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 xith 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
ANALYTICAL CHEMISTRY
1668 nrecioitation. one reaxent solution was added dropwise to the ~~~~~~ ~~
~
~
~
~~~
by centrifuging t&e precipitate suspension, removing the mother liouid by decantation, and taking up the residue in water. A
egpl&ed in prelimincry trial8 made icevident that one washing cycle was ade uate By mean8 o?this.proeedure, it was possible to have a specimen washed, mounted, and inserted within the electron microscope within 1.75 minutes after the reacting solutions were first brought
~~~~~~
~
the examination process. Particles of this appearance were obtained repeatedly with this set of conditions, hut some of the voids were noted to increase in size relative to the rest of the particle while being examined. Thus the oombined oircumstances of evacuated surroundings and electron bombardment were possibly of some significance in changing the appearance of the particles, and the initial particles may have been more d i d than Figure 5, A , indicates. ."
A
B "
...., "
.~" "
marker on the micraxraDhs reprissents 1 micron. Diffraction bands appear on some-of-the mhographs, but these are not an essential part of any interpretations within this study. R
A
Fipul
2. Effect of Temperature at Tim of Formation
on B; mm Sulfate Formed by Direct Mixing of Reagent
Solutions Ba +* solution 10-1 M and in exoeas SO1-- solution IO-' M . A. A t m o m temperature. B . Near hoiling *"int
-L6y.I
-Formed
I. y..ll-
Barium Sulfate
D
-.-
____l._ R e i -"-..I
by Direct Miring of Reagent Solutions
Ba++ eolufion 10-1 M and in excess so,-- 80~ntion. A . 10-1 M . B. 1 0 - 2 M .
c. 10-8 M .
In all other instances the author believes that the particles of barium sulfate were not contaminated or altered in any significant way during observation. This opinion is based upon two observations. Several precipitates of larger particle size were examined in % light microscope, and in each such instance the general appearance was similar to that in the electron microscope except for the relative lack of detail discernible in the light microscope image. Experiments were conducted to examine the particles with the beam intensity as low as possible, then the intensity was increased f a beyond that normally employed for periods of several minutes' duration, and in no instance was any alteration of appearance noted except in the one case discussed above. Therefore, any specimen contamination must have occurred almost instantaneously when the betlm was turned on or prior to that time, and this Beems highly improbable. The influence of reagent solution concentration upon the appearance of the barium sulfate particles is illustrated in the micrographs of Figure 1. In this series, the concentration of
D . 10-4 M
This investigation was conducted during portions of a &year period, most of it within the past year. Each precipitation process was repeated several times t o ensure that truly representative particles were obtained for each set of conditions, In most cases a t least two specimen mounts were prepared from each precipitate suspension, and in several instances four or five mounts were prepared. About 150 precipitations were conducted t o study the features included in this report, and about 350 satisfsc.tory specimen mounts were prepared and examined. The mount was in each instance surveyed with visual examination of the fluorescent screen, and typical areas were photographed. In the various parts of this study, thousands of particles were observed and ahout 350 areas were photographed. Occasionally particles of some different, nontypical appearance were found, but the micrographs accompanying this report are in every instance truly representative of the typical appearance of material formed under each prescribed set of conditions. In one case, Figure 5, A , the particles may have been altered by
"purv.,.
