analysis speed than practically all other uranium methods. Work is continuing on a simple means of further separating uranium from interfering materials. Using two complexing agents consecutively along with two prereduction steps would perhaps be useful, but some other approach, such as solvent extraction, will probably be needed. This further work will be published when completed. LITERATURE CITED
(2) (3)
(a) (5) (6)
(1) Booman, G. L., “Instrument for
Controlled Potential Electrolvsis and Precision Coulometric “In-
(7)
tegration,” U. S. Atomic Energy Commission, IDO-14370 (1956). Carson, W. X., ANAL.CHEV.2 5 , 468 (1953). Duffy, W.E., Tingey, F. H., “Uranium Determnation by the Isotope Dilution Technique,” U. S. Atomic Energy Commission, IDO-14301 (1955). Furman, N. H., Bricker, C. E., Dilts, R. V., A.u.4~.CHEC 2 5 , 482 (1953). Lingane, J. J., Iiyamoto, R. T., Anal. C h i m Acta 13, 465-72 (1955). Nehemias, J. V., Dennis, R. C., Ambo, E., “Calculated Distribution of Fission Product Nuclides,” Univ. of Michigan, - . IP-109,1955. Rodden, C. J., “Analytical Chem-
istry of the Manhattan Project,” pp. 51-77, McGraw-Hill, New
York, 1950. (8) Ibtd., pp. 77-122. (9) Ibzd., pp. 122-35. (10) Ibzd., pp. 596-609. (11) U. S. Atomic Energy Commission, “Chemical Processing and Equipment,” pp. 4-5, U. S.Government Printing Office, Washington, D. C. 1955. RECEIVED for review July 9, 1956. Accepted November 12, 1956. Conference on Analvtical Chemistrv and Aa-olied Spectrosc“opy, Pittsburgh, Pa., hiarch 1956. The Idaho Chemical Processing Plant is operated by Phillips Petroleum Co. for the U. S. Atomic Energy Commission under Contract No. AT( 116-1)-205.
Ferrous and Ceric Ions as Dual Intermediates in Coulometric Titrimetry Effect of Current Density on Titration Efficiency of Electrically Generated Ceric Ions A. JAMES FENTON, Jr., and N. HOWELL FURMAN Frick Chemical laborafory, Princefon University, Princeton,
.The coulometric generation of ceric or of ferrous ions from an acidified solution containing cerous and ferric sulfates offers numerous advantages in connection with oxidations or reductions that require an excess of one reagent followed by back titration. In studying the effect of current density upon the efficiency of titrations performed with ceric ion, approximate maximum and minimum limits were determined. More detailed measurements were made near these limits to find the range of error as a function of current density. Between 1 and 13 ma. per square centimeter, the or apparent efficiency is 99.7% better in the generation of ceric ion. There is a gradual decrease in current efficiency above and below this range.
D
in constant current coulometric titrations mere apparently first used b y Swift and coworkers ( 3 ) . Their work dealt with the generation of cuprous ion or of bromine from aqueous cupric bromide mixtures. I n studying the equilibrium conditions in this system ( 7 ) , they applied i t t o back titrations after generating a n excess of bromine to complete the bromination of aniline. UAL INTERMEDIATES
