Ahmed A. Taha, Ronald D. Grigsby, James R. Johnson, Sherril D. Christian, and Harold E. Affsprung ~h~ University of Oklahoma Norman
I Manometric Apparatus for Vapor and Solution Studies
I
M a n y of the experiments counnor~lyperformed in fresl~n~an and elementary physical chemistry laboratories h:ive in common one unsatisfactory feitture-the procedure uscd in introducing liquid, gas, and vapor samples of linown mass and composit,ion into a closed apparatus. A sample introduction valve wlrich has been used by mass spert,roscopists (1, 2) can be applied to excellent advantage in numerous experiments on the properties of gases, vapors, liquids, solut,ions, and solids. Several applicat,ions of t,hc valve have been descrihed in previous publications (3-8). This article describes a versatile n~anonietricapparatus which can be used, with only minor modifications, in obt,aining all of the following types of dat,a: vapor density and I'VT measurements, vapor pressures of solutiot~sand liquids, dew-point pressures and compositione, solubilit,ies of gases in liquids, solubilities of slightly-miwible liquids, equilibrium ronstants for association reactioris in solutions, interahon of vapors aud gases wit,h solids, and gas and vapor visrosities. Apparatus
The pressure-nieasuring apparatus is shown in Figure 1. The import,ant con~ponentsare a sintered-glass din: of fine porosit,y, with a diameter of 1 or 2 rm, rovered
with mercury to a depth of a few mm, a varuum st,opcork, aud a closed-end manometer (with a t least 1 en1 id). The stopcock used in most of our experiments is a greaseless Teflon-plug vacuum valve (CW850) manufactured by Delmar Scientific Company. The samplc flask attaches to the bask apparatus through a mercurysealed ball joint, which is partially supported by a split rubber stopper fitting into t,he cup. For most of the vapor pressure experiments, we use a flask with a volume in the range 100 to 300 ml. In vapor denshy experiments, it is dci.irable to use %: flask wit,h a volume of a t least 500 ml. The viscosity experiments require a flask having an attached rapillary and large-bore stopcork. An attractive featurc of the assembly is t,hat any of the flasks may be attached easily to the basic apparatus to give a grease-free vacuum system. Thc assembled apparatus is supported on a stand COIIstructed from aluminrun rods fastened to a non-metallic base by support flanges. I n most of the applications, it is desirable to immerse the entire apparatus in water (to t,he level of the disr) ; hence, the base and support rods should he coated with a water-proof paint,. When it is necessary to &&ate a solution in one oft he bulbs, a Teflon- or glass-cnclosed stirrer bar is placed in the bulb prior to assembly, and a magnetic stirrer ie positioned beneat,h the platform and water-bat,h. ( 4 n inexpensive aquarium is adequate for use as a water t ~ e r immersed bath.) Although the n ~ a ~ ~ o n ~ise entirely in water, pressures can be tlet,ermi~ledwith a cat,hetomcter to uearly t,he same precision as i~ obtained when the apparatus is surrounded by air. Procedures and Typical Results
Vapor densit!/ and 1'1-T iileasurements. I'revious rcports from this laboratory have descrihed the use of the merculy-covered disc inlet-valve in vapor density, vapor pressure, adsorption, and solubility experiment? (3-8). Samples of a volatile liquid may he introduced through the disr ~vithsyringes, pipets, or micro-burets which have been calibrated by using the weighing bottle depirted in Figure 1 (6). The bottle may he partially evacuated with an aspirator and weighed hefore and nftcr addition of increments of a liquid. We have found thc 0.2 ml and 2.0 1111 micro-burets manufactured by Roger Gilmoul Industries (nos. S-110-A and S-1200-A) to be suitable for most of the experiments described hcrc. I n determiniug the tot,al volume of the system, either of two procedures may be employed. When flasks snraller than approximately 300 ml are used, it is ronvenient to determine the volume by weighing the 432
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Journal of Chemicol Education
tlpparatus entirely filled with water and empty. In the case of larger systems, the volunie can he determined by addiug a ~ e r i c of s camples of a volatile liquid of known vapor densit,y, using a calibrated syringe or micro-huret, and measuring the pressure i~ftereach addition (3, 6, 7). I n very accurat,e work t.he system volume must be rorreeted for the slight volume rhange which occurs as the level of mercury in t,he manometer varies. The vapor density of unkuowu substances may he determined once the system volume is known. Densit,ies of vapor mixtures may be obt,ained by sddiug volatile liquid mixtures, rather than pure component,^, through the valve. Alternately, one pure compound may be added to t,hc apparatus to a known pressure a d vapor density, whereupon iucrements of the srronrl vompound may br introdu~~ed :w before (5).
vapor sample (at lower initial pressure) on the ot,her side. This is a convenient techuique for obtaining het,ero-association constants and mixed virial coefficients a t pressures less thau about, 300 mm Hg. Vapor pressures oJ solulions and pure liquids. Vapor pressures of liquids may be determined readily by using a. sample flask of rclativcly small volume. 1"igure 3
Figure 3.