ul".SCm."I
'L"L..L8111~"'L"PL."I.I
U U L l l l L r
1V.Iy.z"
by Direct Miring of Reagent Solutions Ba++solution 10-8 Mandiner-a SO,-- solution ' . 0 1 M . A. Right after formation. B. After 1-hour aging near boiling point
V O L U M E 23, N O . 1 1 , N O V E M B E R 1 9 5 1 A
R
Ba+* solution 10 -1 M and in ~ X G L E F P S SO'--solution. A. 10:' M , deht away. B. Same A but 4 day@ later. C. 10-8 M , right sway. D. Same as C, but 4 days latex. E. 10-4 M , right avay. F. S a m o a ~ E , b u r 4 d a ~ s l e t e r
barium chloride was held constant and this reagent was present in excess. As the concentration of sulfate is decreased from 0.1 M , the resultant particle size first inoreases and then decreases. This phenomenon has also been observed with a number of other precipitates. However, the ccysta.Is of barium mlfate are highly imperfect in all oase8. The growth along certain cryst,aI axe8 apparently proceeds a t a rate much greater than that along others. The influence of the specific temperature a t the time of precipitation is illustrated in Figure 2. Although the crystals formed from nearly boiling reagents appear somewhat more perfect than those formed a t room temperature, they are still far from perfect cryst&. Many i t t e m p b were made to precipitate barium sulfate by direct mixing of reagents under various combinations of eonditions; in every case the crystah formed appesred highly imper. ieot. The influence of digestion is indicated in Figures 3 and 4. The hotdigested particles are clearly nothing more than fairly loose aggregates of the particles present a t the start of the aging period. The degree of aggregation is 80 slight that moat of the individual ccyetals are still discernible. It is of significance that there has
1669 been no appreciable change in size of the original, imperfect crystals. The room temperature digestion reveals uo marked change in general shape nor in degree ofperfection. Furthermore, there is no significant change in sire of particle, even after this &day digestion period, except in the case of the particles farmed from the most dilute sulfate solution. With this one exception, these direct electron micrographs lend corroboration to the conclusion of Xolthoff and Noponen (5, 6) that classical Ostwald ripening does not play B dominant role in the aging of barium sulfate. The growth of the smaller crystals, formed from the most dilute sulfate Bolution, msy represent a cme of Ostwald ripening, dthough the lack of perfection even of theee aged particles indicates that this is still not the only factor involved. These observations must not be construed 8 8 contradictory to the classical Ostwald ripening concept, for in reality they lend support to it. This concept is based upon the fact that the solubility of a given substance is a function of the particle siae of the solid phase of that substance. However, the solubility of a substance is approximately constant for crystal sises greater than 1 or 2 miorons (7). Therefore, if the original particles of precipitate are smaller than this siee (Figure 4, E ) , Ostwald ripening does occur during aging, and particles of a larger s h e result. However, when the original crystals are greater than l or 2 miorons, even though imperfect (Figure 4, A and C), aging does not result in an increase in particle size. The highly irregular nature of the crystal shapes makes i t difficult to state just what each size really is, and the dependence of solubility upon particle size is based primarily upon the surface per unit mass rather than upon any cross-section dimensions. Because of these facta, it does not appear feasible to make these Considerations quantitative. However, it appears that Ost.iald ripening occur8 when the crystal size ir small enough to permit ita proper application. The particles of barium sulfate formed under the usual conditions of precipitation are of sizes such that Ostrvald ripening cannot properly occur; this observation k in esfiential agreement with the conclusion of Kolthoff rtnd Noponen (5, 6 ) that such ripening does not commonly play a major role in the aging of barium sulfate. Calcium carbonate particles (8)increase in size upon aging in mother liquid, even when the initially observed size is greater than 1 or 2 miorons. Although there may he no obvious reason for this difference between barium sulfate and calcium carbonate, the crystal sise above which the solubility is reasonably co?stant need not be the same for both substances. The magnitude of the influence of sise upon solubility is known to differ widely from one suhstanee to mother. DIFFUSION MIXING OF REAGENTS
Coprecipitation Studies. Because of the highly imperfect nature of barium sulfate crystals precipitated by direct mixing of reagent solutions, it seemed impossible to observe the influence of coprecipitants upon barium sulirtte crystals formed in this way. Therefore a different method of forming and washing the precipitates was adopted. One reagent solution was placed in a s m 1 1 beaker, a thin (about 150 A.) film of Parlodion was floated on the surface, and 8. drop of the other reagent solution was placed on this film. Two minutes were allowed for the formation of the precipitate as the reagent ions slowly diffused through the thin film. Then the film with the drop was removed with the aid of a glass slide and refloated on the surface of distilled water in another vessel. Ten minutes were allowed for diffusion of mother liquid ions from the drop into the water below; then the drop was dried off and a mounting grid was applied to the film with precipitate t,o ready it for final observation. Because of the much slower rate of mixing reagent solutions, this method would be expected to yield more perfect crystals than the met,hod of direct mixing. Preliminary tests with films of vxiou.~thicknesses, with times allowed for precipitation ranging from 30 seconds t o 2 hours, snd with various times of cleaning, revealed that the conditions
1670
.
