10
Anal. Chem. 1980, 52, 10-15
Measurement of the Solubilities of Slightly Soluble Organic Liquids in Water by Elution Chromatography Frederick P. Schwarz National Bureau of Standards, National Measurement Laboratory, Washington, D.C. 20234
A simple method based on liquid phase elution chromatography is presented for determining the solubilities of organic liquids in water. An inert solid support in a transparent tube is coated with the organic liquid. As this solute is eluted with water, a solute depleted zone develops whlch is different in color than that of the remainder of the support. Measurement of the rate of progress of the boundary of this zone, the flow rate of water, and the mass of solute coated on the support are sufficient to determine the solubility. The method has been tested by measuring the solubilities of benzene, cyclohexene, cyclohexane, 2-heptene, toluene, iodopropane, rndichlorobenzene, 1,l,l-trichloroethane, 1,1,2,2-tetrachIoroethane, and dlbutylphthalate in water at 23.5 k 1.5 O C . These solubility measurements cover a range from 0.296 wt % (1,1,2,24etrachloroethane) to 0.00150 wt % (2-heptene) with an accuracy independent of the solubility of the solute.
Increasing awareness of the harmful effects of organic pollutants on t h e aquatic environment has aroused a keen interest in t h e solubilities of organic substances including organic liquids in water. These data are needed to assess the fate and toxicological effects of organic pollutants in the biosphere. Most of the early aqueous solubility measurements of organic liquids were based on two methods; (i) measuring t h e decrease in volume of a known excess volume of solute after equilibrating it with a known volume of water ( I , 2), (ii) dissolving known volumes of the solute in a known volume of water until cooling of the solution to t h e temperature of interest resulted in the reappearance of t h e solute phase ( 3 , 4 ) . Although these methods are simple and nonspecific, they are limited t o solubility measurements above the 0.01 wt % level. Later and commensurate with t h e development of analytical methodology, solubility measurement methods appeared which were based on analysis of the aqueous phase of saturated solutions by interferometry (5),UV absorption measurements (6-9), gas chromatography ( I O ) , fluorescence analysis ( I I , I 2 ) , and liquid chromatography (13). Examples of other present day methods are head-space techniques based on measuring the solute vapor in equilibrium with its aqueous phase (14)and a light scattering technique based on measuring t h e reduction of light scattered by the solute during solubilization ( 5 ) . In all the more recent methods, solubility measurements below the wt % range require sophisticated instrumentation (7-14). Furthermore some of the methods are limited t o solutes with certain properties, e.g., in the solubility method based on UV absorption, t h e solute must absorb light in the UV region of the spectrum. An improved approach for t h e determination of solubilities would be one incorporating the simplicity and nonspecificity of the earlier methods with the high sensitivity of present day methods. This report presents a n investigation of a solubility measurement method which is simple and may well meet the ideal criteria of high sensitivity and nonspecificity. It is based on elution chromatography where the solute is the stationary phase and water is the mobile phase. A transparent column
is packed with an inert support coated with a known amount o f the solute and then water is forced through the column. As the total volume of water flowing through the column increases, a solute depleted zone, different in color from the stationary phase, develops and increases in length. T h e solubility is calculated from the amount of solute removed from the column, Le., length of the solute depleted zone, and the volume of water passed through the column. This method was applied to ten different organic solutes to determine its sensitivity and specificity.
