June, 1959
PRECIPITATION OF LEADSULFATE AT ROOM TEMPERATURE
817
STUDIES ON FORMATION AND AGING OF PRECIPITATES. XLVI. PRECIPITATION OF LEAD SULFATE AT ROOM TEMPERATURE’,’ BYI. M. KOLTHOFF AND €3.
VAN’T
RIET
Contribution from the School of Chemistry of the University of Minnesota, Minneapolis, illinn. Received December 87, 1968
Lead sulfate crystals were precipitated by rapid mixing of solutions of lead perchlorate and alkali sulfates or sulfuric acid of varying concentrations in polyethylene beakers. Except with potassium sulfate sharp maxima in particle size were found a t intermediate supersaturations. I n equimolar lead perchlorate- sulfate suspensions the maximum occurred at the following hl with ammonium SUIconcentrations of lead sulfate: 6.4 x 10-3 M with sodium, 6.2 x 10-3 M with lithium, 6.6 X fate, 7.0 X 10-3 df with sodium bisulfate, and 6.9 X 10-3 ill with sulfuric acid. At lead sulfate Concentrations greater than those a t the maximum the particle size and the relative degree of perfection decreased rapidly, while the specific surface determined by the radioactive method increased sharply. Specific surfaces of highly imperfect crystals were determined by the radioactive method in a medium of 60-90% ethanol. The same results in exchange experiments were found using radioactive sulfate (S3604) and lead (Pb210). A plot of the log of the induction period t versus the log of the molar concentration cI of lead sulfate was found to be composed of three straight lines, indicating that the usual relation: log t = constant m i n , does not hold for lead sulfate. A t the high supersaturations used in the present study the nucleation reaction rate increases greatly toward the end of the induction period, a result which is in contrast to a homogeneous nucleation reaction as the only source of formation of nuclei.
I n recent years many studies have been made of pensions of the precipitates with solutions conthe kinetics of precipitation of slightly soluble salts taining radioactive lead or sulfate. Induction periods were determined in two differa t relatively low supersaturations. O’Rourke and Johnsona reviewed the theories of nucleation and ent ways. In one method the time elapsed before crystal growth under these conditions. Their con- appearance of a precipitate was determined. The clusion is that nucleation occurs as a homogeneous other method is novel and based on the fact that reaction in relatively weakly supersaturated solu- the dye wool violet is strongly coprecipitated with tions. Systematic studies on solutions of very high lead sulfate. Wool violet was added to the superdegree of supersaturation are lacking in the litera- saturated solutions of lead sulfate a t various periods ture. Christiansen and Nielsen4 determined in- of time after mixing of the reactants. When the duction periods in highly supersaturated solutions dye was added during the induction period a maxiof barium sulfate and of silver chromate, but their mum amount of dye was found in the crystals. This interpretation assuming homogeneous nucleation is amount decreased when the dye was added after not supported by other experimental evidence, as the induction period. Only surface adsorption was shown in a recent study by Nielsem6 Results was observed when the dye was added after conion the precipitation of lead sulfate described in the plete formation of the crystalline precipitate. Thus present paper are also contrary to homogeneous nu- the curve giving the amount of wool violet in a given weight of lend sulfate plotted us. time of addicleation a t relatively large supersaturations. The solubility of lead sulfate is more than ten tion after mixing gives a measure of the induction times greater than that of barium sulfate and a wide period and of the time of completion of crystallizarange of supersaturations can be studied in the pre- tion. cipitation of lead sulfate. In our study supersatuExperimental rated solutions of lead sulfate were prepared by Chemicals.