P. J. Miller
Royal University of Moho Volietta, Msida
Density Gradients in Chemistry Teaching
A
density gradient is a simple device which can be wed to produce interesting visual effects illustrating various chemical principles. There are very few references in the chemical literature to the application of this technique as a teaching aid, and the aim of this article is to outline experiments in which a density gradient might be used to advantage. The author has attempted to present the article so that it progresses from simple principles, involving composition and density, to more advanced experiments which might be used as a basis for small research projects in a neglected field of chemistry. Technique
A density gradient consists of a column of liquid, the composition (and density) of which varies between the top and bottom of the column. Oster, in an excellent review of the subject ( I ) , discusses the theory of density gradients and describes two main methods for setting up a column. Below, briefly outlined, is the method adopted by the author. The apparatus is shown in Figure 1. The denser liquid (I) is placed in vessel A , and vessel B is filled with liquid I1 to about the same level, allowance being made for the difference in density between the two liquids. The taps are opened and the liquid allowed to run at a slow and steady rate down the side of a vertical glass tube. The dimensions of the
LIQUID 1+
apparatus will be determined by the volume capacity of the column required, but if the two vessels A and B are of the same diameter, then liquid will run from vessel B to vessel A at half the rate at which the mixture leaves A. The result will be a linear concentration gradient and an almost linear density gradient. The nature and compositions of liquids I and I1 can be varied to produce a series of columns covering a wide density range and varying gradients. For accurate work the column should be kept at a constant temperature, but for demonstration purposes most gradients are sufficiently stable to be used in the open laboratory, or even on an overhead projector. Densities of Organic Compounds
If a drop of a liquid, which is insoluble in the column liquids and whose density lies within the density range of the column, is added to the column, it mill fall until its density corresponds to that of the surrounding liquid. For most columns the rate of fall is rapid and within 2 min the drop is at its resting position. In one experiment, 20y0 NaCl and water were placed in vessels A and B, respectively. This gave a column with a density range 1.15-1.00. A drop of each of a series of organic liquids was added to the column, and the heights at which the drops came to rest were measured (see the table). No attempt was made to purify the compounds, apart from washing the benzaldehyde with aqueous sodium carbonate to remove any benzoic acid impurity. The densities quoted were those on the labels of the bottles (B.D.H. Chemicals). As can be seen from Figure 2, all the compounds lay fairly close to a straight line relating density to the column height. This type of experiment can be carried out with reasonable accuracy on an overhead projector and can illustrate the following The existence of a, concentlationor density gradient. The different densities of organicliquids. The technique can also be used to confirm the identity of o~gsniccompounds (only one dropis required)or to test theirpurity. Analysis of Aqueous Solutions
I Figure 1 .
Apparotur for the formation of density g r o d i e n b
I
By preparing an organic column using, for example, bromobenzene-benzene 'mixtures, the composition or density of aqueous solutions can be determined. If solutions of bromobenzene in benzene (50% and 15y0 v/v) are placed in vessels A and B, respectively, a column will be produced with a density range 1.200.97. This can be used to analyze aqueous solutions of sodium chloride, the column being calibrated using solutions of known concentrations or density.
in such a column, but more sensitive columns could be prepared using a smaller gradient. Densities of Solids
Density gradients are not restricted to the analysis of liquid drops and can be used to show the relative densities of various solids. Oster, in his review ( I ) , has referred, for example, to the separation of crystals of NaCl and KC1. Reaction Kinefics
An interesting variation of the iodine "clock" reaction Hn02+ 2H+ DENSITY Figure 2. Distribution of various orgonic liquid. (see table1 in a denrity gradient column.