N. J.
Cupious ion n a s generated to titrate the excess bromine ( 3 ) . Takahashi and coworkers (1I) have used ferric-cerous mixtures as dual coulometric intermediates in connection with the estimation of certain organic substances. They inyestigated the determination of micro amounts of oxalic acid and of 2-naphthylamine by generating excess ceric ion, then back titrating the excess with ferrous ion. Their results for oxalic acid n-ere from 2 to 4% high. They studied the effect of variations in the content of cerous ion and of current density as applied in the coulometric determination of hydroquinone with electrolytically generated ceric ions. They found that the current density should be below 0.1263 ma. per square centimeter when the cerous concentration was 0.00144N and below 0.50 ma. per square centimeter Kith a cerous concentration of 0.0048N in order to obtain 95 to 100% current efficiency. I n the present investigation, known amounts of ferrous ion were generated electrically in deaerated solutions under inert atmospheres a t current densities and current levels known from previous experimentation to operate at substantially 1 0 0 ~ otitration efficiency. Ceric ion was generated electrically under identical conditions, except t h a t the current density was varied above and
below that used in generating ferrous ion in the same solution. In this way numerous comparisons could be made of the difference Detn-een the equivalents of ferrous and ceric ions so generated, and the over-all efficiency of the titration with ceric ion was calculated from the difference thus found. The same current level was used in both ferrous and ceric generation, thus simplifying the calculations. Variations in the anodic current density n-eie made by using various anodes of measured areas. Preliminary experiments showed, however, that the presence of phosphoric acid was necessary for the attainment of stable galvanometer readings in the potentiometric indicating system. Kithout phosphoric acid apparent eirois of I to 2% were found, and with fairly large deviations. The phosphoric acid complexes both ferric and ceric ions and greatly improves the sharpness and steadiness of the readings near the equivalence point. Potentiometric titrations made in the generating medium showed clearly the improvement due to phosphoric acid in the form of the graph of e.m.f. us. ml. of reagent. There appears to be improved ease in generation of ceric ion near end points as well a s increase in the speed of reaction of the ceric ion with ferrous because of the VOL. 29, NO. 2, FEBRUARY 1957
221
effects of complexing action of the phosphoric acid. APPARATUS A N D SOLUTIONS
Known electric currents were obtained with the coulometric supply described by Reilley, Cooke, and Furman (IO) and modified b y Reilley, Adams, and Furman (9). The exact current was measured by reading the voltage drop over a precision resistor with a tolerance of =tO,O5% (General Radio Co., Cambridge, Mass.) with the aid of a potentiometer (Leeds & Eorthrup, Catalog No. 7655). Current integration was accomplished by using an integrating motor (Model 903, Electro hlethods, Ltd., Stevenage, Herts, England) which was first used in coulometry by Bett, Nock, and Morris (8). This motor was operated in its range of linear calibration, 12 to 24 volts (1). It was connected in parallel with a standard resistor suitable for the current level at each current range. The motor counter readings were calculated to microequivalents at each calibration point by the following relationship: motor factor (in microequivalents per motor count) = 1000 0.0027)
(F+
where R is the resistance of the parallel resistor in ohms. A fuller description of the integrating motor is given by Parsons, Seaman, and Amick (8). A decade resistance box (Otto Koulf. Berlin) served as a convenikt source of fixed parallel resistances. Generating Medium. This was 0.1M reagent grade ferric alum, 2 X in sulfuric acid. Cerous sulfate octahydrate was added t o t h e ferric solution in a n amount of 8 grams per 100 ml. Fresh portions of this solution were used daily because phosphates a r e precipitated eventually after addition of phosphoric acid. Electrodes. All electrodes used for generating ceric or ferrous ions were of rigid, smooth platinum-10% iridium foil, supported on stout platinum wire sealed in 6-mm. lead-glass tubing. Electrical contact m s made by copper wire fused to t h e protected end of t h e platinum iead wire. The dimensions of electrode and exposed wire were measured and t h e apparent areas were calculated geometrically: Electrode A
R
c
D
Area, Sq. Cm. 2.20 =I=0.02 4.32 2= 0.02 8.13 i 0.02 2.51 i 0.02
These electrodes w r e allowed to stand in concentrated nitric acid or in 0,LY ceric sulfate solutions when not in use. They were pretreated before each set of determinations by anodizing and cathodizing each one in freshly prepared dilute sulfuric acid solution a t a current of approximately 20 ma. for 2 to 5
222
ANALYTICAL CHEMISTRY
minutes. Better response was obtained when such pretreatment was used, The isolated electrode was a 3-inch platinum wire immersed in 3M sulfuric acid in a half-cell. The side arm had a plug of filter paper. Indicating System. T h e sensitive end point procedure (5) was used with a galvanometer having a rated sensitivity of 0.005 pa. per mm. at 1 meter, and a suitable Ayrton shunt. The reference electrode was a lead amalgamlead sulfate half-cell filled with 3M sulfuric acid; a paper plug was used in the side arm to prevent diffusion. The cell potential was -0.300 volt us. a normal hydrogen electrode (K.H.E.). A platinum foil electrode, 1 x 1 cm., was used as the indicating electrode. The preset potential was 0.95 volt relative t o the K.H.E. This setting was based on data obtained by micropotentiometric titrations under the conditions that were to be used in the coulometric titrations. The electrolysis cell was a molded glass jar of 125-ml. capacity, fitted with a rubber stopper through which appropriate openings were drilled for electrodes and gas inlet and outlet tubes. Pure carbon dioxide or nitrogen, washed when necessary with appropriate solutions, was used to deaerate solutions and to protect the surfacc of the liquid. A magnetic bar stirrer
( S o . S-6935 Scientific Glass Co., Bloomfield, K. J.)! regulated with a Variac, was used. The setting was such as to prevent cavitation. PROCEDURE
I n the cell were placed 90 ml. of the cerous-ferric stock solution and 10 nil. of 85% phosphoric acid. After deaeration for 10 to 15 minutes by a stream of carbon dioxide or nitrogen, the gas delivery tube was raked so that its tip was above the surface of the solution to maintain an inert atmosphere. Then ceric or ferrous ion was generated as needed to bring the solution potential to the present value of 0.95 volt. The maximum galvanometer sensitivity was used in the final adjustment. The motor count was noted and recorded as initial count. Ferrous ion was then generated for at least 10 counts, and generally from 20 to 80 counts, because the motor scale can be read only to 1 0 . 0 1 count and a precision of 1 p.p.t. or better n-as desirable. After the generation of ferrous ion at the cathode, the counter was read and noted as ferrous count. Ceric ion was then generated a t one of the four anodes and. after the galvanometer x i s restored to its initial setting a t the end
Table
I.
