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A PHYSICO-CHEMICAL STUDY OF MERCURY NUMEROUS investigations on sodium amalgams have already been carried out, and the literature on this subject has become very extensive. The greater part of this refers to dilute liquid amalgams containing small quantities of sodium. Comparatively little work has been done on the solid amalgams. An excellent résumé of the work up to 1904 is given in Abegg's "Handbuch der Anorganischen Chemie," vol. ii. From all the investigations the conclusion is drawn that several compounds of sodium and mercury exist. The number and composition of these compounds, however, are not known with certainty. Below is given a list of the formulæ suggested by various investigators. The need of further investigation is obvious. It is well known that the ordinary valencies of the metals are not exercised in metallic compounds, and no satisfactory explanation of the nature of the union has been put forward. It was thought that an investigation of the specific volumes might throw light on this problem. A dissertation has recently appeared by Bornemann ("Metallurgie," 1909), in which it is stated that the only methods for fixing with certainty the composition of these metallic compounds are those commonly employed by metallographers, namely: 1. The investigation of the thermal diagram. 2. The study of physical properties other than freezingpoints. 3. The preparation and examination of micrographs. All three methods have been employed in the present work. The Investigation of the Thermal Diagram. Preparation of the Alloys.-The method of preparing sodium amalgams commonly employed, namely, by plunging pieces of sodium under mercury, is unsuitable and unnecessary. Kurnakow melted sodium in a weighed retort, through which a stream of dry hydrogen was passing. A sufficient quantity of sodium was then poured off into an iron cylinder containing a little molten paraffin, and the retort was again weighed. The requisite quantities of mercury were then added to the sodium. * A Paper read before the Faraday Society, March 14, 1911. oil. Mercury was then added to the molten sodium. In the present work the sodium was melted in a current of dry carbon dioxide, and caused to flow into a tube which had been previously weighed, and which was also full of carbon dioxide. The apparatus is shown in Fig. 1. A stick of sodium was placed in A, and the tube heated with a Bunsen burner until the sodium melted; on slightly diminishing the pressure in B the clean metal entered, leaving a shell of oxide in A. When cool, B was quickly stoppered and weighed. The stopper was then replaced by another of rubber, trebly bored. Through the wide central hole a glass tube, closed at its lower end, was passed. This tube contained the thermometer, and also served as a stirrer, the rubber stopper allowing sufficient freedom of movement. This mode of stirring was found necessary, owing to the fact that the amalgams adhere to glass and render it opaque. Through the other holes inlet and exit tubes for carbon dioxide were passed, the former being provided with a stopcock, the latter also serving for the introduction of mercury. before and after. In preparing amalgams in this manner the sodium must be fused and the mercury added at first in small quantities; later, it may be added in large quantities, provided the alloy is kept molten. This was added from a small burette, which was weighed (For ordinary purposes amalgams of all concentrations can be prepared as follows:-The badly oxidised surfaces of a piece of sodium are removed, and the sodium then dropped into ether containing traces of alcohol; ordinary methylated ether diluted with a large quantity of light petroleum answers very well. In a few minutes the in molten paraffin contained in a crucible or test-tube, sodium becomes quite bright; it is removed and immersed heated to 100° C., and the requisite quantity of mercury then added). Determination of the Freezing points.—It will be shown later that the freezing-points extend over a very wide range of temperature, namely, from -47° C. to +360° C. The heating arrangement for temperatures up to 250° C. consisted of a very large boiling-tube filled with olive oil, provided with a stirrer, and illuminated from behind by means of an incandescent burner. For temperatures between 250° C. and 360° C. the tube was heated in a bath of fusible alloy. For temperatures below o° C. the tube was placed in a boiling-tube containing light petroleum, and the latter cooled in a vacuum vessel containing a mixture of solid 85 0°C 700 ab | carbon dioxide and alcohol. The alloy was heated until entirely liquid, then allowed to cool slowly, meanwhile being vigorously stirred. The temperature was read every minute or half minute. In nearly all cases superfusion was observed, and the points determined are the maximum temperatures reached after over-cooling. The thermometers used were small Anschütz normal thermometers, each having a range of 50° C. and graduated in fifths. Temperatures below o° C. were determined with a pentane thermometer graduated in degrees. These thermometers were previously standardised. Corrections for the exposed stem were obviated as far as possible, as these CHEMICAL NEWS, April 21, 1911 Physico-chemical Study of Mercury Sodium Alloys. thermometers passed entirely into the experimental tube, and the latter was well immersed in the oil-bath. At the highest temperature obtained the correction did not exceed 1° C. The amalgam concentrations and temperatures of solidification are given in Table I. Throughout the work the former are expressed in atoms of sodium per 100 atoms of the mixture. The cooling curves of some of the alloys showed a second arrest. This is given under T2 in the table. TABLE I. (continued). 183 No. Conc. T1 (°C.). T2 (°C.). 