EXPERIMENTAL Materials. The selection of an inert solid support material for the columns was limited by the requirement that a visual change, e.g., color change, occurs when the mobile water phase dissolves the solute stationary phase from the support material. Because of its high hydrocarbon adsorbent capacity (13070 by mass) and its color, acid-washed and dimethyldichlorosilanetreated Chromosorb P fulfills this requirement when the index of refraction of the stationary phase 21.4. This adsorbent was, thus, chosen as the solid support for this study. Attempts to use glass beads, which are more opaque in water than in an organic liquid, failed because of their low adsorbant capacity (50.5% by mass). The Chromosorb P was 80-120 mesh, and had a BET surface area of 4-6 m2/g. The n-propyl iodide, 1,1,2,2-tetrachloroethane, dibutylphthalate, and cyclohexene were reagent grade and the benzene was spectral grade. The stated purities of the rn-dichlorobenzene was 98% (mass); l,l,l-trichloroethane, 96%; cyclohexane, 99.9%; 2-heptene, 9970; and toluene, 99.970. All solutes were used without further purification. Distilled water was used as the solvent. Preparation of the Solute Column. The column (hereafter referred to as the solute column) was prepared by filling a tube containing Chromosorb P with the liquid solute and then flowing water through the tube to displace the bulk solute trapped in the void spaces of the column packing. The mass of solute coated onto the Chromosorb P was determined from weighing the tube before and after filling the tube with the solute and from weighing the tube after the bulk solute trapped in the void spaces had been displaced by water. To eliminate the problem of displacing the bulk solute from the column, a simpler alternative solute column preparation method was briefly investigated. This method was based on filling the tube with Chromosorb P already coated with a known mass of liquid solute. The coated Chromosorb P was prepared by mixing a known mass of Chromosorb P with the solute in a beaker, evaporating off any excess solute, and then weighing the solute coated Chromosorb P. However as the tube was being filled with the coated Chromosorb P, small unknown amounts of solute evaporated off the Chromosorb P and the coating appeared non-uniform in the tube. This loss of solute and non-uniformity of the coating introduced significant error in the solubility determinations, (on the order of 15% for benzene) and thus, this method was discontinued. An empty Pyrex or polyethylene tube plugged at one end with glass wool was filled with -1 g of Chromosorb P to -2 cm from the other end and weighed. The tube dimensions were either 3 mm o.d., 2 mm i.d. X 40 cm or 6.4 mm o.d., 4.4 mm i.d. X 15 cm. During the filling procedure, the tube was continually shaken to ensure complete packing of the Chromosorb P in the tube. The 6.4-mm 0.d. columns were capped with a Xytel Swagelok cap which was necessary later to minimize loss of solute from the solute column during handling. Solute evaporation from the 3-mm 0.d.
This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
columns was too small to be detected. After weighing the column, the liquid solute was slowly poured down the column until it appeared at the other, plugged, end of the column. Excess bulk solute at both ends of the Chromosorb P packing was removed with a syringe and the column was carefully weighed. In preparation for the third weighing, it was important, as will be shown, to eliminate the possibility of trapping air in the column. To eliminate this possibility, air in the column above the packing was displaced by water from a syringe and the column was quickly connected to a flowing water reservoir via a Swagelok connector. The water reservoir was either a thick walled 2-L Pyrex vessel or a 50-L stainless steel cylinder, both with an inlet at the top connected to a compressed gas regulator and an outlet valve at the bottom. Water was forced through the solute column by the pressure of the compressed gas. Usually a pressure of 14 kPa above atmospheric was adequate to ensure a flow rate of -0.3 mL/min through the 3-mm 0.d. solute columns and of -1.0 mL/min through the 6.4-mm 0.d. solute columns. Increasing the gas pressure to increase the flow above these rates caused channeling in the columns. After water completely displaced the bulk solute trapped in the void spaces of the column, as evident by the appearance of water droplets at the plugged end of the column, the column was disconnected and excess water at both ends of the packing was removed with a syringe. The column was then weighed. The column and flow system were then reconnected, taking precautions to avoid trapping of air, and approximately 0.5 mL of water was passed through the column. The column was then disconnected and reweighed. This entire procedure was repeated several times passing at least 0.5 mL of water through the column. Thus for each column replicate "third weighings" were obtained. The packings of some of the columns were analyzed after completion of the experiment to compare the composition of the solvent zone to that of the rest of the column. The column was cut apart a t the zone boundary and two 2-cm long segments of the column packing, one just above and one just below the zone boundary, were analyzed for the presence of the solute stationary phase. The analysis consisted of tapping the packing out of the segment into a test tube containing several milliliters of octane and then injecting 1-wL samples of the octane into a gas chromatograph. The gas chromatograph was operated with a 3% SE-30 80/100 Supelcoport column at 80 "C. Retention times were determined from sample injections of the pure solute in octane solutions. Procedure. The solute column was connected to a water reservoir in a manner similar to that for the preparation of the third weighing and the water flow was continued. The water passing through the solute column was collected in a 50-mL graduated buret or a 100-mL graduated cylinder. The measurements were initiated after the appearance of the solute depleted light pink colored zone (hereafter referred to as the solvent zone) on the Chromosorb P. Each measurement consisted of turning off the flow, measuring the length of the zone to the nearest 0.5 mm and the total volume of the water passed through the solute column. Since evaporation might have reduced the amount of solute stationary phase at the beginning of the column, the length and volume measurements were initiated after the solvent zone extended 0.3 cm down the length of the column. Measurements were continued until the solvent zone extended down at least 25% of the column length.