Generations at Different Current Densities
(Separate electrodes used for ferrous and ceric generation) Anodic Current Current Density, &[a. per aiv. weq. Generated" Error, Av. Dev., Electrode S o . of Level, Ferrous Ceric P.P.T. P.P.T. Sq. Cm. Ma. Used Espts. 2 4 3 3 3 4
4
20 22 24 24 25 26 27 28 2s 29 29 30 30 31 33 36 38 46
3
61 00
4
40 34
3
3 2 3 3 2 3 3 3 4 1
C
3
3 4 B
6 3
A
5
3
30 21 15 11 8 4 2
20 00
16 55
64 85 70 65 75
27 85
60 95 T5
40 64 35 90
25
17
28 62 134 100 OS2
High Current Densities 9.18 33. T.5 10 0 38 42 11 0 60 90 11 2 58 58 35 45 11 7 12.2 35.31 12 li 27 1.5
is o
13 1 13 3 13.6 13.9
14 14 15 16 17 21 27
1 4
2 7 4
3 7
69 69
39 98 42 42 41.51 32.45 50 46 82 01 54 16 55 T 1
49 52 58 10 53 06
Low Current Densities 117 5 4 96 92 53 3 '72 60 28 2 60 57 95 1 88 36.21 1.43 31.5'7 1.00 18.63 0.503 7.717 0.256
33.84 38 48 60 91 58 62 35 50 35.40 27.24 69 88 40 13 42 59 41.61 32.61 50 83 82 55 54 45 56 18 50 15
58.75 54 64
117 7 92 59 60 29 58.00 36.28 31,68 18.73 7.829
2.7
2.0
1.6
1.1
0 2 0 7 1 4 2 5 3.3 2 7 3.8 4 0 2.4 4.9 7.3 6.6 5.4 8.5 13
0 5
11
30
0 9 0.5
1 6 1.5
1.1 0.5
0.7 0.3
1.0
1.9 0.9 1.4 2.'0 1.0 2.7 8.5
1.7
0.4
0 6 0.2 0.9
0.1
1.9 3.5 5.4 14.5
0 2
0.5 0.6 0.6 0.9
0.2
a I n all cases ceric count is greater than the ferrous count, which is interpreted as an efficiency less t h a n 1007' in generation of oxidant.
at 100% current efficiency. This posiTable II.
I.:lcctrode Used
Generation of Both Ceric and Ferrous Ions at Same Electrode
S o . of
Current Level,
Espts.
Ma,
Current Density, Ma. er Sq. 8 m .
Av. /-W.Cknerated Ferrous Ceric
Error, P.P.T.
hv. Dev., P.P.T.
