edited by ARDENP. ZIPP
the microscale laboratory Microscale Multiple Extraction-An Experiment for the Microscale Organic Laboratory Steven W. ~nderson,' Philip T. Johns, and Craig D. 6oyd2 University of WiswnsiwWhitewater Whitewater, WI 53190
With several textbooks currently available for the microscale organic laboratory curriculum r 1-6 ,, it is quite surprising to learn that only two (Wilcox (3, and Pavia. ct al... (.4.). )offer exneriments comnarine the efficiencies of single and multiple extractions, and these are only qualitative in nature. Several texts (13,5, 6 )feature experiments in which solvent extraction is used to separate organic mixtures or to determine a distribution constant3( I, 2). To our knowledge, no microscale text has an experiment that quantitatively demonstrates the efficiencv of single versus <iple extraction although several mHcroscaTe experiments exist (6, 9,101. We describe here a microscale experiment, using aqueous benzoic or ~henvlaceticacid, in which a distribution constant (KD)is determined andthe efficiencies of single and multi~leextractions are compared. The volume of solvent required for this experiment is only one-ninth of that for macroscale and the wrresponding time for completion is reduced from two laboratory periods to one. Benzoic acid gives acceptable results but phenylacetic acid is more soluble, has a comparable KDvalue, and provides more reliable data. A solution containing a known amount of the chosen carboxylic acid in water is extracted with dichloromethane. The amount of acid remainine in the aaueous laver is determined by titration with stlndard sodium hydroxide solution and a KDvalue is calculated. The amount of solute extracted can be compared with that extracted by the same volume of dichloromethane divided between two seauential extractions. The amount of solute separated with' this multiple extraction can be compared with the theoretical
-
Chem~stryEduPresented, In pan. at Chem Ea '91, 101hB~enn~a canon Conference, Unwerslty of W1scons1n4shkosh.4-9 A~gust 1991;paper R-P16. 'Author to whom correspondence should be addressed. 'Senior undergraduate research student,Jan.-May 1989. Present address: Sun Chemical Cornration. Northlake. IL 60164. 3The terms oartition coefiicient~(cbnstant , - - -or - -ratio) -,and- distribution - -~ coeh?c,eri (constant or ratio, appear to be s e a nterchangeao y n organc laboratory texts The dlsrr out on constant, KD, wo~lastrinly apply to a nonionic solute (neglecting ionization) in a single definite formdistributed between water and an organic phase (7, 8). 4 ~ utoe the low concentration of sodium hydroxide ( - t 0 ~ - 1 0 ~ mL), we had the students carry out a blank wrrection. For example, in the phenylacetic acid experiment, the average blank volume was 1.4 mL(theauthors obtained 1O . mL) resulting in a 7% increase in the calculated KDvalue. In some cases the blankcorrection could not be used because this led to negative KDvalues. 5Typicalvolumes of sodium hydroxide solution used for extraction of aqueous phenylacetic acid were 23 mL for the standardization, 10 mL for the KD in CH2C12-H20,5 mL for multiple extraction with CH,C12, 4 mLfor the KO in Et20 - H20, and t mL for multiple extraction with Et20. ~
.~~
~
~~~
~
~
~
~
SUNY-Canland cOfllandsNyl-
amount walculated from the KI, value obtained in the first Dan of the exneriment,. The efficiencvof different solvents ie.g., ether) as extraction solvents d s o can be compared. Alternatively, the analysis may be done spectrophotometrically. Experimental Weigh out approximately 244 mg of benzoic acid or 1.09 g of phenylacetic acid on a milligram balance. Dissolve the solid in approximately 75 mL of distilled water, transfer to a 100-mL volumetric flask, and dilute to the mark. Sodium Hydroxide Solution
Prepare a solution by dissolving the appropriate amount of sodium hydroxide in 100 mL of distilled water. Titration occurs best if 240 mg is used for benzoic acid and 160 mg is used for phenylacetic acid. Add 10 mL (for phenylacetic acid) or 12 mL (for benzoic acid) of this solution to a 500mL plastic bottle using a graduated cylinder and bring the total volume up to approximately 500 mL with distilled water. Standardize the sodium hydroxide solution against 1.00 mL of benzoic acid solution or 0.200 mL of phenylacetic acid solution in a 50-mL Erlenmeyer flask containing 5 mL of distilled water and three drops of phenolphthalein indicator5 Distribution Constant in Dichloromethane/ Water