ANALYTICAL CHEMISTRY
finally selected gave reproduoible results, although none of the procedural conditions were critical. Figure 5 shows micrographs of the diffusion-prepared precipitates, from reagent solutions of different concentrations. The most concentrated ones, Figure 5, A , should involve the fastest rate of precipitation, so that the least perfect crystals could he expected. Very possibly this particle as originally formed included much water, 80 that this micrograph may represent merely the residue left upon dehydration within the electron microscope. Some of the crystals in Figure 5 are similar in appearance to Some of those formed by direct mixing. The significant feature of Figure 5 is that micrograph E represents reasonably perfect cryptals which have been repeatedly obtained. Thus it is possible to use precipitates formed under these conditions as a reamnably perfected, reproducible type of crystal to Serve 8 6 a. reference for coprecipitation comparisons. The influence of excess sodium chloride a t the time of precipitation is shown in Figure 6 . The contaminating substance appears, a t least partially, as imperfect growths many iane thick
A
B
. .
; ,;
..
&-.
A B Figure 6 . Effect of S o d i u m Chloride a t Time of Preeipitation of B a r i u m Sulfate by Diffusion Mixing of Reagent Solntions 10-2 M Ba + + end 10 -3 M SO, - -,in presence ofNaCl coneenfration of A 0.0 M, B 10-3 M
upon the crystal surfaces. With the concentration of the foreign electrolyte even larger, some of the particles appeared like Figuie 6, B, hut still less perfect, while others were highly irregular with no apparent similarity to those of the otherwise normal particles, A similar phenomenon appears with sodium nitrate as the ezcess electrolyte available to contribute to contamination (Figurc 7). When the sodium nitrate concentration is manyfold greater t h h that of the proper ions for precipitation, the particles do not even resemble in Bhape the proper barium eulfate crystals (Figurc 7, D). This particular phenomenon is similar to that encountered in the precipitation of benaidine sulfate with excess sodium chloride as the available eoprecipitant, and the explanation map again be based upon adsorption a6 determined by a combination of the Paneth-Fajans-Hahn rule and concentration effects (8ec
..
A
.
8
C
*-~-.--,. . -:. a,
Figure 5. Effect of Concentration of Reactants on B a r i u m Sulfate Formed by Diffusion Mixing of Reagent Solutions A. 10-L M Be + * end 10-1 M SO4-B. lO-IMBa++andlO-*MSOd-C. 10-1 M R a + + s o d 1 0 - 2 M S O < - D. 10-1MBa+*andlO-'MSO.-E. 10-8 M B e + +and 10-8 M SO.--
.---
C D Fie:"re 7. Effect of Sodium Nitrate a t Time of Precipitat ion of B a r i u m Sulfate by Diffusion Mixing of Reagent SOI--, in presence ofNaNOl w)noentr.stio)n o f A 0.0 M, B 10-3 M, C 10-1 M, D 1.0 M
10-1 M B a t + and 10-r M
1671
V O L U M E 2 3 , N C . 11, N O V E M B E R 1 9 5 1 .I ,
..
B
.
..