Large Electrode C 2
4 2 3 2
2 2
26 28 29 30 41 46 61
85 20
3 3 3 3 5 5 7
0:
45 31 90 00
4 5 6 8 1 8 5
22 43 71 46 86 65 106
21 84 57 06
22 21
05
05
82 2
43 71 46 86
88
il
14
65 88 106 6
1 4 0 9 2 0
1 0
38
0 I 0 0 0 0
1 0 1 6
0 7 0 8
l i
0 0 0 9
7 1 9 0 2 5
Small Electrodes R
4 4 3
D D
i 858
1 82 1 99 0 831
5 008
2 086
point a t maximum sensitivity,
20 01 31 20 7 597
1 1
ference, if any, caused by the order in lJ.hich the reagelltsare generated. DISCUSSION
The results and coiiclusions drawn from this study depend to some degree on the assumption that in all cases ferrous ion can be generated electrically and titrated coulometrically with ceric ion a t 100% titration efficiency. This assumption is substantiated by experience iyitli this process in these laboratories and in work of other experimenters. Both higher and l o m r current densities have been used in these other investigations (4, 6). The data which r e r e obtained a t current densities up to 13 ma. per square centimeter (Tables I, 11,and 111) indicate that ceric ion is not generated
I n a nuinher of cases ceiic ion was gcnerated first, folloir.ed by generation of ferrous ion. The general procedure was analogous to that which has been outlined. RESULTS
The results of experiments for regions of high and lorn current density in the generation of ceric ion are tabulated in Table I. Confirmatory data on the electrical generation of both ferrous and ceric ions a t the same electrode and a t tlie same current density are given in Table 11.
Ceric Ion Generated First, Followed by Generation of Ferrous Ion
Current Level, hIa. 20.40
Current Density, M a . per Sq. Cm. 2 51
7.858
1 .82
5,076
2 03 ._
2.096
5 4
Table I11 contains data on cases in
the
(ferrous count - initial count) X motor factor = microequivalents of ferrous ion generated (ceric count - ferrous count) X motor Emtor = microequivalents of oxidant gcnerlzt ecl
Table 111.
20 03 31 25 7 638
0.83
-~ prq. Generated
Diff., Ceric Ferrous, c
Ceric 40 95 67 85 40 73 40 74
Ferrous 40 95 67 85 40 35 10 62
16.36 21.82 10.89
16.34 21.77 10.89
0.I 0.2
17 I 1
17 06
O R
19.65 17.08 23.93
17.06 23.89
0.1 0.2
12.56 8.287
12.50 8.231
0.5 0.7
-.
is 56
0 9 0 3
...
0.4
tive error might be due to other effects such as oxidizable impurities in the reagents or thc instability of the substances that are generated. Pretitration to a preset reference potential might rule out the impurities as the source of the overtitration that is needed JT-hen ceric ion is being generated. However, the wide range of current levels, current densities, and reproducibility at any given level over the large number of test experiments appear to rule out these causes of the uniformly high ceric count. A comparison of the results in Tables I and I1 indicates that the errors in the experiments carried out at various current densities are independent of the current level. Only by changing the current density is the magnitude of the errors changed. The magnitude of these apparent errors would naturally be enhanced if the titration efficiency of ferrous ion were less than 100%. For practical purposes, modeiate changes in current density near tlie upper or lower limits do not cause h i g e changes. Tliose determinations run near the e\trenies of current density show a much greater error. A systematic study of the effect of current density would be very desirable when any novel process of coulometric titration is being developed. As far as is known, the present study is one of the first to be made of the effect of this important variable.
LITERATURE CITED
(1) Amirk, R. M., senior thesis, Princc-
ton University, May 1954. ( 2 ) n e t t , X., Nock, W.,Morris, G., .Inalyst 79, 607 (1954). (3) Buck, R. P., Swift, E. H., ANAL.
CHEM.24, 499 (1952). (4) Cooke, W. D., Furman, S . H., I b i d . , 22, 896 (1950). (5) Cooke, W. D., Reilley, C. N., Furman, IT. H., Zbid., 23, 1662 (1951). ( 6 ) Zbid., 24, 205 (1952). ( 7 ) Farrington, P. S., hieier, D. J., Snift, E. H., Zbid., 2 5 , 591 (1953). (8) Parsons, J. S., Seaman, W.,Amick, R. AI., Zbid., 27, 1754 (1955). (9) Reilley, C. S . , Adams, R. N., Furman, S . H., I b i d . , 24, 1044 (1952). (10) Reilley, C. X., Cooke, W. D., Furman, N. H., I b i d . , 23, 1030 (1951). (11) Takahashi, T., Komoto, K., Sakrirai, H., Rept. Znst. Z n d . Sei., I:nin. Tokyo 5 , 133 (1955).
RECEIVEDfor reviex June 26, 1956. Accepted October 27, 1956. VOL. 29, NO. 2, FEBRUARY 1957
223
