A n IMPROVED BOILINGPOINT APPARATUS HERBERT L. DAVIS Cornell University, Ithaca, New York
An improered modification of the Cottrell boiling-point apparatus has been d&sed and has given complete satisfaction in various student laboratories. A conclusive test for superheating shows the apparatus to be free from this fundamental and persistent error. Tests of the apparatus have s h m it is capable of duplicating the accepted values for the pressure-temperature curve of water from 25' to 100'. Even more mere tests show
reliable results for concentrated solutions of lactic acid and of silver nitrate over a temperature range but slightly more restricted. A convenient arrangement is described for the determination of boiling points at any desired pressure such as 760 mm. The new apparatus is, therefore, suitable for student use as well as for research investigations.
+ + + HE theoretical treatment of the relation between the molecular weight of a dissolved substance and the rise in boiling point produced has been adequately covered for the dilute solutions and the general case, although there are certainly some factors which become of importance in the more concentrated solutions and whose real nature is as yet only dimly guessed. But even for the most favorable systems there is still a chance for improvement in the experimental method of determining the boiling points. Recently an apparatus was devised to give more reliable results with this determination and the present paper is an account of the development of this apparatus together with a discussion of some severe tests to which it has been submitted. The apparatus has proved itself well adapted for general student use as well as for more refined research applications. Following the work of Beckmann in the design of the diierential thermometer and of a boiling-point apparatus, various schemes were proposed for the determination. The most adequate of these was that of Cottrell who realized that reliable results cannot be expected from any method which immerses the bulb of the thermometer directly into the mass of the boiling solution. The reason for this is that there is always more or less superheatiug of a liquid in contact with a hot surface before the bubbles of vapor actually form and rise through the solution. This question of snperheating is the one which requires more attention than any other in the determination of the hoiling points of aqueous solutions and i t seems to arise from two principal causes. The 6 r s t of these is an inherent reluctance of the liquid in contact with the heating surface to form bubbles of vapor. A common remedy for this is the introduction of large roughened surfaces. A second cause is the fact that bubbles formed 3.5 em. under the surface of water will be about 0.1" hotter than if formed a t the surface. The principal problem in the design of a boiling-point apparatus is the removal of this snperheating, bringing the solvent v a ~ o into r true equilibrium with the solu-
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tion under no hydrostatic head. This problem hecomes even more important when one considers the work of Gerlach,' who showed that the boiling point of a solution is affected greatly by the material of the vessel used and the character of the heating surface. Furthermore, the effect of various salts on the superheating in a given vessel was found to be quite erratic and unpredictable, certain salts increasing it, certain ones having a constant effect, while in others the superheating decreased with increasing concentration of the salt. If salts do thus change the superh~atingof a solution in an unknown and specific way, it is nnsafe to assume that this is a constant error throughout a set of determinations, and the only apparatus that may he used for such experiments is one in which there is no superheatiug of the pure water, or at least one in which this superheating is completely removed. In the apparatus of Cottrell which was a t once greatly improved by Washburn and Read2 the attempt was made to relieve this superheating and to achieve equilibrium by causing bubbles of vapor to pump plugs of solution through a pump tube which discharged on the bulh of a Beckmann thermometer The thermometer bulh was thus covered continually by a falling film of solution in contact with the vapor. In a number of such designs of apparatus it has been assumed that equilibrium conditions are achieved because the temperature remains constant. It is obvious that this condition may be fully realized if there be merely a constant amount of superheating of the vapor. Apparently Bancroft and the writer' were the first to devise a test for the existence and magnitude of this error. In essence this test depends on the fact that if any such boiling-point apparatus truly removes the superheating from a pure solvent, G E ~ A C"Boiling H, Points of Salt Solutions and Comparisons
of Boiling Points with Other Properties of Salt Solutions," 2. end. Ckem.. 26,413-530 (1887). WASHBURN AND READ, "The Laws of Concentrated Solutions. VI. The General Boiling-Point Law;' J. Am. C h . Sac., 41,729(1919). a BANCROFT AND DAVIS, "The Boiling Points of Aqueous Solutions." J. Phyr. C h . ,33, 591 (1929).
