to Cm242,Figure 1 shows that the ratio of Ah243 to Cm243must also be high, Hence, the Am243would be the only significant peak a t m:iss-243; therefore, the i9otoliic dilution technique could be applied to our mixture of curium and americiuni without chemical separation. PROCEDURE
-\ known quantity of isotopicallypure was added to an HCl solution in which a sample of irradiated plutonium-aluminum was dissolved. The solution was made 14M in lithium chloride and about C.1Jf in HC1 and was equilibrated with a 2/1 liquid ratio of 3Oy0 Alamine-336 (a tertiary amine nianufactured by General Mills) in diethyl benzene to remove the actinides from lanthanides, other fission products, and aluminum ( 1 ) . Americium and curium were then uartitioned from plutonium by s t r i p , h g into 4.851 HCI. Recovery of americium was about
75%. Approximately 957, of the beta activity, 90 to 99% of the gamma activity, and more than 99.97, of the plutonium were removed. The sample was then evaporated to dryness and redissolved in 1X HXOa, and the mass ratio of to Am243mas determined with a surface emission mass spectrometer. Correction was made for the small amount of present in the sample by purifying and analyzing an unspiked sample by the same method that was used for the spiked samples. One tenth of a microgram of americium was sufficient for the mass spectrometric analysis. h relative standard deviation of = k l . O ~ , was obtained for the analysis (including all purification steps) of seven aliquots of a single >ample. ACKNOWLEDGMENT
The authors acknowledge the work of
W. B. Hess and B. L. Bussey of the
Savannah River Plant who performed the mass spectrometric analysis. LITERATURE CITED
(1) Baybarz, R. D., JTeaver, B., Oak Ridge
Sational Laboratory, Oak Ridge, Tenn., Atomic Energy Conmi. R e p t . ORNL-3185 (1961). (21 Beadle. A. B.. Dance. D. F.. (;lover. K. M., 'Milstead, J., J . Inotg. AV~icl. Chem. 12,359-61 (1960). ( 3 ) Penneman, R. A . , Keenan, T. I0>..41.DSOS
Savannah River Laboratory E. I. du Pont, de Semours and Co. Aiken, S. C. The information cont,ained in this paper was developed during the course of wnrk under contract AT(07-2)-1 with the t-.P. ht,oniic Energy Commission.
Application of Constant Current Potentiometry to Titration of Cobalt with Potassium Ferricyanide SIR. In spite of th: small potential change a t the equivElence point, the volumetric determination of cobalt by oxidation with ferricyanide in ammoniacal solution (2) is>frequently used. This ia primarily becauqe, of the metals commonly present with cobalt in alloys and steels, only mang,anese and, to a le-er extent, chromium interfere. By application of constant current potentiometric end point detection (1, 3, d ) , ferricyanide titrations of cobalt can be performed under the original condition. with a greatly improved break at the end point. This makes plotting titration cur ;es unnecessary. The potential change a t the equivalence point is about 0.5 x olt, some three times greater than when using a conventional platinum-$ .C.E. electrode y t em. EXPERIMENTAL
Apparatus. Potential measurements were made with either a Beckm a n Model H-2 p H meter or with a Leeds & S o r t h r u p Model 7401 p H indicator. T h e electrodes were 0.016in. diameter platinum wire sealed into
soft glass tubing so t h a t about 1 mi. of wire was exposed. T h e polarizing current mas obtained from two 45-volt batteries in series with a 25-megohm resistor. A Sargent Model XV polarogrnph with a rotating platinum electrode was used t o obtain t h e current-voltage curves. Standard Solutions. Solutions of cobalt(I1) sulfate were prepared from reagent grade cobalt sulfate hexahydrate, and mere standardized b y evaporation of 10.00-nil. portions to dryness in porcelain crucibles and ignition a t 500" C. in a muffle furnace. Standard solutions of potassium ferricyanide were prepared by dissolving weighed portions of the recrystallized salt in deionized mater and diluting t o volume in a volumetric flask. These were stored in brown bottles to minimize light decompocition. Comparison of Titration Curves. Titration of 5.00 nil. of 0.1092X cobalt sulfate in approximately 30 ml. of 4;M ammonium hydroxide-1 .M ammonium citrate with 0 . 1 O O O M poTable II.
Table I. Determination of Cobalt and Manganese by Addition of Excess Potassium Ferricyanide and Back-Titration with Cobalt Sulfate - _ _ 11eq. _ taken ~
Co 0.2627
>In
Total-
0.262'7 0.3948 0.39413 0.2627 0.0247 0.2874
?vleq. found, total Ptd. dev. 0.2626 (av. of 8 titns.) f 0 . 0 4 0.3949 (av. of 4 titns.) ~ k 0 . 2 7 0.2868 (av. of 5 titns.) & O . 11
taqsium ferricyanide was performed using two sets of electrodes-a coiiimercial platinum-S.C.E. pair and a polarized platinum pair. Curves are shown in Figure 1. Values for the polarized electrodes are not given with reference t o the saturated calomel, but indicate only potential difference. The shape of a plot of potential us. ferricyanide volume using polarized electrodes may be interpreted by reference to Figure 2. which shows polarograms of solutiona similar to thohe preent before and after the equivalence point in the absence of cobalt. -4fter ferricyanide is added, but before the equivalence point is reached, the potential is governed by oxidation of ferrocyanide a t one electrode and reduction of diqsolved oxygen a t the other (qolid line, Figure 2 ) . If a polarizing rurrent of 1.8 pa. is used, the potential difference between the titration indicator electrodes correqponds to the inter5ection of this plot with the 1.8-pa. value, on the graph. At the equivalence point when a small amount of ferricyanide i present in the yolution. the electrode
Determination of Cobalt Plus Manganese in NBS Standard Samples
52 c o
Meq. taken, Co plus Mn
found,b
31eq. found, av. of Co plus Mn 6 titns. Std. dev 0.4454 0 4452 13 95 f O 07 SBS 168 0 4412 0 4445 41.S5 f 0 10 a NBS Sample 349: 13.957, Co, 0.43'5 Mn, 19.50:; Cr. SBS Sample 168: 41.205% Co, 1.50% l l n , 20.33C; Cr. Correction made for manganese oxidat'ion using S B S values.