64 20'7 234'2 65 19.09 200'4 66 18.84 182.4 156.2 67 18.34 156.2 Discussion of the Freezing-point Diagram. The concentration-temperature diagram obtained by plotting the above results is shown in Fig. 2. It contains nine branches. Starting with pure sodium at A, addition of mercury causes a lowering of the freezing-point until the eutectic-point B is reached. This occurs at a concentration of 85.2 per cent sodium, the eutectic temperature being 21.4° C. Further addition of mercury produces an elevation of freezing-point due to the formation of the compound Na3Hg. On either side of B alloys show the second freezing-point, namely, that of the eutectic 21.4° C. At the point c, concentration 83.4 per cent sodium, the freezing-point is 34'4° C. The cooling curves of alloys in the neighbourhood of c, but containing larger proportions of mercury, show three arrests, the first being above 34'4° C., the second 34'4° C., and the third 21.4° C. The temperature 34'4° C. appears to be that at which the compound Na3Hg undergoes a polymorphic change with evolution of heat. The initial freezing.points continue to rise as the percentage of mercury increases, reaching the maximum temperature of 353° C. at a concentration of 33'3 per cent sodium. Five new phases appear in the construction of this portion of the diagram. There are probably five different compounds of sodium and mercury. It is, however, difficult to decide the composition of these compounds with certainty, as the breaks in the diagram do not occur at concentrations which allow us to assign simple formula to these compounds, except in the case of NaHg2, which occurs at a concentration of 33.3 per cent sodium. The temperatures, concentrations, and probable formulæ of the other phases are given in Table II. The results obtained by Kurnakow and Schüller are given for comparison (Table II.). The portion of the diagram to the left of the maximum point is simpler. The freezingpoints now fall with great rapidity as the mercury concentration increases. Two breaks occur the first at a concentration of 18 per cent sodium and at a temperature of 156° C. this is probably the compound NaHg4; the second occurs at a concentration of 27 per cent sodium, and at a temperature of - 41.6° C., a few degrees below the freezing-point of pure mercury; this is the eutecticpoint due to the lowering of the freezing-points of mercury by the addition of sodium. TABLE II. with the alloy, and removed and replaced by another when necessary. The alloys were made by adding mercury to the molten sodium as in former experiments. The tube containing the alloy was heated in a large beaker of castor oil until the alloy was quite molten. It was then allowed to cool very slowly. The time during Transformation temperature. which the temperature of solidification remained constant Phase. Sodium. Eutectic. Na3Hg. 63°3 118.5 Na3Hg2. NaHg. Na Hgs (?). NaHg2. NaHg4. Eutectic. 38.6 Mercury. Schüller. Sodium. Eutectic. Transformation temperature. was observed. In this way the horizontal and vertical lines of the thermal diagram, Fig. 3, have been obtained. The results are given in Table III. Fig. 3 is the thermal diagram obtained by plotting these results. It will be noticed that the eutectic horizontal at 21.4° C. extends nearly to the axis of temperatures on the right and to a concentration 75 per cent sodium on the left. The horizontal through the transformation temperature 34 4° C. also extends to 75 per cent sodium on the left. The second branch of the diagram has a maximuni at 75°2 per cent sodium. These three facts prove con. clusively that the compound Na3Hg is formed. Fig. 5. Na3Hg. Na3Hg2. NaHg. Na12 Hg13 (?). NaHg2. NaHg4. Eutectic. 38.6 Mercury. Kurnakow. Fig. 4. It is obvious from Table II. that there is excellent agreement between the present work and that of Schüller. The temperatures now obtained are generally intermediate between those of Kurnakow and Schüller. The former used mercury thermometers graduated in tenths for temperatures up to 200° C.; for higher temperatures the thermometers were graduated in degrees. The latter also used mercury thermometers graduated in degrees above 10° C. The lower temperatures were determined with an alcohol thermometer or with a thermo-element. Two reasons may be suggested for these temperature differences (1) the uncertainty of the stem correction; (2) the effect of oxidation. It has already been stated how these were obviated in the present work. The effect of oxidation is to shift the diagram to the left; e.g., a break has been found at a concentration of 63.3 in the present work, whereas that of Schüller was at 61.9. In the course of the work on the specific volumes in the liquid state described in the subsequent pages, a simple method of preparing sodium perfectly free from oxide was found. Consequently, I have re-determined the freezingpoints of alloys containing from 100 per cent sodium to 48 per cent sodium, over which concentrations the greater number of changes occur. The sodium was drawn up into a pipette at a temperature of 130° C., using the apparatus in Fig. 5. The pipette was removed, quickly wrapped in cotton-wool, the lower end dipped beneath the surface of some liquid vaseline contained in the experimental tube, and the sodium allowed to run out of the pipette by opening the stopcock. The sodium thus obtained had not been in contact with air or moisture at any time, and presented an appearance similar to that of mercury. The presence of vaseline prevented adhesion of the metal to glass, consequently an ordinary stirrer of glass rod could be used. The thermometer also could be placed in immediate contact The intersection of the second and third branches of Fig. 3 at the point D is now at a concentration of 73.9 per cent sodium. In the previous diagram, and also in that of Schüller, the point D was at 71.9 per cent. At a concentration of 714 per cent Schüller states that the compound Na,Hg2 is formed. Fig. 3 shows that this is erroneous. The cooling curves of alloys containing from 75 to 63 per cent sodium show as many as three and four arrests. It appears that these solid alloys undergo polymorphic changes with evolution of heat. The horizontal through D also extends beyond 62.5 per cent, hence the formula Na3Hg2 has been assigned to the compound formed between D and E. Alloys along the branch E F show the |