the thermometer reading will be the same whether the thermometer is immersed only in the vapor or is covered by a film of liquid solvent in equilibrium with that same vapor. Measured by this test an apparatus such as that proposed by Mathews4 was found to give a reading 0.4' higher when the thermometer was immersed in the boiling water than wheu the thermometer was raised so that its bulb was in contact.with vapor only. The internal electric heating of this apparatus is not suitable for aqueous solutions of electrolytes because of the inevitable electrolysis. By this test also the modiied Cottrell apparatus was found to give readings from 0.03 to 0.04' higher when water was pumping over the thermometer bulb than wheu the pump tube was stoppered and the bulb was in contact with steam only. This means that, although this apparatus is very much better than any form in which the thennometer dips into the mass of the liquid, it is still not completely satisfactory. The present modification of this apparatus does achieve complete equilibrium, as is shown by the fact that the thermometer reads the same whether the pump tube is operating or is stoppered. These differences of temperature may appear small, but if one considers that 0.03' may be three to four times the temperature rise to be observed in a dilute solution, their importance becomes manifest. Further, it cannot be argued that this superheating is a constant error entering into all the determinations as more salt is added. Exnerience in this laboratom has shown that this error is not a constant one but that salts do change the magnitude of this effect. I t is obviously unwise, then, to start with a known error of unknown magnitude and to carry out a set of determinations in which that error is changing in an unknown manner. The only remedy is to start with a system in which that error is absent, that is, to start with a system in which the superheating is effectively removed and a true equilibrium is attained between the vapor and the pure solvent. Only thus can equilibrium between vapor and solution be assumed. In what is said here we have in mind especially the problem of aqueous solutions, those most often used for class demonstration and student experiments. In the case of the organic solvents the superheating is much less of a problem and it is quite possible that the new apparatus will give results not far diierent from older forms. By using a design which succeeds with the most difficult solutions, one can assure accuracyin the simpler systems. In the modified Cottrell apparatus the pump tube was a funnel inverted and suspended near the bottom of the vessel. Bubbles of steam carried up plugs of liquid on the air-lift or percolator principle through a Y-tube with the tops curved inward to throw the liquid and vapor upon the thermometer somewhat above its bulb, the liquid running down over the bulb to return to the mass of liquid. The pump tube was
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Marnews, "Electrical Heating as a Means of Prevention of Superheating of Liquids." Trans. Am. Elechochern. Soc., 19, 81-90 (1911).
sealed to and supported by an inner glass cylinder sheath which protected the thermometer from external temperature changes and had holes a t the top for the escape of steam to the outer main vessel which in turn connected to the condenser. An ordinary condenser and withdrawal tube completed the apparatus, rubber stoppers or ground glass joints being employed as the purpose and purse of the user dictated. It was believed that the failure of the Cottrell apparatus to remove superheating completely is due primarily to a too brief contact with an insdcient amount of the liquid as well as to the hydrostatic head under which the vapor bubbles are formed. To remedy these defects the superheating is in part relieved by causing the bubbles to pass freely through a large mass of the solution before entering the pump, the rest of the superheating being then removed in that tube. For this purpose the pump tube was shortened and the funnel lengthened so that vapor bubbles after passing freely through about half the liquid are caught in the pump. This modification resulted in complete removal of all superheating from pure water, true equilibrium conditions being achieved. Various designs have been developed and have finally led to the one shown in Figure 1 as the most ~enerallyuseful.* This involves a raised position of * For aid in the driim of theapparatusand for itsconstruction I amdccply indebted to hlr. .\. L . Brandt, thcskilled glass blower of the Baker Laboratory of Chcmiatry. Corncll i'niwnity.
