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Vol. 19, No. 1
A Still for the Purification of Mercury' By Harold Simmons Booth and Newton C. Jones MORLEY CBSMICAL LABORATORY, WESTERN RESERVE UNIVERSITY, CLEWLAND, 0.
A continuous, rugged mercury stil2 of high capacity, based on the accepted principles worked out by others, occupying small space, has been designed, constructed, and operated successfully. HE physical properties of mercury are considerably
T
affected by the presence of traces of foreign metals, seriously interfering with its use in accurate instruments and investigations, and it is imperative, therefore, that mercury be of an unusually high degree of purity. The methods used in the past have consisted in mechanical filtration, chemical purification, and as a last step, physical purification by vacuum distillation. All the forms of apparatus for carrying out this last step have been made of glass, and the frequency with which they have broken has caused endless annoyance. To obviate this difficulty the writers have designed and built (on the basis of the experience of many workers) an unbreakable mercury still of unusually large capacity and yet occupying only a small space in the laboratory. Impurities in Mercury Mercury occurs chiefly as cinnabar (HgS), mixed with metallic oxides, earths, bituminous matter, iron pyrites, arsenical and antimonical compounds, and ores of gold, silver, lead, copper, and zinc. I n the metallurgy, hot cinnabar is
either oxidized in air or retorted with lime and the metallic mercury distilled off. Practically no impurities are completely removed from the metal by simple distillation and therefore they remain in the mercury as it comes on the market. Through fouling in the laboratory any of the following metals may commonly contaminate the mercury: zinc, cadmium, tin, Iead, bismuth, copper, sodium, potassium, silver, gold, platinum, arsenic, and antimony. Any method for purifying mercury must be capable of removing all these metals. Methods of Purification MECHANICAL-Mercury as obtained on the market is contaminated with iron oxides from the flask, dirt, and impurities not removed in the metallurgy. A preliminary cleaning is best accomplished by filtering through a pinhole in filter paper, leather,* bamboo,3 cloth,4chamois,6 or s i k 6 Jena glass filters, recently introduced, are also quite satisfactory, CHEMICAL-The greater portion of the metallic impurities may then be removed by prolonged agitation with mercurous 8
This paper is a minor part o f a thesis sub1 Received July 9, 1926. mitted by N. C. Jones in partial fulfilment of the requirements for the degree of master of arts in chemistry in Western Reserve University.
2. Elektrochcm., 2, 76 (1896). Karsten, 2. Instrumentenk., 8, 135 (1888). Hildebrand, J . Am. Chem. Soc., 31, 933 (1909). Moore, I b i d . , 32, 971 (1910). Patten and Mains, THISJ O U R N A L , 9, 600 (1917).
:Bolton, 4 5 6
. . . . . . . .... .. . . . . . . . . .
List of Parts (First number refers t o part number on 6gures)
I
Figure 1-Assembled
Mercury Still
1-6.35 mm. (1/4 inch) steel pipe, 305 mm. (12 inches) long, thread 140 mm. (51/2 inches) one end, standard thread other end 2-6.35 mni. (1/4 inch) steel pipe, 940 mm. (37 inches) long, thread 50.8 mm. ( 2 inches) one end 3-6.36 mm. (1/4 inch) malleable iron elbow 4-6.35 mm. (I/& inch) malleable iron pipe, 305 mm. (12 inches) long, thread both ends standard b 6 . 