Rapid Determination of the Vapor Pressure of Lubricating Oils and Hydraulic Fluids R. E. Jentoft, A. A. Carlstrom, and T. H. Gouw Chevron Research Co., Richmond, Calif. 94802
THEVAPOR PRESSURE of a lubricating oil or hydraulic fluid is an important criterion in the acceptability of the product in modern, high-performance engines and braking systems. There is a voluminous literature on the determination of the vapor pressure of pure compounds ( I ) . For any particular compound the same results would be obtained irrespective of the method used. The vapor pressure of a mixture is, however, also dependent on the relative amounts of liquid and vapor in equilibrium as this ratio would influence the liquid composition. Results obtained by a bubble point method would, therefore, be quite different from one determined in a boiling point apparatus having a large vapor space relative to the amount of liquid sample present. Ordinary lubricating oils and hydraulic fluids may also contain minute amounts of light materials. As the vapor pressure of the bulk oil is desired, these traces could have a disproportionately large influence on the results ( 2 ) especially if a static method, such as observed on an isoteniscope, is used. If one considers the type of material to be analyzed, high accuracies are not necessary. The described apparatus is used for routine analyses, and emphasis is placed on speed and ease of operation. About two hours suffices to obtain the 4-5 points necessary to construct the pressure-temperature relationship of the liquid. EXPERIMENTAL
Apparatus. A schematic diagram of the vapor pressure apparatus is shown in Figure 1. The principle follows the design of Hickman et ai. (3) with some modifications. The
described apparatus has been developed to determine the vapor pressure of mixtures, while the Hickman tensimeterhypsometer was primarily used to determine the vapor pressure of single high-boiling compounds. The large vapor space in relation to the volume of the liquid sample reduces the influence of the traces of light ends, most of which will appear in the gas phase and as condensate on the walls of the condenser of the boiling vessel. To counteract the possibility of a pressure drop, a direct connection of the manometer to the boiling vessel is used. Because the mercury manometer is not sensitive to small pressure changes, it is only used for pressures of 20 mm Hg or higher. The oil manometer is about 13 times more sensitive and is used for the measurement of lower pressures. The depicted setup obviates the usual problem associated with the use of these manometers at low pressures-Le., the evolution of dissolved gases in the closed reference leg. Dinonyl phthalate, which has a very low vapor pressure at room temperature, is used in both the oil manometer and in the diffusion pump above the reference leg. Problems of contamination are hereby obviated. The dinonyl phthalate used is a heart cut from a distillation on a centrifugal molecular still. The pressure in the system is controlled manually. Initially the apparatus is evacuated. Then nitrogen gas is bled into the apparatus to adjust the pressure. The pressure of the nitrogen in the sidearm is taken as equal to the vapor pressure of the liquid at its boiling temperature. When the pressure is varied, the boiling point is also changed. The result is a series of corresponding pressures and temperatures. The effect of the inert atmosphere upon the vapor pressure is very slight (4). (4) J. R. Partington, “An Advanced Treatise on Physical Chem-
istry,” Vol. 11, Longmans, Green, and Co., London, 1951, pp. (1) A. N. Nesmeyanov, “Vapor Pressure of the Chemical Elements,” Elsevier Publishing Co., Amsterdam, N. Y . , 1963. (2) R. R. Davison, W. H. Smith, Jr., and K. W. Chun, A.Z.Ch.E.J., 13, 590 (1967). (3) K. C. D. Hickman, J. C. Hecker,andN. D. Embree, IND.ENG. CHEM. ANAL.ED., 9, 264 (1937).
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Figure 1. Vapor pressure apparatus 1014
ANALYTICAL CHEMISTRY
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Figure 2. Vapor pressure data
Heating of the sample is carried out with a hot plate connected to a regulated voltage supply. The bottom of the condenser is so constructed that the cold condensate does not drip on the sheathed thermocouple. The temperature of the boiling fluid is measured rather than that of the vapor because the vapor pressure of the bulk fluid is to be correlated with the fluid temperature. The measurement of the liquid temperature is complicated by superheating phenomena. This is of much greater magnitude for a mixture than for a pure component. As a result, the temperature slowly increases until the hot liquid bumps, momentarily dropping the temperature. The cycle is then repeated. This action obscures the “true” boiling temperature of the product; and the use of a boiling stick, a sintered glass bottom on the boiling chamber, or a glass-covered magnetic stirrer is necessary to reduce this phenomenon. Another manifestation of superheating is the dependence of the boiling temperature upon the rate of heating, particularly at low pressures. If the heat input to the oil sample is increased over the minimum required to cause ebullition, the boiling temperature will also increase. No attempt was made ( 5 ) F. D. Rossini, “Selected Values of Physical Constants and
Thermodynamic Properties of Hydrocarbons and Related Compounds,” Carnegie Press, Pittsburgh, Pa., 1957.