Pipet 2.00 mL of the carboxylic acid solution into a 5-mL conical vial and add 2.00 mL of CH2C12.Cap the vial with a septum tightly and shake it vigorously for about 10 s. Allow the layers to separate. With an automatic pipet, carefully remove a 1.00-mL aliquot of the aqueous (upper) layer. Transfer this to a 25- or 50-mL Erlenmeyer flask and titrate in the same manner as for the standardi~ation.4~ Multiple Extraction with Dichloromethane
Pipet another 2.00-mL sample of the carboxylic acid solution into a 5-mL conical vial and extract with 1.00 mL of CH2C12. Prepare a Pasteur fdter pipet (11).Squeeze the pipet bulb to force air from the pipet. Insert the pipet into the vial until it is close to the bottom. Be sure to hold the pipet in a vertical position. Allow the bulb to expand, drawing only the the lower CHzClzlayer into the pipet being careful not to disturb the boundary between the layers. Transfer the CHzClzsolution to a n Erlenmeyer flask. ARer removing all of the first 1.0-mL portion of CHzClz add a second 1.00-mL portion of CH2C12to the vial. Cap and shake again. Allow the layers to separate and remove a 1.00-mL aliquot of the aqueous layer and titrate a s ab0ve.4,~ Distribution Constant in Ether/ Water
Pipet 2.00 mL of the acid solution and 2.00 mL of ether into a 5-mL conical vial. This latter transfer should be done quickly to prevent siphoning. Cap the vial tightly with a septum and shake. Remove 1.00 mL of the aqueous (lower) layer and titrate as (Continued on next pnge)
Volume 70 Number 2
February 1993
A33
the microscale laboratory Multiple Extraction with Ether
Pipet another 2.00-mL sample of the acid solution into the 5-mL conical vial and extract with mL of ether as above, Transfer the entire aqueous layer to a clean 3-mL conical vial. Pipet a second 1.00-mL portion of ether into the 3-mL conical vial. Cap, shake, and allow the layers to separate. Withdraw a 1.00-mL ali uot of the aqueous layer and titrate as before.4% Spectrophotometric Determinationof Distribution Constants
Table 2. Distribution Constant Values for Phenylacetic Acid in Dichloromethane-Water and Ether-WateP
AV~.K [PhCHzCOzH], O~ Af
d
Solvent micro system CHZCIPHZO X CHZCIFHZO X
macro
This work This work
8.3
0.080
11
8,1e
0.0051
5
16.1 23.3e
0.080 0.0051
11 2
EtzGHzO
21 15
0.100 0.00120.0016
-
EtzGH20
X
EtO-HzO
X
-
EtGHz0
Reference
This work This work
X X
17
16 The procedure employed paralleled the titrimetric methods except that after equilibration Yee Table l, footnote a, of the aqueous acid solution with the organic b ~ ~wa b 1.i footnote ~ b. solvent, the aqueous layer Was pipeted to a sil'Molar mncentration of phenyiaceticacid in the original aqueous solution priorto extraction. ica cuvette for subsequent spectral measured ~ Table w 1, footnoted. ment. In order to obtain reasonable absorbances 'see Tablet, footnotee. for the aqueous portion, with a spectmphotometer (in our case, a Perkin-Elmer Lambda 4B), the initial concentrations should be in the range The average microscale KD for benzoic acid in CHzClz of lo4 to 1@ M for benzoic acid and 10-'to lo3 M for Hz0 (Table I), when compared to microscale data obphenylacetic acid. The loss of acid from the aqueous layer tained by a gravimetric method (11, is 60% lower than one was monitored by scanning between 200 and 300 -, alreported (14) value, yet is in accord with a reported range though the most reliable bands we followed were at 227 of values (la.The data obtained by this gravimetic nm for benzoic acid and 257 nm for pheuylacetic acid (12, method might be subject to greater as the values 13). reported in the first instance (14) ranged from 49 to 0. 