3rd A n d Summer S#mp8Zum
- Separatkne
Role of Extraction in Analytical Chemistry GEORGE H. MORRISON
U . S. Atomic Energy Commission and Rutgers University, New Brunswick, N. J.
Analytical liquid-liquid extractions fall largely into three classes: inorganic, organometallic, and organic. Laboratory apparatus and general techniques, including batch and continuous procedures, are reviewed. Many inorganic extractions are dependent upon the formation of undissociated w m plex acids which are soluble in organic solvents. The salting-out effect produced by the presence of cations plays an important role in extraction; in
A
general, the cations of higher valency are more effective. Several simplified theoretical relationships concerning organometallic extractions are presented which emphasize the importance of pH and concentration of reagent in metal extraction. Improved countercurrent distribution instruments permit separation or estimation of closely related organic substances, and are applicable to many complex organic or biochemical problems.
N EXAMINATION of the development of analytical chemistry during the past few decades reveals an increase in popularity of such fields of analysis as chromatography, ion exchange, and electrodepositions. This trend is easily explained when one considers the increasing need for methods of separating complex materials. Liquid-liquid solvent extraction has been known for over one hundred years as a method of separation in analytical procedures; however, it is only recently that this technique has gained prominence equaling that of the more i m p o r t a n t m e t h o d s . The phenomenon is based upon the fact that if a substance is dissolved in a system of two immiscible or slightly miscible liquids, the substance is distributed between the two layers in a definite manner (2). The most important factor in this distribution appears to be the relative concentrations of thebsolute in both solvents and is usually expressed as the d i s t r i b u t ion o r p a r t i t i on. coefficient-Le., the ratio of the activity or concentration in one phase to that in the other (29). In the ideal case, the solute distributes itself between the two liquid phases iri Figure 1- ' Batch ~ I i C r O the ratio of its solubilities in extractor the two phases. This relationship is in turn fundamentally dependent upon other factors such as the molecular species extracted, the presence of certain salts, and acid concentration. Although a study of the distribution behavior of pure substances is one approach to the solution of problems in analytical extractidns, one should bear in mind the effects produced by the presence of all the components in a compley mixture. Consequently, much of the information obtained so far on analytical extractions has been derived from empirical studies of nonideal solutions. In analytical chemistry, extraction is applied prirnaiily as a means of separation. By measuring the radioactivity, optical density, or some other physical property of the solute in the organic layer, it is sometimes possible to make use of the organic layer in a direct determination. The more common procedure, however, is t o separate the extracted solute from the organic
U
layer, either by evaporating to dryness or by back-extracting the solute into water. Standard analytical procedures may then be applied. The use of extraction in quantitative analytical work requires that there be a preferential quantitative separation. The problem of extraction, then, consists in determining the conditions which result in extraction of certain elements, and, equally important, determining substances which will not extract under these conditions. Analytical extractions appear to fall largely into three general classes: inorganic, organometallic, and organic and biochemical. Preparatory to a more detailed treatment of these main types, a discussion of the general techniques and apparatus used in this method of analytical separation follows. APPARATUS AND GENERAL TECHNIQUES
In laboratory work there are three general types of liquidliquid extractions. The simplest is batch extraction, which consists of extracting the solute from one immiscible layer into another by simply shaking the two layers until equilibrium is attained. The layers are allowed to settle and are separated. Discontinuous countercurrent distribution extraction, which permits direct application of the binomial : expansion in order to interpret the results in a quantitative fashion, is the second general t y p e , a n d essentially performs many i n d i v i d u a l extractions rapidly and in s e q u e n c e . The third type, continuous .extraction, makes use of a continuous flow of immiscible solvent through the solution. The s p e n t solvent is p u r i f i e d and re cycled by distillation. Batch extractions Figure 2. Continuous Extractor may be used t o for Use with Solvent Lighter good a d v a n t a g e Than Water
1388
!8
B
1389
