66 Ni, 29 Cu, 2 ,34r0 Mn
NBS Value, %Co 0 54
Sample Weight, Grams 2 000 2 000
NRS 126a
Cu-Xi-Zn alloy, 72 Cu, 18 Xi, 10 Zn
0.136
Hi-Si steel, 367, Xi, 0.47, >In
0.30
7.500 8 000 3 000
3.000
NBS 161
Xi-Cr alloy, 64 Ni, 17 Cr, 15 Fe, 1.29% Mn
0.47
2,000 2.000 2.000
2
ANALYTICAL CHEMISTRY
APPARATUS
Modified tower electrolysis cells, 50-ml. (8).
Analysis of Nickel Alloys and Steels
2 000
NBS 157
cording to these authors, the quantity of nitric acid and of reagent and the time of boiling have an influence 011 color development. As a result, the method is somewhat empirical. An important volumetric method for cobalt is the potentiometric titration with potassium ferricyanide in ammoniacal solution ( I O ) . Although manganese interferes, large quantities of copper, nickel, and iron do not interfere. Hillebrand, Lundell, Hoffman, and Bright state that the outstanding method for the separation of cobalt from nickel is based on the precipitation of cobalt as di-(potassium cobaltinitrite)trihydrate (4). A study of the cobaltinitrite method by Kallmann ( 5 ) indicates that it is selective, can be used for many separations of cobalt, and should be more generally applied. I n the present work, this method of separation is used.
Spec Activity of Deposit,
Countp/Xn./ LIg. 1430 1417 1373 1445 1465 1403 1422 1462 1315 1385 1565 1608 1600 1558 1551 1582 1680 1627 1540 1687 1675 1530 1541
1
S X
c-7
hlg /Counts/ (2% llin Found 6 993 X 0 54 7 058 x 10-4 o 54 7 283 x 0 56 6 920 X 0 53 6 825 X 0 53 7 129 x 10-4 o 55 Av 0 54 7.033 x 0.14 6.839 X 0.14 7,605 x 10-4 0.1~5 7.219 X 0.14 ilv. 0.14 6 390 X 0 32 6 220 X 0 31 !RO x O. 31 6.2.. ,. 1_ 0_ - 4 -~ 6.430 X 0.33 6.447 x 10-4 0.33 6.320 X lo-' 0.32 Av. 0.32 5,953 x 10-4 0.45 6.148 X 0.47 6.501 x 10-4 0.30 5.928 x 10-4 0.45 5,970 X 0.45 6 . , m x 10-4 0.50 6.489 X 0.49 Av. 0.47
Std Dev of Single Abs Measurements, Error of Abs Value, Mean,
%
?
0 012
0 00
0,005
0.004
0 011
0.02
0.025
0.00
Platinum disk anodes, ls/le inch in diameter and 0.005 inch thick, one surface uniformly sand-blasted. Sample changer, Tracerlab PC9D shielded, manual. Scaler, Potter predetermined decade scaler, Model 341. Geiger-Muller tube, Tracerlab TGC-2. Electroanalyzer, Eberbach, rotating electrode. Drying oven, adjustable to 40" C. RIicrobalance. Aluminum absorber, 70 mg. per sq. cni Beckman Model H2 pH meter. Centrifuge, rlinical type, equipped with 50-nil. borosilicate glass tubes. REAGENTS
Radioactive Cobalt Solution. Cobalt-60 n a s purchased from t h e Isotopes Division, IT. S. Atomic Energy Commission, Oak Ridge, Tenn. ( O R S L ) . rippro\imately 1 millicurie of this solution and about 200 mg. of cariier were placed i n a 2-liter volumetric flask a n d diluted t o t h e mark n ith double-distilled water. Standard Cobalt Solution. One gram of spectrographically pure cobalt sponge \vas dissolved in t h e minimum amount of concentrated sulfuric acid and diluted to 1 liter. Matthey cobalt sponge (Johnson, X a t t h e y and Co., Ltd., London) was used. The concentration of this solution was determined by titrating it with a n ethylenediaminetetraacetic acid solution that had been standardized against a known zinc solution (3). Potassium Nitrite Solution, 50% aqueous, was freshly prepared just prior t o use from Merck reagent grade granular potassium nitrite, assay minimum 8770 potassium nitrite. Boric Acid-Potassium Sulfate Buffer Solution, O.1M i n boric acid a n d 0.05M in potassium sulfate.
