November, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
The cured samples were rubbed with solutions of cupric chloride or cobaltous nitrate until stained, and the stains were dissolved off with chloroform or ether. Direct comparison of the absorption spectra of these solutions with those from authentic material again confirmed the presence of dimethyldithiocarbamic acid.
SUMMARY Dimethyldithiocarbamic acid derivatives are formed in the vulcanization of rubber with tetramethylthiuram disul-
1201
fide or with tetramethylthiuram monosulfide and sulfur in the presence or absence of zinc oxide. LITERATURE CITED (1) Bedford and Gray, IND.ENQ.C H ~ M15, . , 720 (1923). (2) Bedford and Sebrell, Ibid., 14, 25 (1922). (3) Cummings and Simmons, Ibid., 20, 1173 (1928). (4) Delepine, Bull. SOC. chim., [4] 3, 651 (1908); Cambi and nasso, Atti. acad. Lincei, [6] 13, 254, 809 (1931). ( 5 ) Hutin, A , , Moni. sci., 7, 193-6 (1917). (6) Jones and Depew, IND. ENQ.CHEM.,23, 1467 (1931).
Cag-
RECEIVED 4ugust 14, 1934
Supersaturation and Crystal Formation in Seeded Solutions Hsw HUAIT I N GAND ~ WARRENL. MCCABE,University of Michigan, Ann Arbor, Mich.
T
however, the solution is capable HE formation of crystals I n continuously cooled, stirred, seeded solufrom solutions is closely of crystallizing spontaneously tions of magnesium sulfate, two reproducible surelated to the phenomeand nucleus formation will occur persaturation curves are obtained: one where new non of supersaturation. Much once the labile region is reached. nuclei are first observed, and the other, lying f a r Ostwald called the line of detheoretical and experimental atther in the supersaturation field, where a sudden tention has been given to the m a r c a t ion b e t ween the two degree of supersaturation supregions the “metastable limit.” increase in the rate of formation of new crystals Miers and his followers (3-1 O), portable by a s o l u t i o n under and a pronounced heat effect are observed. among whom may be mentioned given conditions, to the mechaT h e supersaturation curves are dependent o n Isaac, Jones, Hartley, a n d nism of precipitation from such rate of cooling, speed of stirring, and number and Fouquet, have done work supa solution, and to the effect of the p o r t i n g Ostwald’s metastable size of seed crystals. important variables on such a process, but very little of this l i m i t c o n c e p t . Their experiThe data indicate that both mechanical stimuments covered a large variety of work was done with artificially lus and inoculation effects are of importance in substances in aqueous solutions, seeded solutions. This paper the process of nucleus formation in seeded solureports results of experiments on in organic solvents, and in binary tions. the formation of new crystals in and ternary m i x t u r e s . The a cooling. suDersaturated. seeded Drouess of the crvstallization solution-and on.thermal effects accompanying crystallization was noted by following changes Tn the refractive index of the from such a solution. cooling and crystallizing solution and comparing the index with Seeded solutions were chosen for several reasons. It is a known calibration. Any change in the concentration of a well known that the initial formation of crystal nuclei is solution became immediately known. It was found that a t profoundly influenced by the chance presence of very small definite temperatures sudden changes in concentration took seed crystals of the solute, of a substance isomorphous with it, place, and that such changes corresponded to the formation of or even of foreign dust particles, and is also sensitive to shock. showers of crystals. To check the refractive index experiments It was found that, if the solution was artifically seeded, the an independent method was used: A sample of solution which effects of fortuitous seeding and of minor mechanical effects had been sealed into an ampoule containing fragments of were so far overshadowed by the action of the seeds that the glass, platinum tetrahedra, or other hard bodies to promote results were more reproducible and the experiments under mechanical friction was cooled slowly and the ampoule shaken better control than if unseeded solutions were used. Also, a t regular intervals until crystals were discernible to the naked once crystals have formed in an originally unseeded solution, eye. I n general, these investigators used unseeded soluthe process from that point on is that of crystallization in a tions. Their conclusions can be summarized as follows: seeded solution. Finally, the use of seeded solutions comIn each case the dissolved substance in the solution has a defimercially is a promising method of controlling the size distri- nite supersolubility, which can be represented by a definite tembution of the product of the crystallization, even in cases where perature-concentration curve. This is true for stirred or agithe number of nuclei introduced initially as seeds is small in tated solutions. This curve, called by Miers the “supersolubility curve,” is traceable in each case and is found t o run a proxicomparison with new nuclei formed during the process. mately parallel to the solubility curve. For a solution o?a substance possessing a normal solubility curve and cooled past its PREVIOUS WORK saturation temperature, no crystal formation takes place at temperatures higher than that on the supersolubility curve. Ostwald (11) suggested that the unstable field lying on the When, however, the supersolubility curve is reached, the cooling supersaturation side of the solubility curve may be divided solution will usually crystallize spontaneously. A solution careinto two regions, “metastable” and “labile.” In the meta- fully rotected against mechanical shock, however, can be cooled stable region, inoculation is necessary to start the process of into txe labile region without crystal formation. crystallization; if there are already seed crystals present, Such a supersolubility curve, as first conceived by Miers, crystallization will take place by depositing material on these corresponds to Ostwald’s metastable limit and marks the seeds and new crystals will not form. I n the labile region, sudden change of a supersaturated solution from the metaPresent addreas, Peiyang Engineering College, Tientsin, China. stable state to the labile state. This conception has been
INDUSTRIAL AND ENGINEERING CHEMISTRY
1202
subject to some criticism, especially by de Coppet ( 2 ) and Young (12,. IS), . . and has later been gradually modified to implythe passage from a state in whickcrystaliization is slow and difficult into a state in which crystal formation is rapid and easy. According to de Coppet, the metastable state is the state in which the probability of crystallization in a finite period of time is indefinitely small, and the labile state is one in which such probability is very great; between the two states there is a considerable transition range. Young, on the other hand, concluded from his work on the mechanical stimulus to crystallization that the metastable limit depends on vigor of stirring, shaking, friction, or other mechanical effects; that the whole supersaturated field is
FIGURE 1. DIAGRAM OF APPARATUS labile; and that crystallization may be brought about in any portion of it by the application of an appropriate amount of mechanical energy.
