stream flow was 7 cubic meters per second. I t is convenient to clean the silver gauze stirrer between experiments with a KCN solution. Stirring of the K C N (3 grams of the salt per 100 ml. of HzO) for 1 or 2 minutes, followed by three 5-minute rinses with water, has always produced a bright, active stirrer which gives reproducible results. Actually, in the experiments described here, 2 pieces of gauze were irradiated simultaneously in the reactor. Their activities are identical and a sample and an “acid blank” can be carried out
together. These stirrers show no signs of deterioration after having been used for more than 100 samples or blanks with K C N cleanings between each use. ACKNOWLEDGMENT
We acknowledge the assistance of K. L. Fletcher who helped with many of the preliminary measurements. LITERATURE CITED
(1) Gillespie, A. S., Richter, H. G., ANAL. CHEM.36,2473 (1964). ( 2 ) Gillespie, A. S., Richter, H. G.,
Trans. Am. Nuclear SOC. 5 , No. 2, 273 (1963).
RECEIVEDfor review April 15, 1965. Accepted May 24, 1965. Part of the work was carried out at the Research Triangle Institnte, Durham, S . C., under IJ. S.Atomic Energy Commission, Division of Isotopes Developments Contract No. AT-(40-1)-2513. The remainder of the work was performed at le Centre d’Etitdes Sucleaires de Grenoble, in the Section d’Application des Radioelements, where one of 11s ( I f . G . R . ) was given the freedom to pursrie the atitdy and records here his appreciation of the cooperation from that groiip. H. G. Richter also expresses his appreciation for a 1964-65 Fulbright Grant.
Solvent Extraction Studies with a Hydrochloric Aci d-2 - Ethy I hexa noI Systern K. A. ORLANDINI, M. A. WAHLGREN, and J. BARCLAY‘ Argonne National Laboratory, Argonne, 111.
b The distribution of 45 elements between 83% 2-ethylhexanol-1770 petroleum ether and hydrochloric acid solutions has been determined. A graphic presentation of the extraction data as a function of acid concentration is given, Very effKient extraction of Ti(lV), V(V), Ru(VIII), Sn(lV), Pa(V), Nb(V), Ta(V), Ga(lll), Fe(lll1, Sb(V), Au(lll), As(lll), Np(VI), and Mo(VI) was achieved. O f the remaining elements studied, only W(VI), Zn(ll), U(VI), Tc(VII), Cd(ll), Hg(ll), and Pd(ll) were appreciably extracted. Separations within the group of well extracted elements can b e accomplished by back-extraction utilizing selective reduction, complexation, and variation of acid concentration. The extracted elements are readily stripped into dilute acid solution or water. A number of applications of the system to activation analysis as well as other radiochemical separations, including preparation of some carrier-free samples, are given.
I
ALCOHOLS have not been widely applied as extractants for inorganic species as have ethers, ketones, and more basic extractants such as the high molecular weight amines. Alcohols are often used as diluents for other extractants (2) and as solvents for chelate extractions. Distribution studies between hydrochloric acid and a n immiscible alcohol have been used to determine ionic species in hydrochloric acid solution ( 1 , 3 ) . I n many analyses the samples are solubilized in strong mineral acid MMISCIBLE