-
where pHzO is the density of water. Eliminating V from Equations 1 and 2, t h e coated mass is
m = ( W , - WJ - ( W , - W J / ( 1 - P H , O / P ~ )
W z - W1 = m
y = m / L = ( W , - W , ) / L - ( W , - W,)/L(1-
PH,O/P,)
(4) where L is t h e total length of column packing. Values for p were taken from ref. 16 and are the water and solute densities a t 20 "C except for l,l,l-trichloroethane which was t h a t at 26 "C. T h e temperature variation of p from 20 t o 23.5 "C, caused by thermal expansion, results in less than a 17'0 change / p ~ (16). This was verified by pycnometer in t h e p ~ , ~ ratio measurements at 24 "C. Since t h e stationary phase in the solute column is t h e slightly soluble solute and t h e mobile phase is water, the equilibrium distribution between the two phases is t h e same as between t h e pure solute and its aqueous solubility phase t o a good approximation. At equilibrium, the chemical potential in both phases is equal, PI =
1 s
where p L = p," + RT In a,. a, is the solute activity in the i t h phase and pio is the solute chemical potential under standard conditions. Then, approximating t h e solute activity in t h e aqueous phase by its solubility, S, and in the pure phase by its concentration C1, pl"
+ RT In CL = p s D+ RT I n S
T h e right hand side is constant. If S is in units of volume fraction, then for slightly soluble solutes,
CL/S = p,/S = K
(5)
In chromatography the rate of travel of a solute through a column is
+
d x / d t = ~ ( l KV,/V,)-' where ti is t h e velocity of volume flow through the column and V , and V , are, respectively, the volumes of the stationary and mobile phases. I n t h e solute column t h e velocity of volume flow is zi = ( l / a ) ( d V / d t )
(7)
where a is the average void space cross-sectional area of the column and d V / d t is t h e rate of flow of water through t h e column. The ratio of t h e mobile water phase volume, aL, to t h e stationary solute phase volume, yL/p,, is vs/vln
= y/Psa
(8)
From Equations 5 through 8 dx/dt = dV/dt(l
+ y/aS)-'a-'
(9)
Since S I 0.005 g/cm3, and y/a L 5 g/cm3 for these columns, then t o a good approximation
+ p,Vo
where rn is t h e mass of coated solute, p s is t h e density of the solute, and V , is the void volume. After forcing water through the column to displace the trapped bulk solute, (presumingly the entrapment of air in the void space was avoided) the third weighing W , was
(3)
The coated mass per unit length is
CALCULATIONS T h e determination of the mass of solute coated onto the Chromosorb P was complicated by the presence of bulk solute trapped in void spaces of the packing. The difference between t h e first two weighings, W 2- W1 was, thus,
11
dx/dt = (S/y)dV/dt
(10)
S = rdx/dV
(11)
or
Equation 11 can also be derived from the mass balance between the two phases. If the solute is completely desorbed from a small element of column length, dx, by t h e solubility process, then the mass of solute gained by the solvent is equal
4
I
I
7 4
Ul3
1'A V(XIJME C f
I Mi
Flgum 1. Plr chromrtogram of the octane extract8 ot the rohrant ndroM,zone8of Uw cydohaxmandbdopmpm 3mm 0.d. whb
odumnr
to the mase of solute lost by the column packing material, Sd V = y&, or Equation 11. Integrating Equation 11 yields
S x=-V+b Y
where b is a constant. Least squares fib of the desorbed zone length, x , as a linear function of the measured volume of water, V,were used to calculate values for Sly. Error Analyair. The standard deviation of S in Equation 11, US,is
where u(dx/dV) is determined from the least squares fib and u ( y ) is calculated from the error in the weighing8 o w
=
I(O(W2 - W,)/L)'
+ (dw3- w2)/L(1 - pH@/P&)21'/2
(14) Since the size of the column is such that W z- W1is above 1g and the analytical balance precision is on the order of lo4 g, error in W z- Wlis leee than a percent. Since the ratio p / p , in close to 1.0 and the difference (W3- W2) is on the a r of a tenth of a gram for moat of the solutes studied, the recond term in Equation 13 becomes dominant and (16) dr) cT b(W3 - w2)/L(1 - pHp/p&! where u( Wa- W,)is determined from repeating the third weighing. RESULTS AND DISCUSSION The detsrmination of eolubilities by this method is dew e n t on the development of a zone completely devoid of rduk (eolventzone) in the wlute column, The other portion of the duto column which in contad with the eolute L termed the A u k zone, Since the rolvent mna waa the m e color a I cdumn 4 with water, it wu rruumdthat the eolwnt unw mr drvoid ofamkd rolute, The color oftha other rolute ZOIW, however, remained the m e u the color of a frmhly pmpwd w l u k column, Thir implh that the obwrved rbrrplcw dthr bowd&y betwrm the two lconm rsrulta from dbamtinuity oftbe dub mnantmtion uy. o examino thfr impllortion In more dotdl rt the d drknainowhether the advent !uno oontdnr m y roluk,
