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-
titative measurement of trace quantities of these elements, radioactive tracers were used. The distribution of several of these elements between the mother liquor and solid phase was determined upon the crystallization of different quantities or organic reagent from solution. Finally, the application of the scheme was demonstrated by way of the isolation and subsequent determination of silver in seawater. EXPERIMENTAL
Reagents. Thionalid Solution. h weighed quantity of thionalid (95 to 997, pure, I< & K Laboratories, tJamaica, K. I-.)was dissolved in reagent grade acetone or absolute c.thy1 alcohol. Buffer Solutions. pII 3.5 buffer was of 0.1N acetic acid and the required quantity of sodium hydroxide; p H . 7 buffer n a s prepared from citric acid and disodium phosphate solutions (hIcIlvaines buffer); p H 10 buffer consisted of boric acid and sodium hydroxide, csnch approximately 0.1Jf. Tracers. Most of the tracers described in another reuort (16) were used here. Also studied. were: Hfl*l(IV), Se75(IV),KaZ3iI),RulOG(III) all in HCl; Tals2(V) and \V1s6(VI) in K O H ; and IIg203(II) in HKOa. The same precautions with regard to radionuclide purity and inert -carrier concentration n ere observed as before. Method. T o determine the influCnre of pH upon t h e cocrystallization of various radioelements with thionalid, 10 ml. of lYc thionalid in acetone vas added to 11 nil. of trace element
Table I. Per Cent of Various Elements Cocrystallized with Thionalid at Different Hydrogen Ion Concentrations (98to 99% of Thionalid Crystallized)
Elenient Sa
cs
Sr
Pe 8b Sn
In
T1 Zn
Hg .kg .4u co
Ir Fe Ru
OS RIn
Cr Ta
n-
Zr Hf SC
Y Ce U
PH 0
(1.Y HCI) 3 5
7 10 Per Cent C'ocrvstallized 1.7 1 . 9 10 3 1.7 1 0 19 8 1 . 2 42 7 21 7 39 6 21.6 3 9 78 8 89 7 81 7 8 7 . 5 84 4 95 8 10.3 99.4 100.0 95 7 3.6 6.0 00 0 6 . 9 100 0 98 3 98 5 97 5 99 1 24 8 100 0 100 0 85 8 98 9 97.4 98 3 73 9 1 . 6 85.6 96 3 19 0 85.7 94 6 8 6 7 5 3 57 9 3 2 . 5 56 5 00 0 91.8 94.6 99 7 00 0 8 6 i 5 . 4 96 0 3 i . 3 3 1 . 1 00 0 94.4 98.1 9 5 . 5 91 5 63.5 91.0 1.0 0 5 24.2 6 6 . 7 67 4 77.0 52.9 99 3 21.4 8 . 2 69 2 5.2 9 2 71.7 6.8 7 . 2 11.4 3.3 5 , 7 50.0
solution a t p H 0 (112- HCl), 3.5, 7 , or 10. This solution was evaporated in a steam bath until the original aqueous volume was reached, and then was cooled in a n ice bath for 30 min. Crystals of thionalid containing the microcomponent Tvere collected by filtration and dissolved in alcohol. The recovery of the radioelement was determined b y measuring the gamma-ray activity of the alcoholic solution in a n-elltype scintillation counter and comparing the count with a radioactivity control of the same volume. This solution \vas also analyzed for thionalid content. The distribution of a number of radioelements between solid phase and mother liquor was studied by crystallizing different fractions of thionalid from solution. A sample consisted of 11 ml. of p H 3.5 solution containing radioelement, from 3 to 5 ml. of ethyl alcohol, and 1 ml. of 0.5Yc thionn!id in alcohol. The quantity of alcohol n a s varied between samples to permit the crystallization of various quantities of the organic reagent. Samples maintained at 0" C. n-ere shaken to encourage crystallization and u-ere filtered at different times after the appearance of crystals. The crystalline phase 1%-as analyzed for the quantity of mdioelement and thionalid as described. Spectrophotometric Determination of Thionalid. T h e distribution of thionalid between solid phase and mother liquor was determined photometrically n i t h a Beckman spectrophotometer. T h e maximum spectral absorption occurred a t 286 mp Rhen thionalid was dissolved in absolute ethyl alcohol. The absorbance of 5.0, 10.0, 20.0, and 40.0 p p.m. of thionnlid in this solvent mas 0.222, 0.450, 0.920, and 1.80 and conformed to Beer's law.
