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)
the microscale laboratory
7
Densities (g/mL) of Different Substances
Alcohols
Rotate
B
Pull Apart
U
.
:
A -
B Cut, and Seal off Here I
Methanol Ethanol RPropanol ~Butanol RHeptanol Expta
0.803
0.783
0.806
0.810
0.823
LIP
0.791
0.789
0.803
0.810
0.822
Solutions (Wvol%)) Sugar Expta
5% 1.024
1.038
1.073
1.147
1.222
LIP
1.017
1.038
1.081
1.177
1.236
10%
20%
40%
60%
'Experimental values. b~iteraturevalues from CRC Handbook 01 Chemistry and Physics; CRC Press: Boca Raton, FL, 1988.
displaced from inside the pycnometer when it is filled (Part D of figure). Procedure to Determine Density or Specific Gravity
View of ihe construction of a micropycnometer Preparation
Hold a Pasteur pipet (Flint Glass, 5-in. capillary pipet) in an oxidizing (nonluminous~Bunsen flame 1cm awav from the recedi@ or tapering part of the pipet stem (part A of figure).Rotate the body of the pipet over the flame for even heat distribution. When the glass has bewme sufficiently soft so that it can be pulled without much effort,remove it from the flame and gently but immediately pull the two ends apart (Part B of figure) in order to form a 10-25 cm thin capillary (Part C of figure),with a bulb a t one end and the main body of the pipet at the other. Keep the stretched pipet taut until the capillary and the small bulb become rigid. At about 0.5 cm from the rear end of the bulb, heat the capillary strongly to detach it &om the main body of the pipet (Part C of figure). Save the cut off pipet (that now has a capillary stem) for later use as the delivery pipet to transfer the given solution to the micropycnometer after its fused end is cut open. Heat the just-sealed part of the pycnometer bulb strongly to form a smooth, rounded end. Cut the opposite end of the pycnometer to give a stem about 1an long. This stem serves as the mouth of the pycnometer. Fire polish the opening. Attach a rubber pipet bulb to the delivery pipet. The width of the capillary opening must be smaller than the opening of the mouth of the pycnometer. This will ensure the easy passage of air being A38
Journal of Chemical Education
At first weigh the empty micropycnometer.Then use the capillary pipet to fill the pycnometer to the top with the desired liquid and record its weight. The difference between this mass and that when it was empty gives the mass ofthe liquid in the pycnometer. The liquid is removed from the pycnometer with the same delivery pipet. The pycnometer is rinsed twice with acetone (if unknown is an organic liquid) and several times with distilled water. Finally, the pycnometer is filled with distilled water, weighed and the mass of the water determined. Using the density of water at the given temperature, the volume of water (and thus of the pycnometer) is calculated. The density or the specific gravity of the unknown liquid can now be determined. This microdevice yields results that compare favorably with those obtained using commercial pycnometers and correspond closely to literature values. (See table.) The device also can be used to determine the densities of different % (wlv) solutions of inorganic salts. It also works well for an experiment we have developed in which students identify an unknown alcohol or estimate the concentration of an unknown solution from a plot of density against the number of carbon atoms in alwhols or against % concentration of the solutions, respectively. Students enjoy making their own pycnometers. Literature Cited 1. Safran, 2.;Rke, R M.: Foateq J. C. Miuaffak Dewml C h e m i n v Inborntory with &kctad Mmm& Eqxr-imntt: Wilily: New Yark,1993. 2. Nelson, J. H.;Kemp, K C . I n b a m l o ~ E ~ p ~ " l sCh.rnkhy for *ha C o n t m l s e k l p : Rent-Hall: Englewaod C m , NJ, 1977. 3.Berm, J. A.Inborntory Man& for F u n h n i d s o f Chamistry. 3rd ed. Wlley: New York,1988. 4. pasto, D.:J o h n m , C . ;Milk, M.E%pnm"* and Tenhniques in O l g a n i e C h P m k f ~ ; Rentice-Hall:Englewmd C W s , NJ, 1992.
Acknowledgment
We appreciate the help of several students from Chem 114 (advanced general chemistry) and especially of Nicole Casey, a sophomore, who performed the laboratory before it was introduced into general chemistry laboratory.