1. Our student values ranged from 3.4-7.1 in CHzClz Hz0 Results and Discussion and from 1.0 to 22 in EbO / H.O. For ~henvlaceticacid. our student KDvalues ('fable >) ranged From 5.4-11 in Results of distribution constants for eight semesters of CHzClfizO and from 9.4 to 25 in EkO/H20. No micmscale our organic laboratory, including previous macroscale data have been reported for pheuylacetic acid. data, are presented in Tables 1and 2. Both carboxylic acids The microscale data obtained for phenylacetic acid in exhibit good agreement between t h e K ~ values determined Et20/H20compares favorably with the corresponding titrimetricall~and s~=trophotometrically. To our howlmacroscale Kn values (16. 17). In s h a r ~contrast, the edge, no microscale determinations have been made in macroscale ~ o v a l u e for s bdnzoic acid in bbth CH2C1fi20 and Et10/H90 Et.,O/H,O. - . . are more than twice as large as those determined by microscale (57% higher for Table 1. Distribution Constant Values for Benzoic Acid in DichloromethaneCHzC12/H20 and 58% higher for Water and Ether-WateP Et20/Hz0. At first, we thought this contradiction Ave. Kob [PhCOzH],mMC nd Solvent system micro macro Reference could be reconciled by considering that fraction cuts, or an incomplete transfers during extraction, would be magnified X This work on the microscale level. For example, if X This work two d m ~ of s the aqueous layer (-0.1 mL) were lost prior to kxtractioi, this would X 14 decrease the calculated KDby approxiX 15 mately 5%.Aloss of 0.5 mL would lead to X This work a 25% reduction in the calculated KD value. If this were the case, it is not apX This work parent why the KD values obtained for X This work phenylacetic acid (Table2) do not show a X This work similar variation from macroscale to microscale. 24-38 1-14 - EtzGHzO X 16 We are at a loss for a reasonable expla'All values are at or close10 25 'C.Unlessotherwise specified, the mnfidenca limits (atthe95% ievel) on nation of the two-fold difference between the student measured microscale )6's are + 78%. b ~ ~ i n as e dthe ratio of the mncentration of caboxylic acid in the organic solvem tothe mncentration of the macmscale and microscale Kn values caboxylic acid In the aqueous layer (7.8). for benzoic acid that also can a&ommoCMiiiimolarmncentration of benzoic acid in the onginai aqueous solution pnor to emaction. date the results for phenylacetic acid. d~umberof determinations. This difference cannot be attributed to 'Determined by UV-VIS spectroscopy; details in expenmental section. The confidence limits on measurethe benzoic acid concentration because ment (at the 95% ievel) are + 6%.
(Continued o n page ,436)
A34
Journal of Chemical Education
the microscale laboratory Table 3. Comparison of Extraction Efficienciesfor Carboxylic Acids in Dichloromethane-Water and Ether-Water I
Acid
Method
na
single. %
multiple, %
% of
single, %
multiple, %
theoreticalb Benzoic
Micro 42 82 90 Macro 40 68 74 Phenylacetic Micro 11 90 94 'Number of determinations. b ~ omultiple r extractions,based upon the KOvalues listed in Tables 1 and 2. this was identical for both the macroscale and microscale procedures and the concentrations of benzoic acid and phenylacetic acid only differ by a factor of four. Clearly, the factor or factors contributine to this difference are independent of the solvent systems employed. The fact that the KOvalues determined both titrimetricallv and s~ectroohotoketrically are in good accord, rules okt titrakon e m r . Furthermore, the good ameement between the microscale and macroscale K; values for phenylacetic acid (Table 21 indicates that our observation is unique to the benzoic acid system. The objectives of this experiment, to illustrate quantitatively the differences between single and multiple extractions and to compare extraction solvent efficiencies, have been met (Table 3). The percentage variation is small, but the volume differences in sodium hydroxide5 clearly indicate the greater efficiency of multiple extractions over single extraction. Similarly, the volume differences and the calculated Kn values demonstrate that ether is a better ~~~~~-ex--traction solvent than dichloromethane for the chosen carboxylic acids. ~
~