-Merck Analytical Reagent grade potasrapid mixing under conditions of rapid stirring of sium, sodium and ammonium sulfates were used to prepare equimolar solutions of sulfates and lead perchlo- 0.05 14 stock solutions in twice distilled water. These were rate. Under standardized and reproducible condi- filtered through fine glass filters before use in precipit,ation tions of precipitation the effect of concentration esperiments. “Chemically pure” lithium sulfate of Riedel Haen and anhydrous sodium perchlorate of G. F. Smith and of the specific nature of the reactants on the de Co. were used to make stock solutions 0.10 and 2.00 M , characteristics of the precipitates was studied. respectively, in these reagents. Lead perchlorate mas prepared according to the directions The habit of crystalline fresh precipitates was observed microscopically and the approximate size of Hershenson, et al.7 Reagent grade yellow lead oxide of Chem. Co. was used to neutralize 70% Ba!cer estimated from these observations. Specific sur- General Analytical Reagent grade perchloric acid. After filtration faces were determined by a modification of the through a fine glass filter the solution appeared to be strongly method of Paneth6 using exchange with the radio- buffered a t pH cu. 5.0. Perchloric acid was added to lower active isotopes P b 2 1 0 and S3504. Also, the appar- the p H to 3.0 and then the solution was diluted to a concentration of 1 M in lead. Unless ot.herwise stated nll reac,tant ent degree of perfection was determined by measur- solutions were acidified to a p H of 3.0 with perchloric acid. ing t.he apparent rate of exchange of aqueous susRadioactive Materials .-Pb2’0 (Ra-D) was separat,ed from deposits in old radon bulbs in which this lead isotope accumulates. The deposits were dissolved in a mixture of dilute nitric and hydrofluoric acid. .4ftcr adjusting the Presented before the 134th A.C.S. Meeting, September, 1958,Chicago, pH to 2 with dilute sodium hydroside, Ra-E and Ra-F were Ill. removed by extract’ing twice with 0.005y0 dithizone ill car(2) This investigation was supported by a grant froin the Office of bon tetrachloride. Ext.raction of lead 18 insignificant a t this Ordnance Research. pH. The aqueous layer containing Ra-D was adjusted (3) J. D. O’Roiirke and R . .4. Johnson, Anal. Chem., ‘27, 1699 (1955). to pH 10.5 with 1 N ammonia. After addition of dithizone (4) J. A. Christiansen and A . E. Nielsen, Acta Cliim. Scand., 6 , 673 the lead dithizonate wts est,ractecl at this pH in cnrbon
(1) From a Doctor’s Thesis, submitted by Bartholoineus van’t Riet t o the Graduate School of the University of Minnesota, 1957.
(1951).
( 5 ) A. E. Nielsen, J . Colloid Sei., 10,570 (1955). (0) F. Paneth, 2. Eleklrochem., 28, 113 (1922).
(7) H. M. Hershenson, M. E. Smith and D. N . H I I I I I C J ., Am. Chen. Soc., 76, 507 (1955).
I. M. KOLTHOFF AND B. VAN’T RIET
818
Vol. 63
0.001 0.003 0.005 0.007 0.010 Initial molarity of lead and sulfate in mixture. Fig. 2.-Relation between particle size and concentration in mixtures of solutions of lead perchlorate and of various sulfate: I, H,SO,; 11, Na&3Od; 111, NaHS04; IV, KzSO4.