Drop Heights for a Series of Organic Liquids Density (wt./ml a t 20PC) (h) aminbbeniene (c) fluombenzene (d) acetophenone (e) ethyiphenylacetate (f) bensaidehyde (g) bensyialcohol (h) ethylbenzoate (i) diethyloxalate ( j ) methylhensoate (k) chlorobenzene
Height (em)
1.023 1 .02,i 1.03 1.031-1.034 1.046 1.043-1.048 1.046-1.049 1,077-1 .OX0 1.087-1.089 1.105-1.108
Analysis of Binary Mixtures of Organic Compounds
The analysis of organic mixtures can be carried out using an aqueous density gradient column. Such a technique would find application in the determination of boiling point-composition diagrams (3) or in following distillation processes. Figure 3 shows the calibration curve for a column prepared from 50% w/w sucrose (liquid I) and water (liquid 11). Mixtures of nitrobenzene and benzene containing up to 60% v/v benzene could be analyzed
+ 21-
-+
2H20
+ I1
can be observed by filling a column with the reaction mixture at a uniform concentration, except for one reactant, the concentration of which varies linearly throughout the column. NaCl is also added to produce a density gradient which stabilizes the much smaller concentration gradient of the reactant.
Experimental A mixture was prepared containing 25 mi of 0.25 M H2S04, 12.5 ml of 0.1 M KI, 5 ml of 0.05 M NsnSIOs, 10 ml of water, and starch indicator. This was added to both tubes A and B i n the appar& tus shown in Figure 1 and the tap 1 wwas shut. Water (10 ml) was then added to vessel B and 5 ml of 20% NaCl to vessel A. Finally, 5 ml of 1 volume HnOl was added quickly to vessel A, the stirrer and a stopwatch were started, and taps 1 and 2 were opened so that a 50-ml buret was filled during 2-3 mi". After about 11 min a deep blue color appetsmd a t the bottom of the buret and gradually spread up the column. The boundary was sufficiently sham to allow the height of the color ( h )to he read at regular intervaisbf time, t, (see Fig. 4). The above emerimental ~rocedureshould lead to a. linear HJh concentration egadient in the buret, so that the buret reading is proportional to the peroxide concentration. Also ~~~~~~
-
1 rate of reaction rr -
t
therefore a plot of llt against the buret reading should confirm that the reaction is first order in peroxide (Fig. 5). rate
m
LHsOnl
Figure 4. Time taken for the color to appear for the different concentrotions (buret reading.) of reacting specie..
% BENZENE Figure 3.
Cdibrotion curve for a sucrose density grodieht.
HIO, + 2H+
+ 21-
-
2H10
+h
a, different [I-]; b, different [HIO,]; c, different [I-] ond [HzOd
Volume 49, Number 4, April 7 972
/
279
71.8
. 1.
. 1.1
- 0, -3LE
10
A
ALE RATE (min-'I Figure 5. The relationship between the concentration (buret reading1 of reacting species and the rate of the clock reaction. a, [I-], scale, I; b, H2Oescalc 11; c. [I-] = [ H 2 O 2 ] , s ~ ~11.l e
In a similar experiment., the peroxide concentration was kept constant and the iodide concentrstion was varied linearly throughout the length of the column. The variation of the height of the color boundary with time was again recorded (Fig. 4). A plot of l/time against height (Fig. 5 ) showed that rate of reaction = [I-] Finally, s, column was prepared in which [I-] = [HIOJ, and the concentration of these two vaned linearly throughout the oolumn. I n this case
2 = time
(buret reading)n
i.e., rate = 11-1 [H2021(see Fig. 5). As can he seen from the graphs, despite obvious sources of error, the results are fairly good. A column can be placed on an overhead projector to allow students to take their own readings and plot graphs during the experiment. The varying salt concentration does not seem to adversely effect the above experiment. Solvent Extraction
A recent article (3) states that "little work has been published on the rate at which equilibrium is'reached in liquid-liquid solvent extraction systems." The densky gradient technique allows for the continuous observation of the extraction process using very simple apparatus; and should afford interesting project work for students in a field which has been neglected, but is now beginning to receive increasing at,tention. If one component of a binary mixture of organic compounds is soluble in water and a drop of the mixture is added to an aqueous column, the water soluble component will be extracted by the aqueous phase. The residual organic phase will change in composition and density and the rate of the extraction can therefore be measured by following the rise or fall of the drop in the column. Systems that have been studied by the author include the extraction of alcohols, phenols, diethylether, and a ketone from 1-methylnaphthalene. It was previously reported (8that the extractionof 1-pentanol from fluorobenzene by aqueous sodium chloride follows first-order kinetics, being proportional to the concentration of the alcohol. However, at very low 280
/
Journol o f Chemical Education
20
30
TIME (MIN) Figure 6. Tho oqueow concentrotion of voriovr solutes from dropr of 1-mothylnophthdene shown b y movement of the dropr. 0, pentan-2-one; b, diethylether; c, pentan-1-01; d, pentan-2-01; e. butan-1-01; f, 2-methylpropan-1 -01; g, 2-methylpropon-2-01; h, blank.