1 Present address, University of California, Berkeley, Calif.
1148
ANALYTICAL CHEMISTRY
solution, while the optimum conditions for specific separations occur in the p H range 0-12 \\here masking and p H control are most effective I n radiochemical procedures, a preliminary separation step is often used to eliminate the bulk of interfering activity and consequent radiation hazard in a simple, remote operation. The hydrochloric acid-alcohol system t o be described is very suitable for this type of separation because of the rapid equilibrium, ease of manipulation, and high solute capacity. I n the case of sequential multielement analysis, the separated fractions are free in most cases of excess reagents and salts which may interfere in a following analytical step Many of the elements covered in the present study extract into various oxygenated solvents such as ethers and ketones, but 2-ethylhexanol is superior to these reagents in several respects. The low solubility (0.33y0 in concentrated HCl) of the alcohol is a distinct advantage over ether and ketone systems. Having a flash point of 85” C., and an azeotrope with water boiling at 99” C., gives the alcohol system stability as well as ease of evaporation. The low vapor pressure a t room temperature permits one to carry on experiments without significant solvent loss and with low flammability hazard. The latter is of particular concern when working with highly radioactive samples. Of some 45 elements studied, 14 were well extracted (75-9975) while nine were extracted in the 30-60Oj, range and would be considered interferences. The alkali metals, alkaline earths, and rare earths were not extracted appreciably. Nickel, bismuth, indium, scandium, silver, and copper were extracted 10%
and less; iridium (IV), ruthenium (111), chromium (111), manganese (11), and yttrium, 1% and less. EXPERIMENTAL
Reagents. 2 Ethyl-1-hexanol (practical grade) was obtained from RIatheson, Coleman, and Bell. This grade has a n assay of 98yc by gas chromatography, the remainder consisting of isomers. -411 other chemicals and solvents were reagent grade or equivalent except n-hexanol which was Eastman practical grade. Procedure. Distributions were determined using purified radioactive tracers. All extractions and Ire-equilibrations were carried out in 40-ml. centrifuge cones using battery-driven mechanical stirrers. Although phases separated well, centrifugation was employed to eliminate any suspended droplets in either phase. Aliquots were pipetted into small polyethylene bottles or dried on planchets for counting. Single channel and niultichannel analyzers were used for gamma counting, and gas proportional detectors for alpha and beta counting. Petroleum ether was used as diluent for all alcohols studied. The 2-ethylhexanol for all extractions was diluted to 83% by volume for ease of phase separation, unless otherwise noted. A small amount of cryhtalline KMnOc (30-40 mg.) was added three times during the evtraction of V, Ru, Xi), Au, Mo, and Sb tracers in order to oxidize these elements to their hi her oxidation states. Cr, Ce, hln, an! Bi could not be maintained in the maximum valence state in the alcohol-HC1 system. The organic phase was pre-equilibrated in all cases. Equal volumes (10 ml.) of organic and aqueous phases were used for the extractions. The distribution coefficients (D)given are the ratio of radioactive count rates of equal
'2M
rn
kCe
-
Pr
83% 2 Ethylhexanol-17% Petroleum Ether -Hydrochloric Acid
Nd
Pm
Sm
: m
:Eu
-
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Cm
Rk
Cf
Es
Fm
Md
No
Lw
:la Pu
Figure 1.
aliquots of organic and aqueous phases. Based on studies of extraction time dependence for several typical elements, a 10-minute equilibration time was used for all extractions. RESULTS AND DISCUSSION
Figure 1 shows log-log plots of the extraction data. The main coordinates as shown on the model figure are distribution coefficient D and mean ionic activity a, as determined from molality and standard activity coefficients. Molarity and percent extraction are also indicated on the model. The alkali metals show no appreciable extraction. Less than 1% of Li, Na, and K was extracted from strong HCL. Ag(1) has a D about 10 times higher, 8% having been extracted from strong HCI. The distribution of HCl is included to indicate the significant extraction of H + from concentrated acid solution, giving an organic phase concentration equivalent to 4-11 HCL. This affects the distribution behavior of elements extracted from strong acid solutions. Significant quantities of bivalent elements were extracted by this system. Zn, Cd, Hg, and Pd with maximum extractions between 40 and 50y0 are considered interferences over the useful range of acidities, whereas Co would be an interference a t 9JI HCI or higher. Cu and M n can be easily removed from well estracted elements by organic phase scrubbing. Of the trivalent elements G a , Fe, Xu, and As(II1) show high distributions (>%yo). Insignificant amounts of
Am
Distribution coefficient vs. ionic activity of HCI
h l , In, and the rare earths were extracted. G a and Fe were extracted together under oxidizing conditions; however, reduction of Fe(I1I) to F e ( I I ) , which is poorly extracted, facilitates the separation from gallium. Gold was highly extracted (>goy0) even in the lower acid range where distributions of most elements are low. Gold eutractions were run in the presence of KMnOc because of the tendency of gold to be partially reduced in the simple system. The high extraction of Ga and Fe makes possible separations from associated elements including Co, Cu, Mn, Ni, In, V, and Al. The curves in Figure 2 represent the dependence of the distribution coefficient upon the concentration of the alcohol in the organic phase, with HCl concentration as a parameter. These curves are useful in maximizing differences in behavior for specific separations. Gallium and iron have similar alcohol dependence. The alcohol dependence for gold was studied a t 6.U HCI acid and found to be similar to Fe but with a higher range of distribution coefficients. The family Ti, Zr, and Hf shows increasing extractability with decreasing ionic radius. Ti is well extracted (>91%) above 11J4 HCI. Separation factors in this subgroup of elements are not high. However, Sn(IV) shows a different dependence on acid strength, and separation factors between Sn and Hf, or Sn and Zr, a t 8M HC1 are good. Sn is also readily separated from In, 15, and Cu which are poorly extracted in this system.