Radioactivity of solutions waa measured after evaporation by exposure to infrared radiation of a neutralized aliquot portion. Counting was done using a thin mica window Geiger counter TCG2-1B-84 connected to a conventional scaler unit. Method of Precipitation.-In order to get reproducible precipitates the reactant solutions must be mixed rapidly. Generally 50 ml. of each solution (pH 3.0) was used to give the reported concentrations in the mixtures. Either the sulfate or the lead solution was poured within one second into the other reactant solution from graduates of 50 ml. content. Rapid mixing was carried out in a 450-ml. polyethylene beaker in which a perpendicularly bent silicone coated glass rod was stirred at a rate of 500 r.p.m. In experiments in which the concentrations of lead and sulfate exceeded 0.05 M in the mixture the sulfate solution was poured within one-half of a second into the lead solution from a 50-ml. beaker, or vice versa. With rapid mixing no difference in the properties of the crystals was observed between “direct” and “reverse” precipitation. Generally the experiments were carried out at 25 f 0.1”. Crystal growth was allowed to proceed in stirred mixtures of the reactants. When the concentration of the reactants Af the precipitation was virtuwas greater than 2.5 X ally complete within 5 minutes and the precipitate then was examined under the microscope. Under the specified experimental conditions deviations from mean particle length were found to be small. As an example, a size distribution curve is illustrated in Fig. 1. The precipitate waa formed from a mixture 5 X 10-8 M in df in lead perchlorate under sodium sulfate and 5 X the specified conditions. Appreciable deviations from mean size were observed if precipitation occurred in non-stirred solutions, or when glass beakers, or even silicone coated glass beakers were used. Precipitates in glass beakers contained agglomerates of very small crystals, which prohibited a precise count. Change of Supersaturation During Crystal Growth.-The following methods were used. ( a ) At a given time after mixing the supersaturation was suddenly decreased to saturation conditions by addition of a complexing agent, e.g., E.D.T.A., acetate or by dilution with 0.001 N perchloric acid solution. ( b ) At s given time after mising the supersaturation was suddenly increased to a value at which a different crystal habit is observed upon direct mixing. Effect of pH.-Because appreciable hydrolysis to basic lead salt occurs in unacidified lead perchlorate solutions, the effect of pH on particle size was studied in mixtures of solutions of lead perchlorate and sodium sulfate. Pronounced variations in size were observed with mistures of pH higher than 3.2. Unless stated otherwise the pH of the mixtures was 3.0.
tetrachloride, from which the lead was extracted again with aqueous 0.1 N perchloric acid. The extractions were done a short time before each experiment. Pbz10has a half-life of 22 years; it decays to Bizlo. The pparticles given off are too soft t o be measured with an ordinary Geiger counter. The daughter element Bizlodecays with a half-life of 5 days to PozlO; p-particles of high energy are emitted which can be measured with a Geiger counter. In exchange experiments using Pb210 the quantity of this isotope can be measured by determination of the equilibrium concentration of Bill’ in the sample. This build-up requires a waiting period of 15 days a t least. Correction was made for non-attainment of equilibrium. Radioactive S36 was obtained from Oak Ridge in the form of HzS3504 in 0.4ml. of 0.89 N hydrochloric acid (no inactive sulfur). The half-life of S35 is 87.1 days (p-decay). The energy of the p-particles is 0.167 Mev., and sufficiently high to be measured with the Geiger counter. The activity of the solution was 10 mcuries on arrival. The solution was diluted 10,000-fold with twice distilled water. The low concentration of sulfuric acid in the stock solution eliminated the possibility of precipitation of lead sulfate on addition of the radioactive solution t o saturated solutions of lead sulfate, even if they contained a large excess of lead ions. Microscopic and Radioactive Measurements.-Precipitates were observed under a Spencer microscope and photographs were made using a Bausch and Lomb eyepiece camera. Length measurements on crystals were made, using a Leitw micrometer eyepiece which was calibrated with a grating on A slide.
Results Particle Length as a Function of the Concentraof solutions of tion of the Reactants.-Mixtures lead perchlorate and of sulfuric acid or alkali sulfate were made in concentrations varying from 0.001 to 0.1 M lead and sulfate in the mixtures. After precipitation from stirred solutions the length of the crystals was measured. Some of the results are presented graphically in Fig. 2. Under the same conditions the curves in Fig. 2 obtained with lithium and ammonium sulfate, MOdiuni bisulfate and sulfuric acid were very similar to those found with sodium sulfate. At a concentration of 1.25 X 111rhombohedra were observed (Fig. 3A). Increasing the concentration to 2.5 X 10-3M (Fig. 3B) and 5 X M (Fig. 3C) resulted in a deformation of the rhombohedra to rectangular crosses with preferred growth in one direction, while an increase t o 6 x 10-3 M with sodium sulfate as a precipitant produced crystals composed of obtuse angle crosses which reacted a maximum length a t a Concentration of 6.4 x 10-3M (Fig. 3D). The length decreased sharply with increasing con-
2olt-dddfar 10 0
20 30 40 50 60 70 Length of the particles (N). Fig. 1.-Size distribution of lead sulfate crystals. 10
120 i ,g 100
}
.M 13
g
80
2j
20
0
.