3 5 mm. (1/4 inch) malleable iron coupling (2 required) 6-6.35 mm. (1/4 inch) gas or air cock ( 2 required) 7-101.5 X 228.5 mm. (4 X 9 inches) cast-iron blind flange, drill and t a p as indicated 8-101.5 mm. (4 inches) cast iron or steel companion flange 9-101.5 mm. (4 inches) steel nipple, 114.2 mm. (41/9 inches) long, thread one end standard 10-31.7 X 101.5 mm. ( 1 1 / 4 X 4 inches) steel floor flange, not drilled for screw holes 11-31.7 mm. (ll,', inches) steel pipe, 750 mm. (29l/g inches) long, thread one end standard 12-19.05 mm. (8/4 inch) malleable iron pipe, 940 mm. (37 inches) long, thread 60.8 mm. (2 inches) one end 13-101.5 mm. (4 inches) malleable iron nipple, 76.2 mm. (3 inches) long 14-50.8 X 101.5 mm. (2 X 4 inches) malleable iron reducing coupling 15-50.8 mm. ('2 inches) malleable iron nipple, 50.8 mm. (2 inches) long 16-19.05 X 50.8 mm. (8/4 X 2 inches) malleable iron reducing coupling 17-6.35 X 19.05 mm. (I/, X 1/4 inch) malleable iron reducing coupling 18-6.35 mm. (1/4 inch) malleable iron pipe, 786 mm. (29 inches) long, thread one end standard 19-31.7 mm. ( l l / r inches) malleable iron nipple, 101.5 mm. (4 inches) long 2 b 3 1 . 7 mm. ( 1 1 / 4 inches) malleable iron cap 21-3.2 mm. ( 1 / ~ inch) malleable iron pipe, 152.4 mm. (6 inches) long, thread one end standard 22-31.7 X 101.5 mm. (11/4 X 4 inches) steel floor flange (4 required) 23-31.7 mm. (11/4 inches) steel pipe, 457 mm. (18 inches) long, thread 305 mm. (12 inches) one end, thread standard other end (2 required) 24-50.8 X 50.8 X 31.7 mm. (2 X 2 X l1/4 inches) malleable iron tee (2 required) 25-50.8 mm. (2 inch) malleable iron nipple, 152.4 mm. (6 inches) long 26-Ring gas burner, about 66 mm. (25,s' inches) inside diameter
INDUSTRIAL AND ENGl'NEERING CHEMISTRY
January, 1927
nitrate solution to which a little nitric acid has been added to prevent the formation of an insoluble basic mercurous nitrate. The mercury is placed a n inch deep in a series of suction flasks, covered with an equal volume of the mercurous nitrate solution, and air is bubbled through by suction for at least a week. The solution is changed daily. To further the purification and to remove the mercurous nitrate solution preliminary to drying the mercury, it is then run through the "tower." I n the apparatus of Meyer' the mercury flows in 6 a thin stream through a fine capillary into a long tube containing dilute (about 1:20) nitric acid or acid livered bright and dry. Atomizers of leather, chamois, cloth, silk, or bamboo have been u s e d s t o s p e e d the process of r u n n i n g mercury through the solution since numerous streams are provided instead of simply one Metals above mercury in the electromotive s e r i e s 1.8 displace the mercury Figure 2-Cap from its salt: silver, gold, and the platinum metals are not removed by this chem: ical treatment. Other met'hods of this kind have been proposed. MohrQ agitated impure mercury with warm dilute sulfuric acid, but the results are less satisfactory than with nitric acid. Ulex and Wildio used ferric chloride. However, the mercury becomes subdivided into a mass of droplets which do not run together, and subsequent treatment is necessary. Briihl" employed a mixture of dilutesulfuric acid and dilute potassium, dichromate, but the formation of insoluble mercury chromate is a disadvantage. Betteli2 claims to obtain very pure mercury by agitation with potassium cyanide and sodium peroxide. The process requires from 5 to 14 days and is expensive. Using the cell
n
amalgam
1
HgNOs ("0,)
1
pure Hg
in combination with the Meyer tower, Patten and Mains6 claim rapid and excellent purification of mercury. The apparatus is unwieldy, however, and appears to be easily breakable. BannerjeeI3 removed lead by rubbing with garlic ! Forbes14 shook mercury with oxygenated charcoal to oxidize base metals and then filtered to remove scums of oxide and charcoal. Dunstanls treated mercury with warm hydrochloric acid to remove zinc. Violettei6 steam-distilled mercury. Craftsi7 drew air through a long column of mercury'to oxidize 2 . anal. Chem., 2, 241 (1863). T h e capillary or atomizer should deliver the mercury beneath the surface of the solution, otherwise the gas film adsorbed by the droplets pass7
8
ing through the air delays contact with the solution. 9 A n n . , 26, 222 (1838). 10 Ibid., 60, 210 (1846). 1 1 Ber., 12, 204, 576 (1879). 1 2 Chem. News, 97, 158 (1908). I J 2. anorg. Cbem., 83, 113 (1913). 14 Cbern. News, 106, 74 (1912). 's Phil. Mag., [51 29, 367 (1890). 15 Crmpl. rend., 31, 546 (1850). 17 Bull. SOC. chim., 29, 856 (1888).