EXPERIMENTAL PROCEDURE The general experimental method was as follows: A solution (usually about 900 to 1000 grams), free from solid phase and of a known concentration, was allowed to cool a t a definite rate and constant stirring speed. When the saturation concentration was reached, seed crystals, of known weight and size, were introduced, and the cooling was continued. The first appearance of new crystals was noted, as shown by the Tyndall effect developed by the new crystals in a strong beam of light. Any heat effects were also noticed by following the temperature-time curve of the cooling solution. Throughout the experiment the solution was protected from dust inoculation. APPARATUS. The apparatus consisted of a batch crystallizing unit in which the solution was allowed to cool, and a OF TABLEI. RESULTS
SALT SATN. RGN C O N C N . ~TEMP. O
c.
Vol. 26, No. 11
circulation system for the cooling medium. The apparatus is 1: shown diagrammatically in Figure The cooling medium was mineral oil. It was drawn from the reservoir, 1, by the Viking rotary pump, 4, and flowed through strainer 10, cooler 5 , and pipe 19. A part of the oil flowed through pipe 20 into jacket 2, and the rest was returned to the reservoir through by-pass 21. The oil from the jacket returned to the reservoir through the glass tube, 22. Reservoir 1 was a 20 X 20 X 30 em. rectangular tank of galvanized iron, with an outlet near the bottom. It was provided Kith a brass stirrer, 7, a 250-watt immersion heater, 15, regulated by the rheostat, 18, a brass cooling coil, 6, and a 15-cm. glass funnel, 14, through which the oil, heated externally, was transferred t o the reservoir. The temperature of the oil was measured by a calibrated thermometer, 13, reading to 0.1" C. Strainer 10 was constructe3 of standard pipe flanges and fittings and contained a 32-mesh steel screen. The purpose of cooler 5 was t o prevent a rise in temperature of the oil due to the friction of the pum The batch crystallizing unit consisted o f 'an oil jacket, 2, inside of which was placed a crystallizer, 3, containing the solution. Jacket 2 was an inverted bell jar, 20 cm. in diameter and 30 cm. deep, with a steel cover, 23, held in place by screws and wing nuts. Openings were provided in the cover for the insertion of the crystallizer, 3, a 60-watt heating bulb, 16, a brass stirrer, 9, and a calibrated thermometer, 12, reading to 0.1' C. Crystallizer 3 was a one-liter beaker held in position by the steel cover, 24, which was fastened to cover 23 of the jacket by screws and wing nuts. Crystallizer 3 was provided with a calibrated thermometer, 11, reading to 0.05" C., a belt-driven glass stirrer, 8, and an opening, 25, through which seed crystals could be introduced and samples of solution removed. Stirrer 8 passed through a mercury seal, 17. Although the crystallizer was not designed to be vapor-tight, the use of the cover and the mercury sea1 protected the solution in the crystallizer from c o n t a m i n a t i o n and dust inoculation during the course of the experiment. A hundred-watt incandescent lamp (not shown) was placed behind the jacket for illumination.
PRELIMINARY EXPERIMEXTS. A number of preliminary runs were made with unseeded solutions. Two salts were used-magnesium sulfate heptahydrate and copper sulfate pentahydrate. The results are shown in Table I. It is evident that both magnesium sulfate and copper sulfate solutions are capable of being supersaturated to a considerable extent. In runs D, E, and F the solutions were found to crystallize, undoubtedly because initially they contained a few small crystals that had not been dissolved completely. This gave evidence of the effectiveness of inoculation in greatly diminishing the degree of supersaturation and in starting crystallization, even in well-stirred solutions. The unseeded runs were followed by a second series of preliminary experiments on both salts in which seeding was done a t the saturation temperature and crystallization was always found to take place. The magnesium sulfate runs showed that the temperature a t which crystals first appeared, and the temperatures a t which the heat effect began, reached the maximum, and ended, could all be reproduced within the
P R E L I M I N A R Y RCXS WITHOUT SEEDING
STIRRINQ SPEED
REMARK8
R . p . m. RCN8; C O O L I N G RATE, 0.1" C. PER 6 MIN. Run stopped a t 28" C. No crystallization was obsvd. R u n stopped a t 30' C. No crystallization was obavd. Run stopped a t 27.9' C. N o crystallization was obsvd. The s o h was then left overnight, without stirring, until its temp. fell to that of the room (about 24O C . ) . Still no crystallization was obsvd. by t h e next noon. A crystal of MgSOa.7HzO was then introduced and crystallization started within 3 min. Solid phase had not been completely dissolved. Numerous new crystals appeared a t 32.5' Solid phase bad not been completely diesolved. New crystals appeared a t 33.6' C. Solid phase had not been completely dissolved. New crystals appeared a t 32.8'' c.