the funnel portion of the pump tube and an elongated portion to catch as many bubbles as possible. The funnel should be made as large as can conveniently be placed inside the main tube since escaping bubbles do no pumping. Experience has shown that a single ann pump tube as shown is as effective as the Y-tube of the older apparatus, is easier to make, and is very much less fragile. The purpose of the bulb blown in the main tube is to minimize volume changes of the solution with heating and with additions of salt as these volume changes do have a slight effect on the readings obtained. Ordinarily the apparatus will be filled so that the liquid when hot shall come about to the outlet of the sheath tube, the size shown requiring about 70 cc. Sealing the condenser to the main tube makes an awkward and fragile apparatus and where rubber stoppers are to be avoided ground glass joints are preferred. The outer apparatus may well be of Pyrex, the pump tube of ordinary soft glass, and the condenser an ordinary 20 cm. condenser. If the soft glass condenser be ground into the main tube it will be found that its greater expansion will make with the Pyrex apparatus an especially firm, tight joint. The sheath tube may well have near the top one or two holes for the escape of steam and the apparatus should be set up so that the pump tube is on the opposite side to the reflux from the condenser. Smoother operation is achieved with the help of some of the standard anti-bumping devices. After trying out indentations in the bottom of the main tube, and various objects such as beads, silicon carbide crystals, and boiling tubes, we find that most effective of all is to seal through the waU of the apparatus a short length of platinum. This is heated directly by the flame of a small-sized burner and makes for excellent bubble formation and quiet boiling. I t is convenient to hold the apparatus by a suitable clamp. Below it a piece of asbestos board is placed on a ring and the flame plays on the apparatus through a 1-cm. hole in this board. An asbestos cylinder surrounding the burner and a glass cylinder around the main apparatus minimize temperature fluctuations due to stray air current.
that their instructors report that the determination of molecnlar weights by boiling-point elevations is one feature of the laboratory which is adequately provided for. In addition to serving these student needs, the apparatus has been applied to research problems in which the tests have been quite severe. Mr. A. L. Avens and Mr. W. B. Kunz of the Cornell Laboratory have attached such an apparatus to a manometer. By means of a vacuum pump the whole system is evacuated, heat is applied, and boiling achieved with pure water as low as 25'. Good agreement with the accepted values for the pressure-temperature curve of water is realized over the entire range of 25-100". We know of no equally simple and accurate method of determining these values for any pure solvent. An even more exacting test has been the use of aqueous lactic acid solutions of concentrations up to twenty per cent. Such systems are notorious for terrific bumping even in open beakers. The use of the platinum wire has minimized this and has permitted pressure-temperature runs on such systems from 25' to 100". Below 40" the boiliug is not quite so steady as could be wished in these lactic acid solutions, but above 40' the constancy of the reading is quite satisfactory. An easier problem was met in the case of silver nitrate solutious, the data for which will be published later by Mr. Avens, while Mr. Kunz will report on the lactic acid solutions. We have made another arrangement of the apparatus that will interest many people. I t was desired to measure the boiling-point rise of some fairly concentrated aqueous salt solutions without the necessity of correcting for the varying barometer. Duplicate sets of apparatus were joined through their condensers to a common three-liter bottle provided also with a tube carrying a stopcock. Both pieces of apparatus were filled with water, heated to boiling, and the temperature readings taken, the whole system being open to air. From the observed barometer reading it was possible to calculate for each Beckmann what the reading would be if the external pressure were 760 mm. In the actual runs one apparatus was filled with water and the other with the salt solution. After both were heated to boiliug, pressure was applied through the stopcock until the apparatus containing FURTHER TESTS AND APPLICATIONS OF THE APPARATUS only water showed the reading calculated for 760 mm. Smce this apparatus was 6rst designed and used for The reading on the oth& thermometer minus the the determination of the boiling-point rise due to com- readmg calculated for 760 mm. gives the rise in boiling mon salts in aqueous solutious, it bas been in constant point due to the solute present-in a system very use in the regular laboratory sections in physical definitely boilmg under 760 mm. pressure. I t is not chemistry a t CorneU with perfect success. This ex- generally realized that a thermometer thus used is perience has been confirmed by a number of other far more sensitive than the ordinary barometer as a laboratories.* Students achieve very satisfactory re- pressure indicator. This procedure avoids the necessults with the apparatus, 6nd it durable and convenient, sity of correcting for change in the barometer and is and obtain constancy and reproducibility of results, so probably the most accurate method of determining * Far about two years the Fisher Sdentific Company has been the boiling-point rise in solutions. Data for some salt solutions will be reported later by Mr. L. P. Gonld. making and selling a satisfadory form of this apparatus.