*
June, 1959
Fig. 3.-Photographs
PRECIPITATION OF LEADSULFATE AT ROOM TEMPERATURE
819
of lead sulfate crystals, 53Ox; the concentration and kind of reactants are given in text.
I. M. KOLTHOFF AND B.
820
4 8 12 16 20 24 Hr. of contact with radioactive soln. Fig. &-Exchange between 205 mg. of lead sulfate and 53 ml. of 5.0 X 10-6 M sulfuric acid: I, solution 80% ethanol; 11, solution 90% ethanol.
$0.6
-
VAN'T
RIET
Vol. 63
centration (Fig. 3E: 7 X 10+ M ; F: 0.01 M lead sulfate). With potassium sulfate as a precipitant no sharp maximum in particle length was found (curve IV, Fig. 2). Also, the crystal habit was different from those observed a t the same lead sulfate concentration with the other sulfates. Figure 3G and H show crystals from solutions 5 X low3M and 0.01 M in lead sulfate, respectively, with potassium sulfate as the precipitant. Crystals from solutions of the same lead sulfate concentrations, but with sodium sulfate as the precipitant, are reproduced ill Fig. 3C and F, respectively. Specific Surface of Fresh Precipitates.-The radioactive method of determining the specific surface of lead sulfate in equilibrium with its saturated solution originates with Paneth.6 Extensive studies by Kolthoff and Rosenblum* on the exchange of radioactive lead between lead solutions and solid lead sulfate showed that as a result of recrystallizations the extent of exchange becomes greater than that which corresponds to the original surface and it increases with time of shaking. I n order to minimize the effect of recrystallization the exchange experiments were carried out in media which were SO-SO% in ethanol. The apparent exchange was calculated from the following relationship
m
L
.e
d
0.4
-5 a Y
d
5 0.2
4 6 8 10 12 14 Molarity of lead and sulfate in mixture ( X 1000). Fig. &-Surface exchange of lead sulfate as a function of the concentration in the precipitation mixtures: I, NarS04; 11, KrSOa.
6 12 18 24 30 36 43 48 Hr. of contact with radioactive soln. Fig. 6.-Apparent rate of exchange between lead sulfate and aqueous solutions of sodium sulfate. Precipitates from mixtures of solutions of sodium sulfate and lead perchlorate of the following concentrations in the mixture: I, 0.01; 11, 0.0075; 111, 0.0063 $1. Mixtures of potassium sulfate and lead perchlorate solutiona were used of the following concentrations in the mixture: IV, 0.01; V, 0.005; VI, 0.003 41.
(Mso1 = mmoles of tagged lead or sulfate in solution which contains all the initial radioactivity; Mprec= mmoles of solid lead sulfate; A. = initial activity of an aliquot portion of solution; A t = activity of a same aliquot of solution after time t ) . As an example, the variation with time of the extent of exchange between 205 mg. of a 20 minutes old precipitate separated from a mixture which was 12 X M in lead sulfate (with sodium sulfate as precipitant) and 50 ml. of 5.00 X low5M radioactive sulfuric acid in 80 and 90% ethanol, respectively, is given in Fig. 4. Because of rupture of crystals upon continuous shaking of the suspensions during exchange experiments the vessels containing crystals and radioactive solution were swirled repeatedly by hand for a few seconds before centrifugation and sampling of the supernatant liquid for activity measurements. The exchange extrapolated to time zero (Fig. 4) yielded a surface corresponding to 0.66 and 0.63 mole % of sulfate in 80 and 90% alcohol, respectively. It is of interest to mention that the same values were found when radioactive lead instead of sulfate was used in the same alcoholic media containing radioactive lead as lead perchlorate. This result has been found with all precipitates tested and agrees with results obtained by Stow and Spinksg with lead sulfate. Specific surfaces of 20 minutes old precipitates obtained with sodium and potassium sulfate respectively as precipitants from mixtures containing (8) I. M. Kolthoff and C. Rosenblum, J. A m . Chem. Soc., 55, 2656 (1933); 56, 1264, 1658 (1934); 57, 597, 607, 2573, 2577 (1935): 88, 116, 171 (1936). (9) R. M.Stow and J. W. T.Spinks, Canadian J. o/ Chem., 33, 938 0955).
