Measurement of Molecular Weight in Organic Solvents Thermoelectric Method GUY B. TAYLOR AI\D MAURICE B. HALL Experimental Station, E. I . d u Pont d e Nemours & Co., Inc., Wilmington, Del. weights of substances unstable at the boiling points of their solvents, and provides a large extension in the number of useful organic solvents over those having sufficiently stable boiling points for ebullioscopy. Solvents do not require rigorous purification, and in favorable cases a complete measurement may be made with as little as 5 mg. of material.
The object of the investigation was to measure molecular weights in organic solvents by a procedure hitherto reported as useful only for water as the solvent. A method was developed for measuring molecular weights with the same precision as advanced ebullioscopic procedures but near room temperature. It is useful for measuring the molecular
THE
Exactly the opposite is true of the modified apparatus described here. Couples of the new type readily hold pendent drops of organic liquids but not water.
proposal by Hill (6) of observing the temperature difference between two different solutions or between a solution and solvent when opposing thermopile junctions are wet viith the two in an atmosphere saturated with the solvent vapor was considerably improved by Baldes (1-S), who substituted a single couple for the thermopile. The method yields information on osmotic behavior reflected in vapor pressure differences, and hence on the molecular weight of the solute. I t appears to have been used exclusively for aqueous systems (6, 10, 12-16), For molecular weight measurement where the chamber is presumed to have its atmosphere saturated with the pure solvent vapor, with solvent on one junction of the couple and a solution of the nonvolatile substance under test on the other, the underl?-ing principle is t h a t of the ordinary ebullioscopic method. The present paper is concerned with a n adaptation of this method to organic solvents and use of the modified technique in determining molecular weights. The authors' constructions of fine-\Fire couples as described by Baldes and Johnson (S), comprising tiny circular loops held in a horizontal plane, failed to support organic liquid drops although readily holding water.
APPARATUS
The two forms of apparatus constructed are shown in Figure 1.
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Figure 2.
Wiring Diagram for Thermoelectric 3Iethod 1.
2. 3. 4.
5. 6. 7. 8. 9. 10.
Figure 1.
i Galvanometer Knives of double-pole double-throw switch Thermocouple in cell Fixed resistance, 0.001 ohm D r y cell battery Milliammeter, 0-5 ma. Switch Variable rheostat Point-o-lite lamp Meter scale marked in rnm.
The lead-in wires are of pure S o . 18 gage copper and arp brought through the walls of the glass cell by flattening short portions and then sealing through Nonex brand glass, which in turn is attached t o the Pyrex brand glass of the cell through a graded seal. The thermocouple, supported horizontally from A to I), is made of 2-mil wire, B to C" constantan, A t o B and C to D Manganin for cell I and copper for cell 11. To make the junctions a t B and C the upper electrode of a spot welder is shaped to fit into stamped-out thin platinum cups about 1 mm. in diameter and the fine wires are held against the bottom of a cup during welding by the flat lower electrode. The wire on either side of both junctions a chemically inis coated with Teflon tetrafluoroethylene resin (4), ert nonn-etting material, t o prevent liquid running along the wire. The resin is applied from an aqueous dispersion and subsequently baked a t about 300" C. At the t'op of the cell are ti70 small, unlubricated ground joints for mounting droppers (capacity 1 ml. or less) equipped with small rubber squeeze bulbs and fine tips, so arranged that by turning the tips may be brought directly over the cups on the junctions and about 5 mm. above them. The cell is mounted in a thermostated water bath capable of control to 0.003' C. Type I was used in a bath a t 20" C. 11-hen the room was about 25' and hence there was no danger of solvent dew on its exposed part above the water level. Type I1 was operated a t 30" C., but because the part above water level is negligible, it can be operated a t any convenient temperature. The lead wires from bhe bottom of this cell are coated with waterproof resin. The electrical circuit (Figure 2) is the same as given by Baldes
Modified Baldes Apparatus for Molecular Weight
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ANALYTICAL CHEMISTRY
948 ( 3 )with a Leeds b: S o r t h r u p high sensitivity galvanometer ( l y p e 2284, resistance 14.3 ohms, rated at 0.05 microvolt per mm. on a scale at. 1-meter distance) as the registering instrument for the couple. The resistanw of the galvanometer is almut equal to that of thc rest of the circuit. It was found necessary in practice t o mount the galvanometer firnil>- on a pier free from vibration, and t o take considerahle care that no extraneous e.m.f. was generated anywhere in t,he circuit. latter was awomplished only whrn all metal except t o p p ~ i ~ rcbmoved from the circuit and all connections (rscept at thrx galvanometer itself) s-err soldered with a soldri. pix-ing i i o (>,in.i'.against cwppcr.
PROCEDURE
Pure solvent is placed in thc cell to a levrl about 1.5 cm. hr.lo\v the horizontal thermocouple wire and both droppers arc filled with solvent. The assembled cell is placed in the bath. The lead-in wires are soldered to the circuit with the double-pole. double-throw switch open. When the cell and cmntenta are :tt bath temperahre, about half :in hour after mounting, t h r tlroppers are turned with their tips directly over the junctions and several drops of solvcut arc tlvlivered to each. It is then hest to t u n the droppr.r tips awaJ-. .41ternate drops will hang below the cup and be w e p t away by the subsequent drop. The size of the drop does not affecst obsrrvations, even when the drop on one cou le is large and pendent and on the other considerably smaller a n f n o t pendent. .If'ter about 2 minutes readings are taken with the galvanometer by throwing in the switch, first one polarity :inti then the o t h r . The difference in t,lir two readings is rec*ortl(das thr blank correction and is to b e subtracted from all subsequt.nt observations. I n measurements with organic solvents there is a n initial disturbancc. when putting on the drops, then deflections are steady enough for repeat readings ovei' a pwiod of 5 or 10 minutes, though there prohablp is always a c-ontinuous downdrift with time. In a t>-pical case tlle deflec*tions were followed to 80% of the initial valur in 5 hours on :I slightly curved line. If a drop is pcndei!t a?,thcs start,, it will fall off in 21 hours because of the in('rease in size. The drop on t,he solvent junction does not change in visible size during this period. T o calibrate the galvanometer readings in centimeters in terms of microvolts per centimrter, the batter s connected to the test taIr 1.5 minutes, ten or more drops of solution are delivcwd in mi'cession to its junction anti galvanonicter ot)swvatioii,
