1665
V O L U M E 23, N O . 11, N O V E M B E R 1 9 5 1 tions, 0.01 ,\- and 0.001 iV, were prepared by dissolving the appropriate quantity of the salt in oxygen-free water. The solutions were transferred under carbon dioxide to a storage vessel, and stored under carbon dioxide. A solution so prepared maintained its strength without appreciable change for 2 weeks. As it seemed inadvisable to prepare more dilute solutions, it was necessary to take very small volumes of the 0.001 N solution as samples for the smaller ranges of concentration. For this purpose, a l-ml. syringe with an attached capillary was used as a weight pipet. Samples as low as 10 mg. of solution could be weighed with an accuracy of 1%. Evaporation from the capilIarftip was negligible. A saturated solution (ea. 12%) of cerous sulfate u-as prepared from G. F. Smith Chemical Co. reagent grade cerous sulfate.
RESULTS
The results of the titrations carried out by the above procedure are shown in Table 11. ACKNOWLEDGMENT
The authors gratefully acknowledge the financial aid which made this work possible. The work was begun while one (W. D. C.) held a postdoctoral fellowship from the Atomic Energy Commission and the National Research Council. 4 subsequent fe1lowship from the Eugene Higgins Trust Fund enabled the study t o be continued and completed.
PROCEDURE
LITERATURE CITED
A solution of the desired volume was made approximately 0.1 N in respect to cerous ion in 2 to 3 N sulfuric acid. These con-
(1) Furman, N. H., Cooke, IT.D., atid Reilley, C. Y., .INAL. CHEM,;
centrations were not critical. Carbon dioxide was bubbled through the solution for a few minutes to remove any dissolved oxygen. As this solution had a potential of 0.71 volt, and the indicator electrode was set a t 0.95 volt, an oxidation diffusion current of the cerous ion was set up. The indicator current was adjusted to below 0.0005 microampere by generating ceric ion to raise the voltage of the solution. The timer waa reset to zero, and the unknown ferrous solution was added. Ceric ion was then generated until the current was again below 0.0005 microampere. Rather than titrating exactly to zero current in the preliminary adjustment, an arbitrary current value close to zero could be chosen as the reference point. If the sample was titrated just to the end point, additional samples could be added to the same solution.
23, 945 (1951). (2) Heyrovsk?, J., “Polarographie,” p. 419, Vienna, Julius Springer, 1941. (3) Kolthoff, I. M., and Lingane, J. J., “Polarography,” p. 447, New York, IntersciencePublishers, 1941. (4) Ibid., p. 462. ( 6 ) Muller, Erich, “Electrometrische Massanalyse,” 6th Aufl., p. 90,
Dresden, T. Steinkopff, 1942. (6) Reilley, C. N., Cooke, TV. D., and Furman, N. H., - ~ N A L . CHEX. 23, 1030 (1951). (7) West, P. TV., Ibid., 23,51 11951). RECEIVED > l a r c h 17, 1931.
Coulometric Titration of Microgram Quantities of Vanadium in Uranium N. HOWELL FURMAN, CHARLES N. REILLEY, AND W. DONALD COOKE’ Princeton University, Princeton, X . J. This investigation was undertaken to test the application of the coulometric titration process to the estimation of milligram and microgram amounts of vanadium in the presence of macro quantities of uranium. With judicious choice of reagents for the prior reduction or oxidation of the substance that is to be determined, a high degree of accuracy may be attained. In the microgram range a sufficiently sensitive method of determining the end point must be adopted. By opposing a suitable potential to a cell composed of an indicator electrode and a reference electrode and titrating to zero current with a galvanometer of high sensitivity, indication is achieved that is capable of almost unlimited sensitivity. This procedure may be considered from the viewpoint of an amperometric or potentiometric titration. The galvanometer sensitivity is so chosen that no significant amount of material is used by the indicating process itself.