concentration there appears some deviation from firsborder kinetics, and this might be due to the slight solubility of fluorobenzene in water. Experimental A column was prepared using water (liquid 11) and 4% NaCl (liquid I ) , and a drop of I-methylnaphthdene was then added, to observe the rate of fall of a drop when no extraction was taking place, and to find the Level (h,) a t which a drop of pure solvent stops. Next was added drops of I-methylnaphthalene containing 5% ," v ,l v of various solutes. and the rate of fall (or rise) of these drops was followed using a cathetometer. Typical results are shown in Figure 6. The difference in height between a drop of pure solvent (h,) and a drop of solution (h,) is proportional to the amount of solute present a t any time, 1. The relationship between the rate of loss of solute ( - deldl) and solute concentration can therefore he determined (see helow). When the rate of extraction is rapid, an in the case of the lower aliphatic dcohols (see Fig. 6), the process can be more easily studied by following the rate of extraction by a, dilute aqueous solution of the alcohol, instead of water. A solution of 4y0 2-methylpropan-1-01 in water was prepared. and to 80 ml portions of this solution was added 20 ml of 20% NltCl (to give Liquid I) and 20 ml of water (to give liquid 11). A column was prepared from these liquids and maintained at 22.5"C. When 8. drop of 1-methylnaphthalene (d = 1.023) was added to the prepared column, i t fell rapidly, due to gravitation effects, and then started to rise as i t extracted the alcohol from the aqueous column. Conversely, when a drop of methylnaphthalene containing 8.37' v/v 2-methylpropan-1-01 was added to the column it fell rapidly due t o gravity and then continued to fall as the alcohol was extracted from the organic phase into t.he aqueous column. Eventually, when equilibrium was established in both drops, they stopped a t the same level (within 0.1 mm of each other). The progress of the two drops is shown in Figure 7. Results
For a first-order process (n = 1)
kt
ho - h, = log,, hr-h, 2.303
TlME (MIN) Figure 7. The opprooch to equilibrium in the system 1-methylnaphthalene; 2-methylpropan-1 d:oqueous NaCi.
and aplot of loglo l/(ht - h,) against time, t, should give a straight line. As mentioned previously this was observed in some cases. However, many of the above systems did not show first-order kinetics, and the order was therefore found by the tangent method. A graph of (h, - h,) was plotted against time' (e.g., Figs. 6 and 7) and tangents to the curve were drawn to give values of d(ht - h,)/dt at diierent (h, - h,). The slope of a subsequent graph of loglo d(ht - h,)/dt against loglo (ht - h,) gave the order (n) of the process. This was found to be 3/2.