IIIT2i-F
ZOC
2'2U
300
WATEH IML)
rI#w2. ~ s n t z o r n b n g l h u ah n o t k n o f t o w v o k n w o l ~ pmdthrough a m o d . knurwodvm WIUI y = 0.0140 0,008 g / m (-04=), a 6,4-mm od. knsrnr odum WMy 0.046 0,001 g/om (a a 33.mm ), 0.d. bdopropuw edun,WW, y = 0.0303 O.OO06 glm (-+-+-), urd 6 . h O d kdopropuw ookmn Uum 0.100 0.006 (-A-A-)
*
* *
several eolute columnswere cut apart at theii eo114 boundubr by a razor and segmenta of each zone were anal@. The analysis conaisted of extracting the solute on the chnvmarorb P packing of the segmenta with octane and injectin0 umpbr of the octane extracts into a gas chromatograph. In Figure 1are shown typical gas chromatagramsof the octane exttacu of a 3-mm 0.d. cyclohexene column and a 6.4-mm 0.d. iodopropane column. Ae shown in Figure 1, a compwieon of the peak areas yields a ratio of 90:l for the cyclohexenecolumn and a ratio of MOA for the iodopropane column (ICIOU a cutting width of only -0.1 mm. These ratim may be minimum valuea since any solute detected in the solventm e could have resulted from sliiht mixing of the zones during the cutting procedure. Apparently for the eolute columnr, tbe solvent eone contains very little solute and the zone boudary is quite sharp at least in the solubility range of 0.0213 wt Z (cyclohexene)to 0.107 wt 9% (iodopropane). For a given solute column, the developmentof the dwt zone as given by the increase in zone length, x , in dependent on the volume of solvent, V,and on the cortad mur ofdub per unit length, y , via Equation 12. Thin ir rhown in pirurs 2 where the variablex in plotted ae a functii of V for benrars and iodopropane columns with different y d u m . The &h values of y were from the 6.4-mm 0.d. columnr and the lower values, from the 3.0-mm 0.d. oolwnno. Since the l h u i t y between x and V ie maintained throughout the 1of tln column, as shown in Figure 2 for the 6.4-mm ad.cdumm, the coated mase distribution,y, must remain the aune 111 tln solute zone during the solubhtion proms, 81mllu m d t a were observed for other d u b columnr. The InOF y in the solute zone with mpect to x and the dwptmon OF the zone boundary imply that the rdubili#tion proar rwmt take place at the zone boundary urd rrtumtioa h rtwlml within a very short distance of the m e bouadrry. The localization of the dubilityproom m i pd&cWo&Ir the theoretical plate collcdpt ofchromrbgnphy. A t&ombrl plate in defined aa the l ~ t u d i n rwklth l in a e d u a a n m in which the mobile phm h in aqulliMmwitb thr#kaug phaw. The hebht spuivalont to a homW phblhwlrl k on the order of wwrd putlo1 matogtrphlcef€ldency(17). Slna of the Chromomorb P b 0.16 mm Covin a thlclmwr d -&a mm, -tin mnb whom rolublliution trtVr phtm QPWS r&rp(dn of a fraction of a mllllmotw) md tmah Cathroughout the roluk PMH ot tb,d ~ ,
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
13
Table I. Solubilities of Liquid Hydrocarbons in Water solute benzene
Y, d c m
R
0,0140I 0.0003 0.0154 t 0.0005 0.0124 = 0.0002 0.0464"i 0.0010
0.099
0.0526'
t
0.0017
dx/dV, cm/mL
S (23.5
0.995 0.999 0.971
0.134 i 0.002 0.122 I0.004 0.147 2 0.005 0.0415 t 0.0023
0.188 0.188
0.182 i 0.007 0.193 I0.011
0.997
0.0362
0.190 i 0.007
0.997
0.00443 z 0.00005
0.0052 i 0.0002
t
0.0007
1.5 ' C ) , wt %
S wt 70 literature
0.005 0.009
0.186 at 25 " C b 0.186 at 25 " C c
5
i
cyclohexane
0.01 22
cyclohexene
0.0125 i 0,0001
0.998
0.0225 i 0.0003
0.0281 i 0.0005
2-heptene
0.0146 i 0.0001 0.0135 i 0.0002
0.992 0.998
0.0196 0.0009 0,00111= 0.00002
0.0286 = 0.0013 0.00150 i 0.00003
toluene
0.0160
t
0.0001
0.997
0.0419
0.0009
0.0670
:0.0015
0.0137
i
0.0001
1.000
0.0482 = 0.0002
0.0660
f
0.0005
i
6.4-mm 0 . d . solute columns,
Ref, 7 .