Figure 2.
Coupled
opporotur.
L)rvi;~tions of gases : ~ n dvapors from ideality may also hc studied with a n apparatus ~mlst~rocted by joining two romplctc syetems through a greaseless valve, such as the .\Ianost.at Corporat,ion #7R-425-01 Teflon needle-valve ( I .2 One side of the romhined apparatus is evacuatecl, and a gas is added to the other side. The initial pressure and volume of the gas can then be determined, ~:orresponclingto a lrnown temperature. The connecting valve is opcnrd to allox a small sample of gas t o cnt,er the evacuated side of the nyst,rm. Simultaneous values of pressure and volume a t constant t>cnrperature can he det,ermined for the gas in the two parts of the apparat,us. The met,hod ?an be used to illustrate the v:tlidity of Royle's law for g a e s suvh as argon and nit,rogcn. Alternately, dcvintions from t,he ideal gae li~wscan be infcrred from observed changes in the sum of the pressure-volume produrts determined for the two isolated gas samples. The double apparatus may also 1~ used t.o add awurat,ely measured quantitics of a vapor or gas on onc side of the apparatus t o a gas or
Preswre-solubility doto of air in methonol at 25".
shows the dependence of preesure on volume of liquicl methanol added to an evacuated system having a volume of about 291 ml. Note that the liquid san~ples need not he added with great precision, since the vapor pressure is ealculat,ed hy extrapolating the nearly horizontal portion of the p versus v curve t o zero volume. In t,he case of mixturcs of knowu composition, the dewpoint pressure can be obtained by noting the point at) whirh a break first occurs in a plot. of p versus v. It i , ~ desirable to use a bulb of relat,ively large volume for de~v-point.measurenirnts, sirwe otherwise saturation may he retlrhed after only a small quautity of solution has been added. Vapor pressures of mixtures may be determined by using a small-volume bulb and continuing the addition of samples well beyoud the condensat,ion point,. In arrumte worlr it is necessary to correct. for the pressure produced hy air which is initially prescut in the liquid san~ples(see below). Solubilities of gases in lirluids. Henry's law roust~ants for air in pure liquids may be determined from the same type of data used i n inferring vapor pressures. I n Figure 3, the inrrease in pressure occurring a t volumes greatly in excess of the saturat,ion volume is due solely to t,he dissolved gases delivered through the valve along with the methanol. From t,he slope of the curve a t large volumes, it is not difficultt o calculate the quantity of air dissolved in the rnet,hanol,equilibrated at. the prevailing atmoapherir pressure. Henry's law constants have been calculated for air in several common organic liquids using this t,erhnique; the values obtained agree reasonably well with t,hose reported in the literature. We have also ohtaiued reliable values of air soluhilities by an alternate proredure. A large sample of a pure liquid is int~rodueedint,o the flask and the syst,em is evacuated to the vapor pressure of the liquid. Air samples are added volumctrirally, using a 5 or 10 mI, gas-tight syringe. The technique may be applied in studies of the solubilit~iesof gases other than air; t,he gas syringe may he filled either from a gas bottle or direct,ly from a tank with a suitable reducingvalve. h'utual solubilities oJ liquids. If a pure liquid is Volume 43, Number 8, August 1966
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placed in the sample flask and the system is evacuated until air and other gases have been expelled, it is a simple matter to determine the solubility of a volatile solute as a function of partial pressure. I n Figure 4,
conclusion that water exists primarily in monomeric and dimeric form (solvated) in nitrobenzene a t 25"; the equilibrium constant for the reaction
*
Figure 4.
Pressure-solubility doto of benzene in woter at 2.P.
