Cocrystallization of Ultramicro Quantities of Iron and Other Elements with Oxine Generated in Situ H. SHIPMAN and Physical Chemistry Branch, U. S. Naval Radiological Defense laboratory,
HERBERT V. WEISS and WILLIAM Nuclear
b The cocrystallization of iron in trace quantities with 8-quinolinol formed homogeneously in solution b y the hydrolysis of 8-acetoxyquinoline was studied. The distribution of microcomponent between the solid phase and the mother liquor was profoundly affected b y the degree of supersaturation. After relief from supersaturation, a constant distribution coefficient was calculated by an equation which was predicated upon the Doerner-Hoskins equilibrium. The cocrystallization of other elements was also examined with an arbitrarily established set of conditions. Plutonium, cerium, and praseodymium were quantitatively recovered from solution.
I
process in which the entire quantity of both the organic reagent and trace element are present initially, the distribution of microcomponent between the crystalline phase and mother liquor in certain cases (6) obeys the Doerner-Hoskins logarithmic distribution law (2): The microcomponent-carrier ratio of the crystal surface is proportional to the microcomponent-carrier ratio of the solution. Accordingly, the expression: N THE COCRYSTALLIZATIOK
Jf/
C
log - = x log Mi Ci is derived where X and C represent the microcomponent and carrier, i and f indicate initial and final quantities in solution, and X is the logarithmic distribution coefficient. This study examined the cocrystallization process in which the crystallizing compound is generated in situ and thereby promotes its own crystallization. The opportunity was provided by the recent description of the homogeneous formation of 8-quinolinol in solution by the hydrolysis of 8-acetoxyquinoline (3). Consider a solution that contains the parent compound and an inorganic microcomponent capable of reacting with the hydrolytic product. When the water-soluble 8-acetoxyquinoline hydrolyzes in slightly alkaline medium, a condition in which 8-quinolinol possesses limited solubility, crystallization of 8quinolinol occurs as its solubility is exceeded. f
1010
ANALYTICAL CHEMISTRY
With further hydrolysis, the rate of crystallization is governed b y the rate of hydrolysis until the supply of the precursor compound is exhausted. As in the mechanism of coprecipitation by Doerner and Hoskins, the surface of the crystallized material is considered to consist of 8-quinolinol and 8-quinolinate 1%-hichare in equilibrium with their counterparts in solution. This relationship can be expressed by: (8-Quinolinate) on surface (8-Quinolinol) on surface (8-quinolinate) in solution (8-quinolinol) in solution
=
dC
kM
(3)
S
where k is the distribution coefficient and
S represents the solubility of 8-quinolinol. Integration of this expression with respect to the initial and final quantities in solution gives :
M
(4)
= M,e-kCiS
The applicability of this expression was studied in the cocrystallization of
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I
I
I
I
I
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iron n-ith 8-quinolinol. Iron was used because it combines with 8-quinolinol a t the p H selected for the crystallization. Further, a preliminary examination indicated that the incorporation of this microcomponent proceeded a t a rate which afforded close examination of the process. A general survey was also made to determine application of this organic system to the separation of minute traces of elements from solution. EXPERIMENTAL
(2)
where K is a proportionality constant. With this equilibrium and the inverse relationship between the carrier solubility and the efficiency of cocrystalliaation, the removal of microcomponent from solution with the crystallized 8quinolinol can be expressed by:
- dM _
Sun Francisco 24, Calif.
