Paul F. Thelander Lee A. Hasledalen a n d Maurice M. Kreevoy University of Minnesota ~inneap&s,MN 55455
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Use of a pH Difference to Pump an Anion across a Non-aqueous Phase
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A student experiment or lecture demonstration
This article describes a n exneriment to illustrate one of the central ideas of thermodyn&ics-the amount of work that must he done to bring about a particular transformation depends not only on the energy content of the initial and final states but also on the degrees of dispersion of those states. This principle has extremely widespread and important consequences. T h e degree of dispersion appears as entropy; however. the term "entronv" ." is not used in the rest of this article, and formalism is generally minimized in order to maintain accessibility to first-year students. For more advanced students the instructor may choose to introduce the concent of e n t r o w and the formalism which is readilv available & standard physical chemistry textbooks. s he kxperiment may be performed by students or may be done as a lecture demonstration. The central problem of waste reuse or disposal is that waste products are widely distributed in low concentrations, while their reuse or even safe disposal usually requires them to be localized in a small volume. The same problem is crucial to the utilization of widely dispersed natural resources. Indeed, today's waste may be tomorrow's resource. (Copper mining wastes from the beginning of this century are currently being reextracted to eet a second cron of comer out of them.) The processes of coilection a t a codstant i e i p e r a t u r e involves a necessarv minimum of work. the macrnitude of which can be calculated hy the methods ofthcrmc&namin ( I I. In practice we are usually not a i efficient as we rould he, because high thermodynamic efficiency usually requires slow, labor-ihtensive processes, and we usually do a gwd deal more than the minimum required work. This article describes a student experiment which illustrates these principles with respect to the dye Orange 11, (I). Orange I1 is a structural analog of one of the more common constituents of detergents, dodecylbenzene sulfonate, (11).
The dye is used because its concentration is determined easily.
The Experiment We start with a dilute aqueous solution of the dye at pH 6 7 . This dye is equilibrated with a solution of a high molecular weight secondary amine, RzNH, chosen so that euen its salts ore essentially water-insoluble. The solvent is tetrachloroethylene, whieh is widely used as a dry-cleaningsolvent.Amines of thisstructure are moderately strong bases (2). However, the proton cannot he extracted into the water-immisciblehalocarbon phase without also extractingan anion. Otherwise an unacceptable charge separation would occur. Thus the reaction which occurs is represented by eqn. 1, where A- is the anionic oortion of I. and B is the amine. (Ions are not separated in hnlwarhm mrdm ) The more a e d prwnt ithe Iwer the pH) the murr H+ and A- nre tranrlrrred to the hahxarbun phase This transfer is quantitatively shown by
in which K is an equilibrium constant, and the bracketed quantities are activities at equilibrium;that is, escaping tendencies. (Activities are determined by concentrations. In dilute aqueous solutions they are aooraximatelv to concentrations. In other solutions ..orowrtional . this gay not be at all true, hut it will still he true that in two solutions of identical composition the activities of the constituents will be the same.) The halocarbon solution of the amine salt of the dye anion is stripped by equilibrating it with a smaller volume of aqueous NazCOs solution, called aq. 2. Such a solution is basic, so that [Hf] is very low, shifting the equilibrium shown in eqn. (1)to the left. The new equilibri- is shown in eqn. 3.