To check the validity of this result for other systems the rate equation was integrated
and graphs were plotted of t against (h, - h,)-%. Straight lines were obtained in many instances (e.g., Fig. 8). I n the systems where one is studying the approach to equilibrium (Fig. 7) rather than complete extraction (Fig. 6), (hl - h,) does not represent the concentration of solute remaining at any time, t. However, the above procedure is still relevant as -d(ht - h,)/dt represents the rate of approach to equilibrium. Discussion
The experiments involving extraction processes serve many useful purposes. Besides illustrating solvent extraction and an equilibrium process, both of which can be shown on an overhead projector, they give students experience with calculations in kinetics. Also, they provide the basis for a discussion of the nature of the interphase between two liquids and the factors effecting the rate of transfer of a solute between two liquid phases (4, 5). The diierence in the rate of extraction of o- and p-nitrophenol from an organic phase (Fig. 9) could lead to a consideration of interand intramolecular H-bonding and solute-solvent interactions. Similar experiments to the above could be used to
T l M E (MIN) Figure 8. Tho reciprocal rqvore root of the distance from equilibrium, *(he h,J-'/a ploned against time for the system 1-methylnaphthalene:2-methylpropon-1-ol:NaCIII.
-
I
I
2
6
10
0
100
200
14 TIME (MINI . SCbLE I 300TIME (MIN) SCALE U
Figure 9. The extraction of nitrophenols from 1-methylnophtholene b y water and 0.05 M NoOH. o, o-nitrophenol-HsO l w 3 e Ill; b, p-nitro~ h ~ ~ o C IsH d e ~ I);0 c, o-nitrophenol-0.05 M N a O H I x d e I); d. p-nitrophenol-0.05 M N a O H Issale I).
follow the rate of transfer of a solute from an aqueous drop to an organic column. The studies could also be extended to observe solvent extraction with chemical reaction, for example, the extraction of nitrophenols with aqueous NaOH (Fig. 9), and also the effect of surface active agents on the liquid-liquid interface. Craig (4) in 1956 considered there was no quantitative data to be found in the literature on the effect of surface active agents on the rate of interchange of a solute between two immiscible phases. I n the experiments described above, the drops of liquid were only approximately the same size, and a better quantitative relationship between the extraction rate constants (k) for different systems could be obtained by using drops of identical diameter. Also, a more rigorous treatment would allow for change in the size of the drop during the extraction process. Again, it must be remembered that the mechanism of the transfer process and even diffusion coefficients can change with concentration. Volume
49, Number 4, April 1972
/
281
Conclusions
The aim of the author is not to describe specific experiments involving density gradient techniques, but to illustrate the possible versatility of this inexpensive device as a teaching aid. Some other applications are described in a review by Oster (1) and in addition, density gradients have been mentioned in connection with demonstrations of diffusion (6, 7), density (8),and the minimizing of surface energy of drops by the formation of perfect spheres (9). Also, a discontinuous density gradient has been used to demonstrate the different densities of various materials (lo).
282
/
lournol of Chemical Education
Literature Cited (1) OBTER, G., A N D YAMAIIOTO, M., them. ~ m .6,3 , 2 5 7 (1963). ( 2 ) MILLER, P. J.. Sch. Sei. Rw.. to be published. (3) Hamswoen, J.. AND TDCR,D. G., Tmns. Faladsv SOC..6 6 , 2526
W.
(1970). (4) C n ~ r o L. , c., *no C n ~ r a D., , ( ~ d i t wW ~ S S . ~ R ~A,), B ~ '. ' ~ ~ of Organic Chemistry. Val. 111. Part I." 2nd ed.. Interaoienee, New York. 1956, p p 157-161.
(5) H ~ W T BT. B ,G., nao ~ * A. mw., J.SOC.o h m . rnd. L O ~ O 5~ 1, , 285 (1932). A.). "Techniaue of Organic ( 6 ) G n a ~ A c x ,B.. (Ed&?: WEIBSBEROEB, Chemistry. VOI.I. Part I," rnterselenoe, N ~ Wyork, 1949, P. 572. ( 7 ) C o ~ ~ n C. n ,R.. AND BEBT,H. E.. J. CXEM.EDUC.. 46, 758 ( 1 0 6 s ) . (8) Harson. C. G.. Srh.Sn'.Reu..51, 681 (1970). ( 0 ) H*s~an.o.E. J., "The Screen Projection of Chemioal Experiments:' Melbourne University Press,Victoria, 1953, p. 227. 10) D m u n ,R. E.. J. CHBM.EDUC..13,589 (1936).
~
h
~
i
