Topics in..
.
Chemical Instrumentation feature Edited by 5.
Z. LEWIN, New York University, New York 3, N.Y.
These articles, most of which are to be contributed by guest authors, are intended to serve the readers of this JOURNAL by calling attention to new developments i n the theory, design, or availability of chemical laboratory instrumentation, or by presenting useful insights and explanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques.
Thus, (solid)
XXVI. Instrumentation for Osmometry (cant.)
Advances are carrently being made in the development of new membrane materials of lower permeability limits. However, conventiondly used organocolloid membranes show permeability to solut,es of molecular weight below about 20,000. When it is desired to know the osmotic pressnre of solutions containing solutes of low molecular size, indirect methods must be used to measure other colligative properties of the solutions. Nearly all of the methods are designed to measure a temperature effect which is of small msgnitude. Further, precision in such a ease diminishes as the molecular weight of the sample increases, to a practical upper limit of not much beyond 1000. Examination of Table I is instructive (4). The greater magnitude of the physical effect produced in osmosis (col. 4 ) is in marked contrast to that in ehullioseopy or cryoscopy. Table I. Comparison of Calculated Boiling Point Elevation, Freezing Point Depression, a n d Osmotic Pressure for Polymers in Benzene
.
,Trrn~udelrare ar-ailalde r~iuiirini: type 0-62 stiirine rod ~ i t eslranulneter h as either 2 1x1 or 0.2 ml ram~,lss. The Portshle Model lee-salt mixture in imbalance detcetor. iustrument is oalil~rared t o r e d ~ l i fyue Deivar reells in ti,? ranm from U t o :JUOlI h l m k 111 Refrigerator milliusmds. l'lir l l u ~ l e l G-81 Lnrtrument is s1,oun in iriy,,re 17. Xodel CY-1 ~ l ~ ~~ e ~~ i c ~e~a p i ~d~ i b r ~ a t i o nof l .4C ~ bridge ~ ~t j t hampli~ i ~ sample volume of 2 ml or 0.2 ml. .\luremirring rod tier snll mmo-amp elnll eompaitmem t o h e w rarnide%at 6*C above fh? fPm,,erat,,re of ciiolinp "ill1 meter. llstil is also prorrded as part of tile instrument. Tile iurrrnmenr >rrlmo\rnin Figure 18. I f ir crlilirat~d for 301l0 m i i l i ~ ~ m ursnee. ls DC Bridge with D C S&rnpLesire is a i ~ s f1 "11. Xvdel CT-I bfodel CY-2 ir calibrated ior U-I'C, ~ I D I I PCY-2 I amplifier and null is callllrated i u r u-:30~10 milli~,r,,l"l./ meter. k p Time inatmment ir illurrrated in lFi.ore 1Y. I microscope cull1 ararr ir also available and ir s1,oi.n in 1 i m m 20." A sample volume of 2 07 0.2 1n1. is rrOsmette Thermoeleolrio Device Rapid rilm.tion of Battery opersted D C stirring rod ibridge m t h meter as quired. The instrtiment reads d i r w t l i for t h e ranpe from 0 r u XOOO mlllurimldsnoedereotor. m o l d Time inrtrumenr ir lllurlrateil in *.igure 21.
* offer s system f a r the observation of the freering point of nanoliter ssmpies. 1n these instruments a cold pram ir inroryorased wti, mamlfaet,,rers f u r v i a i d observation of the f r e e i n g point; s thermistor probe is again employed for temperature moniroring. Fnr a rwieir of microscup? sceesaoriea and miororeupy s e Lewn, 3. Z.. J. C a w . ED., 42, A566, k610, A775, 4855, A045 (1966).
a
(Conlinued on paye .4301)
A302 / lourno1 o f Chemical Education
pal units mounted on a. common base. The sample chamber assembly contains the various elements of the osmameter, while the control unit houses a Wheatstone bridge, null detector and heater input control circuit,. A sketch of the sample chamber assemhly is shown in Figure 23. I n this instrument two thermistor beads are suspended in a precisely thermostated chamber sat,urnted with solvent vapor. The thermistor beads are two legs of a Wheatstone bridge. The bridge is initially hslanced wit,h solvent drops placed on each thermistor bead. By means of a syringe the measuring thermistor is "washed" with the sample and a drop of solution is then placed on the measuring thermistor; also a drop oi solvent is
Figure 13. Advanced Inrtruments. Inc., cision Research ormemeter.
Hi-Pre-
placed on the reference thermistor. Solvent condenses on the solution drop, because of the lower vapor pressure of the solution. This randensbtion warms the measuring thermistor and produces a bridge imbalance. The bridge is rebalanced when eqidibriurn is reached, and
Figure 14. Advanced Inrtrurnentt, Inc., liter omorneter.
