A SENSITITT ELECTRICdL PRECISIOS -APPARATUS FOR THE DETER3IIShTIOS O F THE R O I L I S G POIST ELEVATIOS J. S . PL-IlICT. A S D MILDRED 11, H I C K S
Among the experimental methods which have been employed for determining free energy data in aqueous and non-aqueous solutions those involving the lowering of the freezing point and of the m p o r preqsure of the solvent, and the electromotive force? of concentration cells have been most frequently used. These methods are all capable of high accuracy and precision, but each is limited by the experinicntal contlitionp. The freezing point method is conveniently applicable t o only a relatively fen- qo!vcnts. While the vapor pressure and electromotive force methodq may he employed a t any temperature, the former is limited to the study of n few sol\-ents and volatile solutes, because of the difficulty in finding suitable ahsorbents for the vapor; the latter can only be employed for solutions of electrolytes in any solvent when perfectly reversible electrodes are obtainable. From the standpoint of thermodynamics the principle of the boiling point elevation ranks equally with that of the freezing point method as a means of measuring the activity and free energy relations of dissolved substances. It has the one decided advantage that it can be applied t o solutions in all solvents. Except as a means of determining molecular weights of dissolved subqtances, the use of the boiling point method has. heen neglected for many reasons. Among these arc ( I ) the unavoidable superheating of the liquid and vapor; ( 2 ) uncontrollable fluctuations in the barometric pressure during a series of determinations: ( 3 ) imperfections in the Beckmann thermometer, which involve uncertainty in the correction for the emergent mercury thread, the friction in the capillary, the mechanical and thermal hysteresis of the thermometer bulb, and the effect of change of pressure on the bulb; (4) radiation of heat t o and from the bulb: (5) introduction of impurities from the air or stopper and changes in concentration due t o the presence of the solvent in the vapor phase and to the pure liquid returning from the condenser. Until recent years all boiling point elevation determinations have been made with the thermometer bulb immersed in the solvent or solution. The same equilibrium temperature is obtained for the pure solvmt when the thermometer is immersed in the hoiling liquid as when immersed in the vapor. This is due t o the fact that the condensing vapor covers the thermometer bulb with a film of the solvent xvhich in contact n-ith the vapor insures ideal conditions for temperature equilibrium. Tbis is not true, however, n-hen the thermometer bulb is imrnersed in the vapor ahove a boiling solution. It was Cottrell' who first conceived the ingenious idea of actually pumping up the boiling liquid and allowing it t o flow over the thermometer bulb which 1
Cottrell: J. -1m. Chem. SOC., 41, 721 11919).
ELECTRICAL APPARATTS FOR B O I L I S G P O I S T E L E T A T I O N
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was suspcnded in the vapor phase. I n principle, the pump consisted of a small inverted glass funnel with a long stem extending to above the bulb. The vapor forming below the funnel carries with it through the bulb the boiling liquid and pours it in a thin film over the bulb. During transit superheating is removed and the solution film on the bulb is in temperature equilibrium with the vapor. The principle involved in the Cottrell apparatus has since been employed by Kashburn and Read.' Their apparatus and the technique employed were designed to eliminate as completely as possible all source$ of error. They avoided the necessity of making corrections for changes in barometric pressure by using two similar boiling flasks, one of which contained the boiling solution under investigation and the other the pure boiling solvent. They attenipted t o avoid superheating and t o insure quiet boiling hy placing in each boiling flask a small quantity of fragments of porcelain. Both they and Cottrell employed small gas flames for heating. I n this paper we have attempted to develop an apparatus capable of even greater precision and yet one in which the former ohjectionable features are entirely eliminated. T o this end n e have further modified the apparatus of TYashburn and Read in such a way as not only t o employ electrical heating, but also to substitute for the Beckmann thermometer a delicate thermocouple. The potentials thus read give values n.hich are proportional to the differences between the boiling points of the solution and solvent. The apparatus tlevieed is shown in Fig. I . The boiling flask C is provided with a ground-glass