V O L U M E 2 2 , NO. 11, N O V E M B E R 1 9 5 0 when the distribution coefficient is large, for in such cases a few extractions will effect quantitative separation. The usual apparatus for a batch extraction is a separatory funnel. I n extracting from one liquid into a lighter liquid it is n e c e w y , when using a separatory funnel, to remove the heavier liquid from the ’funnel after each extraction before removing the extracting Lolvent. Kirk and Danielson ($1) avoid this double removal by means of the apparatus shown in Figure 1. This extractor was designed for working- with small volumes, but the design could probably he applied to larger volumes. Discontinuous countercurrent distribution extractors are particularly useful in the separation of substances whose distribution coefficients are of the same order of magnitude and consequently have found greatest application in the field of organic and biochemical separations (33). A series of separatory funnels may be used for this technique; however, certain special devices are more convenient (6, 6 ) . Continuous extractions are used when the distribution coefficient is relatively small, so that a large numb6r of batch extractions would normally be necessary to effect q u a n t i t a t i v e separation. hlost continuous extraction devices operate on the same general principle, which consists of distilling the e\tr:tcting solvent from a boiler flask Figure 3. Continuand condensing it and passing it ous Extractor for through the solution being exUse with Solvent tracted. The extracting liquid sepaHeavier Than Water rates out and flows back into the rereiving flask, where it is again evaporated and recycled while the extracted solute remains in the receiving flask. As the extracting solvent passes through the solution fritted glass disks, small orifices, baffles, or stirrers may be used to bring the two immiscible layers in closer contact. Indeed, the efficiency of any continuous extraction apparatus depends greatly upon area of contact developed between the two liquids. Figure 2 is a modified tall form of Friedrich extractor designed by Heberling (16) and is used for extraction with a solvent lighter than the solution being stripped. This apparatus is particularly useful for the ether extraction of aqueous solutions of inorganic substances. With solvents heavier than the solution being treated, the extractor in Figure 3 may be used (SO). One type of extractor may be converted to the dther simply by changing the tube inside the jacket. An auviliary technique used with batch extractions to effect quantitative separations of elements is that of washing. The combined organic phases from several extractions of the original aqueous phase contain practically all of the element desired and possibly some of the impurities that have been extracted to a much smaller extent, depending upon their relative distribution coefficients. This combined organic phase, when shaken with one or more small portions of a fresh aqueous phase containing the optimum acid concentration, salting agent, etc., will result in a redistribution of the impurities, as well as the major component, between the two phases. Under optimum conditions most of the element desired will remain in the organic layer; however, the bulk of the impurities will be back-extracted to the fresh aqueous layer. This technique is analogous in Wany respects to the washing step in a gravimetric precipitation procedure. With the proper conditions most of the impurities can be removed by this washing technique with negligible loss of the main component, thereby resulting in a selective separation.
Many times special organic solvents do not lend themselves to repeated distillations. Certain treatments involving substantial losses may be prohibitive. A simple technique of ‘[stripping” the organic phase of the extracted material may be effected by washing with a fresh aqueous phase containing the proper reagents, thereby causing the extracted material to favor the aqueous portion. Several of these stripping treatments, using relatively small aliquots, should result in quantitative removal of the extract from the organic phase, thereby permitting it to be recycled again. Hixson and Miller ( 1 7 ) suggest the use of this technique in industrial extractions of metals where the cost of organic solvents is a determining factor. The use of extraction techniques in spot test analysis offers a means of separating an ion from its interferences which might prove valuable in a manner similar to the use of masking agents. West and Carlton (37) have devised an extraction pipet, prepared from a capillary droper pipet. In practice the extraction involves the addition of a few drops of extractant to one or more drops of the test solution. Mixing is accomplished by drawing the liquids into the pipet and then expelling them, repeating the procedure several times, quickly. The organic phase may then be evaporated and analyzed by conventional spot test methods. INORGANIC EXTRACTIONS
The majority of the extractions of this type involve the extraction of a metal salt from an acid solution into an immiscible organic solrent. High acid concentration has been shown to be a dominant factor in many systems, and in many instances an undissociated complex acid formed hy the metal salt and the acid is the species responsible for extraction into the organic phase. In the case of iron, one of the most studied of the extraction systems,’the formation of chloroferric acid (HFeCL), ,the extracted species, excluding solvation, is particularly favored at an initial hydrochloric acid concentration of 6 formal, using diethyl ether as the extracting solvent (8). Furman and Morrison ( 1 2 ) have shown that if part of the chloride ion concentration involved in the process of formation of chloroferric acid is supplied as a soluhle chloride salt, varying degrees of extraction are effected, depending upon the nature of the cation used. In general, at equal chloride concentrations, the cations of higher valency are more effective, as would be expected according to salting-out theory ( 7 ) . This effect is illustratrd in Tables I and 11. Tn the absence of added hydrochloric wid, using aluminum chloride as a salting agent, the iron does distribute slightly. This effect may be explained by dissociation of the hydratpd aluminum ions [AI(HzO), ‘1 to furnish the nrecled fi
Table I. Effect of Ammonium Chloride and Hydrochloric Acid on Ferric Chloride Extraction Using Diethyl Ether Chloride Concentration, Moles/I.iter NHICI HCI . . 5 0 2 5
4.9
> O 0
2 5 0 1u
Distribution Coefficient, Corg ./Caq.