105
base metals. This method causes a great deal of mercury to become subdivided and removal of base metal is incomplete.l* Reduction of the purified mercury compounds to free the mercury as required by such methods is tedious and expensive, and is wholly impractical for large amounts of mercury. Mercury which has been run through the Meyer tower is then completely dried in small beakers in the oven a t 110" to 120' C. for 2 hours before distillation. Distillation of Mercury
Distillation must be the final step in the purification of mercury. Ordinary distillation is accompanied by much spurting of the mercury and the distillate is not free from such metals as zinc, cadmium, lead, tin, etc.I9 I n order to lower the temperature of distillation, WeinholdZodesigned a vacuum still in which the descent of mercury vapors into a Sprengel column automatically preserves a Torricellian vacuum after residual gases have been carried out. Distillation then proceeds with no suggestion of spurting or bumping, but metals with appreciable vapor pressures like zinc distil over even when present to one part in ten thousand.21 HulettI8 has shown that if a small amount of air is bubbled through the mercury during distillation all oxidizable metals may be removed by a single distillation, and non-oxidizable metals, as silver, gold, and the platinum metals, may be removed by two or three distillations. R e q u i r e m e n t s of a Mercury Still
The usual glass mercury stills are slow in operation, are easily broken by the continual shaking of the heavy liquid, and are difficult to repair. It seemed d e s i r a b l e , therefore, to construct a simple continuous still based on the results of previous workers, obviating the difficulties encountered by them but embodying their principles and fulfilling in addition the following requirements:
8
,
'
Figure 3-Stillhead
1-The apparatus-shouldibe rugged and non-breakable. 2-The material should be resistant to hot mercury. Glassenameled iron and steel are such materials; experience has shown that the steel jets in Langmuir pumps do corrode and it seemed advisable that all parts coming in contact with distilled mercury should be glass-enameled. 3-For cheapness and ease of assembly, standard pipe fittings should be used. 4-It should be easily taken apart for cleaning. 5-The design should be as compact as possible. 6-It should be automatic in action. 7-Provision should be made for bubbling air through the boiling mercury to oxidize base metals. 8-A good vacuum should be easily maintained. 9-Only a small amount of mercury should be required for operation. 10-It should have an economical heating arrangement. 11-The suction line should be so constructed as to eliminate the chance of backing up of water (from water pump) into the still, thus causing a n explosion. 12-Distillation should be sufficiently rapid. Assembly of the Still
The still built to conform to these specifications is shown in Figure 1. It consists of six essential parts, the cap carrying Hulett, Phys. R m ,33, 312 (1911). Meyer, Ber , 20, 498 (1887). 20 Carl's Repert , 9, 69 (1873). 2 1 Hulett and Minchin, Phys. R e v , 21, 388 (1905). 18
INDUSTRIAL AND ENGINEERING CHEMISTRY
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the suction line and air inlet (Figure 2), the stillhead (Figure 3) the condenser and reservoir (Figure 4), the reserve supply reservoir (Figure 5), the compensating cup a t the base (Figure 6), and the supports (Figure 7). These parts are mounted and assembled concentrically so as to occupy the least possible space. A few precautions must be observed. Since oil and mercury in the vapor phase together form solid mercury emulsions, all fittings must be freed from oil either by treating for 2 to 3 hours in strong boiling lye solution or by heating to
Vol. 19, No. 1
(No. 300). This union allows the head to be readily taken apart for cleaning. Pipe joints were tried a t this point but were found to leak badly on reassembly. The stillhead might have been insulated with asbestos or magnesia, but the rate of distillation was so rapid that this was thought unnecessary. The condenser and reservoir unit is set in the lower support and pipe 18is screwed in. The assembled head unit is 21 slipped down over the condenser pipe 12 and through the tee of the upper support. The compensating cup (Figure 6) is inserted under the bottom of pipe 18. The vacuum Figure 6-Compensating line is made of glass and connects CUP by means of pressure tubing to the stopcock a t A . Connections to the reservoir and manometer are all glass. Operation of the Still
Figure &Condenser a n d Reservoir
Figure &Reserve Supply Reservoir
redness. Joints must be drawn up securely with litharge and glycerol cement,22and must then be heated to drive out all excess glycerol. Wherever welding is done to eliminate leaks, the parts must be of steel or cast iron, as malleable iron does not weld satisfactorily. CAP (Figure 2)-Pipe 2 is screwed up through the central