Table II. Logarithmic Distribution Coefficients (A) for the Cocrystallization of Various Elements with Thionalid at p H 3.5
Recovered in Crystal Phase MirroThionalid, component,
7%
75
6.2 16.1 27.8 27.5 (1.0 84.4
36 2 43 8 45 3 62 5 74.0 87 2
Gold 7.7 3.3 1.9 1.2 1.1 1.1
Osmium 7.2 33.5 71.0 92.6
20 22 37 71
5 0 6 2
2.9 0.61 0.38 0.48
2.5 4.0 9.2 56.0 87.0 89.5
Tantalum 17 3 21 5 24.9 43.1 66.1 71,2
6 5 7.1 3 .6 0.68 0,53 0.57
7.5 23.0 51.8 62.5 76 8 87.0
Silver 22.0 24.1 32.5 46.0 65.5 82.2
:3 , 2 1.0 0.54 0.61 0.73 0.85
Mercury 5 2 70 0 19 1 37 5 56.3 88.7
30 0 35.2 36.7 39.0 47.1 87.3
7.3 4.1 2.1 1.1 0.77 0.95
Indium
RESULTS AND DISCUSSION
The recovery of the tracer quantities of elements from solutions of varying hydrogen ion concentration with crystallized thionalid is shown in Table I. A wide variety of elements was carried efficiently while only small to moderate amounts of the other elements were carried b y the solid phase. (In these experiments 98 to 99% of the thionalid was crystallized.) While the cocrystallization of trace elements with thionalid lacks specificity, some selectivity is achieved by performing the crystallization at a n appropriate hydrogen ion concentration. The distribution of trace element between the mother liquor and crystal phase a t p H 3.5 is shown in Table I1 for Au, Os, Ta, Ag, Hg, and In. The crystallization in these cases was produced rapidly so that the effect of diffusion of ions within the crystal and of recrystallization was negligible. This type of crystallization normally favors t h e Doerner-Hoskins logarithmic distribution (IO) for systems in which the microcomponent is internally incorporated within the crystal. Logarithmic distribution coefficients (A) were cal-
A"
3 0 10 0 40 5 46.8 71.6 80.8
37.2 45.2 71 0 77.7 91 .o 94.0
15.7 5.7 2.4 2.4 1.9 1 .7
total tracer tracer in solution total carrier log carrier in solution
.
culated and, in general, X was inversely related to the fraction of thionalid recovered from solution. This relationship is probably attributable to lessening in the degree of supersaturation with the progression of crystallization. A previous investigation (15 ) showed t h a t as supersaturation is relieved, the distribution coefficient diminishes and finally attains a constant value upon the complete elimination of supersaturation. JJ7ith the exception of Au and In, the solid phase is not enriched with tracer over the entire range of thionalid recoveries (Table 11). (A distribution coefficient greater than 1 indicates enrichment.) To determine whether, VOL 34, NO. 8, JULY 1962
1013
Table 111. Solubility in Water and 1 : 2 Ethyl Alcohol-Water a t 0" C. and pH 3.5 of Thionalid and of Silver and Gold Thiona la tes
Solubility, hlolee/Liter AlcoholWater Water 3.34 X 1 32 X 10-3
Compound Thionalid Silver thjonalate (AgTn) 2.00 X 2.73 X 10-6 Gold thionalate (A4~T2) 2.93 X 10" 3.24 X 10" a T represents the thionalid moiety.
Table IV. The Relation of Standing Time a t 0 ' C. and Reagent Concentration to the Cocrystallization of Silver in Seawater with Thionalid
(The reagent in 1 ml. of acetone was added to 50 ml. of seawter) Thionalid Standing Silver Added, Time, Recovered, hlg. Hours % 6 4 88.8 10 4 90.4 15 4 91.0 20 4 91.2 10 20 92.3 10
60
92.6
in fact, the nonenriched systems mere examples of exception to Fajans Rule under the conditions of crystallization, ~olubilitymeasurements were made of one such system, silver thionalate, and compared with gold thionalate Solid thionalates of known specific radioactivity and crystalline thionalid were stirred in water or 1:2 ethyl alcoholwater solutions a t pH 3.5 and 0" C. for 8 hours. The solubility was determined by radiometrically measuring the inorganic component in the ssturnted thionalate solution. T h r thionalid solubility was estimated spectrophotometrically. (Stoichiometry of the thionalates was established by eight and radioactivity measurements ) The results (Table 111) show that only the thionalid solubility is mnrkedlj affected bj- alcohol. JIoreowr, thc solubility of the thionalates in either sollition is of the same magnitiidp and suhstantially niorr insoluble than thionnlitl. These datn clearly indicate that in the cocrvstallization of sih er with thionalid, Fajans Rule is not obeyed The rule in the current context States that if trace elcment and orgnnic reagent combine to form R compound whoqe insolubility is greater than that of the reagent, crystallized reagent should be rnriched with trace element. Sotnble evreptions to this rule havr been reported, such as the failure of radium to coprecipitate from solution with calcium sulfate ( 5 ) . Thc rvplanation offered by Hahn, which ma\- also apply in the current situation, 1014
ANALYTICAL CHEMISTRY
is heteromorphism and crystallographic incompatibility between the carrier and microcomponent compounds. DETERMINATION OF SILVER I N SEAWATER
Cocrptallization with thionalid was applied to the quantitative analysis of silver in seawater. To determine the silver content of seawater a preliminary isolation step is required because of the macro salt content and the submicro silver concentration. Several methods of isolation have been described which depend upon coprecipitation with inorganic carriers, and the silver concentration has been variously estimated as 0.3 ( d ) , 0.15 ( 7 ) , and