f
Ref. 15.
I
' Ref. 14. Ref.
Ref. 13.
10.
0.179 i 0.001 at 25 ' C d 0.178 I 0.005 a t 20 " C c 0.0055 i 0.002 at 24 ' ~ f 0.0213 i 0.0014 at 24 ' ~ f 0.0015 i 0.0001 at 25 ' C f 0.0623 i 0.0004 at 26 " C g
0.0006 il
Ref. 6.
Table 11. Solubilities of Liquid Heteroatom Substituted Hydrocarbons in Water solute ni - d ic h 1or o be nze ne
dcm 0 0204 i 0.0005
6.4-mm 0.d. solute column.
S (23.5 I 1 . 5 " C ) , wt 7% 0.0149 F 0.0011
0.998 1.000 0.970 0.944 0.998 0.995 0.994
0.00746 = 0.00008 0.0107 = 0.0001 0.0350 = 0.0021 0.116 i 0.001 0.0182 0.0004 0.0549 t 0.0038 0.00079 t 0.00002
0.0144 0.0003 0.107i 0.005 0.106 i 0.007 0.296 = 0.008 0.115 z 0.003 0.120 = 0.007 0.00183 0.00006
S wt '6 literature
0.0123 at 25 " C b
0.0193 i 0.0004 I I - pr o p y 1 iodide 0.0997' z 0.0047 0.0303 I0.0006 1,1,2,2-tetrachloroethane 0.0255 = 0.0006 1,1,1-trichloroethane 0.0630' i 0.0006 0.0219 i 0.0002 dibutylphthalate 0.0232 i 0.0005 a
0.980
dx/dV, cm/mL 0.00729 = 0.00053
R
v,
Ref. 2.
Ref. 1.
Ref. 18.
f
0.107 at 25 'jCc 0.103 a t 30 " C c 0.287 a t 20 " C d 0.132 at 2 0 " C d 0.126 at 35 " C d < 0 . 0 1 at 20 ' C e
Ref. 1 9
Table IIIA. Typical Data for Tables I and I1 solute benzenea cyclohexeneb tolueneb ni -dichlorobenzene a U;,
was
Wl
1.0665 t 0.9597 t 0.9130 t 1.5115
w,, g 0.0001 0.0001 0.0001 0.0001
- 8 g and column o d . was 6.4 cm.
It'> - "t':, g 0.0690 = 0.0016 0.0838 I0.0005 0.0592 2 0.0007 0.1626 = 0.0037 It-, was
p
(20 - C ) , g/mL 0.8790 0.8102 0.8669 1.288
L , cm
11.9 40.8
38.1 41.0
Y, Gicm
0.0464 0.0146 0.0137 0.0193
i i i i
0.0010 0.0001 0.0001
0.0004
- 5 g and column 0 . d . was 3.0 cm.
T o examine the dependence of Equation 12 on solubility and the nonspecifity of the method, it was applied to solute columns of alkane, alkene, and aromatic hydrocarbons and to solute columns of iodine, chlorine, and oxygen substituted hydrocarbons. The results are shown in Table I for the pure hydrocarbons and in Table I1 for the substituted hydrocarbons. Data for some of the determinations in Tables I and I1 are presented in Tables IIIA and IIIB. Values for dx/dV and R , the correlation coefficient, were determined by least squares fits of Equation 12 to the x and V measurements. The correlation coefficient indicates how well the points fit Equation 12. A correlation coefficient of 1.000 corresponds to a perfect fit. T h e error in the values for d x / d V was determined by the least squares fits and the error in the values for y by repeated determinations of the third column weighing. The columns were not thermostated but run at a thermostated room temperature of 23.5 f 1.5 "C. T h e solubility runs were continued without interruption over time periods varying from several hours to ten days. It was observed t h a t it was necessary for several milliliters (benzene column) to 1.5 L (2-heptene column) of solvent to pass through the column before the solvent zone appeared. This delay could result from contamination of the solvent with small quantities of solute adsorbed onto the column to reservoir connection.