tapped hole in the blind flange 7; pipe 1 is screwed down through the other hole in the flange. Tapers on the pipe threads make these joints tight. Remaining parts are screwed on as indicated. The inside and outside of pipe 2 and the under face of the flange 7 are then glass-enameled. STILLHEAD (Figure 3)-In order to have the joint vacuumtight, flange 8 is screwed by machine onto the steel nipple 9; floor flange 10 is likewise screwed onto steel pipe 11. The flange 10 is then welded into the nipple and the screwed joints (8 t o 9, 10 to 11) are also welded in, care being taken to avoid pinholes in the welds. This procedure has been found necessary since ordinary screwed joints cannot be made free from small leaks, if they are heated. CONDENSER AND RESERVOIR (Figure 4)-Parts 13, 14, 15, 16 are screwed together as indicated; pipe 12 is screwed in tightly; coupling 17 is screwed on. Pipe 18, glass-enameled on the inside and for 20 cm. on the outside of the lower end, is not screwed in until the unit rests in its support. The inside of the unit (lacking pipe 18) is glass-enameled. RESERVESUPPLYRESERVOIR(Figure 6)-The reservoir M is a one-liter aspirator bottle (Will Corporation No. 3032). Glass tubes 0, X,and N are fitted in with rubber stoppers. The unit is supported on a shelf behind the still. COMPENSATING CUP (Figure 6)-Parts are screwed together as shown. The entire inside is then glass-enameled. SUPPORTS (Figure 7)-The pipes 23 pass through a 12-cm. steel lath wall, and are made fast by the flange plates 22 on either side of the wall. The length of the screw permits of considerable adjustment. The tees are then screwed on. The lower support is 90 cm. from the floor and the upper support is 69 cm. above this. Adjustment of the height of the top of the nipple 25 from the lower support to 85 cm. is made by using 5.1-cm. (2-inch) nipples, 10.15, 12.7, or 15.25 cm. (4,5, 6 inches) long in the upper tee. The gas burner, 26, is supported around this nipple.
The cap and stillhead are bolted together a t the flange union, using a 2.5-mm. (1/16-inch) Janos asbestos ring gasket 22
Neville, J . Phys. Chem., 80, 1181 ( 1 9 2 6 ) .
Mercury to be distilled is placed in the reservoir, R, with the stopcock N slightly open to the suction, thus raising the mercury until M is almost completely filled, when N is closed. When, during the operation of the still, the level in R falls below that shown, air is admitted to the partial vacuum in M by means of the capillary, X, and the mercury flows down the tube 0 until the tip of X is again covered. While the still is in operation the stopcock N is kept closed. Suction applied at A by means of an oil or water pump draws mercury up in the annular space between the tubes 11 and 12 into the still, 8. At the same time pure mercury placed in the compensating cup, E, is drawn up into the condenser tube, 12. Air is admitted by means of the tube 1 with a forged capillary tip, T,which, with control by the
24
4
\
Figure 7-Supports
stopcock 6, allows small bubbles of air to enter the still through the boiling mercury. Heat is applied under the still, S, by means of a ring gas burner or by means of an electric heating arrangement as described by Hulett.ls The mercury vapors pass down the condenser tube, 12, with the air current and condense, at the same time warming the rising mercury in the outside tube, 11. While this does provide a heat exchange, condensing vapors warming the rising column of cold mercury, the concentric arrangement was used primarily to economize space. The distillate is collected a t G. I n order to prevent mercury vapors from passing out the vacuum line, it is made with the lower end just above the level of the mercury in 12. Greater condensing surface is also provided by this arrangement. A glass tube, T, connected to the vacuum line, is immersed in the mercury in R. The level in the still can thus be observed directly and its proper control assured. The still is capable of distilling over 5 kg. of mercury per hour, Mercury in the reservoir R becomes too hot, however, a t this rate, and the writers have found 3 kg. per hour best. With the reservoir M full, the still runs without attention for 3 to 4 hours. If a good welding outfit were available to anyone building this s t a the construction could be made much simpler and a t less expense. The cost of the still as made by the wiiters was about $15.00, not including their labor. No tests
January, 1927
INDUSTRIAL A N D ENGINEERING CHEMISTRY
of the purity of the product, other than the observation that the resulting mercury was brilliantly clean, were made because this still follows the principles already shown by others to produce pure mercury.
107
Acknowledgment The authors wish to express their thanks to the Ferro Enameling Company of Cleveland for their kindness in enameling parts.
Determination of Thoria in Tungsten Filaments’ By Dorothy H. Brophy and Charles Van Brunt GENERAL ELECTRIC Co., SCHSNBCTADY. N. Y.