I
a
-
June, 1959
PRECIPITATION OF LEADSULFATE AT ROOM TEMPERATURE
varying amounts of lead sulfate are given in Fig. 5 . The differences between sodium and potassium sulfate are given in Fig. 5. The difference between sodium and potassium sulfate is strikingly noticeable. Varying the initial concentration of lead sulfate from 0.005 t o 0.01 M gave a surface exchange of 0.030 and 0.59 mole yo of the solid, respectively, with sodium sulfate and of 0.19 and 0.22% with potassium sulfate as precipitant. This large difference in specific surface of crystals obtained with either sodium or potassium sulfate as precipitant is in qualitative agreement with the results presented in Figs. 2 and 3. Degree of Perfection of Precipitates.-DebyeSchemer X-ray photographs of crystals formed a t all concentrations of reactants gave the normal pattern of orthorhombic lead sulfate. In order to distinguish between different degrees of perfection of crystals obtained a t different supersaturations the rate of exchange between radioactive sulfate or lead in solution and solid lead sulfate was determined. After establishment of surface equilibrium the rate of additional exchange is a measure of the rate of recrystallization which indicates the relative degree of perfection of the crystals. I n comparing rates of exchange of various precipitates the weight of precipitate, volume and concentration of the radioactive solutions and method of treatment were kept the same. Under these conditions the surface areas exposed to the solution by the precipitates are different because the specific surface of the various precipitates is different,. Rates of exchange were determined using 0.65 mmole of lead sulfate and 50 ml. of an aqueous A l in tagged sodium sulfate and solution M in perchloric acid. After completion of precipitation the crystals were separated by centrifugation, washed twice with M perchloric acid and once with inactive d l sodium sulfate solution. The centrifuged precipitates were 20 minutes old before being in contact with the radioactive solution. The mixtures were shaken in 100-ml. bottles in a horizontal shaker with a stroke of 4 cm. The exchange as a function of time is given in Fig. 6 for a variety of precipitates. Exchange with precipitates from solutions 0.005 M in lead perchlorate and in sodium sulfate was only 0.5 mole % sulfate after 3 days. Rates of exchange also were determined between lead sulfate and A l tagged lead perchlorate in aqueous M perchloric acid. I n these experiments the final washing of the crystals was made with inactive 0.001 M lead perchlorate instead of sodium sulfate solution. Rates of exchange were approximately the same as those found in sodium sulfate solutions containing radioactive sulfur. Precipitates from mixtures 0.01 M in lead perchlorate and 0.01 d l in sodium sulfate gave 100% exchange after 24 hours of contact between crystals and solution. Portions of this precipitate were aged for four hours in various media and the rate of exchange determined. The results, plotted in Fig. 7, show that aging is promoted by increasing concentration of sulfate in the aging medium while an excess of 0.01 M lead inhibits the aging. The influence of adsorbed wool violet on the rate
82 1
4 8 12 lli ‘LO 24 38 82 Hr. of contact with radioactive soln. Fig. 7.-Effect of aging on the rate of exchange: I, fresh precipitate (20 minutes old); 11, 111, I V and V, exchange after 4 hours of aging in the following media, respectively; 11, 0.01 AI lead perchlorate; 111, saturated lead sulfate in 0.001 AI perchloric acid; IV, 0.011 M ; V, 0.10 M sodium sulfate.