Nano-
the change in resistance is noted. This change in resistance is a relative quantity dependent both upon the solvent and the probe. A different thermistor probe is used for aqueous than for non-aqueous solutions. Care must be taken in the choice of solvents as some solvents will attack the robe elements. The instrument is calibrated with a solution of a. standard compound. Data on an unknown is then compared against the data obtained with t,he standard. The instrument may be used thus to determine molecular weights, asmolalities, osmotic concentrations, activity coefficients, etc., depending upon the type of initial calibration. I t is capable of (Conlinued n p a p A308)
A304
/
Journol of Chemical Educotion
operating with anumher of solvents. The sensitivity of the instrument varies with the solvent being used. Solvents of low heat of vaporization, such a? benzene,
Figure 15.
reference arm thermistor bead. The thermostat probe is also mounted in the sample chamber assembly and each operating temperature requires a separate
Americon lnrhvment Co.. Inc., model 5-2050 freezing paint depression appardur.
toluene, carbon tetrachloride, ete., show a greater sensitivity than those of higher heat of vaporization, such as water. I n the sample chamber assembly, sir syringe guides are placed around the thermistor probes so that five samples may be run sequentially. One syringe channel is retained for adding pure solvent t o the
thermostatic control element. Because the instrument operates a t m isothermal temperature, it is free of complexities associated with the phase transitions of freezing. Two models are offered. Model 301A may be ordered with thermostat and (Conlinved on page A308)
thermistor probe set for any of Lhe following temperature: 25, 37, 60 or 65°C. Model 302 offers the same operating temperature choices as u~ellas 100 and 130°C. Intermediate temperature ranges are provided on request. In both models the temperatore of the ehamher is cantrolled to within +0.00IoC.
Bibliography (1) ROLPSON, F. B.,and Col,~., H., Anal. Chem., 36, 888 (1064).
Figure 16.
J., ''Prineiple~of Polymer Chemistry," Corndl University Press, Ithaca, New York, 1953. (3) sTEnnEn, E,, Z, phVsik, them,, ~ 1 9 0 , l(1942).
/
Journd o f Chemical Educofion
\
Americon lnrtrurnent Ca., Inc ... Micro.romple freezing point depression opporafur.
( 2 ) FLORY,P.
A308
(4) FLORY, P. J., op. cit., p. 272. (5) RAY, N. H., F a d a y Soe., 48, 809 (1952). - .-- ,. (6) RAFIKOY,S. R., PAVLOVA, S. A., and IVERDOKHI~EBOVA, I. I., "Determination oi Molecular Weight and Polydispersity of High Polymers," Ihniel Davey & Co., Inc., New York, 1965. (7) Laboratory Bulletin #3, May 1963, Methodist Ilospitd, Minneapolis, Minnesota.
Addresses of Manufacturers
An slphabetieal listing of the addresses of the manufacturers oi these instruments (Continued on page ASIO)
and suppliers of membranes for osmumett.y follows. These instruments are generslly sold directly by the mnnufscturer m r l not through laboratory supply houses.
Figure 17. Osmometer.
Firke Asroclotes, In..
Model G-62
Figure 18. mometer
Firke Asociater, Inc. Mark Ill or-
Figure 19. lndurtriol lnrtruments, Inc., model CY-2 osmometer.
Figure 20. lndustricl lnrtrumentr. Inc., model CY-3 thermoelectric microscope cold stage with AC Wheatstone bridge for meorwing temperature irerirtoncel of thermistor detector.
(Continued on page ,1313)
A310
/
Journol of Chemicol Education
1' Figure 21. Precision Systems, Ormette loboro. tory otmometer.
Figure 22.
Mechrolob vopor prerrureormometer, rerier 300.
Georgia Avo., Silver Spnng, llarylsnd
7na1n
Figure 23. Block diagram of Mechrolob vopor pressure ormameter: A1 Foam, 81 Aluminum block, C I Chamber, D l Syringe guide, El Thermistor probe. Fl Syringe in down lloodingl position, G,Hl Solvent cup and wick.
A31 2
/
Journal o f Chemical Education
ArRo Lab. Inc., 1107 West Jefferson St., Joliet, Illinois. Dohrmann Instruments Co., 990 Varian St.. S m Carlos. Calif. 94070. Fiske ~ssociates,'Inc., Quaker Highway, Uxbridge, Mass. 01605. Hallikainen Instruments, 750 National Court, Richmond, Calif. Industrial Instruments, Inc., 89 Com-
meree Road, Cedar Grove, Essex County, New Jersey. Meehrolab Products: F & 11 Scientific Carp., Div. of Hewlett-Packard, Rte 41 and Starr Road. Avondsle. Pa.. 19311 Precisian Svstems. 6 Cornell Rbad.'~raminghrtm, Mass. 01704. Carl Schleicher and Schuell Co., Keene, New Hampshire. Welch Scientific Co., 1515 Sedgwiek St., Chicago 10. Illinois.