M A Q N E S I U M S U L F A T I HEPTAHYDRATE
A
146.6 175 175
33.7 41.3 41.3
About 230 About 230 About 230
D
152 154.5 153.5
35.3 36.0 35.7
About 2301 About 230 About 230
B C E F
c.
COPPER SCLFATE PENTAHYDRATE R U N S ;
G H a
C O O L I N Q R A T E , 0.3'
C . PER 6 M I N .
Run stopped at 30' C. No crystallization was obsvd. Run stopped a t 30° C. No crystallization was obsvd. The aoln. was left undisturbed, withput stirring for 7 days. Room temp. during these days was about 22-23O C. Still there was no crystaihzatlon until a duS0c.5H20 crystal was introduced. In parts of hydrated salt per 100 parts of solvent water. 54.2 54.2
39.7 39.7
146 147
November, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
limits of experimental error. This reproducibility was gratifying; the most annoying feature in work involving spontaneous crystallization is the great difficulty in obtaining check results on duplicate experiments. Analogous results were obtained in the copper sulfate runs except that the heat effects were not so prominent as in the case of the magnesium sulfate. MAINRUNS. The main part of the experimental work consisted in determining the so-called supersolubility curve for seeded solutions of magnesium sulfate and investigating the effects of the five variables-amount of seed crystals, size of seed crystals, rate of cooling, speed of stirring, and initial concentration of solution-on the location of this curve. PROPERTIES OF MATERIAL.Only one salt -magnesium sulfate heptahydrate-was used in the main part of the work. It was chosen for two reasons: first, its solubility-temperature coefficient is large, and hence it is not necessary to cool its solution very far in order to obtain sufficient amount of product for a screen analysis if such analysis is desired; and secondly, its pronounced heat effect helps to establish the supersolubility curve. It was important that the solubility of magnesium sulfate be accurately known if supersaturations were to be determined. It was found that the solubility data given in the literature were not consistent, and that appreciable errors could result if some of these data were accepted. Accordingly, new determinations of the solubility of magnesium sulfate in the temperature range of 20" to 45" C. were made. For this purpose the crystallizer shown in Figure 1 was used. Both undersaturated solutions and supersaturated solutions were agitated a t constant temperature with excess heptahydrate crystals until equilibrium was obtained. This work on solubility is to be reported in detail in a separate communication. The following empirical formula was found to express the solubility of magnesium sulfate closely between 30"and 45 C. : S = 0.00360t S
- 0.3140t a + 12.140t
- 43.80
where S = MgSO4?H20soly., parts/100 parts solvent water t = temp., C.
PREPARATION OF SOLUTIONS.I n preparing a solution, an amount of c. P. magnesium sulfate heptahydrate was weighed and dissolved in an amount of water such that the concentration of the resulting solution was somewhat higher than that corresponding to the temperature chosen for investigation. After the salt had been dissolved, the solution was transferred to a one-liter volumetric flask. Usually six such flasks of solution were prepared a t a time. The concentration of the solution in each flask was determined by analysis, weighing the sulfate as barium sulfate. From the known concentration the amount of water that should be added per 1000 grams of solution to adjust the concentration to any desired point was calculated. PREPARATION OF SEEDCRYSTALS.The seed crystals were taken from the products of the preliminary experiments. After being screened into different sizes, they were washed with alcohol saturated with magnesium sulfate heptahydrate. The screening of the seed crystals was carried out with Tyler standard testing sieves. The intermediate screens were used in conjunction with the usual sieves. For each run a flask containing solution was PROCEDURE. weighed and warmed in a water bath to 10" or 15" C. above the saturation temperature of the solution so that the solid phase mas completely dissolved. Mineral oil, heated to about 5" C. above the saturation temperature of the solution, was transferred to the reservoir and jacket. The solution in the flask was then poured into the crystallizer which had previously been cleaned, dried, and weighed. The weight of the solution was determined by weighing the crystallizer plus
1203
TABLE11. TYPICAL DATASHEET
FOR
RUN38
Concn. of soln. = 203.27 parts MgS04,7Hz0/100 parts solvent H20 Hz0 required to convert concn. to 188.30 (satn. temp., 38.95' C.) = 73.2 g./1000 g. s o h . Volumetric flask soln. = 1172.72 g. Crystall/zer soln. = 1224.85g. Volumetric flask residue = 280.35 g. Crystallizer 312.75 g. S o h . used = 912.37g. Soln. used = 911.90g. Soln. used (av. weight) = 912.13 g. Distd. Hx0 added = 68.70g. Total soln. of desired concn. = 978.83g. Seeding: Size of seed crystals = 28 mesh per inch (11 mesh per cm.) Weight of seed crystals (0.2g./lOOO g. s o h ) = 0.1958g. Cryatsllizer soln. seeds = 312.75 978.83 0.20 = 1291.78 g. Crystallizer mother liquor crop at end of run = 1288.05 g. Loss due to evapn. = 3.73 g. Stirrer beaker I: Final = 712.74g. Initial = 710.14 g. Hs0 condensed on cover = 2.60 g. Dry crop paper = 75.13 g. Paper = 2.50 g. __ Dry crop = 72.63 g. Seed crystals used = g. 72.43 g. Net dry crop = SPBED
++
++
-
+
+
+
+
+
+
+
0.20
OF
TIME (P,M.)
TEMPERATURE Crystallizer Jacket Reservoir
c.
O
c.
c.