Sample" TBS 349
VOL. 35,
NO. 9 ,
AUGUST 1963
1313
0.
0. v)
t J
0
>
0.
0.1 1
1
4.00
,
1
1
1
,
1
1
5 00 ML. O.IM K,Fe(CFD.
1
1
6.00
'Figure 1. Comparison of potential readings using polarized electrodes (0)with conventional Pt4.C.E. pair (A) in titration of cobalt(l1) with ferricyanide in ammoniacal citrate medium
VOLTS VS. S. C. E. Figure 2. Polarograms in 4M ammonia-1 M ammonium citrate a t rotating platinum electrode
_ . - - 2- X -- 2 X
10-4M K,Fe(CN)e 10-'M KdFe(CN)s
Polarizing current, 1.8 p a .
reduction process becomes that of ferricyanide to ferrocyanide, and the ~jotentialdifference between the indicator electrodes drops to a value approaching zero as the concentration of ferricyanide increases (dashed line, Figure 2). The oxidation step remains that of ferrocyanide to ferricyanide. Polarograms of 2 X 10-~11cobalt sulfate and of 2 X 10-4.11 pentammineaquocobalt(II1) chloride t,aken with a rotating platinum electrode under the same conditions ( 4 X ammonium hydroxide-111 ammonium citrate) hhowed that cobalt(I1) oxidation did not occur a t a pot'ential less positive than supporting electrolyte oxidation, about 0.88 volt. Furthermore, reduction of t,he cobalt(II1) ammine complex occurs a t a potential about 0.2 volt more negative than the reduct'ion of oxygen. Thu,., the cobalt(I1)-cobak(II1) couple does not enter into either of the potential determining steps. Removal of &solved oxygen in titration solution> hp flushing with nitrogen caused reduction of cobalt(lI1) t o become one of the potential controlling -teps and increased the potential change a t the equivalence point by 0.2 volt. However, the break is great enough to make this atep unne(-essary. Procedure for the Determination of Cobalt in Alloys and Steels. If nianganese is present': add a n excess of ferricyanide t o the dissolved alloy sample and back-titrate with standard cobalt sulfate. This is necessary because manganese is oxidized along 13 14
ANALYTICAL CHEMISTRY
with cobalt t o t h e +3 oxidation state, but a t a rate so slow t h a t direct titration is impractical. Keigh samples of a cize requiring about 4 nil. of 0.1X ferricyanide into 125-mI. conical flasks. +4dd 10 ml. of a 2 : 1 miyture of concentrated HC1 and HSOs. After the sample is dissolved, add 10 ml. of 72y' perchloric acid and heat to fumes of HClO4. L-se a fume eradicator connected to a water aspirator or a hood designed for perchloric acid vapors. Precipitates of molybdic or tungstic oxides which may appear at this time will not interfere and may be ignored. If chromium i3 present. remove by volatilization a; chromyl chloride by adding several 50to i'j-ing. portions of sodium chloride. Cool, add 25 ml. of water, and boil 5 minuteq. Cool, add water to bring the .ample volume to about 50 ml.. and pour the sample 1%-ithconstant qtirring into a 250-rnl. beaker containing 25 ml. of concentrated ammonium hydroxide, 50 ml. of 111 ammonium citrate, and 5.00 nil of 0.111 pota-sium ferricyanide. Stir for 5 minutes t o effect complete manganese oxidation. Titrate nith standard 0 . 1 X cobalt sulfate to a large change in potential, adding the last few drop. of titrating bolution slowly. RESULTS A N D DISCUSSION
Results of a series of titrations of known amounts of cobalt and manganese using the back-titration procedure are shown in Table I.
-t 2 X 10-4M
KaFe(CN)e
L-pon initial application of this method to some SBS alloys, results for cobalt ranged from 1 to 37, high because of partial chromium(II1) oxidation by ferricyanide. The most rapid and effective method for elimination of this interference is remora1 of the chromium by volatilization as chromyl chloride ( 5 ) . Result< for the determination of cobalt pluq inanganebe in two XBS samples after chromium remoTal are given in Table 11. In the.e titrations, a calrulated correction \vas made for niangane*e oxidation. using XBS value>for the per cent manganrie. In practice manganeqe can con\ enientlj be determined independently by the Lingane-Karplus method and cobalt determined by diff errn ce . LITERATURE CITED
il
Bishop, Edmund, d n a l y s t 85, 422
(19601. ( 2 ) Dickens, Peter, Ifasssen, Gerd, drch. Eisenhueltenw. 9, 487 (1935). ( 3 ; Dutoit, Pierre, ll-eisse, G . I-.>J . Chim. Phys. 9, 578 (1911). ( 4 ) ReilleJ-, C. S . ,Cocilte, IT. D., Furman, S . H., -4s.t~.CHEM.23, 1223 (1951). ( 5 J h i i t h , F. tT.> I N D . ESG. CHESI., r \ S . t L . E D . 10, 360 (1038).
BYROXK R i T O L H V I L
Departrnent of Chemistry Uiiiversit? of Wisconsin 1Iadison 6, W s .
Work Tvas supported in part by the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation.