Germanium is not included because of its well known chloride extraction into a variety of organic solvents. The alcohol dependence of T i and Zr is unique among the elements studied IOoo
I
I%
5%
25%
100%
Figure 2. Distribution coefficient of Fe(lll) vs. molarity of alcohol in organic phase VOL. 37, NO. 9, AUGUST 1965
1 1 49
IC Z r 12M HC
%“
I.c
0.I D
0.01
0.001
0.0001
I
I
0.I I I%
-M !
_L
I .o
6.,
ROH I
t
5%
1
8
25%
1
-
100%
Figure 3. Distribution coefficient vs. molarity of alcohol in organic phase
because of the distinct shifts a t 9.26 HC1 (Figure 3). Zirconium goes through a transition in the alcohol dependence curves from 9 to 12M HC1. The maxima in the dependence curves for Ti and Zr a t 50% alcohol suggests the use of this alcohol concentration rather than 83% strength. Sn(1V) shows a strong and also uniform alcohol dependence a t all acidities. Distribution coefficients of greater than 2000 for Sb(V) and 10,000 for Pa(V) were observed. The high distribution (>9970) for vanadium(V) contrasted to low distributions for vanadium(1V) and (111) enables ready separation of vanadium from well extracted elements, including group (V) elements. Tantalum is not stable in hydrochloric acid and tends to be precipitated out of solution, and in the case of tracer concentrations is quantitatively adsorbed on the glass surroundings. The T a curve in Figure 1 shows the extraction in the stated HCl concentrations with 0.2M HF present. Since T a is well
Table I.
Element Zn(I1) Ga(II1) Fe(II1) Sn(1V) Nb(V) W(V1)
extracted from H F . HC1 solutions the possibilities of separation from many elements is evident. This behavior has also been studied by Casey and Maddock (1). Figure 4 shows the alcohol dependence curves for Sb(V), Nb(V), Pa(V), and V(V) a t 12 and 5M HCl. Noteworthy is the overall low dependence of vanadium and the shift in the antimony curve in 5M HC1. h very efficient separation of Sb(V) and from Sn(1V) can be achieved on this basis. The distribution coefficient of antimony between 12.11 HC1 and 0.32W (57’) alcohol in petroleum ether is 125 (99+ %) while distribution of Sn(1V) under these conditions is less than 0.1 or 9%. Among the hexavalent and heptavalent elements Mo(V1) and Np(V1) have good distributions (75%) in strong HCl while Tc(S’I1) shows a maximum extraction of 67%. Tungsten with a maximum extraction of 57% is considered a high level interference. Np (VI) is easily reduced to lower oxidation states which are poorly extracted. 140 also is poorly extracted in its lower oxidation states and is readily reduced by stannous chloride. Cr(V1) has a tendency to be well extracted in this system. However, this element is rapidly reduced to the nonextractable Cr(II1) even in the presence of KMnOl and may easily be separated from other well eytracted elements such as V, Ti, Fe, Xb, and Sn. Ru(II1) and (IV) are not appreciably extracted from HCl, but in the presence of an oxidizing agent (K.\In04) Ru extracts quite well (7548%) from 1 to 3M HC1. At higher acidities the extraction falls off and remains between 50 and 7oyob. Extraction of Ru from dilute HC1 offers the possibility of separating this element from many others. Effect of Macro Amounts of Element on Distribution. Experiments were r u n to determine the effect of macro concentrations of a n element upon its distribution. Examples are given in Figure 5. Distribution COefficients remain near maximum well into the range of analytical con-