CRYSTALLImR
STIRRER
R . p . m. 145
40.1 40.8 43.40 40.0 40.8 42.80 41.1 40.1 42.40 148 42.10 40.1 41.0 1:oo 40.0 40.4 148 41.80 05 41.50 39.8 39.8 10 39.4 41.20 39.2 148 15 39.2 40.95 38.9 20 38.8 145 40.80 38.5 25 38.5 40.30 38.3 30 38.3 40.00 38.0 146 35 38.1 39.70 37.8 40 148 37.8 39.40" 37.5 45 37.5 39.10 37.2 50 146 37.2 38.80 37.0 55 37.2 38.50 38.7 2:oo 147 36.8 38.5 38.20b 05 38.5 38.2 37.90 10 148 38.2 35.9 37.80 15 38.1 37.30 35.7 20 146 35.4 37.00 35.4 25 35.0 35.0 38.70 30 148 34.7 34.8 38.40C 35 34.4 34.4 38.25 40 41 38.23 42 38.20 38.20 43 44 38.17 148 34.1 34.2 38.15d 45 38.12 48 38.10 47 38.10 48 49 38.05 33.7 33.8 38.02 50 38.00 51 35.97 52 35.92 53 54 35.87 33.4 148 33.6 35.82 55 32.9 33.3 35.50e 3:oo 32.8 145 33.0 35.20 05 32.8 32.7 34.90 10 34.8 148 33.0 34.75 15 35.4 33.9 34.75 20 148 34.4 34.1 34.S0f 25 34.0 34.1 34.80 30 148 34.1 34.0 34.80 35 33.9 34.1 40 34.80 4 Seed crystals added a t 39.50' C. b New crystals appear at 38.20' C. C Heat effect starts a t 38.50° C.; decided increase of crystallization. obscured. d Max. heat effect a t 38.15O C.; very numerous crystals; stirrer completely 12:45 50 55
Heat effect ends a t 35.50' C.
I To attain equilibrium.
solution, and checked by weighing the flask plus residue. A weight of distilled water necessary to dilute the solution to the desired concentration was added. The crystallizer was assembled, the stirrers and circulating pump were started, and the rate of oil flow was regulated. The illuminating lamp behind the jacket was turned on, as well as the immersion heater in the reservoir, the heating bulb in the jacket, and the cooling water. The preparations for the run required from 2 to 3 hours. By adjusting the rheostat and flow of cooling water through the reservoir, the desired rate of cooling was maintained. Such manual control proved to be
1204
INDUSTRIAL AND ENGINEERING CHEMISTRl
satisfactory, and the temperature of the solution could easily be regulated within *0.05' C. The speed of the stirrer in the crystallizer could be controlled within *2 r. p. m. In the meantime, seed crystals of the desired size and weight were weighed out. The seeds were introduced into the crystallizer when the solution reached the saturation temperature. When the rate of cooling was under control, the following data were taken. 1. The temperatures of the crystallizer, jacket, and reservoir (every 5 minutes). 2. The r. p. m. of the stirrer in the Crystallizer (every ten minuta). 3. T h e t e m p e r a ture of the solution at which crystals first appeared spontaneously. 4. The temperatures of the solution at which the heat effect started, reached the maximum, and ended. During the heat effect the temperatures were recorded more frequently.
The first spontaneous a p p e a r a n c e of crystals could be deCRAMS OF SEEDS PER loo0 GRAMS SOLUTION tected by s t o p p i n g FIGURE2. EFFECTOF WEIGHTOF the stirring tempoSEEDCRYSTALS rarily +o let the seed c r y s t a l s settle and observing what happened. Illuminated slightly obliquely by the hundred-watt lamp behind the jacket, the new crystals could be recognized by the Tyndall effect as brightly sparkling particles. After the seed crystals had been added, stirring was stopped frequently to locate the temperature a t which new crystals first appeared. At the end of the run the temperature of the solution was kept constant for half an hour to insure equilibrium conditions. Then the crystallizer was quickly taken out, wiped clean of oil, and weighed to determine loss by evaporation. The crop was separated from the mother liquor in a centrifuge, washed with alcoholic magnesium sulfate solution, and dried. The run proper required from 3 to 4.5 hours, depending on the rate of cooling and the extent to which the solution was cooled. TYPICAL DATASHEET. The data taken during run 38 are shown in Table 11. The heat effect began at 36.50' C. (corrected, 36.43'), accompanied immediately by a copious shower of new crystals, reached the maximum a t 36.15' C. (corrected, 36.07"), and from there lingered for more than 10 minutes until the end a t 35.50" C. (corrected, 35.42'), after which the temperature of the crystallizer was again under control. During the heat effect, however, the temperature of the jacket was still allowed to decrease a t the usual rate of 0.3" C. per 5 minutes. There was also a uniform lag of temperature between the crystallizer and the jacket outside the limits of the heat effect. The loss of weight due to evaporation of water was 3.73 grams. Evaporation was observed to take place a t the very beginning of the run when the solution was hot. The initial concentration of the solution was corrected for this evaporation. This evaporation was unimportant because the loss was about the same for every run and the total amount of solution used was relatively great. Solutions having the same concentration before the evaporation did not, therefore, differ appreciably in their corrected concentrations after the evaporation. CALCULATIONOF A TYPICAL RUN. The calculations carried out on the data of run 38 are shown in Table 11.