Figure 8. Effect of S o d i u m N i t r a t e a t Time of Preeipitat i o n of B a r i u m Sulfate by Diffusion Mixing of Reagent Solutions lo-' M R r . 1 0 - , M S O i - - . a n d 5 X 1 O ~ ' M H T . I . i n o r ~ c n e r n f N e N O i
liquid results in no increase in particle size except when the particles a t the start of the aging period are very small (less than about 1 micron). This observation is in essential agreement with both t,he classical Ostwald ripening concept and the conclusion of Kolthoff and Noponen that Ostwald ripening does not normally play % dominant role in the aging of barium sulfate. Hot digestion results in Some agglomeration of the particles present when the aging period is initiated. Reasonably perfect crystals of barium sulfate are formed when very dilute reagent solutions are mixed slowly by ionic diffunian through a thin membrane. The presence of foreign elect.rolyte (eodium chloride, sodium nitrate, ferric chloride) a t the time of precipitation results in tho appearance of protuberances upon the crystal surfaces. In the presence of extreme concentration, the otherwise normal development of barium sulfate crystals is prohibited. It is suggested that these bumps, which are due in some manner to the presence of foreign electrolyte a t the time of precipitittian, are incorporated within the irregular crystals formed by direct mixing of reagent solutions. LmERATURE
CITED
(1) Fischer, R. B., J . Chem. Education, 24,484 (1947). (2) Fischer, R. B., and Ferpuson, B. L.. Pmc. I n d . Acad. Sci., 60. 145 (1951). (3) Fisoher, R. B., and Simonsen. S. H., ANAL. CHEM.,20. 1107 (19481. (4) Fischer, R. B.. and Sprapue, R. S.,Anal. chim. Acta. 5, 98 (19511. ( 5 ) Kolthoff, I. M.. and Noponen, G. E., J . Am. C h a . Soc., 60. 499 (1938).
EPgure 9. Elfeat of Ferric Chloride at T i m e of Preoipitation of Barium Sulfate b y Diffusion Mixing of Reagent Solutions 1 0 - ' M R a ' + , 1 0 - 2 M SO.--,aod5 x lO-?MHCl,inp~...,.,fFeCla concentration of A 0.0 M , B 5 X 10-1 M
4 ) . A eompmison of the coprecipitation effects of 1M sodium nitrntr and of l 1M sodium chloride as revealed by the electron micrographs indicates that the former is more serious, a comparison in :agreement with the relative solubilities of barium chloride and barium nitrate as interpreted by the Pmeth-Fajans-Hahn rule. The contaminating influence of sodium nitrate is illustrated again in Figure 8, this time in 5 medium acidified with hydrochloric acid. The appearance of the rough edges again marks the presence of excess foreign electrolyte a t the time of precipitation. Figuie 9 shows the similar effect of ferric chloride as an available cuntnminant; this comparison also involves an acid medium, so t,hat any possible interference from hydrous ferric oxide is eliminated. The eract composition and significance of the coprecipitant structure are not clear. A solid solution of eopreeipitating material aould conceivably cause a spotty eruption of the continuous ionic structure. The adsorption of impurities may prevent the smooth growth of the crystals, so that the surfaces appear erupted. I t may be that surface-adsorbed impurities on the highly imperfect crystals formed by direct mixing of reagent solutions are to some extent incorporated within the crystal structure as the irregular crystals rapidly form. Thus an adsorption process may lead to an internal solid solution of impurity as well as to an inclusion. SUMMARY
Barium sulfate crystals formed hy direct mixing of reagent solutions are very imperfect. The crystal size of barium sulfate formed by direct mixing of reagent solutions increases, then decreases, 8 s the concentration of sulfate solution is decreased from 0.1 M and the barium ion cancentration is maintained constant. Although crystals formed a t elevated temperatures are imperfect, they are more perfect than those formed a t room temperature. Room temperature aging of the particles in contact with mother
REOEITED December 20, 1950. Contribution 522 from the Chemiatry Department of Indiana University.
Calomel Electrode and Cell-Correction I n connection with the article on "Simple Calomel Eleetrodr and Cell for Polarographic Analysis" [Brunner, A. E.,Jr., and COPPER W I R E
PLATINUM W I R E
f
I
5 MM,
5 MM
i:
0 MM.
f"" I1
3MM.
Means, P. B., Jr., ANAL.CHEM.,23, 1525 (1951)], the diagram printed herewith should have been used, instead of the one t h a t was published.