All the solutes measured in this study were selected on the basis of the availability of their known solubilities and on the basis that their index of refraction be greater than 1.39 (20 "C). Knowledge of the solubilities was important for testing the method's accuracy. An index of refraction 21.39 (20 "C) was required in order to distinguish visually the two zones. For example, attempts to determine the solubility of 1-hexene failed because the proximity of its index of refraction of 1.3821 (20 O C ) to that of water made it difficult to distinguish visually the two zones. For comparison the literature values of the solubilities are presented in Tables I and 11. The dependence of Equation 12 on the solubility and the nonspecificity of the method is reflected in the close agreement between the experimental and literature solubility values for most of the solutes studied. This agreement was quite good even with those literature solubilities which were determined in the aqueous phase by more complicated methods. T h e method's accuracy extended over a range of more than two orders of magnitude, from 16.0 f 0.3 ppm for 2-heptene up to 2960 f 80 ppm for 1,1,2,2-tetrachloroethane. The standard deviations of the experimental solubility values in Tables I and I1 vary from 1% for toluene up to 7 % for m-dichlorobenzene with a mean standard deviation of 3.0%. There are two major sources of random error; (1) variations in the experimental dx/dV, and (2) error in the
14
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
ends of the column to hold the packing in place, would reduce this deviation. The source of error in determining y was the determination of the mass of bulk solute trapped in the void space of the column. This error, cr(y), Equation 15, increases as ps, the solute density, approaches pH20 the density of water. If the adsorbed solute mass could be determined by an alternative method, this deviation might be eliminated. In the solute column, the coated solute film must be held in place by partial entrapment in the pores of the Chromosorb P particles. A pore volume of the particles (hereafter referred to as the accommodation volume per unit column length) can be calculated via
Table IIIB. Typical Data for the Determination of dx/du in Table I benzene, y = 0.0140 i 0.0003 g/cm V, mL X , cm 5.1 16.7 27.0 33.1 39.4 48.2
5.0 7.0 16.0 20.0 23.0 25.0
0.50 1.90 3.35 4.25 5.05 6.20
benzene, y = 0.0464 0.001 0 g/cm
benzene, y = 0.0154 f; 0.0005 g/cm V, mL X,cm
i
1.90 2.10 3.10 3.65 4.00 4.40
benzene, y = 0.0526 0.0017 glcm
V, mL
X , cm
V, mL
X,cm
25.0 40.0 44.0 54.0 70.0 80.0 95.0 130.0 135.0 160.0 178.0
1.95 2.55 2.70 3.05 3.70 4.05 4.60 5.90 6.05 6.90 9.10
14.0 17.5 32.2 44.8 62.8 74.4 87.7 100.1 114.9 136.1
1.00 1.30 2.50 3.40 4.80 5.70 6.80 7.50 8.40 9.70
vp= Y/P,
i
Values for Vp calculated from the given y values for the 3-mm 0.d. solute columns of Tables I and I1 are presented in Table IV. With the exception of dibutylphthalate all the values of V , are between 0.0141 mL/cm and 0.0191 mL/cm with an average value of 0.0164 f ,0014 mL/cm. Since this variation of Vp from substance to substance is the same as that for the three benzene columns, it must reflect variation in the packing of the column, and not be characteristic of the solutes. If the packing could be made more uniform from column to column, then presumably Vp would be constant and y could be calculated from Equation 16. This would simplify the method by eliminating the three columns weighings and reduce the error in the solubility determination. The invariance in V , from solute to solute reflects the inertness of the Chromosorb P solid support toward the solute stationary phase. Any interaction between the solute and Chromosorb P would presumably vary from solute to solute and V,, which is also the volume of solute adsorbed per unit column length, would vary independently of ps. This noninteraction between the solid support and the stationary phase has been also observed in gas chromatography a t high stationary phase loadings (20).