36 32
ii
1
2
3
4
5
6
Hr. of contact with radioactive s o h . Fig. %-Exchange in the presence of the dye wool violet: I, no wool violet; 11, 6 mg. W. V./liter (adsorption 1770 of saturation value); 111, 30 mg. W. V./liter (adsorption 45% of saturation value); IV, 90 mg. W. V./liter (adsorption 78% of saturation value); V, 200 mg. W.V./liter (adsorption loo$&surface saturation, the adsorption was 4.55 mg. W. V./g. lead sulfate).
of exchange was determined by addition of varying amounts of the dye to the suspension before adding radioactive sulfate. These experiments were done in aqueous solutions and the measurements of small rates of exchange in sulfate exchange experiments could be done accurately only in solutions with low concentrations of sulfate. The precipitates were prepared by mixing 33.5 ml. of 0.022 M lead perchlorate with 33.5 ml. of 0.02 14‘sodium sulfate (pH 3.0). Two minutes after mixing and stirring wool violet solution (4 g. of W. V./liter) was added in varying amounts. Five minut,es after W. V. addition and stirring the solution was made radioactive by addition of 0.6 ml. of H2S*O4stock solution and the exchange was determined after various periods of time. From tbe resiilts in Pig. 8 it i p clear that
I. M. KOLTHOFF AND B. VAN'T RIET
822 I .G 1.4
1.2
5
1.0
i
3 0.8 m
0.6 0.4 0.2 0
0.6 0.7 0.8 0.9 1.0 Log molarity of lead sulfate in mixture (+ 3.0). Fig. 9.-Induction periods in precipitation of lead sulfate with sodium sulfate as precipitant.
even very small amounts of W. V. inhibit exchange. However, in agreement with previous results'O complete coverage of the surface with dye is necessary to prevent exchange as a result of recrystallization. Induction Periods (I.P.) .-In determinations of the I.P. a t relatively high concentrations the wool violet method was found to give practically the same results as observations of the first appearance of turbidity. The W. V. method was very useful in determination of I. P. in solutions of relatively low supersaturations. In those cases it was difficult to observe the exact time of turbidity in the opaque polyethylene beaker. In Fig. 9 the log of I.P. in seconds is plotted us. the log of concentration of PbS04 in the original mixtures of lead perchlorate and sodium sulfate. The reproducibility of the determinations was *0.5 second up to I.P.'s of 10 seconds; longer I. P.'s were measured with a precision of 3t5.0%. The supersaturated mixtures were prepared from the same stock solutions of reactants which had a p H of 2.8. At concentration A in Fig. 9 the shape of the crystals changes from rectangular to obtuse angle crosses. At concentration B the inaximum particle length is attained. The following empirical relations are derived from Fig. 9: rectangular crosses are formed (Ci varies between 3 and 5 X M) I.P. = kl X
nl
~ i - ~ l ;
1.7
0.07
Obtuse angle Crosses are formed (ci between 5 2nd 0.8 X 10-3AT) I.P.
=
ki
X
c~-"Z;
T L ~=
2.8 & 0.15
Rapid decrease of particle leiigth with increase of c, (ci greater than 6.8 X M) I.P. =
k3
X
ci-*a;
n3 = approximately 8.0
The molar concentration of lead sulfate in the origin:tl mixtiire is denot,ed by ci. (10) ( 1935,.