Ratio of Distribution Coefficient in Mixed Acids to Distribution Coefficient in HCI Alone Distribution 8.2.V HC1, 8.2M HC1, 8.2M HC1, Coefficient HC1
0.06M H F 1 0 1 0 0 63 1 0 0 13
1.2M “ 0 3 0 56 0 39 0 34 0 39 3 0
9.6M HC1, 0.06M HF
9.6M HC1, 1.2M HXOS 1 0
0 85
Zr(IV)
0 02
1 150
ANALYTICAL CHEMISTRY
0 47
1.3M H2S04 1 63 1 2 1 14 1 34 06 1 35 9.6M HC1, 1.3M H z S O ~ 0 03
8.2M HCl 0 42 770
270
2 89
62
1 44
9.6M HC1 0 46
I
01
I
10
6 #
MOLARITY o f ALCOHOL in ORGANIC PHASE I > 5 I% > 25% 100% 1 I%
Figure 4. Distribution coefficient of pentavalent elements vs. molarity of alcohol in organic phase pa(W -O-,.Sb(V) -0-, V(V) -A-, Nb(V) -0-,
-.-A-6
centration for the elements shown. Noteworthy is the close similarity between Sn(1V) and Mo(V1) under these conditions. It was observed in the extraction of macro amounts of vanadium from 9-12M HCl that V(V) in the 2-ethylhexanol phase was notably more stable to reduction than V(V) in aqueous HC1. Effect of Acids and Anions. The effect of HC1 containing moderate amounts of hydrofluoric, nitric, and sulfuric acids upon the distribution coefficient was determined for a representative group of elements (Table I). Qualitatively, hydrofluoric acid lowers extractions with varying effectiveness. Hydrofluoric acid at 0.06M showed little effect upon Ga estraction but strongly decreased Zr extraction. Sulfuric acid strongly decreases Zr extraction at the acidity shown and could be used to enhance separation factors where Zr is an interference. Kitric acid triples the extraction of Kb(V) a t 1.2M HKO,, 8.2M HC1 while decreasing Ga and Fe extraction at the same acidity. An interesting extension of the HC1
Table II.
Applications of 2-Ethylhexanol in Petroleum Ether as Extractant
Element Fe(II1)
Separated from Co, V, Mn, Cu, Sc, Ni, Cr, Al, Mg
Aqueous phase 6-8M HC1
Ga
Fe
Nb, Sb(V)
Zr, Hf
6-8M IICl reducing agent (SnC12) 6-8M HC1
Nb
Sb
Nb
Fe, V
VV)
Ti, Cr, Al, Sc
Ti(IV) Ta
v, Sc
Pa Sn(1V) Au(II1) Ag (carrier free tracer) Sb(V)
Hf, Zr, Th Hf, Zr, Cu, In, Bi Many elements
Sample Meteorites, steel
Detn. of Zr in Hf using Nbg7 daught,er activity Purification of tracers
Sepn. of V48 from Cyclotron irradiated Ti foils Trace analysis of Th, for Hf, Zr, Ta by neutron activation
Detn. of trace Sb in metals and fission products
system is the pronounced effect of KI on the extraction of Ag. Carrier free Ag tracer is extracted about 85% from 1 N HC1 containing a small amount of KI. At higher concentrations of Ag, the extraction is decreased. Effect of Alcoholic Type. T h e alcohol dependence curves for the elements studied showed a complex distribution response to changes in the alcohol concentration. Contrasting behavior is noted between regions where the distribution coefficient is relatively insensitive to changes- e.g., Figure 3, T i 8.5M HC1; Figure 2, Fe 10.3M HC1-and regions where extraction increases rapidly with moderate alcohol concentration changes (Figure 2,
+
+-
Zrlml I I M H C L
+I
Pa, Hf, Zr
Sn, Bi, Cu, Zn
9-llM HCl reducing agent 5M HC1 mild reducing agent 6-8M HCl KMLIPU’O, Warm 12M HC1 3-5M HC1 0.5-1M HF 7-8M HCl 7.5M HC1 3M HCl 1M HCl, K I 12M HC1