m-dichlorobenzene, i 0.004 g/cm X , cm V, mL y = 0.0193
22 140 180 27 0 310 331 350 407 437 777 849 892 927 1172 1198 1259 1282
(16)
0.30 0.65 0.85 1.70 1.90 2.10 2.25 2.85 3.10 5.55 6.05 6.45 6.70 8.40 8.60 9.20 9.30
CONCLUSION
determination of the adsorbed solute mass per unit length, y. Deviations in the former resulted from inhomogenity in the column packing and produced a mean standard deviation of 3.0% for all the solubility determinations with a deviation as high as 7% in the case of rn-dichlorobenzene, Better packing techniques, such as affixing rigid porous plugs to both
The elution chromatography method has been successfully applied to the determination of the solubilities of ten different organic liquids in water. In these determinations, the method has been shown to be nonspecific, sensitive to the ppm level, accurate, and fairly precise. The average 3% standard deviation of the solubility determinations by this method was the same as those of more complex methods (10-13). If the weighing portion of the procedure could be eliminated, this deviation can be reduced. The possibility of eliminating the weighing steps has been shown to be possible for identically packed solute columns. All that would be required is the density of the solute and some known accommodation volume for the packing. Since the determination of the solubilities by this method
Table IV. Calculated Accommodation Volumes per Unit Column Length solute benzene cyclohexane cyclohexene 2-heptene toluene m-dichlorobenzene n-propyl iodide 1,1,2,2-tetrachloroethane 1,1,l-trichloroethane dibutylphthalate
Y, g/cm
0.0140 i 0.0003 0.0154 i 0.0005 0.0124 i 0.0002 0.0122 i 0.0005 0.0125 i 0.0001 0.0146 i 0.0001 0.0135 i 0.0002 0.0160 i 0.0001 0.0137 i ,0.0001 0.0204 0.0005 0.0193 t 0.0004 0.0303 t 0.0006 0.0255 0.0006 0.0219 t 0.0002 0.0232 0.0005 _+
_+
_+
~ ~ ( "C), 2 0 g/mL 0.8790 0.7791 0.8102 0.7034 0.8669 1.288 1.747 1.600 1.325 (26 " C ) 1.047
Vp, mL/cm
0.0159
?
0.0003
0.0175 i 0.0006 0.0149 i 0.0002 0.0157 i 0.0006 0.0154 t 0.0001 0.0180 i 0.0001 0.0191 i 0.0003 0.0185 i 0.0001 0.0158 i 0.0001 0.0158 i 0.0004 0.0158 i 0.0003 0.0173 i 0.0004 0.0159 i 0.0004 0.0165 f 0.0002 0.0222 i 0.0004
Anal. Chem. 1980, 5 2 , 15-19
H. Klevens, J . Phys. Colloid Chern., 5 4 , 283 (1960). D. Arnold, C. Plank, and E. Erickson, Chem. fng. Data Ser., 3, 253 (1958). P. Wauchope and F. Getzen, J . Chem. fng. D,rta. 17, 38 (1972). C. McAuliffe, J . Phys. Chem., 70, 1274 (1966). F. P. Schwarz. J . Chem. €ng. Data 22, 273 (1977) D. Mackay and Wan Ying Shiu, J . Chem. €ng. Data, 22, 399 (1977). W. May, S . Wasik, and D. Freeman, Anal. Chern., 50, 997 (1978). R . Brown and S . Wasik. J . Res. Nat!. Bur Stand. Sect A . (45) 78. 453 (1974). R. S. Stearns, H. Oppenheimer, E. Simon and W. Harkins, J . Chem. Phys., 15, 496 (1947). "Handbook of Chemistry and Physics", 41st ed., Chemical Rubber Publishing Company, Cleveland, Ohio 1960. J. Calvin Giddings, "Dynamics of Chromatography, f'ari 1, Principles and Theory", Marcel Dekker, New York, 1965, p 277. A. E.van Arkel and S. E.Vles, Recl. Trav. Chim. Pays-Bas, 55, 407 (1936). A. Michaelis, Ber. Dtsch. Chem. Ges.. 46, 3683 (1913). R. J. Laub and R. L. Pecsok. "Physicochemical Applications of Gas Chromatography", John Wiley and Sons, New York, 1978, p 33.
was unaffected by the chemical composition of the solute, this method is apparently nonspecific. However more determinations would have to be made to test this implication. If the method is nonspecific then t h e chemical composition of the solute need not be known for the application of this method. Furthermore, the method is amenable to those solutes which may be toxic; the columns can be easier filled in a hood, sealed u p and weighed if necessary, and then run in a hood. To extend the application of the method to solutes with indices of refraction less than 1.4, other methods of zone differentiation could be used such as visible fluorescence excitation. The method also avoids several disadvantages of other methods (1-15). The formation of emulsions is a problem with those methods relying on vigorous shaking of the water-solute mixture to ensure saturation ( I , 2,4412). Furthermore despite the vigorous shaking, the achievement of saturation is not always assured. The accuracy of the method is independent of the solubility of the solute as with other methods (1-15). This elution chromatography method provides a simple, inexpensive, and direct means to measure the solubilities of a wide variety of organic compounds.