IN
RECENT years the determination of small quantities of vanadium has become increasingly important ( 2 , 4). 91though a coulometric procedure has been applied to macro quantities of vanadium (S),no work has yet appeared on application to micro quantities of vanadium. The coulometric method has its greatest advantages in the microgram region due to ease of addition of reagent, elimination of reagent impurities, and the fact that the addition of reagent does not dilute the solution ( 1 ) . Since the development of a sensitive end-point procedure ( I ) , the microgram vanadium titration seemed feasible in view of the work by Parks and hgazzi ( 4 )and Gale and Mosher (8). The principle of the “carrier ion” method of coulometric analysis is illustrated by Figure 1, A . 1
Present address, Cornel1 University, Ithaoa. N. Y.
The electrons are carried to or from the generator electrode with the help of an added intermediate ion. This carrier ion, converted a t the generator electrode to another valence state, then travels throughout the solution and reacts with the substance to be determined. If a platinum electrode in a solution containing ammonium metavanadate and sulfuric acid is subjected to a polarizing potential, a curve such as ABGHIQJ results. The wave a t B is attributed to the reduction of metavanadate to vanadyl ions, the wave at H to the reduction of vanadyl ions to vanadous ions, and the wave a t I to the reduction of hydronium ions If a current of value indicated by N is forced through the solution, the generator electrode will attain a potential indicated by K. As the supply of metavanadate ions becomes depleted during such an analysis, the voltage will drop to a value indicated by L in Figure 1, B. At this time the current is due in a small part to electrol.ytic reduction of metavanadate ions and to a larger extent to the reduction of vanadyl ions forming vanadous ions, which in being stirred through the solution will encounter and react with metavanadate ions to give vanadyl ions. Thus essentially the electrons are carried by the
1666
ANALYTICAL CHEMISTRY
ions throughout the solution to the metavanadate ions; this is equivalent to making the whole solution act as a generating electrode. However, many ions do not have another oxidation-reduction state to assist the electrode in carrying electrons to or from the electrode. In such case6 after a short period of generation, the voltage of the electrode will have changed to a value where some hydrogen will be liberated, as there is no second wave such as H . This liberation of hydrogen leads to high results.
Table I.
Titration of Diluted Metavanadate Stock Solution (0.05360 N )
Vanadium, hfg. Taken Found 13.57 13.58 13.58 13.58
Error,
%
5.453 5 I454 5.452 5.454
2.447
Generator Ma. Current, Approx.
+0.1 +O.l +O.l
53
+O.l +O.l
25
+o. 1
+0.1
Jlicrogranis
hIicrograma
21.89
21.74 21.62
J
.. a
z
l
-0.7 -1.2 -1.6 -1.2 -1.5 -1.5
21.5*5
21.63 21.56 21.56
W
a 1
1.8
APPARATUS L
K
X
Y
E
I
/.
W
a a
I
3
--
N---------r
0 .
-A A
X
K
L
0
R
The constant-current circuit described elsewhere (5) was used in conjunction with the sensitive end-point procedure recently developed (1). The solution to be titrated was held in a 100-ml. uncovered beaker and stirred by a magnetic stirrer. The generator cathode was a platinum foil 18 sq. cm. and the anode was a platinum wire dipped into a 6 N sulfuric acid medium in a separated compartment which made contact with the solution proper through a sintered- lass disk. The sulfuric acid in the compartment was kept a t a aeight above the solution to prevent diffusion of the solution to be titrated into the anode compartment. The indicator circuit consisted of a platinum wire electrode kept a t a potential of 0.95 volt on the hydrogen scale by bucking the reference electrode with the required external potential. The current was measured by means of a galvanometer (sensitivity 0.0001 microampere per mm.) in conjunction with an Ayrton shunt. This full sensitivity was never required. The reference electrode consisted of a mixture of mercury, mercurous sulfate, and sulfuric acid and gave a potential of 0.73 volt on the hydrogen scale. The complete wiring and discussion of this endpoint procedure may be found in reference (1). The current in the generator circuit was measured by obtaining the voltage drop across a standard resistor by means of a Leeds & Northrup student-type potentiometer. The time was measured by a type 5-6 timer manufactured by the Standard Time Co.