Vol. 26, No. 11
( A ) Theoretical Yield. Before the run: MgSO4.7HpO in soln. = (978.83)(166.30) 31 611 .53 grams 266.30 Solvent water = 367.30 grama After the run: Solvent water evapd. = 3.73 grams Solvent water in mother liquor = 363.57 grams Soly. at 34.80" C. (cor., 34.72 ) is 149.44 grams MgS0d.7HI0 per 100 grams - solvent water. (149.44)(363.57) MgSO4.7HpO in mother liquor = 100 _.. 542.5 grams Theoretical yield = 611.5 - 542.5 = 69.0 grams Actual dry crop = 73.43 grams ( B ) Ga between Saturation Temperature and the Temperature at Which d y s l a l s First Appeared Spontaneously ( Ah). From ( A ) ,cor. initia1 concn. of soln. = (611.53)UOO) = 168.1 363.57 parts MgS0,.7Hz0/100 parts solvent water. Satn. temp. = 39.47 C. The average concentration of all the runs made at this particular temperature is 168.15, corresponding to a saturation temperature of 39.48" C. In computing the gaps, this average concentration and saturation temperature were used. Temperature (tl) at which crystals first appeared = 38.20 (cor., 38.14) O C. At1 = 39.48 - 38.14 1.34' c. (C) . Gap between Saturation Temperature and the Temperature at Whzch the Rate of Spontaneous Crystallization Suddenly Increased and Heat Effect Started ( A h ) .
Temperature (k) at which heat effect started = 36.43' C. At, = 39.48 - 36.43 3.05' c.
RESULTS I n the main series of experiments, sixty-seven runs were made, All but nine of these were satisfactory. Each pair of runs w e r e d u p l i c a t e s . The results of one run of each pair a r e g i v e n i n T a b l e I11 in which the concentrations of the solutions a r e e x p r e s s e d i n p a r t s MgSO4.7HzO p e r 100 parts solvent water, Atl and Ah retain their p r e v i o u s meanings, and all temperatures a r e corrected values. Duplic a t e r u n s c h e c k e d so closely that it is unnecessary to show all runs in Table 111. The At values were reproducible t o *0.03' C. COMPARISON OF ACTUAL AND THEORETICAL YIELDS. OJO a13 SIZE OF SCREEN OPENING IN CM. Although this work does FIGURE3. EFFECTOF SIZE OF not consider seriously the yield obtained from each SEEDCRYSTALS run. a comDarison of the actual yields with the theoretical yields calcuiated from the new solubility data was made with a view to establishing, by means of material balances, the reliability of the concentration data. Of the fifty runs compared, seventeen showed an error of within 5 per cent; twenty-two, an error of from 5 to 10 per cent; and the other eleven a n error of over 10 but not exceeding 13 per cent. Because of the large amount of water of hydration in the product, the yield is very sensitive to small
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1934
TABLE111.
EXPERIMEXTAL
DATAON iMgS04.7HzO
1205
SOLUTION"
TBMP~RATURD
RUN
No.
COR. CONCN. b
Satn.
c.
Final O
c.
First appearance of crystals
c.
Inoreaaed crystallization#
c.
168.15 38.14 36.43 39.48 34.72 168.15 38.50 36.88 39.48 34.72 150.60 33.91 32.40 34.98 29.89 150.60 34.42 32.76 34.98 29.89 150.60 33.56 31.60 34.98 29.89 150.60 34.98 29.89 33.91 32.00 42.91 197.15 45.45 44.37 40.98 197.15 42.66 45.45 44.27 40.98 43.22 197.15 45.45 40.98 44.92 37.18 168.15 39.48 34.72 38.65 150.60 34.98 29.89 34.02 32.40 150.60 31.30 34.98 29.89 33.46 197.15 42.71 45.45 40.98 44.32 197.15 45.45 40.98 44.72 43.42 197.15 45.45 40.98 44.67 43.32 168.15 39.48 37.13 34.72 38.65 197.15 45.45 43.47 40.98 44.67 197.15 45.45 43 I47 40.98 44.67 168.15 39.48 37.15 34.72 38.65 150.60 34.98 32.35 34.12 29.89 150.60 34.98 32.30 29.89 34.12 150.60 34.98 32.15 29.89 33.81 168.15 39.48 36.17 34.72 38.09 197.15 45.45 43.08 40.98 44.87 168.15 39.48 37.10 31.90 38.55 168.15 39.48 37.84 31.90 39.06 197.15 45,45 43. 83 ... 44.77 197.15 45.45 44.43 ... 45.21 168.15 39.48 37.138 38.65 Check runs are not given. b After average evaporation of all runs. Concentrations are C Start of heat effect. d Heat effect not prominent.
38 40 42 46 48 50 53 55 57 59 61 63 65 67 69 77 79 81 83 85 87 90 92 94 98 99 100 101 102
-
...
Max. heat effect
E n d of heat effect
c.
0
c.
36.07 36,33 31.50d 32. OOd 31.30 31.65 42.61 42.35 42.81d 36.88 32.00 30.54 42.30 43.32 43.12 37.03 43.27 43.37 36.58 31.85 31.75 31.25d 35.77 43.69 36.58 37.14
35.42 35.10 30.34 31.35 30.64 30.79 42.05 41.70 42.30 36.12 31.50
30:?3d
3 0 : i5
...
4i:29 43.06 42.61 36.38 42.71 42.86 35.92 31.30 30.89 30.24 35.02 43.06 35.17 36.44
...