% Extracted
17.6 0.376
94.6 26.6
... ..
1 niole of HCI i m s r n t for c\ er? mole of Fe
-
Table 11. Effect of Aluminum Chloride and Hydrochloric Acid on Ferric Chloride Extraction Using Diethyl Ether Chloride Concentration,. hloles/Liter AlCll HCI” 5.0 3.5 2.5 4.9 0.1
a
Distribution Coefficient, Cow . /Cas.
% Extracted
94.6 93.3 16.4 0.6 5.0 ... 5.0 1.0 97.1 4.0 2.0 08.4 3.0 3.0 99 2 Initial HCI concentration of 6 M gives optimum extraction. 17.6 14.0 0.196 0 006 33.0 66.1 116.
1390
ANALYTICAL CHEMISTRY
protons. .4t a total chloride ion concentration of 6 molar, various combinations of acid and salt appear to be equally effective in attaining almost complete extraction. This salting-out effect may be explained in part by the pronounced effect of'added salt on the activity of the distributing species. Similarly, one of the primary factors in salting out is a characteristic constancy of binding of a part of t,he water by some ions, the extent of which is directly proportional to the number of ions. The bound water is protmblg removed as a shell of oriented water dipoles around the ion and thus becomes unavailable as solvent molecules in which the undissociated chloroferric acid could be dispersed
3
0.4
0.1
R
5
0
2-0 t
the removal of cerium (46). The high degree of extraction by butyl phosphate over a wide range of conditions suggests the formation of R compound between the solvent and cerium(1V) nitrate. Templeton, Rothschild, and Hall (36) have investigated the distribution of thorium nitrate between water and rertain esters and have proposed an explanation of the nature of the distribution equilibrium. The solubility of uranyl nitrate in ether has been used for years for the separation of uranium from many elements (31 ). Recently, however, Hecht and Grunwald (16) have used ammonium.nitrate as a salting agent to effect complete separation of uranium in the analysis of ores, and Rodden (32)hm described a procedure for the continuous eutraction of uranyl nitrate using ammonium nitrate. The insolubility of the perchlorates of the alkali metals in various organic solvents has furnished a convenirnt method for the separation and determination of potassium (38). Various organic solvents were examincd and cAthyl acetate appears to be one of the best, the solubility of potassium perchlorate being least and that of sodium perchlorate greatest. The fact that larious metal perchlorates are soluble in organic solvents presents many possibilities for future study. Solid extractions have similarly been used to separate calcium and magnesium sulfates ( 3 4 ) .
0
I-
24.4 -
H
He
K
Li Be
z-03
B C N O F N e
0 (L-0.) 0
"
-I
@
2.4F HCI
+
@
2 . 4 F HCI
+ NH+CI
CaCI,
Na Me
Ca Sc X : V " . C k
Cs Ba La
Figure 4.
CHLORIDE
FORMALITY
Mn
Fe,& N i / C u
...............
Zn I
tV Ta W
Re OI
;
Xe
'*.....:.