to determine the maximum effect of rate of heating upon the boiling point, but increases of 3-5” F have been observed. A medium rate of heating which causes rapid but steady boiling is desired in all cases. Because of these factors and because of the type of products involved, results are only accurate to within a few degrees. This suffices for most purposes. RESULTS
Figure 2 shows vapor pressure data obtained with this apparatus. The octadecane line has been constructed from published data (5). The circles on this line represent the measured points obtained in this study. The bottom line has been obtained on a 2-ethylbutoxypolysiloxane-based hydraulic fluid. The center line has been constructed through the points obtained on a petroleum-derived hydraulic fluid. The two sets of data for this line were obtained with an interval of more than one year between the two series of measurements. The excellent correlations in all these cases attest to the reproducibility of this method. RECEIVED for review January 29, 1968. Accepted February 14, 1968.
Modified Molecular Sieve Reflux Extractor for Efficient Dust-Free Dehydrations David S. Rulison, Paul Arthur,’ and K. Darrell Berlin* Department of Chemistry, Oklahoma State Unisersity, Stillwater, Okla. 76074
REMOVAL OF TRACES of moisture from organic solvents and solutions can be of prime importance as, for example, in studying reaction kinetics, performing nonaqueous amperometric titrations, and in the trace analysis of complex ions in which water may compete for a coordination site. Many other ramifications of an efficient dehydration technique are evident. In a previous report ( I ) a general method of dehydration via an approach employing a Soxhlet extractor was described. However, it has been discovered that traces of dust from the molecular sieve may catalyze certain reactions of organic molecules during the dehydration process. To illustrate, methyl ethyl ketone acquires a light yellow color (presumably a result of an aldol condensation) when attempts were made to prepare an anhydrous sample. The apparatus illustrated in Figure 1 provides a continuous recycling system which circumvents the difficulty of contamination of the anhydrous solvent by products arising from a catalytic influence of dust from the molecular sieve. Checks were made on solvent purity (after dehydration) by gas-liquid chromatographic (GLC) analysis employing a hydrogen flame detector. EXPERIMENTAL Apparatus. Figure 1 shows the modified apparatus; the left-hand portion of the diagram is similar to the apparatus already described ( I ) . In the present modification the liquid siphons and empties into flask B, instead of returning to the original flask. Thus any molecular sieve (Molecular Sieve
Deceased.
* To whom inquiries should be addressed. (1) P. Arthur, W. M. Haynes, and L. P. Varga, ANAL.CHEM., 38, 1630 (1966).
4A-Linde Co., Division of Union Carbide Corp.) dust and other impurities, do not contaminate the material in flask A . Parts C and D constitute a device for regulating the heating rate of flask B. A platinum band is mechanically held around the shaft of part D,which is a float. The position at which the platinum band is to be placed must be determined experimentally. In part C, two platinum wires (22 gauge) are attached so that their lower ends bend under the edges of the tube which fits around the shaft of part C. Part E is a one-way glass valve which allows flow only from the condenser F to flask A . The upper ends of the platinum wires of part C are connected to a relay which controls the heating of flask B. Two types of relays have been used successfully: a Supersensitive Relay No. 4-5300 available from American Instrument Co., Inc., Silver Spring, Md., and a Thermocap Industrial Control Relay available from Niagara Electron Laboratories, Andover, N. Y . Operation. The apparatus is normally filled through port G. Solvent is then distilled through condenser F into flask A until the latter is about two thirds full. Flask B is then filled to approximately the same level. The apparatus is ready for operation. If it is desired to dry a solution, the solute must be added to flask A before the drying operation is begun. Flask A is heated by means of a heating mantle until the solvent boils. Vapors from the solvent rise and condense so that solvent drops onto the molecular sieve bed. After passing through the bed, the solvent siphons into flask B when a sufficiently high level is attained in the Soxhlet extractor. This process continues until the float D is elevated to the required level to cause the platinum band to make contact with the two platinum wires of part C, thus actuating the relay and turning on the heating mantle under flask B. The vapors from the boiling solvent in flask B enter condenser F, after which the condensed liquid passes through the one-way valve into flask A which is heated continuously. VOL 40, NO. 6, MAY 1968
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