Size
Weight COOLINQ per 1000 g. RATEPER STIRRINQ 5 *WIN. SPEED soln. At1
M,esh/ zn. 28 14 48 48 28 14 28 28 28 28 28 28 28 28 14 28 28 28 28 28 28 48 28 48 48 48 48 48 48
Grams
c.
0.20 0.20 0.20 1.oo 0.20 0.20 0.20 0.05 1.00 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.05 0.20 0.20 0.05 1.00 0.05 1.00 0.20
0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.20 0.20 0.50 0.50 0.20 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.50 0.30 0.30 0.30 0.30 0.30 0.30
R . p. m.
c.
146 146 146 146 146 146 145 145 146 146 146 146 146 146 146 201 201 261 262 26 1 201 146 147 148 147 147 147 146 147
1.34 0.98 1.07 0.56 1.42 1.07 1.08 1.18 0.53 0.83 0.96 1.52 1.13 0.73 0.78 0.83 0.78 0.78 0.83 0.86 0.86 1.17 1.39 0.58 0.93 0.42 0.68 0.24 0.83
At1 a
c.
3.06 2.60 2.58 2.22 3.38 2.98 2.54 2.79 2.23 2.30 2.58 3.68 2.74 2.03 2.13 2.35 1.98 1.98 2.33 2.63 2.68 2.83 3.31 1.47 2.38 1.64 1.62 1.02 2.10
in parts MgS04.7Hz0/100 parts aolvent water.
changes in temperature and concentration, so that the agreement between theoretical and actual yields is satisfactory. GENERAL BEH.4VIOR OF THE SOLUTION DURING A RUN. The general behavior during a run, which was observed in all the magnesium sulfate solutions, is briefly as follows: After the seed crystals had been added at the saturation temperature, the solution continued to cool a t the usual rate and the seed crystals increased somewhat in size until a certain temperature was reached, when new crystals first appeared. From this temperature the solution continued to cool a t the usual rate. The seed crystals, as well as the crystals recently born, grew in size, but a t the same time new nuclei were formed. This nucleus formation took place very slowly in most of the runs, but in a few of them (runs 46 and 99, for example) in which the size of seed crystals used was small (48 mesh per inch, or 19 mesh per cm.) and the amount was relatively large (1.0 gram per 1000 grams of solution), nucleus formation proceeded more rapidly. The weight of material crystallized up to this point was small in comparison with the theoretical weight demanded by the solubility difference, and the solution remained supersaturated. Then another temperature was reached where there was a sudden increase in the rate of crystal formation; many new crystals were formed throughout the solution until there was such a multitude of them that the glass stirrer in the crystallizer was completely obscured. Accompanying this increase in crystallization rate was a heat effect, during which the cooling rate of the solution was retarded, although the temperature of the surrounding jacket was allowed to decrease a t the required rate. In most of the runs, in which the nucleus formation preceding the temperature of sudden promotion of crystallization was slow, it was found that the heat effect was pronounced and had a maximum lasting for several minutes; for instance, in run 56 there was a maximum heat effect at 42.35' C. that lasted 4 minutes. On the other hand, for those runs in which the nucleus formation preceding the temperature of promoted crystallization had been relatively rapid, the heat effect was barely discernible and had no definite maximum. For example, in run 99 the heat effect started a t 37.84' C.; thenceforth the rate of cooling was but slightly decreased-
from 0.30' C. per 5 minutes to 0.25-0.28' C. per 5 minutesuntil the end of the heat effect a t 36.44' C., and no definite maximum heat effect could be recognized. After the end of the heat effect the rate of cooling of the solution could be brought into control again. Somewhere during or, more likely, after the interval of promoted crystallization the solution seemed to be relieved of its supersaturation. EFFECT OF VARIABLES ON Atl AND At2. The effects of the following variables were studied: (1) weight of seed crystals, (2) size of seed crystals, (3) rate of cooling, (4) stirring speed, and ( 5 ) concentration of solution. Three values were chosen for each variable; thus, the weight of seed crystals was 0.05, 0.20, or 1.00 gram per 1000 grams of solution used; the size of seed crystals was 48, 28, or 14 meshes per inch (19, 11, or 5.5 meshes per cm.); the rate of cooling was 0.20', 0.30", or 0.50' C . per 5 minutes; the stirring speed was 146, 201, or 261 r. p. m.; a n d t h e concentration of solution was 150.60, 168.15, o r 1 9 7 . 1 5 p a r t s of MgS04.7HzO p e r 100 parts of solvent water. The results of the FIGURE 4. EFFECT OF RATEOF effects of these variCOOLING ables on the temperature of first appearance of crystals ( t l ) and on the temperature of increased crystallization ( t z ) are plotted in Figures 2 to 5 . All concentrat,ions are in parts of MgS04.7Hz0per 100 parts of solvent water. EFFECT OF WEIGHTOF SEEDCRYSTALS.When the other factors are kept constant, an increase in the weight of seed crystals tends to increase both tl and tP,and as a consequence
1206
INDUSTRIAL AND ENGINEERING CHEMISTRY
both At, and At2 are decreased. As shown in Figure 2 the gaps are, roughly, straight-line functions of the weight of seed crystals used; below 0.2 gram seed per 1000 grams solution there is a slight upward curvature. EFFECT OF SIZEOF SEEDCRYSTALS.As shown in Figure 3 the effect of the size of seed crystals is rather peculiar in that seed crystals having a size of 28 mesh per inch (11 mesh per -cm.) lower b o t h tl and tz to a minimum. The gaps Atl and Ata f o r seed crystals of this size have maximum values, T-I h i 1 e seed crystals of either smaller or larger size produce narrower gaps.