aration of the standard curve has been published ( 8 ) and a more ComDlete
lowed) until the solution is basic. I n order to reduce the volume and to elim-
( 9 )'
water, discard the washings, and combine the precipitates from the tubes. Khile stirring the hydroxide slurry, add glacial acetic acid until the precipitate just dissolves. Use Black Ribbon S and S paper to filter the acid solution into a 400-ml. beaker. This removes some manganese which was converted to manganese dioxide by air oxidation in the basic solution. Heat the solution nearly to boiling and add half its volume of hot 50% potassium nitrite. A precipitate of yellow potassium cobaltinitrite forms in a short time. After half an hour centrifuge in 50-ml. borosilicate glass tubes, using more than one tube if necessary. Wash the precipitate with 570 potassium nitrite acidified with a little acetic acid, and dissolve by warming with 1 to 2 nil. of 1 to 9 sulfuric acid. If the solution is turbid a t this point. it may be due to silica. I n this case, evaporate the solution to dryness in a 50-ml. beaker, take up the residue in hot, dilute sulfuric acid, and remove the silica by filtration. If the cobaltinitrite precipitate n-as not distinctly yellon or if the sample contained manganese, make a second precipitation. If manganese is present in the final plating bath it will be anodically deposited as manganese dioxide, and is deposited more readily than cobalt 18'1. Transfer the acidic solution to a 100ml. beaker, and add 20 nil. of buffer solution and O.jM sodium hydroxide to pH 7.8. The volume should be about 50 ml. D r y the clean sand-blasted platinum disk anode for 15 minutes a t 40" and
Determination in Steels or Alloys. Dissolve a sample containing about 10 mg. of cobalt b y adding hydrochloric acid and warming on a hot plate. About 30 ml. of concentrated hydrochloric acid per half gram sample are usually needed. T h e last remaining residue may be dissolved in nitric acid, if necessary. Copper alloys require nitric acid with a little hydrochloric acid. Add 10 nil. of the radioactive cobalt solution to the dissolved sample. Mix well. Boil off gases such as nitrogen dioxide, cool, and remove silica by filtration. Evaporate to a sirup and proceed as follows, according to the type of material being analyzed. A. STEELSAND FERROUS ALLOTS. N o s t of the large excess of iron must be removed, or basic ferric salts m-ill precipitate \vhen the potassium nitrite is added. Make the solution 6 to 8iV with respect to hydrochloric acid. Mix v-ell. The volume should be 100 to 175 ml. Transfer to a 250-ml. separator!: funnel, add 7.5 ml. of ethyl ether, stopper. and shake vigorously. Iron is extracted into the ether layer as ferric chloride. Remove the ether layer and repeat the extraction. Boil dissolved ether from the aqueous layer. ilgain evaporate to a sirup, and proceed with Procedure B. E. SOKFERROUS ALLOYS(less than 5% iron). -4dd 30% sodium hydroxide to the solution from the evaporation (or from the ether extraction if A was fol-
PROCEDURE
Preparation of Standard Curve. Prepare a standard curve b y adding a given volunie of radioactive cobalt solution t o each of several inactive cobalt solutions. Buffer each solution a t a p H of 7.6 t o 7.8 with boric acidborate and place in a modified tower electrolysis cell fitted with a tared, sandblasted platinum disk anode. Electrolyze the solution, using a rotating cathode. This produces thin, uniform deposits of hydrated cobaltic oxide. Low temperature drying of these black deposits gives a product which can be neighed as cobaltic oxide trihydrate. An aluminum absorber screens out beta radiation, so that only the gamma activity of deposits is measured and a n y error due to self-absorption of beta particles is eliminated. All samples were counted for about 20 minutes-Le., in most cases for a total of about 40,000 counts. From the weight and activity of deposits, the specific activity is calculated. A straight line is obtained when the reciprocal of the specific activity is plotted against the weight of cobalt that is present in the solution before the active solution is added.