Combining eqns. 2 and 3, and rearranging, we get eqn. 4
for the ratio of anion aetivitv in the striuuine . . .. solution (aq 2) to that in theloading solution (aq 1jineschcaseoftprequilihrati"n with the organic phase. If (ore 2) has the same composition as (org 1). then IlBlorg l)IIHHfA-(~,rp. 2)1,IB(org211[RH+A-iorg1)Il is unity, and eqn. 5 results
How may org 2 and org 1 be made to be the same? By treating a large volume of organic phase either with very small volumes of aq 1and aq 2, or keeping the A- concentrationsin aq 1and 2 very small, or hoth. This maximizes ([A-(aq 2)]I[A-(aq I)])whieh is, therefore, designated as limitinp. because it minimizes the concentrationchanges in the organic phase.- he process can be repeated, cyclically, in order to transfer an appreciable amount of A- from aq 1 to aq 2. Any concentration change in the organic phase would make the concentration of B(org 1)smaller than the concentration of B(org Z), and the concentration of BHfA-(org 2) smaller than the concentration of BHtA-(org I), because B is converted to BH'A- in loading and the reverse is true in stripping. The quantitative relation between concentrations and activities in an organic phase of this sort are very uncertain, hut the direction of change is clear. A decrease in concentration must produce a decrease in activity in a homogeneous system, and an increase in concentration must produce an increase in activity. Even in a heterogenous system an increase in the amount of one constituent cannot decrease its activity, hut can, at best, be achieved at constant activity (3).By stripping a finite amount of the dye from the organic phase, the quantity ([B(org l)][BHtA-(org 2)]/[B(org2)][B+A-(orgI)]]will generally be made smaller than unity. There is no way it can be made larger. However loading and stripping infinitely small portions of dye in each cycle also maximizes the amount of human labor required to transfer a given amount of dye from aq 1 to aq 2, and makes the process very slow. The ifreducible "price," in potential ability to do work, for transforming one mole of some substance from one form, or situation, to another is called the molar Gihhs free energy of the transformation, in honor of J. Willard Gihbs, who first identitied it asa useful fundion. It is symbolized by AG. Far calculation purposes it turns out to he useful to quote such "prices" in logarithmic units. If the whole process is wried out reversibly; that is, by means of successive, infinitesimally small changes in the concentrationsin the weakly polar phase; it will have cost no more than the rock-bottom price, which is AG for transfer of one mole of H+ from the near neutral dye solution to the basic carbonate solution, RT In ([H+(aql)]/[Hf(aq 2))). The real practice will always fall somewhat short of this ideal for obvious practical reasons, and the price paid will be higher. The first law of thermodynamics can he paraphrased, "You cannot get something for nothingJ9Thesecond,which is illustrated by the present experiment, can be psrephrased as, "You generally cannot break even, either." It may he noted that nothing perteiningto the weakly polar phase appears in eqn. (5). The limiting result is independent of the organic Volume 57. Number 7, July 1980 1 509
phase, which is used as a sort of selectivememhrane. In principle we could use an organic phase without the amine. In practice, however, the m i n e enormousfv ,facilitates the oracess. because the coniueate arid oforange I1 wmld not he meavurablq soluble in theorganic phase without it. Thus, practically. A- would not be transfered at any useful rate. To operate successfully,the m i n e must not transfer carbonate or the sodium salt of Orange I1 hack into the dilute aqueous phase because otherwise equilibrium would be reached with all the concentrations equal in the two sets of aqueous phases. It would also he helpful if the organic phase could he designed so as to minimize the chanees in activitv that accomoanv concentration chanees. ~r&esseslike that describei h e k have their main oresent use in the mineral industry where they serve to roncentrate and purify uranium, copper, and other metals. They appear to have a very intereating potential in the liquid waste treatment field. They arecalled liquid ion exchange processes. ~
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Directions and Questions to the Student Dissolve about 0.16 g of didodecylaminel in 20 ml of tetrachlomethylene to give an amine concentration of about 10VM. Dissolve about 0.10 g of Orange I1 in 240 mlaf distilled waterto make a solution about 10VM. By diluting 5.0 ml of M Orange I1 solution of 500 ml with distilled water, make up a M solution of Orange 11. Prepare 100 ml of a0.2 M Na~C03solution in water. By dissolving 2.1 g of solid NazC03 in amixture of 5 ml of 10-3M Orange I1 and 90 ml of water, make a solution which is 5 X 10-6M in Orange I1 and 0.2 M M and in NazCOz. Make up 10 ml of solutions which have 2 X M of Orange I1 and 0.2 M of Na2C03by diluting 4.0 and 2.0 ml, M Orange I1 and 0.2 M NazC08solution to respectively,of 5 X 10.0 ml with 0.2 M NazCOs in water. Store these solutions in labelled test tubes. Now measure the pH of your M Orange I1 solution, preferably with a pH meter; with pH paper if no meter is availabe. If the pH is above 6, add dilute (0.1 M) HCI dropwise till it falls to 6 or below. Now make up, by dilution of this solution with distilled water M and 1 X M Orange 10 ml each of solutions containing 3 X 11. Store 10 ml samples of 10-5M, 3 X 10-6M and 1X 10-6M Orange I1 in labelled test tubes. llriny any availsl,le photoelectric speetrophotometer or colorimeter, measure theabuurlmcr, A, or percent transmission. T, oftheorange I1 nolurions, 5 X l ~ ~2 X flo-", , and 1 0 - M with02 M Na2C03, and IO-iAf.3 x IW"f,and 10-fiM withwt NnCOratawavelengrh of 515 nm. Use as references pure water or 0.2 molar NazC03, cohesponding to the solvent for the Orange II. Use the same cell for all the orange TI solutions, and the same c;ll far all the reference solutions throughout without interchange. The Beer-Lambert Law requires that A or T h e governed by eqn. (6a) or (6b)