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The kinetics of hydrolysis was studied and a series of experiments x a s performed in which the recovery of iron n-ith solid 8-quinolinol was measured a t various stages of hydrolysis. Reagents. 8 - Acetoxyquinoline (Burdick and Jackson Laboratories, Muskegon, Llich.), dissolved in nbutyl alcohol (AR) a t a concentration of 220 mg. per ml.. prepared within a half-hour before use. Ammonium carbonate (AR), saturated solution in distilled water, was added to samples to neutralize the acetic acid formed in the hydrolysis of 8-acetoxyquinoline. The hydrogen ion concentration of samples was thereby maintained constant a t 8.4-8.6. p H 4.5 buffer solution, 8 grams of ammonium acetate (purified crystal) and 8 ml. of glacial acetic acid (AR) diluted to 2 liters with distilled water. 8-Quinolinol (Eastman Kodak) : crystals. Tracers. T h e following radioactive tracers were used: Ce 144-Pr144(I I I), Sc46( I I I), In (II I), ~91(111),s ~ ~ I sb124(111), I ) , ~e59(111), ;21n54(11), z ~ ~ I I C) P, ~ I I I ) , ~~195(111), Cs137(I), Ir1g2(IV), and CoGo(II)all in HC1 solution. Nb95(V) and Zrg5(IV) were supplied as the oxalate. Zr95 was seoarated from the Sb95 just before use Os'91 vias in N i O H solution. Srg5(II), AgllO(I), T1204(I), U237(VI), and P U ~ ~ ~ (were I V )in dilute nitric acid. ,411 the tracers were obtained commercially except U237 and Pu237, which were formed by the reactions U236 (n,y)U237 a t the Materials Testing Reactor and U*35(~~,2n)Pu*37 in the University of California 60-inch cyclotron. The purity of the y-ray emitters was established by -pray pulse-height analysis. Some of the tracers contained inert carrier, and the quantities of carrier were limited so that their concentrations in a sample were 10-8 gram per ml. or less.
(4.
TIME lYlN,
Figure 1. The hydrolysis of 8-acetoxyquinoline a t three temperatures
-
-z
CRVST4ILL N E
7
TIME
-
Figure 2. Schematic representation of events in the crystallization of 8-quinolinol from solution upon its formation in situ from the hydrolysis of 8-acetoxyquinoline
Method. Samples consisted of 1 ml. of a radioactive tracer solution, 1 ml. of ammonium carbonate solution, and 50 ml. of n-ater. T h e y were equilibrated i n test tubes for 15 minutes in a constant temperature b a t h maintained a t either O.O', 25.0', or 40.0" i. 0.1" C. Then to each was
Table 1.
added 1 ml. of the 8-acetoxyquinoline solution. During an experiment, samples were stirred constantly. To induce crystallization, unless otherwise stated, solutions were seeded with 0.39 0.15 mg. of oxine a t a predetermined time after hydrolysis began. These times were 8, 12, and 30 minutes for experi-
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Cocrystallization of Iron with Oxine-from Incipient Crystallization to Termination of Experiment
Hydrolysis Time (Min.)
MI
Fraction of Iron Carried
Distribution 8-Quinolinol Coefficient, 8-Quinolinol in Solution K8s = Crystallized (Mg./ -1n(l - Ma*) (Mg.) Sample)" CdS CJ
0"
37 .O 41.0 45.8 22.3 10.0
95.0 140.0 200.0 260.0 320.0 380.0 455.0
0.085
0.6
0.084 0.138 0.194 0 .294d 0.322 0.370 0.391 0.449 0.446 0.484 0.495
0.8
14.25 14.50 16.25 20.0 30.0 45.1 60.0 80.0
0.085
11.75 12.75 15.0 25.0 40.0 60.0
0.093 0.112 0 . 194d 0.214 0.219 0,229
0.088 0.234 0 .360d 0.382 0.393 0.390 0.414
8.8 10.5 41.gd 56.5 77.5 101.5 114.9 125 3 132.1 137.8
Errore
c. 33.5 38.7 32.2 36.8 20.9 20.9 20.9 20.9 20.9 20.9 20.9 20.9
25" C. 60.0 58.1 55.7 40.9 40.9 40.9 111.1 40.9 119.5 40.9 2.8 5.5 15.2 43.1d 72.4 97.8
2.7 5.4 41.4d 69.3 87.5 93.7
Standard
0.058
AO.017
0.067 0.052 0 .Oil 0.061 0.073 0.0'73 Av. 0.065
1 0 .009 f0,005 f0.005 10,004 10.005 f 0 .005 f0.007
0.048 0,040 0.029 0.047 Av. 0.041
f 0 .019 10.011 AO.009 f0.008 A0.012
40" C. 96.6 102.3 75.6 75.6 75.6 75.6
A0.030 0.069 0.052 f0,018 f0.015 0.064 10.021 Av. 0.062 Calculated from the rate of hydrolysis and the quantity of crystallized 8-quinolinol. - Md, Subscript (8s) represents data corrected for supersaturation. Thus, M,, =
m,
c,, = c - C d .