I. AI. Koltlioff and C. Rosenblurn, J. A m . Chern. Soc., 57, 607
Vol. 63
Sudden Changes of Supersaturation during Precipitation.-Supersaturated solutions of lead sulfate were diluted rapidly during and after the induction. A mixture which was 0.0075 M in both lead perchlorate and sodium sulfate had an I. P. of 4.5 seconds. The resulting crystals had a length of about 10 p. When the mixture was diluted rapidly 2 to 3 seconds after preparation to a concentration of 0.005 M lead sulfate the length of the crystals was 55 p after complete precipitation, the same length as of crystals precipitated from a mixture which was originally 0.005 M in lead sulfate. When the 0.0075 M mixture was diluted to 0.05 M 4 seconds after its preparation the crystal length was 30 p, and when diluted after 4.5 or more seconds approximately 10 p in length. Therefore, dilution after termination of the induction period did not affect the size of the crystals. The effect of sudden increase or decrease of supersaturation on crystal habit was studied by making solutions more or less supersaturated after the induction period. The crystals present a t the end of the induction period formed the center of crystals on which additional growth took place in directions found a t original supersaturations identical with the supersaturation after the sudden change. For details on these experiments and photographs the reader is referred to the thesis of the junior author.' Discussion The interesting observation can be made that the center of all the crystals has the habit of a rhombohedron. At low supersaturations the final crystals are composed of more or less perfect rhombohedra (Fig. 3A). With increasing supersaturation the rate of crystal growth increases and rectangular crosses are observed (Fig. 3B and C). When the supersaturation is further increased, using sodium sulfate as precipitant, another preferred growth is observed, resulting in the formation of obtuse angle crosses which reach a maximum length a t an original concentration of 6.4 X M in lead sulfate (Fig. 3D) in the original mixture. This concentration for maximum length is sharply defined and is equal to 6.2 X M with lithium A4 with ammonium sulfate, sulfate, 6.6 X M with sodium bisulfate, and 6.9 X 7.0 X loM3 M with sulfuric acid. Above this concentratioii the rate of formation of nuclei becomes so great that the size of the crystals decreases sharply, but they are obtuse angle crosses, or mixtures of these with needles. Dilution of a precipitating mixture before the end of the induction period affects the crystal size. The size corresponds closely to that observed when the original mixture was a t the same concentration as the diluted mixture. This is no longer true when dilution occurs just before, or at, or after the end of the induction period. These results indicate that the majority of nuclei are formed in the later stages of the I. P. and that under our experimental conditions of high supersaturation the homogeneous nucleation cannot be the only source of formation of nuclei. Apparently, nuclei formed during the I. P. promote the formation of new nuclei. From Fig. 9 it is evident that the usually valid relation between indiiction period t and the concen-
d
c
%
REDUCTION O F
Julie, 1969
DICHROMtlTE I O N BY THALLOUS I O N INDUCED BY a-I1ADIATIOh'
t,rtltioii ci of lead sulfate in the original mixture: t = constant X cp does not hold. In the concentration range between 3 and 5 X M' n is 1.7, i t increases to 2.8 in the region between 5 and G.8 X M (maximum particle length in Fig. 2) and it becomes about 8 a t higher concentrations. Nielsen6 plotted all the known data for the relation between log t and log concentration in the precipitation of barium sulfate. It is of interest to note that this plot was also composed of three straight lines, although the slopes varied in a different way from those in Fig. 9. This complicated relationship is probably accounted for by the fact that the induction period involves both a rate of nucleation and a rate of growth and that the order of growth reaction varies with the concentration of lead sulfate in the supersaturated solution. With potassium sulfate as a precipitant the ob-
823
servations are quite different from those with tho other sulfates studied. Potassium sulfate can form a double salt with lead sulfate" and a t higher concentrations double salt formation may account for the different behavior found with potassium sulfate as a precipitant. While not reported in the experimental part it may be mentioned that the effect of potassium also is observed when part of the sodium sulfate is replaced with potassium sulfate. If lead perchlorate is replaced with lead nitrate, similar particle length curves are obtained as described in the present paper with deviating behavior of potassium sulfate from the other sulfates studied. Acknowledgment is made to Dr. P. R. O'Connor for help in the performance of radioactive measurements, and to Dr. K. R. Lawless for help in making the photographs. (11) 1%.Randall and D. L. Shaw, ibid., 67,427 (1935).