Vol. 26. No. 11
solutions, as shown by the preliminary experiments described above, the At1 gaps were only rom 0.24' to 1.42" C. 2. Their position depends on several variables, such as weight of seeds, size of seeds, rate of cooling, speed of stirring, and concentration of solution. 3. They do not correspond to a copious shower of crystals or to a sharp drop in supersaturation. The t 2 curves differ from Miers' supersolubility curves in these respects: 1. They are dependent on the same variables that effect the positions of the tl curves. 2. There is nucleus formation before the t z curve is reached. Once rate of cooling, speed of stirring, and amount and size of seed crystals have been fixed, however, both the t l and the tz curves are definitely located. EXPL.4NATION
OF THE
EFFECTS OF
VARIABLES ON
GAPS.
Reasonable qualitative explanations can be given for most of EFFECTO F RATE the results obtained. Two main effects can be visualized: OF COOLISG. Figure 1. The work of Young ( l a , IS) has shown how the mechanical 4 indicates t h a t , a s stimulus of impacts in a su ersaturated solution brings about the rate of cooling is nucleus formation, and that t i e more violent are the impacts the less supersaturation is required t o bring about preci itation. increased, b o t h At 2. It is well known that, in the absence of mecganical stimuand Atz are increased, lus, nucleus formation can be brought about by inoculating the fairly rapidly a t first, solution with seed crystals of the solute, rovided the solution is h a highly superbut very slowly above supersaturated. A seed cr stal falling !trou a cooling rate of 0.3' saturated solution leaves bezind a trail of smalfseed crystals. C. per 5 minutes.
E F F E C TOF STIRSPEED.Figure 5 shows that increasing FIGURE 5 . EFFECTOF STIRRING SPEED the s t i r r i n g s p e e d from 146 to200r.p.m. decreases the values of Atl and At2. Above 200 r. p: m., however, stirring speed has no effect. EFFECTOF CONCENTRATION OF SOLUTION. As is evident from Figures 2 to 5 , the tendency for an increase in concentration is to decrease both Atl and At,. The At2 curves are similar in general shape to their respective At, curves, but the At, gaps are larger than the corresponding At, gaps. I n Figure 6 the results on rate of cooling are plotted, in the form of Miers' supersolubility curves. The curves connecting both the temperatures of first appearance of crystals ( t l TEMPERATURE IN O C . curves) and the temperatures of sudden promotion of crysCOOLINQ RAT= COOLINQ RATE tallization (t2 curves) are approximately parallel to the solut~ CURYE PEIR 5 MIN. ti C U R V l PER 5 MIN. c. c. bility curve but tend to converge toward i t as the concen4 1 0.2 0.2 tration increases. Similar curves for the results on rate of 5 2 0.3 0.3 6 0.5 3 0.5 stirring and size and number of seed crystals can easily be prepared from Figures 3, 5 , and 6. FIGURE 6. EFFECTOF RATEOF COOLING PLOTTED AS SOLUBILITY AND SUPERSOLUBILITY CURVES DISCUSSION OF RESULTS The above facts do not, of course, deny the possibility of The experimental data indicate clearly that the concept of a nucleus formation in unseeded, quiet supersaturated soluunique supersolubility curve for seeded solutions must be tions. The evidence is, however, that a higher supersaturamodified. Two curves were found, one referred to as the ti tion is necessary to obtain nucleus formation in such a case curve where new nuclei mere first observed, and the other as than when mechanical or inoculation effects are used. the tz curve, where a pronounced heat effect and copious evohfechanical stimulus in the present experiments was prolution of nuclei were found. vided by the impact of the stirrer on the crystals, by the imIt should be noted that in all of the experiments continuous pact of the crystals on the walls of the crystallizer, and by the uniform cooling was used until the cooling rate was disturbed collisions of the crystals with each other. by the heat effect accompanying the rapid formation of many The true nature of the inoculating action is unknown. It new nuclei. It does not follow that, if the cooling is stopped seems reasonable to suppose, however, that it is due to the or the rate of cooling otherwise varied a t some point between effect of the surface of the crystal on the solution in contact the saturation curve and the ti or tz curves, there may not be with the supersaturated solution. nucleus formation before the tl or tz curves are reached. FurEFFECTOF WEIGHTOF SEEDCRYSTALS.When the weight ther experiments are to be made to investigate this point. of seed crystals is increased while the size remains constant, The tl curves differ from Miers' supersolubility curve in both mechanical and inoculating effects increase, since the these respects : number of collisions and surface both increase. It is to be expected, therefore, that the increase in weight of seeds should 1. They are quite close to the solubility curve. Although MgS04.7Hg0 has the power of forming highly supersaturated decrease the gaps, This conclusion is checked by Figure 2. STIRRING SPEED R.P.M.
RING
O
November, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
EFFECTOF COOLING RATE. The rate of cooling has no appreciable influence on the inoculating or mechanical effects, but the temperature decrease may be too fast for the nucleus formation to keep up, and higher cooling rates would result in wider gaps. Figure 4 shows such an effect. EFFECTOF STIRRING SPEED. The obvious effect of increasing stirring speed is to increase the mechanical effect and therefore to decrease the gap. Such an effect is shown in Figure 5 . The fact that beyond 200 r. p. m. the speed of stirring has no effect is consistent with the known fact ( I ) that a stirrer reaches a limit in effectiveness as the speed is increased. There is probably a change in inoculation effect with stirrer speed, especially a t the lower speeds, where the seed crystals are not dispersed through the solution as uniformly as a t higher speeds. The fact that the 28EFFECT OF SIZEOF SEEDCRY~TALS. mesh-per-inch (11-mesh-per-cm.) seeds gave wider gaps than did larger or smaller crystals (Figure 3) is peculiar. KO simple explanation of this effect is available.