stopper L into which is sealed a test tube of sufficient diameter t o admit one of the arms of the thermocouple S. X g1a.s tube J sealed into the flask a t 11-supForts the heating Coil of platinum wire T-. B ic a removable vapor pump by nieans of which the boiling liquid is sprayed through the openings S and over the test tube 11. The openings H and I permit both the circulation of the boiling liquid over the heating coil T' and the exit of vapor bubbles which insures a rapid mixing throughout the boiling liquid. h glass sheath E sealed in a t F prevents the cooling of the boiling liquid flowing through the vents S by the cool liquid returning from the condenser K. The capillary tube D is used for withdrawing samples for analysis. The wires P are the leads which connect the nholc or a part of the thermoelement with the potentiometer circuit, T o insure rapid adjustment of the temperature between the element C and the boiling liquid the tube S is not only charged with naphthalene vapor, hut it is also surroucded n-ith mercury in JI t o a depth of two or three inches. ,J is a small glacs tube CBI lead of the heating coil T' which ic wound about .I, the second lead enters the flask at IT7 and is completely insulated from the first by a narrow glass tube. Since twin boiling vessels are used in this differential method of determining the boiling point elevation, it is necessary that the two heating coils shall have the same resistance. For this reason each coil was made of exactly 30 ems. of uniform s o . 26 platinum wire, Kashburn and Read: J. Am. Chem. Soc., 41, 729 (1919'.
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J. h-,P E A R C E AKD MILDRED JI. HICKS
ELECTRICAL APPARATUS FOR BOILISG P O I S T ELEVATION
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were mounted on a sheet iron base and earthed. K i t h the apparatus thus described it is possible t o detect potential readings to 0 . I microvolt. The thermocouple was made in strict accord with the specifications laid down b y White.' I n boiling point elevation determinations of this kind it is necessary t h a t the thermocouple be calibrated for a temperature interval close t o the boiling point of the liquid t o he used as solvent. I n this work the procedure consisted in placing one arm of the element in pure boiling benzene an? the other in piire boiling alcohol. Each of these boiling liquids was connected
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5
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2.5
-x FIG.2
in turn with a large Dewar flask filled with moist conductivity water ice by one of tIyo reference thermocouples. From the two reference elements we obtained the exact boiling temperatiires of the two liquids: from the difference in these hoiling points we calculated the change in temperature in degrees per microvolt on the standard thermocouple. Benzene which had been shaken with concentrated sulphuric acid until thiophene free was further purified hy repeated crystalization and fractional distillation. X large mass of naphthalene was repeatedly crystallized by first fusing and allowing t o cool slowly: in each step the last IOO C.C. of the liquid was rejected. A long series of soliitions of naphthalene were made up by weight. The angle of refraction of each solution at ~ 5 ~ i o . 0was 1 then determined for sodium light by means of a large Pulfrich refractometer capable of being read by vernier t o 0.1minute. The refractometer readings were then plotted on a large scale as ordinaies against the mol-fractions of naphthalene as abscissae. Khite: J. Am. Chem. SOC., 36, 2293, 2313 (1914).
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J. K. P E A R C E AND MILDRED 31. H I C K S
The equilibrium sample was expelled from the boiling vessel into a clean, dry, glass-stoppered flask, quickly cooled t o 2 j 0 and the angle of refraction determined. The composition of the solution was read directly from the plot, This method of analysis proved t o be very rapid; the data obtained were checked by the pycnometer method and have been found to be very accurate, The data obtained in this test of the apparatus are plotted in Fig. 2 . Owing t o the fact that one of our reference thermocouples proved to be defective in a later experiment we do not wish t o publish the calculated boiling point elevations. The smoothness of the curve and the uniform position of the various points on the curve speaks for the accuracy of the method. I n passing we may say that the curre iq identical in form with that given by Kashhurn and Read. TI-e hope that the method suggested may prove of value t 3 others. Physical Ciiemzsf, ij L a b o m f o r g , The State c1121 ersitij of Ioii'a.