A
=d
( ~ - +A. )
log(TdT) = $(A-)
(64
+C
(6b) in e k b of the two series. In eqn. (6) r is the molar absorption coefficient, a parameter measuring the effectiveness of the substance in absorbing light at the wavelength used, 1 is the path length of the cell containing the Orange 11, and A. or C is a correction for the possibly nonidentical transmittance of the two cells. T is the transmittance of a cell containing Orange 11, measured against a reference cell, of transmittance, To. C is log(T,lT) where T is the percent transmittance of the Orange I1 cell, measured against the reference cell, with the same solution in both cells. Equation 6 requires that in each series a plot of A or log(TdT) against (A-) be linear with a slope of 61 and an intercept of A. or C. Make such plots and determinethe two values of €1 and of A.or C. The two values of A,or C should be small and very similar, as they are measures of the same cell imperfections. The two values of €1 may not he identical, due to changes in the ionization of the phenolic --OH of Orange 11. Save these parameters to use them later in determining the concentrations of Orange I1 solutions. You should get a n t value around 1.8 X 104for the solutions with pH < 6, and around 1.3 X 10' for the NavCO. ..solutions. Measure out 164 ml of aqueous IWSM Orange 11 solution into a 200 i,r 250 ml sepnrntory funnel, add the 20 ml uf lo4 M amine aolution, and shake thoroughly. After the amine sulution has settled u, 'A commercial secondary amine, Ameen 2-C, obtained from Armak. Industrial Chemicals Division. Box 1805. Chicaeo. " . IL 60690. and having coco-derived fatty groups, serves this purpose very well after one recrystallization of 4 g of amine from 25 ml of absolute
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% I tis not necessary that these solutions have exactly the specified concentrations, but it is necessary that the Orange I1 concentration he known, with uncertainties no greater than a few percent.
510 1 Journal of Chemical Education
the bottom, leaving hoth solutions clear, carefully draw it off into a labelled test tube. If the aqueous phase daes not become dear (clear does not mean colorless) in -10 min, separate the layers anyway, and shake the aqueous layer with a gram or so of paraffin shavings. These will absorh any trace of the organic phase tbat may be dispersed in the aqueous phase. When the paraffin shavings are filtered off the aqueous layer should be clear, and you can pmceed. Then transfer the aqueous Orange I1 solution to a clean flask, and save it. Now replace the loaded amine solution in the separatory funnel and shake it thoroughly with 5.0 ml of 0.2 M NazC03. Allow the two layers to separate, then carefully drain out the organic layer and pour off the clear aqueous layer, saving hoth. Note the color of the m i n e solution before the original extraction, after extraction, and again after stripping. Measure the pH of the 10W M Orange I1 solution you have ertracted with the amine solution. (The pH of thin solution should be a little higher than that of the 10-I M Orange I1 solution before the extraction.) Measure the pH of the 0.2 molar NazC03 solution. A pH meter is ~referahlefor these measurements.. hut DH naver - . . can be used. Usine the same colorimeter as before. measure A or T far the extrarted.0range 11 solutions. 1)ilute 3.0 kl of the NazCO3 stripping solutions to 10 ml and measure i t s Tor A value. Use eqn. 6 and your previidy determined parameters to convert each of these u, eoncentrations. Determine the amount of Orange I1 you started with and the amount you can account for by combining the unextmcted residual and the stripping solution. When you calculate the number of moles of A- in the stripping solution, remember tbat there were 5.0 ml of this solution. even thoueh vou have lost some in manioulation. Also recall the dihrion. dud&; from its color d,, you think there is any appreriable amount of Orange 11 left in the m i n e sdution? (Your final total should be w;thin about 15%of the amount you started with.) In dilute aqueous solution, ratios of A- concentrations (CA-)are good approximations to ratios of A- activities. Calculate CA-(aq ~)/CA-(aq 1) and compare it with [H+(aq I)] [H+(aq 2)]. You can expect to get a value around 40 for CA-(aq 2)ICa-(aq 1). Why ia it so much lower than its limiting value? In principle, how could this experiment be modified to get closer to the limiting value? Is this practical with your experiment? Would it be practical, in a large scale water purification operation, to use a much smaller volume of NazC03 solution in each stage of stripping? How would this affect the efficiency of operations? Calculate the chanee .. in DHvou should have observed in vaur dilute Orange I1 solutions, on extraction, i n n the decrease in the Orange I1 roncentration. Compare this to the observed decrease and explain. Du we get something for nothing here? (Hint: there is a big reservoir of C02 in the atmosphere.) A slogan sometimes used by waste disposal experts is "dilution is the solution to pollution." Comment both favorably and unfavorably on this slogan, on the basis of the present experiment.