1207
LITERATURE CITED (1) Badger, W. L., and McCabe, W. L., "Elements of Chemical Engineering," pp. 481-2, McGraw-Hill Book Co., New York, 1931. (2) Coppet, L. C. de, Ann. chim. phys., 10,457 (1911). (3) Fouquet, G., Compt. rend., 150, 280 (1910). (4) Hartley, H., Jones, B. M., and Hutchinson, G. A,, J . Chem. Soc., 93, 825 (1908). (5) Hartley, H., and Thomas, X. G., Ibid., 89, 1013 (1906). (6) Isaac, F., Ibid., 93, 384 (1908). (7) Jones, B. M., Ibid., 93, 1739 (1908); 95, 1672 (1909). ( 8 ) Jones, B. M., and Shah, P. G., Ibid., 103, 1043 (1913). (9) Miers, H. A., J. I n s t . Metals, 37, 331 (1927). (10) Miers, H . A., and Isaac, F., Proc. Rou. SOC. (London), 798, 322 (1907); 82-4, 184 (1910); J . Cheni. SOC.,89. 413 (1906); 93, 927 (1908). (11) Ostwald, W., 2. physib. Chem., 22, 289 (1897). (12) Young, S. W., J. -4m. Chem. SOC.,33, 148 (1911j. (13) Young, S. W., and Van Sicklen, TV. F., J. A m . Chenr. Soc., 35, 1067 (1913).
RECEIVED July
2, 1934. Extracted from a thesis submitted by Hsii Huai Ting in partial fulfilment of the requirements for the degree of doctor of philosophy, University of Michigan.
Solubility of Magnesium Sulfate Heptahydrate Hsu HUAI TING~ AND WARRENL. MCCABE,University of Michigan, Ann Arbor, Mich. H a u e r (2020), Van d e r H e i d e URING experiments on New data on the solubility of magnesium sul( I o ) , G u t h r i e (9)s f i t a d (61, the c r y s t a l l i z a t i o n of fate hepkhydrate in the temperature range 290 magnesium sulfate hepand Weston (21). The results to 45" C. are reported. The data are consistent of t h e s e investigations a r e tahydrate, trouble was experienced in o b t a i n i n g the yields with recent accurate data at 25' and indicate plotted as individual points in called for by the solubility of significant errors in older data in the range 2" Figure 1. The second group of solubilithis material at the end temperato 40" C . ture of the process. Crystalline ties contains those determined crops from 50 to 100 per cent larger than theoretical were by Basch (11, Grunewald ( 8 ) , Van Klooster ( I @ , Blasdale obtained. The difficulty was traced to the solubility data ( Z ) , Takegami ( l 7 ) , Jackman and Brown ( I I ) , and Schnellwhich were taken from the literature. It was found that the back and Rosin (16). These data were grouped together in data available on the solubility of magnesium sulfate hepta- the last seven lines of Table I. The determinations are in hydrate were, with the exception of those at 25" C., old and satisfactory agreement. The average of those of Grunewald, inconsistent with themselves and with the results of recent Van Klooster, Blasdale, Takegami, and Schnellback and determinations carried out a t 25" C. The discrepancies were Rosin is 26.66 parts magnesium sulfate per 100 parts water, easily apparent when the data were expressed in parts of This result is plotted as point A in Figure 1, which is a comanhydrous magnesium sulfate per 100 parts of solution, and posite point of good accuracy. It will be noted that most were greatly magnified when the solubilities were expressed, of the older solubilities are definitely high in comparison with for the purpose of crystallization calculations, in parts of hy- point A. The dotted line of Figure 1 shows the solubility curve recdrated salt per 100 parts of free water, To reduce the uncertainity in such calculations, the solubility of magnesium ommended by Bronsted in the International Critical Tables sulfate pentahydrate was determined experimentally over a (3). It gives considerable weight to the 25' C. data, but it passes somewhat above point A of Figure 1. temperature range of 30" to 45" C.
D
EXISTINGDATA Magnesium sulfate heptahydrate is stable over a temperature range of 1.8" (6) to 48.4' C. (4). The available solubility data in this temperature range are shown in I- The data fa11 into two groups: The first group consists mostly of older data obtained at various temperatures; the second group consists of recent data deExcept for one Point from Weston (21) termined a t 25" a t 30' c. there are no recent data a t any temperature other than 25" C. In the first group are points determined by Gay Lussac ( 7 ) , Loewe1 ( I % , Tobler (18), Schiff (15), Mulder (141, Von
*
Present address, Peiyang Engineering College, Tientsin, China.
c.
EXPERIMENTAL PROCEDURE Although constructed for another purpose, a glass batch crystallizer was used to determine new solubility data. The apparatus is described elsewhere ( 1 3 ) . spite of the fact that the equipment differed from the usual solubility apparatus, it served its purpose well. The efficient agitation insured rapid approach to equilibrium, and the equilibriurn could be approached from either the supersaturated or the undersaturated direction. In those experiments where the approach was from supersaturation, a warm concentrated solution was cooled in the crystallizer and inoculated with seed crystals. After cooling to a definite temperature a t which the solubility was desired,