RECEIVED for review July 30. 1979. Accepted September 24, 1979. This work has been supported by the Office of Environmental Measurements a t the hational Bureau of Standards. Certain commercial equipment, instruments, and materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material, instruments, or equipment identified are necessarily the best available for t h e purpose.
LITERATURE CITED (1) (2) (3) (4) (5) (6)
15
A. Rex, 2 . Phys. Chem., 55,355 (1906). A. Klemenc and M. Low, Red. Trav. Chim. Pays-Bas, 49, 629 (1930). V. Rothmund, 2 . Phys. Chern., 26, 433 (1898). H. Fuhner. Chem. Ber., 5 7 , 510 (1924). P. M. Gross and J. H. Saylor. J . A m . Chem. Soc.. 5 3 , 1745 (1931). R. L. Bohon and W. F. Claussen, J . Am. Chem. SOC.,73, 1571 (1951).
Liquid Chromatography Columns of Microparticle Amberlite XAD-2 Robert G. Baurn,' Rolf Saetre,2 and Frederick F. Cantwell" Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Small particle size cuts of the nonionic adsorbent Amberlite XAD-2 have been obtained by grinding, sieving, and solvent elutriation. Columns, 15 cm long, packed by a stirred slurry packing technique with 3.6-8.4 pm XAD-2 yield efficiencies of greater than 20 000 plates per meter. Although permeabilities are smaller than those available with commercial reverse-phase bonded packings, necessitating slower solvent flow rates, the XAD-2 provides separation times adequate for most analyses and it can be used at all pH values including dilute sodium hydroxide.
Amberlite XAD-2 resin is a macroporous styrene-divinyl benzene copolymer which has been shown to be a versatile liquid chromatographic packing. It is a hydrophobic adsorbent, since it is not wetted with water ( I ) , with a specific surface area of about 330 m 2 / g ( 2 ) . Although XAD-2 is nonionic, it adsorbs both neutral and ionic species (3-5). Columns of XAD-2 have been used to analyze preservatives (3) and active drugs ( 4 ) in pharmaceutical syrups and to separate a variety of organic acids (61, bases (7), nitro- and P r e s e n t address: Searle Laboratories, Box 5110. Chicago, Ill.
60680. *Present address: Esso Resources, 339 5 0 t h Avenue A l b e r t a T 2 G 2V3.
S.E., Calgary,
0003-2700/80/0352-0015$01 .OO/O
chlorophenols ( B ) , and aromatic compounds (9). T h e chromatographic retention of peptides and amino acids has been investigated on XAD-2 ( I O ) , and t.hat of nucleic acid hydrolysates, purines, pyrimidines, nucleosides, and nucleotides has been studied on both XAD-2 ( 1 1 ) and a closely related adsorbent, Amberlite XAD-4 (12). The dependence of retention volume of acids and bases on mobile phase p H has been well characterized and demonstrates retention of both neutral and ionic conjugate acid-base species ( 4 , 5 , 23). Amberlite XAD-2 is obtained commercially as hard, white spheres, nominally 20 to 50 mesh (14) which must be ground to a smaller particle size for use in analytical chromatography. T h e skeletal density of the highly cross-linked styrene-divinylbenzene matrix is 1.07 g/mL ((14).Swelling of the resin in water is nil. Swelling in nonaqueous solvents has not been extensively studied, although 3 % swelling has been claimed in methanol (I51 and solvent uptake studies ( I , 9) suggest a similar figure for solvents such as acetonitrile and ethanol. Other physical properties of chromatographic interest have been summarized ( 2 ) . As a chromatographic packing, XAD-2 is distinguished by its low cost, wide useful p H range (pH 0 to 14), compatibility with virtually all solvents, and relatively strong adsorbent properties. Because it possesses a hydrophobic: surface, XAD-2 finds use in many of t h e same separations that are done on reverse-phase bonded packings such as octadecylsilyl silica C 1979 American Chemical Society
