Steven R. Kurtz, Owen T. Anderson and Bennett R. Wllleford, Jr. Bucknell University Lewisburg, Pennsylvania 17837
A New Simple Apparatus for the Measurement of Dipole Moments
T h e dipole moment of a molecule is often an important structural characteristic. Experimental measurement of this property i n t h e elementary physical chemistry laboratory provides t h e student with a n understanding of t h e various ~ h v s i c a concents l involved: i t also reauires extensive treat&nt of t h e ekperimental'data and a n assessment of t h e orobable error in t h e calculated auantities. Eauioment limi.. tations, particularly a t small colleges, a r e sometimes severe. Although a commercial apparatus is available,' i t is expensive a n d more sophisticated t h a n is necessary for instructional purposes. Many laboratories have constructed their own ao. . paratux, using published circuits ( I ) or modifying a war surplus frequency meter ( 2 ) or commercial apparatus (3).We descrihehere a n apparatus utilizing m o d e r ~ c i r c u i t r ywhich we have constructed from components which are readily available a n d cheap. We have found t h a t undergraduates can quickly obtain d a t a which a r e accurate, reproducible, a n d precise with this apparatus. T h e theory of dipole moment measurements a n d their calculation from experimental d a t a are described elsewhere (4).
RELPXATION
/CAPACITANCE
CELL
I
OSCILLATOR
MFREUNCY 1 CWNTER
Figure 1. Block diagram of dipole moment apparatus
Apparatus A block diagram of the apparatus is shown in Figure 1. Most dipole moments are determined by measuring the dielectric constants of a series of solutions in a nonoolar solvent. The dielectric constant is related to the eaoacitance oEa condenser with the oarticular solution . t,etwen the platrs. A persistent pwhlem is that stray capacitance is frrquentl; d t h e r m e order dmngmtude as t h e e l l enpacitanre.The measured capacitance will he given by the equation
c, = ec, + c,
(1)
where c is the dielectric constant of the substance in the cell, C, is the capacitance of the empty cell, and C, is all other capacitance not sffected by emptying or filling the cell. Any convenient cell may be used (4); the measurements reported were made with a 25-ml cell with C, about 25 pieafarad~.~ The frequency counter can be any available instrument capable of measuring frequencies up to about 100 kHz with an accuracy of 'Kahl Scientific Instrument Corp., Box 1166, El Cajon, California 92022 or General Radio Co., Concord, Massachusetts 01742. 2Model 2TN25. Balshaugh Laboratories, South Hingham, Massachusetts 02043. 3Heathkit Model 1B-1100, Heath Co., Benton Harbor, Michigan 49022. We used Southwest Instruments Model UDI-I and FC-3 in klt iurm, Sc.64.2988 lL9421:Hdstrand. (71 Hsiuentadt. I. F..and Kumler. W G . . Z Phyra. Chem.. 82.428 (19291.