HE tendency of tungsten to form complexes with iron, manganese, silicon, phosphorus, and other elements, and the lack of complete information as to their genesis, character, and analytical behavior necessitate a careful checking of all proposed methods for the determination of the purity of tungsten. This, in turn, calls for synthetic mixtures of known composition, and, in many cases, of a history similar to that of those met in the field. Owing to the pressing demands of the industrial situation, the research here reported was not carried very far forward when the need for it first appeared, and, indeed, has not yet been satisfactorily completed. This, perhaps, is largely because evidence rapidly accrued from other sources showed that practically all impurities were, or readily could be, eliminated in manufacture, combining, as this did, crystallization, precipitation in strong acid solution, and finally, treatment in hydrogen and in vacuo a t a temperature high enough to volatilize nearly all other metals. Kew and more specific problems accompanied the discovery that the deliberate addition of certain refractory oxides would prevent “offsetting” or destructiye crystallization of the filaments in use. Later, by a process of survival, thoria came to be used exclusively for this purpose. Within recent years, systematic research has been made on the effect of small additions of many elements. Methods of determination of small quantities of specific metals, usually singly, sometimes in pairs, in the presence of relatively large quantities of tungsten, have been devised. Papers on molybdenum2 and boron3 have already been published. The present paper deals mainly with thoria, by far the most important of the additions from a practicd standpoint, and the only one which has so far withstood the test of service in lamps. The most obvious method-solution in nitro-hydrofluoric acid and precipitation as hydroxide-was quickly abandoned, because as a rule only very small quantities of material, sometimes a single lamp filament, are available and because the precipitate retains tungsten, necessitating retreatment. Alternative dry methods, involving selective volatilization of the tungsten as chloride, promised manipulative advantages in dealing with minute quantities. The reactions involved, however, have required much study, in the light of which the procedure has been modified from time to time. The earliest attempts were based on the fact that tungsten metal is volatilized by moderate heating in a current of Clz, whereas ThOz is not. This, of course, involved the assumption that the thorium is present as oxide in the filament. As the accepted theory of its functioning in the filament involves its presence as oxide, and as the percentage of oxide is usually the thing sought, however, this was not regarded
T
1 Presented at the Intersectional Meeting of the American Chemical Society at Niagara Falls, N. Y., January 29, 1926. Received July 1, 1926. 2 Hall, J . Am. Chcm. SOC., 44, 1462 (1922). 8 Brophy, I b i d . , 41, 1856 (1925).
as important. But unfortunately, the matter does not rest here. I n collaboration with M. C. Lamar, it was soon discovered (1912) that the unsatisfactory results yielded by this method were due to the complete volatilization of T h o z as chloride in a current of Clz carrying WCI6, the latter being changed to oxychloride, which also is volatile. This action of the WC16 is analogous to that of Sic14 on many oxides, except that in the case of SiC14 no oxychloride is formed, but non-volatile SiOz, or a silicate of the oxide acted upon. The oxychlorides of tungsten appear to have no effect as chlorinating agents, and admixture of oxygen or air with the Clz effectually prevented the volatilization of the thoria with the tungsten when the two were not mixed but merely placed one after the other in the heated tube. Misgivings were felt, however, in regard to the application of this method to massive or filament tungsten containing thoria. The reaction with tungsten is somewhat violent, and temperature control therefore difficult. Also mechanical losses seemed possible. Actually, widely varying results were obtained on presumably homogeneous material. It was therefore deemed best to burn all samples to oxide in air or oxygen before chlorination. Carefully checked on mixtures of known content, this method gave excellent results, I t s disadvantage is that it is very tedious, many hours being required for complete volatilization. Pinagel’s work4 had shown that volatilization in pure HC1 gas gave excellent results for the separation and estimation of minute quantities of silica in tungsten trioxide, and trial seemed to confirm the idea that thoria, in common with other refractory oxides, was unaffected by this treatment, while the removal of the tungsten was complete in a small fraction of the time required with chlorine. This method then came into very general use among chemists engaged in lamp filament work. Late in 1924, when much filament analysis, demanding a high degree of accuracy, came into our hands, it fell under suspicion. It was found that satisfactory “checks” could not always be obtained. I n practice, the heating in the gas was usually terminated as soon as the residue in the quartz boat appeared white. On trial, however, a further loss oocurred on prolonging the treatment. This was a t first assumed to be due to residual tungsten, but later it was shown that thoria was being chlorinated and volatilized. The rate is relatively slow, however, and to this must be ascribed in part the useful service given in the past by this method. The data upon which this conclusion is based are given in Table I. The addition of oxygen to the HC1 in about equal proportion prevented loss (Table 11). All the samples in Table I1 are residues actually obtained in analyses of tungsten wire. I n practice, i t is customary to make the first weighing as soon as the residue appears white, but the second weighing not infrequently shows a decided loss, due, no doubt, to traces 4
Inaugural Dissertation. Bern, 1904.