V O L T A G E
Figure 1. Polarization Curves A . Initially B . Near end point
Figure 1 also illustrates the fact that higher currents could not be utilized without liberation of hydrogen (point Q in Figure 1, A ) , even though such a second oxidation state did exist. The use of higher currents, with subsequent shorter titration time, may often be attained without the liberation of hydrogen by the addition of a proper carrier ion. The addition of ferric sulfate in this case causes the polarization curve to become ABCDE in Figure 1, A. Here a much higher current, indicated by P , can flow without liberation of hydrogen even when the concentration of the unknown has been almost completely exhausted (Figure 1, B). At this higher current value, only a small fraction of the current goes into the direct reduction of metavanadate ions and a much larger part goes into the reduction of ferric ions to ferrous ions, which in turn are stirred through the solution and reduce the metavanadate ions. A sufficient concentration of the carrier must be added so that the diffusion limit (point F ) occurs at n current higher than that to be used (point P ) . The various volumetric methods for determining vanadium are discussed in two recent papers (8, 4 ) . The method described here is similar to the method of Gale and lfosher except that, in the tests for complete oxidation of vanadium by permanganate, the standard metavanadates were reduced by sulfite prior to reoxidation rather than by alcohol.
Table 11. Titrations of Diluted Metavanadate Solutions in Presence of Uranium Uranium, Mg. 58.5 44.6 45.0 47.3 31.0 27.0 16.5 12.0
Taken Vanadium, 7
436,8 218.4 21.89 21.89 2.19 2.19 2.19 2.19
Vanadium Found,
Error,
Y
%
438.0 218.9 21.73 21.71 2.15 2.18 1.99 2.13
+o
3 +0.2 -0.7 -0.8 -1.8 -0.5 -9.1 -2.7
Generator Current, Ma. Approx. 1 8 1.8 1.8 1.8 2.5 2.5 2.5 2.5
PROCEDURE
The 10Gml. beaker is partially filled with a solution mixture containing 10 ml. of 0.5 ferric sulfate and 50 ml. of 6 N sulfuric acid. The resulting solution is first titrated to the reference potential coulometrically. At the correct potential, no indicator current is indicated on the galvanometer. Then the unknown is added, the timer set to zero, and the titration started. During the titration the voltage across the standard resistor is occasionally noted and the progress of the titration observed by closin the indicator circuit. Near the end point the current is addef in small increments and the galvanometer observed. Equilibrium is reached rapidly (close to the natural period of the galvanometer). When the galvanometer reaches zero, the time record is noted. A new sample may be now added to this same reaction mixture and generator current again added to bring the indicator current to zero. As many as six small samples have
V O L U M E 23, NO. 1 1 , N O V E M B E R 1 9 5 1 been added one after the other in this fashion with no noticeable effect upon the results. RESULTS
Table I gives the results obtained for the titration of fractions of an ammonium metavanadate solution standardized against ferrous ammonium sulfate, which in turn was standardized by means of Bureau of Standards potassium bichromatr.
Table 111. Titrations of Metavanadate after Reduction by Sodium Sulfite Followed by Reoxidation with Potassium Permanganate Taken 1 ranium, Vanadium,
Vanadium I ound,
Error,
Y
5
F
137.4
137.5 137.4 137 3
-~
.\ gI 2(1
.