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Discussion In the present experiment CA-(aq 2 ) l C ~ - ( a q1) falls far short of its limiting value because the condition ofreversibility is not met. That condition reouires that thestrionine should not appreciably change the concentration of t h e dye in the organic phase. Reversihility could h e more nearly approached by using a stripping solution of lower pH, or by stripping with a laree number of verv small volumes of carbonate solution. re~oabingwith fresh; aqueous dye solution between eacl; stripvine. However, either of these either reduces the amount of d i e (as distinguished from its concentration) collected in t h e stripping solution, or increases the amount of manipulation t h a i m u s t he done, or hoth. A close approach to the thermodynamic limit can he made hy using the organic phaseas a membrane,supported on porous polypropylene, and continuously passing one solution over the top of the memhrane, the other over the hottom. Mechanically, the device is essentially the same a s a dialysis cell. with the imnreenated nolvnronvlene renlacine the dialysis'membrane. ~ h e m e m h i a n then e acts as apumG, with the A- beine pushed U D its concentration differential bv the transfer if^+ down-its concentration differential, just &, the bulk organic phase does in the present experiment (4). In that case the condition of reversihility is t h a t the concentration of dye on t h e two sides of the membrane be equal. Again, it is
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clear that no net transfer will occur if this is true. However, with a membrane device i t is possible to approach this condition while still collectiug appreciable amounts of dye, by usine a large surface area and a very thin membrane. Such a deviee is the equivalent of a separatory funnel device in which a verv large number of small volumes of loading and stripping solu&ms'are used, with the organic hein# reloaded herween each stripping. . - - Yet the mechanical work of repeated loading and stripping is minimized, and the rate of transfer under near-reversible conditions is maximized. This process is thought to be one way that living cell membranes do their work (5). As we are compelled to use more and more widely dispersed resources, ultimately recovering the rare metals from seawater, such devices will become more and more attractive by comparison with current technology. Good candidates for early application include uranium and copper recovery from sulfuric acid leaching solutions. Both of these are currently practiced with mixer-settlers, which are the large scale industrial equivalent of separatory funnels. The chemistry of one method for uranium recovery is similar to that described in this experiment, with [ U O Z ( S O ~ ) ~playing ]~the role of A(6). The chemistry of the copper recovery system is a little different, with the H+ flowing counter to Cu2". I t is shown in equs. (7) and (8)(7).
The general structure of RH is 111, OH N/oH
m with alkyl or aryl groups in positions X and Y. Literature Cited H.. he Second Law:'mford Univeraity~rra..New Y d ,N.Y.. 1965. pp. 77-83 (b) leuis, G. N., and Randall, M., '"Thermcdynamics," (revised by Pitzer. K. S.,and Brewer I.,) MCGTBY-HillBDDk CO.,New York, N.Y.,1361pp. 352.364. (2) Streitwiener, Jr., A., and Hedhmck. C. H.. "Introduction to Organic Chcmietry," 773-777. MaemiUenPuMiahingCo..lne.,NewYork,N.Y., 1976,~~. 131 Moore, W.J.,"PhysicalChemlatry(1_4thEd..Prentiee-Hsll,Ine.,EngleamdCliffa,NJ., 1972. pp. 258-259. (OCussler,E L..A.ICh.E Jalrmol, 17.1300(1971). 15) Stein, W. D.,"The Movement of Moleeule~AnolsCeU Membranes," AeademicPreas, N o r York,N.Y.. 1967. (6) (a) Anon.,"Uranium~Fieldfor 0rganies:'Chem. ondEng. Nous, 34.2590 (1956). (b) Allen. K.A.. J. Phrs. Chem., 60.. 239 (1956): 60.943 (1956). (7) A g e s , D. W.. House, J. E., swanson,R. R., and Dlabniek, J....I nonr. soe Mining Engineers. 191 (1966).
(11
(a) Bent,
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