REDUCTION O F DICHROMATE ION BY THALLOUS ION INDUCED BY 7-RADIATION BY THOMAS J. SWORSKI~ Chemistry Division, Oak Ridge National Laboratorg,2 Oak Ridge, Tennessee Received December 67, 1958
Reduction of dichromate ion in air-saturated 0.4 M sulfuric acid is induced by y-radiation with G(Cr+++) 7 1 / 3 [ 2 G ~ 1f~ z GR - GO=]. G(Cr+++)is increased t o 1 / 3 [ 2 G ~ ~ 0 ~GH Goa] by addition of thallous ion. Thallous ion IS oxidized by OH radical yielding thallium(I1) ion which reduces dichromate ion with concomitant production of thallic ion. Neither dichromate ion nor the intermediate chromium(V) and chromium(1V) ions oxidize thallous ion to thallium(I1) ion. Chromic ion a t concentrations as high as 10-2 M . has no measurable effect on G(Cr+++) either in the presence or absence of thallous ion a t concentrations as low as 10-4 M . The specific reaction rate ~ c ~ + + * ,iso H so low that (a) chromic ion a t concentrations as high as 10-2 M does not measurably decrease GH*O* and (b) ~TI+,oH/~c~+++,oH cannot be evaluated even with values as high as 100 for (Cr+++)/(Tl+).
+, +
Introduction G(Ce+++)*for the reduction of ceric ion in sulfuric acid solutions has been postulated4 to be equal to 2GHZoz GH - GOHaccording to amechanism in which H atom reduces ceric ion and OH radical oxidizes cerous ion
+
Ce4+ Ce+++
+ H +Ce+++ + H + + OH ---f Ce4++ OH-
reduction of dichromate ion has been presenteds as evidence that the mechanisms for reduction of dichromate ion and ceric ion are identical. To further elucidate the mechaniSm for reduction of dichromate ion, the radiation chemistry of thallous dichromate solutions was investigated. Experimental Procedure
(1) (2)
Water was purified by a procedure previously establishedg This mechanism was evidenced5 by concomitant in this Laboratory. The purest chemicals a v a k b l e were used without further purification. Solutions were irmdioxidation of radioactive cerous ion during reduction ated with cobalt, ?-radiation of homogeneous intensity disof ceric ion. G(Ce+++) is increaseda*' to ~ G H , o ~ tribution provided by a cylindrical source.10 A 2-cm. cylinGH GOHby addition of thallous ion according to drical cell could be placed inside of the source positio.ier of the cobalt source.10 The rate of energy abqorption in solu. the sequence of reactions tion was determined by use of the ferrous sulfate dosimeter TI+ OH -+ TI++ OH(3) in the same irradiation cell. G(Fe+++)of 1.5.6 was deterCe4+ T l + +--+ Ce+++ T l + + + mined" for the dosimeter by a calorimetric calibration in this (4) Laboratory. The oxidation of radioactive chromic ion during Dichromate ion reduction as a function of energy absorbed was followed in each solution through use of intermittent (1) Union Carbide Nuclear Company, P. 0. Box 324, Tuxedo, exposures. The optical density of the dichromate ion soluNew York. tion in the irradiated cylindrical cell was measured after (2) Operated for the United States Atomic Energy Commission b y each period of irradiation with a Cary model 11 recording Union Csrbide Nuclear Company. Cylindrical cells were obtained from 13) The 100 e.v. yield# of the intermediates U, OH, H P and HZOZ spectrophotometer.
+
+
+ +
+ +
are denoted by OH, GOH, GHZand G H ~ O Z .The 100 e.v. yield of products of irradiation is denoted by G(product1. (4) A. 0. Allen, Radinfion RPaeaxh, 1, 85 (1954). ( 5 ) G. E.Challenger and B. J. Masters, J . d m . Chem. S o r . , 77, 1063 (1955). (6) T. J. Sworski. ibid., 77,4689 (1955). (7) T. J. Sworski,Radiation Resoarch, 4, 483 (1950).
(8) M. Lefort and 1,I.Lederer. Compt. rend., 242, 2458 (19%). (9) C. J. Hochanadel, THIS,JOURNAL, 66, 587 (1952). (10)J. A. Ghormloy and C. J. Hochanadel, Rev. Sci. I n s t i . , 22, 473 (1951). (11) C. J. Hoohansdel and J.
(1953).
A. Ghormley, J . Chrrn. P h ~ p . .21, 880