The standard error calculated by the usual formulas ( 1 ) considered the determinate errors of counting ( 1 yo)and 8-quinolinol determination (2a/,). The value of the first experimental interval after relief from supersaturation.
ments performed at 40°, 25", and 0" C., respectively. At certain times after the appearance of crystals, the samples were filtered rapidly through coarse-porosity sintered glass. Filtrates of the completely hydrolyzed samples were reserved for 8-quinolinol solubility measurement. Other filtrates were discarded. The crystalline phase was dissolved in approximately 10 ml. of dilute acetic acid and the solution was collected in a test tube for the radioactivity measurement. After adjustment of the solution volume, a count was taken in a well scintillation counter for a duration sufficient to confine the counting error to 1% or less. T o determine the recovery of microcomponent, this count was compared with that of a radioactive control diluted t o the same volume with dilute acetic acid. Following radioactivity measurement, the solution was diluted with p H 4.5 acetate buffer and spectrophotometrically analyzed for 8-quinohol. For 8-quinolinol solubility determinations, the fully hydrolyzed filtrates were similarly diluted. Measurements were made in a Beckman spectrophotometer at 252 mp, the wavelength of maximum absorption. The absorbance of 8-quinolinol at concentrationsof 1.0,2.0, and 4.0 p.p.m. was 0.216,0.443, and 0.905, respectively, with adherence t o Beer's Law. The standard deviation of measurements was about 2%. RESULTS
The kinetics of hydrolysis (Figure 1) for 8-acetoxyquinoline was determined indirectly by measurement of the quantity of 8-quinolinol recovered in the solid phase at various intervals after crystallization. This information, together with the solubility of 8-quinolinol at the respective temperature, could be used to determine the apparent quantity of reagent hydrolyzed. (The solubility of oxine a t 0, 25O, and 40' C. was 39.4 0.2, 76.4 f 1.5,and 142.6 i. 0.0 mg. per 100 ml.) Cocrystallization Characteristics. T h e cocrystallization of F e was studied during the course of hydrolysis. T h e distribution coefficients were calculated from the experimental d a t a after relief from supersaturation and n ere relatively constant (Table I) The influence of seeding vcith Fquinolinol upon t h e distribution was evaluated b y permitting three sample; containing F e tracer t o hydrolyze for 25 minutes at 25' C. Two samples were seeded a t 9 and 13 minutes after t h e beginning of hydrolysis and a third sample was allowed to crystallize spontaneously (18 minutes). Xt the end of the hydrolysis period, the quantity of crystals recovered from each of the samples was measured to be 65.5 mg. On the other hand, Fe cocrystallized to the extent of 19.2, 29.3, and 42.6% as the time between the
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VOL. 34, NO. 8, JULY 1962
101 1
initiation of hydrolysis and crystallization lengthened. The recovery of a variety of elements that cocrystallized with 8-quinolinol a t 25” C. was surveyed. The seed time was 12 minutes and crystals n-ere collected 20 minutes after the start of the experiment. -4wide range of recoveries !vas obtained from values as lorn as 47, for Cs, Ir, and Co to quantitativeness for Ce, Pr, and Pu. DISCUSSION
The sequence of events during the isothermal hydrolysis of 8-acetox.yquinoline is shown schematically in Figure 2 . As this reagent hydrolyzes, the sohition becomes supersatursted m-ith 8quinolinol. Crystallization occurs spontaneously a t critical supersaturation, or i t is induced by seeding during the period of supersaturation n-ith crystals of 8-quinohol. Supersaturation is relieved more or less rapidly, and upon its elimination, the concentration of 8quinolinol in solution remains constant with time. During this stage of the process, the crystallization of 8-quinolinol from solution proceeds a t the rate of parentcompound hydrolysis. Distribution coefficients were not calculated for the data obtained during the period of supersaturation. The csamination of the applicability of the Doerner-Hoskins equilibrium to this system is permitted only whcn the solubility of 8-quinolinol in the mother liquor is constant. Clearly this prereq-