Table
Sample KO.
NBS 161
KBS 126a
NBS 162
Weight of Deposits, Grams 0.632 1.340 1.460 0.511 0,637 2.237 2.148 0.624 1.434 0.507 2.069 2.189 0.635 0.609 0.727 1.182 1.534 1.141
0.553
870
0.565 1.582 0.692
Counting Data for Table
I
Counts per Minute Total 1090 2223 2384 1004 1222 3479 3372 1103 2406 861 3455 3445 1005 892 1047 1669 1155 2293 1646
0.766
NBS 157
II.
912 103gb 54lb
Dead time 5 6 6 3 4 rn c
I
3 6
2c
+
2 3 4 5 4 6
5 3 3 3 1
Bkgrnd. - 50 - 50 - 51 - 62 - 62 - 66 - 66 - 49 - 49 - 52 - 52 - 52 - 52 - 47 - 47 - 50 - 50 - 50 - 50 - 49 - 49 - 186 - 186
Effi~.~ Set 2 1047 2180 1 2248 - 91 862 - 83 1068 - 96 3420 0 3313 0 978 - 79 2302 - 61 81 1 0 0 3410 3400 0 1005 0 871 23 26 1030 1621 0 1109 0 2249 0 1601 0 787 37 827 39 2190 1334 557 913
Specific Activity 1680 1627 1540 1687 1675 1530 1541 1565
1608 1600 1558 1551 1582 1430 1417 1373 1445 1465 1403 1422 1462 1385 1311
a Efficiency corrections compensate for day-to-day variations in detector unit or for re lacement of Geiger tube. Counted with Tracerlab TGC-8 tube.
The detailed procedure for the prepVOL. 29, NO. 1, JANUARY 1957
3
weigh to the nearest 0.002 mg. With the disk in place in the electrolysis cell, introduce the active buffered solution. Electrolyze for 40 minutes at 1.5 to 1.8 volts at room temperature. Remove the still active solution through the glass side arm of the cell. Other deposits may be obtained from this solution, if desired. Rinse out the cell with doubledistilled water, and remove the disk, which nom contains oxide deposit. Wash the deposit with double-distilled water and remove adhering water droplets with a piece of filter paper. Allow the deposits to dry in air until no water is visible. Place in a 40” oven for 2.5 hours and weigh the cobaltic oxide trihydrate deposits. Deposits usually weigh about 1 to 2 mg. Determine the activity of the deposit by placing the disk in a planchet in position in the sample changer. Use a TO mg. per sq. cm. aluminum absorber. Correct for dead time, background, and efficiency on the observed activity and calculate the specific activity. All samples were counted for about 20 minutes. Table I s h o w that the method is satisfactory for a series of representative samples which vary considerably in composition.
standing is 30 minutes instead of the 24 hours that is usually recommended to ensure complete precipitation (4). This reduction is possible because a large percentage of the cobalt is precipitated in the first half hour and the isotope dilution technique does not require isolation of all the cobalt. Thus, even double precipitations may be made within a reasonably short time. Two or three deposits of cobaltic oxide trihydrate may be made from the same cobalt solution. The precipitation of potassium cobaltinitrite is satisfactory for the separation of cobalt from large amounts of iron, although several precipitations may be necessary (1, 7 ) . If the iron present in an alloy weighs a t least ten times as much as the cobalt, i t is convenient to perform a t least one ether extraction of iron before the precipitation. The volume of the solution was kept a t a minimum in order to increase the rate of cobaltinitrite precipitation. The combination of the two processes for reduction of volume (evaporation and sodium hydroxide treatment) was more rapid and convenient than either alone.