Fa Ra Ae
Effect of Salting Agents on Extraction of iMolybdenum
It has been observed for some time that the distribution ratio of iron between aqueous solutions and an organic layer varies with total iron concentration. Nachtrieb and Fryxell (27) explain this anomaly by suggesting a sclf-salting-out effect by the iron, which is consistent with the above study. A similar study on the distribution of gallium (HGaClr) reveals the same effect (98). Because the analytical chemist is confronted with the separation of inorganic substances present in a particular anionic medium, these inorganic extractions are so arranged in this discussion. In extractions performed in the course of an analytical procedure, relatively large amounts of $certain foreign cations, as well as the cation being extracted, act as salting agents and consequently enhance the separation. Among recent studies on extractions in the chloride system is the extraction of antimonic chloride from hydrochloric acid solutions by Edwards and Voigt (9). SbV can be quantitatively separated from Sb"' by this method. Garwin and Hixon (13) have succeeded in separating cobalt and nickel by extraction M ith capryl alcohol from solutions of hydrochloric acid or calcium chloride. An investigation of the effect of salting agents on the extraction of molybdenum from chloride solutions by Morrison and Taylor (46)further suhstantiates the important role of the higher valent cations. This effect is illustrated in Figure 4. In the bromide system, conditions for the extraction of bromoauric arid into various organic solvents have been determined by McBryde and Yoe ( 2 3 ) , who also studied the behavior of ferric bromide and of the platinum metals. Undoubtedly there are many possibilities for extractions in this system that tire not particularly effective in the chloi ide system. Extraction from nitrate solutions has resulted in a method for
C1 A
Tc Ru Rh Pd Ag Cd In Sn/SbjTe I
...... TOTAL
S
I
-I 2
-14
A I Si P
.
K
0
0 J
17.............
Cc Pr
Nd Pa SD
Eu Gd Tb Dy Ho
Et Tm Yb Lu
Th Pa U Np Pu Am Cm 97 98
Figure 5.
Elements Extracted from Chloride Solutions Solid blocks.
Broken blocke.
Complete or good extraction Fair extraction
A characteristic property of heteropoly acids is their solubility in organic solvents. Thus, the possibility presents itself of effecting inorganic separations by the formation of a complex acid of this type, followed by extraction. A number of elements are capable of forming this type of molecule, and the method developed by Berenblum and Chain ( 1 ) for phosphorus, based on H
He
LL Be
B C N O F N e
Na Mb
A I Si P
K Ca
Sc Ti V
Rb Sr Y Cs
Ct
Mn
U
Fe Co
Ni
Cu 2t1 Ga Ge As
S
C1 A
.SI
Br Kr
Zr Cb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I
h La tif Ta W
Xe
Hg TI pb Bi Po At Rn
Re
Fa Ra Ac
Ck PI-
m Figure 6.
NL Pm
Sm Eu Gd Tb
Dy Ho Er Tm Yb Lu
Pa U Np Pu Am Cm 97 98 Elements Extracted frQm Bromide Solutions
1391
V O L U M E 2 2 , NO. 11, N O V E M B E R 1 9 5 0 He
H
Li
B C N O F N c
Be
Ai S i P
Na M&
Ca Sc Ti V
K
Cr
Mn Fe Go Ni Cu Zn
Rb S r Y
Zr Cb Mo Tc
Cs Ba La
HI Ta W
Ru Rh Pd A8
Re os Ir
Pt
S C1 A
ill deriving a general expression for the extractability-Le., the ratio of the amount of metal extracted as complex into the organic phase t o that remaining in the aqueous phase: The instability constant of the complex:
Ga Ge As Se Br K r
Cd
Ih Sn
Sb Te I
The Brfinsted dissociation constant of the reagent:
Xe
(3)
Au Hg TI Pb Bi PO .At Rn
Partition coefficients :
Fa Ra Ac
0
Ce R
Nd Pm
Sm Eu Gd Tb Dy
Th P a u Np Pu
Figure 7 .
Am
Ha
Er Tm Yb Lu
and
C m 97 98
(5)
Elements Extracted from Nitrate Solutions
the extraction of phosphomolybdic acid, is an example of this c1:tss of inorganic extractions. Figures 5, 6, 7, and 8 indicate most of the elements that have already been extracted into organic solvents under various conditions, using a particular anionic system. By the proper choice of acid concentration, salting agents, organic solvents, valence state, etc., separations may be further effected within any onk system.
IO0
n
80
W
tV
< a
ORGANOMETALLIC EXTRACTIONS
FI
8-
The separution of niet>alsin the form of organometallic complexes from aqueous solutions is based on the preferential solubility of the chelate compounds in a particular immiscible organic is solvent. When, therefore, an aqueous solution of a cation, Mn+, shaken up with a solution of the organic reagent, H R , in a solvent inimiscible with water, the folloll ing equilihria nrc in operation:
60
J