uisite m s not achieved (Figure 2 ) . The concentration of 8-quinolinol in solution changed from the time of incipient crystallization until supersaturation was relieved. Interestingly, a relatively large fractionof the microcomponent cocrystallized during this period. Conceivably. the metal-8-quinolinol is concentrated in the crystal phase during the nucleation process. This suggestion n ould account for the observed effect of supersnturation when the direct relation between numbers of nuclei formed and drgree of supersaturation ( 5 )is considered. After the elimination of supersaturation, quantities of microcomponent continue to be incorporatcd bJ- the crystal phase. dccording to Klein and Gordon (41, new nuclei are not formed during cryrtallization from homogeneous solution after supersaturntion is relieved. This fact, togethrr uith constant solubility during this stage of the process, should be reflected by a constant distribution cocffirient, if the equilibrium model applies. The first experimental interval after which supersaturation is knon n to be relieved (Figurc l), thcrefore, was considered the starting point in calculating the recovery of 8-quinolinolnnd microcomponent in the solid phase. T h i l e the magnitude of the standard error is large, particularly a t the higher temperatures, it is clear that over a range of 8-quinolinol recovcries the distribution coefficient is rensonab’y constant. Moreover, while the solubility of 8-quinolinol
is vastly different a t the three temperatures studied, the coefficients are consistent viith one another. These results, therefore, substantiate the applicability of the Doerner-Hoskins equilibrium for the system described. lTTith regard to the recovery of elements from very dilute solution by cocrystallization with 8-quinolinol, it should be emphasized that the conditions ivere arbitrarily established to give a general indication of the effectiveness of the method. In addition t o providing for the efficient collection of Pu, Pr, and Ce from solution, it is probable that by adjustment of variables such as supersaturation, reagent concentration, temperature, and hydrolysis time, the recovery of other elements can be enhanced and even brought within the region of quantitativeness. LITERATURE CITED
f l ) Cook. G. B.. Duncan. J. F.. “Modern Radiochemical Practice,” p 60, Clarendon-Press, Oxford, 1E152. ( 2 ) Doerner, H. il., H oskins. 11- H., J .
Am. C h m . SOC.47,662 (1925). (3) Gordon, L., Salesin, E. D., Talanta 4, 75 (1960). ( 4 ) Klein. D. H.. Gordon. L.. lbid.. 1.
CHEM.32, 4; ( i )Wish, L., 1
RECEIVEDfor review February 23, 1963. Iccepted May 11, 1962.
Cocrystallization of Ultramicro Quantities of Elements with Thionalid Determination of Silver in Seawater MlNG G. LA1 and HERBERT V. WEISS Nuclear and Physical Chemistry Branch, U . S. Naval Radiological Defense faborafory, Son Francisco 24, Calif.
b The cocrystallization of ultramicro quantities of 2 7 diverse elements with thionalid was investigated b y radiotracer techniques. Under optimum p H conditions, greater than 90% of each of the following elements was recovered from solution: Au, Os, To, In, Hg, Ag, W, Zn, Sn, TI, Co, Ir, Ru, Mn, Cr, and Hf. Limited distribution studies revealed that the DoernerHoskins distribution was inapplicable and that Fajans Rule was not obeyed. Information derived from these experiments was applied to the isolation 1012
ANALYTICAL CHEMISTRY
and determination of silver in seawater. After radiometric correction for its yield, the concentration of silver in seawater was determined to be 0.1 45 f 0.006 pg. per liter.
A
concerned viith the separation of ultramicro quantities of elements from solution by cocrystallization with organic precipitants is in progress (11-15). This report describes a study with the organic precipitant thionalid selected because of GENERAL PROGRAM
its inherent insolubility in &rater and its ability to combine with a variety of metallic elements to form compounds more insoluble than itself (16). This difference in solubility between the reagent and the nietal compound satisfied Fajans Rule (S), a condition t h a t normally favors efficient recovery of B microcomponent from solution b y the cocrystallization process. The cocrystallization of 27 diverse elements with thionalid was studied under conditions of controlled hydrogen ion concentration. T o permit the quan-