partial pressure of benzene is plotted against volume of benzene introduced through the disc to a 190 ml sample of pure liquid water a t 25', with a vapor volume of 101 ml. The calculated solubility of benzene in water (0.022 mole/l) compares favorably with the values reported by Andrews and Keefer (9) and Bohon and Claussen (10) (0.0223 and 0.0229 mole/l, respectively). The linearity of the p versus v curve provides an excellent illustration of the validity of Henry's law in dilute solution. Pressure corrections may be made for the slight lowering of the solvent vapor pressure caused by the presence of the solute. Association reactions in solution. By using the technique described in the previous section, it is possible to study self-association and hetero-association reactions in dilute solution. The data are particularly easy to interpret if a non-volatile or slightly volatile solvent is employed. Figure 5 is a plot of partial pressure of water
has been calculated to be 1.41 0.10 molal-'. Taha (8) has used the technique described above to study several hydrogen-bonding reactions involving two interacting solutes dissolved in a non-volatile solvent. Interaction of gases and vapors with solids. Adsorption isotherms may be obtained by placing an adsorbent in the flask, evacuating the system, and adding gas or vapor samples through the valve. The vapor volume must be determined with somewhat greater accuracy in adsorption studies than in most of the experiments described in the preceding sections. However, it is not difficult t o make a "blank" run in the absence of adsorbent. Fogel and co-workers (12) have used the apparatus to study the formation of hydrates and pyridine adducts of complex salts of transition metals. I n these experiments, it is frequently necessary to allow several days for the attainment of equilibrium; however, excellent values of equilibrium constants and thermodynamic parameters can be obtained with the method. Gas and vapor viscosities. The basic apparatus can he used in viscosity measurements by attaching a Pyrex capillary to a sample flask. The outer end of the capillary is joined to a large-bore stopcock which connects to a vacuum system. A gas or vapor sample is introduced through the valve; the stopcock attached to the capillary is opened to vacuum; and the pressure is measured as a function of time. Salzberg has discussed a method for analyzing flow-rate data as well as the procedure for selecting a proper size capillary in viscosity experiments (13). Miscellaneous experiments. Several other types of experiments may be performed with the apparatus. Reaction rates may be measured for condensed phases in which a volatile reactant or product is involved. Gas phase kinetics may be investigated for systems in which there is a change in the number of moles during the course of the reaction. The double apparatus can be used in gaseous diiusion experiments. Chemical analyses based on evolution, sorption, or reaction of gaseous molecules can be performed by using the basic methods presented here. The technique and apparatus may also be used in introducing known pressures and concentrations of volatile species int,o vapor or solution spectrophotometer cells. Summary
Figure 5.
Pressure-solubility data of woter in nitrobenzene at 25-.
versus volume of water added to 180.5 g of nitrobenzene in the bulb; the vapor volume of the apparatus used in obtaining these data was 163 ml. When a slight correction is made for the amount of water remaining in the vapor phase after equilibration, the data can be converted into sets of simultaneous values of water partial pressure or activity a t known molal concentration in solution. Numerical methods have been described for calculating association constants from water solubility data (11). The data shown in Figure 5 support the 434
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Journal o f Chemical Education
The techniques described in this article have been useful in obtaining accurate physical and chemical data of many types. The apparatus is simple and inexpensive-the components of the assembly, excluding burets and pipets, temperature-controlling equipment and cathetometer, can be purchased for approximately $30. We believe the procedures we have described will find wide applicability in both research and instructional laboratories. We acknowledge the support of grants from the Office of Saline Water, Department of the Interior (No. 14-010001-543), and the National Institutes of Health (No. GM08972).
Literature Cited (1) TAYLOR, R. C., AND YOUNG, W. S., Ind. Eng. Chem. (Anal. Ed.), 17, 811 (1945). R. J., Anal. Chem., 22, 1337 (2) PURDY,K. M., AND HAFLRIS, (1950). (3) C&ST&, S. D., APFSPRUNG, H. E., AND LING, C., J. CHEM. EDTIC.,40,323 (1963). S. D., AND AFXSPRUNG, H. E., Anal. Chin. (4) CHRISTUN, Acta, 29, 586 (1963). 5. D., AFFSPRUNG, If. E., AND LING, C., J. (5) CHRISTIAN, C h m . Soe., 2378 (1965). 5. D., Chem. Analyat, 54, 119 (1965). (6) CERISTIAN, (7) LING,C., "The Determination of Hetero-association Con-
stants by a New Vapor Density Method," PhD Dissertation, University of Oklahoma, 1964. (8) TAHA,A,, "A New Method for the Study of Hydrogen Bonding in Solutions of Volatile Compounds in Nonvolatile, Nonpolar Solvents," PhD Dissertation, University of Oklahoma, 1965. L. J., AND K E E ~R., M., J . Am. Chem. Soe., (9) ANDREWS, 71, 3645 (1949). W. F., J. Am. Chem. Soe., (10) BOHON,R. L., AND CLAUSSEN, 73, 1571 (1951). (11) JOHNSON, J. R., CHRISTIAN,S. D., AND AFFSPRUNG, H. E., J. Chem. Sac., 77, (1966). (12) FOGEL,N., AND LEWIS, C., Unpublkhed work. (13) SALZBERG, H. W., J . CHEM.EDUC.,42,663 (1966).
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