Fe(II1) 6M HCl). Experiments using n-hexanol, n-heptanol, 2-ethylhexanol, n-octanol, and n-decanol indicate that significant differences are observed only in certain regions of high alcohol dependence. Fe(II1) in 10.3M HCl was extracted to nearly the same extent by the five alcohols a t 50% concentration. I n 6 M HCl and 50% alcohol, 2-ethylhexanol was least effective (D = 1.7) and n-hexanol most effective (D = 11.5). Similar behavior occurs with Sn(1V) in 5 M HC1 with 50Oj, alcohols and with T i in 12M HC1 and 20% alcohol. A comparison of 2-ethylhexanol and n-hexanol in the extraction of Pa(V) from 5 M HC1 showed no difference over the range of 70-100% alcohol. The studies indicate that an alcohol-type effect may be observed where the 2-ethylhexanol concentration dependence curve exhibits linear regions. The use of different alcohols can enhance the separation of elements under certain conditions. Typical Applications. Table I1 shows some typical applications using 2-ethylhexanol in petroleum ether as the extractant. For these radiochemical applications, milligram amounts of carrier for gravimetric measurement were not added since the separation could be made quantitative (> 98%). The use of inactive carriers is recommended to eliminate adsorption losses. By utilizing differences in distribution response to the various parameters, separations can be readily optimized for the particular sample material of interest. A typical application of the 2-ethylhexanol-HC1 system is the extraction of high levels of radioactive protactinium-
Organic phase 2-Ethylhexanolpetroleum ether 83% ROH L,
30% ROH 11
50% ROH 83’30 ROH 11
83% ROH
2,570 ROH
233 from a thorium matrix in the determination of trace amounts of hafnium and zirconium by neutron activation. Vanadium has been separated as carrier free V48from deuteron irradiated titanium foils. This separation is performed in 7.5M HCl where in the presence of KMn04, 94% of the V(V) and about 13% of the Ti(1V) are extracted. The Ti(1V) is scrubbed out of the organic phase with 7.5M HCl. Alternatively, Ti(1V) may be extracted from vanadium in 12M HC1 where vanadium is easily reduced to the poorly extracted IV state. An example of separation of a three element combination is the case of processing zirconium-niobium tracer which contained antimony impurity. Niobium and antimony are readily extracted from zirconium in 7.5111 HCl by 30% alcohol. The Nb-Sb combination is scrubbed once’ for removal of traces of zirconium, then antimony is back-extracted into a separate aqueous phase of 9M HCl containing a suitable reducing agent such as stannous chloride. The niobium is back-extracted into water containing HzOz. LITERATURE CITED
(1) Casey: A. T., Maddock, A. G., J. Inorg. hucE. Chem. 10, 289 (1959). ( 2 ) Moore, F. L., “Liquid-Liquid Extrac-
tion with High Molecular Weight Amines,” Nuclear Science Series Monograph NAS-NS-3101, 41 (1960). (3) Scherff, H., Herrmann, G., Proc.
Protactznzum Chemistry Symp., Gatlinburg, Tenn., TID-7675,105 (1963). RECEIVED for review March 18, 1965. Accepted J u n e 9, 1965. 148th hleeting,
ACS, Chicago, Ill., August 1964. Based on work performed under the auspices of the U. S.Atomic Energy Commission, VOL. 37, NO. 9, AUGUST 1965
1 151