+0.1 0.0 -0.1
Generator Current, .\la. Approx. 2.5 2.5 2.5
“0
1667 during a dilution or directly to the titration vessel when the ammonium metavanadate was added. Table I11 shows the results of reducing the synthetic mixtures by sodium sulfite followed by subsequent reoxidation with potassium permanganate. This served as a test for the complete oxidation of a solution containing mixed valence states of vanadium. Reduction was accomplished by addition of a slight excess of 0.1 AT sodium sulfite followed by dropwise addition of enough 0.1 N potassium permanganate to give a definite pink color which persists for 5 minutes. Then a 2-ml. portion of 1% ’ sodium azide was added to destroy the excess permanganate, and the solution was allowed to stand for 5 minutes. The sample was then introduced into the titration vessel and titrated. ACKNOWLEDGMENT
The contribution of one of the authors (W. D. C.) to this work was made possible through support from the Eugene Higgins Trust Fund. LITERATURE CITED
Table 11 shows the results for synthetic mixtures of uranium (1.1) sulfate and ammonium metavanadat,e a t various dilutions. The uranium(V1) sulfate was prepared h y heating 10 grams of uranium(T’1) nitrate x i t h 5 ml. of concent,rated sulfuric acid to fumes twice, and finally diluting t o 100 ml. Fract,ions of thie stock solution were added t o thP ammonium mrtavanadate
(1) Cooke, W. D., Reilley, 23, 1662 (1951).
C. N., and Furman, N. H.. ANAL.CHEM.,
(2) Gale, R. H., and Mosher, Eve, Ibid., 22, 942 (1950). (3) Oelsen, W., and Gobbels, P., Stahl u. Eisen, 69,33 (1949). (4)Parks, T. D., and Agazzi, E. J., . ~ N A L .CHEY.,22, 1179 (1950). ( 5 ) Reilley, C. N., Cooke. W. D., and Furman. N. H., Zbid.. 23. 1030 (1951).
REXEIVED .4prll 7, 1951
Precipitation of Barium Sulfate Investigation by Electron Microscopy ROBERT B. FISCHER, Indiana University, Bloomington, Ind. This investigation was undertaken to obtain direct information with the electron microscope concerning the mechanisms of aging of barium sulfate and of coprecipitation of other materials with it. The aging of barium sulfate crystals in contact with mother liquid does not result in any significant change in crystal size nor in apparent degree of perfection unless the particle size at the start of the aging period is well under 1 micron, in which case
T
HE precipitation of barium sulfate has been studied widelv
because of the continuing practical importance of the graviriietric method for determining sulfate arid because many factors regarding purification and perfection of precipitates are encountered. Kolthoff and Noponen ( 5 , 6) have studied in detail the aging of precipitated barium sulfate in contact with mother liquids of varied compositions. They have presented evidence that the freshly precipitated material is highly imperfect, and that the gradual perfection which follows is not predominantly due to Ostwald ripening. This perfection is rather shown to be primarily the result of recrystallizations occurring within the liquid film surrounding the particles and not through the bulk of the solution. The incorporation of impurities within a mass of precipitated barium sulfate has been attributed to a combination of several active mechanisms. A double ion layer is probably adsorbed on the crystal surfaces, yet the quantity of impurity is generally too great with respect to the surface area for this t o be the chief factor. Occlusion of mother liquid within the pockets as small crystals coalesce is recognized as a factor of considerable come-
Ostwald ripening occurs. A slight agglomeration occurs in some instances. Coprecipitation involves the formation of surface protuberances upon barium sulfate crystals formed very slowly by a process of diffusion mixing of reagent solutions. These results clarify and amplify earlier published reports on the precipitation of barium sulfate and other substances, and the micrographs obtained are of use from a teaching standpoint.
quence. Walton and JValden ( 8 , 9 )have shown by means of x-ray diffraction studies that the contamination of barium sulfate by univalent cations and by water involves a solid solution of contaminants within the crystalline barium sulfate; chemical analyses reported by these investigators reveal that the precipitate when contaminated, for example, by lithium contains in essence the species barium sulfate, lithium bisulfate, lithium sulfate, and water. An attempt has been made in the present investigation to employ the electron microscope for the observation of precipitated particles of barium sulfate, both fresh and aged and both reason-. ably pure and highly contaminated, and to look for direct indication of the aging and the coprecipitation processes. The electron microscope has been used in the investigation of several other precipitation processes (1-4). DIRECT MIXING OF REAGENTS
Standard 0.1 M solutions of barium chloride and potassium sulfate mere prepared, and the more dilute solutions of these reagents were then prepared by quantitative dilution. For each