DISCUSSION
ACKNOWLEDGMENT
I n the separation of cobalt by the cobaltinitrite precipitation, the time of
The authors wish to acknowledge aid to Darnel1 Salyer in the form of a
predoctoral fellowship from the Cincinnati Chemical Works. LITERATURE CITED
(1) Faleev, P. V., Zaoodskaya Lab. 8 , 381 (1939). (2) Hague, J. L., Maczkowske, E. E., Bright, H. A , , J . Research iVatl. Bur. Standards 53, 353 (1954). (3) Harris, \Ir. F., Sweet, T. R . , ANAL. CHEM.26, 1649 (1954). (4) Hillebrand, W. F., Lundell, G. E. F., Hoffman, J. I., Bright, H. A., “Applied Inorganic Analysis,” 2nd ed., pp. 419-20, Wiley, Xew York, 1953. ( 5 ) Kallmann, s.,ANAL.CHERI.22, 1519 (1950). (6) Lundell, G. E. F , Hoffman, J. I . , Bright, H. -4., “Chemical Analysis of Iron and Steel.” a . 334. Wilev. Sew York, 1931. (7) Nikolow. C.. Przemusl Chem. 17, 46 ’ (i933j. ’ (8) Salver, D.. Sweet, T. R., 4 x . 4 ~ . CHEM.28, 61 (1956). (9) Theurer, K . , Sweet, T. R., Ibzd., 25, 120 (1953). [lo) Young, R. S.,“Industrial Inorganic .4nalysis,” pp. 77, 82, Wiley, New Tork, 1953. i l l ) Young, R . S.,Pinkney, E. T., Dick, R., IND. ENG. CHEJI., ANAL.ED. 18, 474 (1946). I
I
I ,
RECEIVED for review February 2 i , 1956. Accepted October 1 1 , 1956.
Chronopotentiometric Analysis in Fused Lithium Chloride-Potassium Chloride H. A. LAITINEN and W. S. FERGUSON Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, 111.
b
This chronopotentiometric investigation was prompted by the need for analytical procedures that are directly applicable in molten systems a t high temperature. With platinum microelectrodes, measurements upon the chlorides of bismuth(lll), cadmium(ll), silver(l), and copper(1) in a eutectic mixture of lithium chloride-potassium chloride a t 450” C. showed the theoretical relationship among applied current density, concentration, and transition time. The mixtures bismuth(lll)-silver( I) and bismuth(ll1)-copper(1) also gave the expected results. For times less than 5 seconds the mass transport was not As the complicated b y convection. diffusion field of the electrodes used was not physically constrained, the response of very small electrodes departed from linear diffusion control The diffusion even within 5 seconds. coefficients of bismuth(lll), cadmium-
4
ANALYTICAL CHEMISTRY
(II),silver(l), and copper(1) were, respectively, 0.6, 1.7, 2.6, and 3.5 X 10-5 sq. cm. per second. The chronopotentiometric method of analysis in fused salt solutions using electrodes without physically constrained diffusion fields was performed with an accuracy within f.2.6‘70. procedures capable of direct application to molten systerns a t high temperature are of increasing importance in present-day technology. S e e d for such procedures is evident, for example, in metallurgical slagging, the manufacture of glasses. metallurgy and electrorefining, and the use of fused salt solvent systems in some types of atomic reactors. Potentiometry (9, 12, 17, 20) and polarography (1, 2, 6, 15, 14) have been applied to analysis in molten systems. SALPTICAL
The chronopotentiometric method has inherent advantages which warrant its application to analysis in melts. Analysis by potentiometry is limited to a single component of a mixture, whereas chronopotentiometry is applicable to the simultaneous determination of several components. At high temperature the polarographic method must in general be carried out a t a solid microelectrode under conditions such that mass transport is a t best a mixture of diffusion and convection. Furthermore, the product of the polarographic process is frequently a solid metal of dendritic form which grows out into the solution, causing the limiting current to be variable n-ith time and nonreproducible. The chronopotentiometric method can, in contrast, be carried out under such conditions that (virtually) all mass transport is by linear diffusion, and the total amount of electrolysis product