Acknowledgment The authors are grateful to E. J. Drexler, K. West, and K. Romary for their insightful comments on this work. We extend our appreciation to the State of Wisconsin for a n equipment grant, through the UW Laboratory Modernization Program that has allowed conversion of the organic laboratories to microscale. S. W. A. is indebted to the PEW Charitable Trust for facilitating his participation in the Third Annual Summer Institute on Microscale Organic Labortory Techniques presented by the Department of Chemistry a t Bowdoin College, Brunswick, ME, 19-23 June 1989. Finally, we thank the many students who subjected this experiment to their scrutiny and brought anomalies to light. Literature Cited 1. Mayo, D. W ; Pike, R. M.; Butcher, S. S. Mlcmsmk Omanle Labomtory 2nd ed.; Wiley: New Yark, 1989: pp 80- 84. 2. wieamn, K L. ~~~~h ~mosmb~ ~ ~ r u cneath: ~ ~~ ~ i~ , , h~ ton, MA, 1989: pp 116137. 3. Wi1mx.C. F . E ~ m ~ m ~ t t 1 O g ~ n ~ C k m i s f ~ : A S m l l - S aM b Aap pd ihh ; New Yark, 1988; pp 93-95. 4. Pa"%, D. L.; Lampman. G. M.; Kriz, G. M . ; Engel, R. G . h f d u c f i o n to 01gonle Labomtory %hnlgm3:A M i i i l a A p p i h ; Saunders: Chicago, 1990.p 46. 5. Nimitz, J. S. Ermrimnla in 01gnnle chpmratry: Fmm Mkrosmk to M m s o o l n ; Rentice-Hall: Englewod Cliffs,NJ.1'331;pp 59-61. 6. Radig, 0. R.; He& Jr, C. E.; Clark, A. K. w i e Chemistry Laboratow: aandard and M l w a m l e Exmhmlmlts; Saunde18:Philadelphia, 1990; pp 45-62. 7 . Irving, H . M. N. H.PureAppl. C k m 1810,22,109-114. 8. Laitinen, H. A ; H-s, W. E. Ckmleal Analysis; 2nd ed.; McCrccw-ml: NeuYork, 1975: 427. 9. yip,^.?; ~ s ~ t o~n. ~, . ~ r g o n t e c h e r n r in a h~y l r ~ o b o m ~ ovrsyn;~~a .s t r a n d : ~ e u Y d , 1979: pp 3 4 4 5 .
A36
Journal of Chemical Education
% of theoreticalb ~~-~~ ~
98
92
95
~~
97
.
12 S l h c n v m . R M Hsorlpr. G C . . .Monrll. T C S w r m m m c ldmr,ruaruxn or Ory o n r . C r m p o ~ n d a : J l h c dWllry : UPV Ymk. 1'331;p 3 U I 13 HdI) E C C.: W h o m . F C. J Chem Sa 1815. 107. 1058-1070. 14 Sundm.C E 20th (ireat IakcsRemmai Mrcunu~.rfhrA m c n r a n C h r n ~ ~ ~ a l S o o n v Maryurm I h v c r s n y . ~ l w a u k i eW I . 2 4 JU& 1986. paper 1.~2 15. Plkc, R M RPmsrka from a a c ~ e m nnt ih. Tnwd Annual S ~ m m c rlnartturc on hLmrcalvihgarur Iaboraro~T e c h m y v e ~BowdmnCollcgc. Hmnaun&ME. I9 21 June 1989 16. Forbea, C. S.;Anderson,GOW. InI~lfe~lf~lftioml C h t i i l T d I P ~Wwshbum,E. ~; W.Ed., Mecraw-Hill: New York,1928: Vol.3,p 429. 17. Dermer, 0. C,Oermer, V. H. J. h r Chem. Sac 1843.65. 1653.1654,
Construction of a Micropycnometer for Determination of Density and Specific Gravity of Liquids and Solutions Mono M. Singh, Zvi Szafran, and Ronald M. pike' Merrimack College N. Andover, MA 01845
The determination of densities or specific gravities of solids, liquids, andlor solutions is one of the important laboratory assimments in the eeneral chemistrv curriculum. The most &nunon method &d for liquids &d solutions is based on the measurements of mass and volume. Usuallv. the volume of the solution or liquid is determined using; graduated cylinder. ~ i w tor , svrinee (13).More accurate determinatibns can k m i d e using a pycnometerorspecific gravity bottle. However, commercial ~vcnorneters/s~ccific gravity bottles are prohibitively expe&ive ($30-$96/each) for use in general chemistry laboratories. These methods oRen use large volumes of liquids or solutions, resulting in the accumulation of sienificant amount of waste material. A typical class of l ~ c s t u d e n t s a t Merrimack College generated about 3 L of oreanic liauid waste for disposal ( 1 0 k per ~ student for eachof threktria h In an effort to decrease the volumes of oreanic liauids (and the large volume of the waste . ~ or~ solutions ~ t ~ required ; produced), we have our students construct a micropycnometer from a n inexpensive Pasteur pipet. In addition to decreasing the liquid volumes drastically (from -30 mL to less than 1mL), this laboratory introduces the technique of making capillaries using.microburners,. glass cutting. -. and f r e polishing (4).
-
'A fotthcoming book by Singh, M. M.; Pike. R. M.; Szafran, 2. including this technique on Advanced Miuoscale General Chemistry Laboratorywill be published in 1993-94. (Continued on page A38)
