Topics in..
. Chemical lnstrumentution
feature
Edifed by S. Z. LEWIN. N e w York University, N e w York 3, N. Y. These articles, most of rohkh are to be contributed by pat aulhors, are inlended lo serve the reaa!ers of Uia JOURNALby calling allation lo new developments i n the lhemy, design, or auaiIabilily of chemical labmatmy imtmmenldwn, o i by presenting useful insights and explanations of t0Pi.a lhal are of practical imporlance to Uwse who use, pr leach the use of, modern instrumenlalion and instrumenlal techniques.
XXIII. Design and Construction of Glass VUCUUN~ S~~tem~---~onc~uded -
Roger E. Rondeau, Air Force Moterials laboratory, Wright-Patterson ~ i r ~ o r cBare, e Ohio
Transfer of Gases TJnlike 1 . l ~ forepump and the dilflttiion ~ n ~ n the ~ p Toepler , pump is not int,ended lo prod~~c a ehigh vnrmnn. Its principal function is to transfer gases in the vemnml syst,em to a vessel where the gases ran he measured and/or rollect,ed. The mosl. popular and most oompaot. Toepler pump is shown in Figwe 4. The gas to be transferred enters the piston chamber throngh the inlet tnbe, A . Air is then admitted t,hrough tnhe C and forces t,he mercury through t,he lower dip t,,lbe and into t,he piston chamber. As fhe mercury riaes, i L closes the inlet t t ~ h eand opens the outlet tube hy pushing up on t,he two Hont, valves a t A and R therehy allowing the gases to How t,hrongh the exhaust. When the mercury level reached the float valve : ~ B, t stopcock C is opened to a mechanical pump which evanlat,es t,he nearly empty mercury reservoir. As the mercury level in the pist,on chamber is lowered, float, valve A is opened, thereby allowing the incoming gas to occupy bhe vwmnlnl vrenterl hy the receding mercury. With :i
Figure 6. The Rodder Instrument Company Verthe Toepler Pump. SpecificrrllyAdopted far the Tronder of Small Quantities of Go,. sion of
Figure 5. Design of o Monvolly Operated Toepler Pump. 1. Gar Outlet. 2. Piston Chombor. 3. Air/Vacuum. 4. Hg Reservoir. 5. Go, Inlet.
Figure 4. Schematic Diagram Illustrating Design Principles of the Automatic Toepler Pump.
500-cc piston chamber and s. 30-eeond piston cycle, such a pump will evacuate a Miter volume t o 0.1 percent of original gas in approximately 10 minutes. The manual operation of the Toepler pump is extremely tedious and for this reason it is desirable to operatesuch pumps automatically. A magnetic leak valve a t C and a valve relay box connected to Lhe tungsten wires at D, E, and F will make the pwnp i d l y automatic. By
adjusting the speed of the piston stroke8 with tho stopcock a t C, the mercury is allowed to rise slowly until i t creates n spark at D and makes electrical contact het,ween the wires, D and F. This rontact closes a relay which st,ops the air entering through Cand turnson a mechaniral pump. The mercury level now falls in !,he piston chamber and rises in the reservoir lrnbil it reaches E and establishes electrical contact between E and F. This r.ant,srt shorts out the relay, shuts off the mechanical pump, opens the relay leak, : u ~ dallows nir to enter the reservoir chamber again. The cycle is repeated a ~ ~ t r r ndieally. A relay control box is available from Delmar Scientific (Maywood, Illinois) for approximately $65. The disadvantage of such an arrangcment is that the spark a t D may affect t,he gases being pumped through B. This problem can be circumvented by con~nectinga solenoid valve to a repeat-cycle timer and setting the desired stroke time. No connections are needed a t D, E, and F because the rise and fall of mercury is dependent only on the pumping time and the air leak time. Whether the Toepler pump is operated manually or automatically, extreme care shodd he taken to force the mercury into (Conlint~eda page A512)
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A51 1
the piston ehamber very slowly, especially when the vacuum in the chamber hecomes high. A violent rush of air into the reservoir will cause s. rush of mercury inlo the piston chamber; snd this can break it. If extreme precaution to ensure safety is more important thsn speed, the bobtom of the dip tube in the reservoir chamber
Figwe7.
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can be constricted to prevent the nrernwy rush into the piston chamber. The above type of Toepler pump can be porchssed from H. S. Martin Co. (Evanston, Ill.) and Delmar Scientific for approximately $85, for a 500-cr pomp. A simpler, m a m d l y operated pump, as shown in Figure 5, is available f u r almut half t,his
Diogrom Summmriring Ranger of Useful Functioning of the CommonTypesof Vacuum Gouges.
lournol of Chemical Educofion
price fromEck & Krebs (Long Island City, New York). A Toepler pump designed for quaatitatively transferring gas samples weighing less thsn 40 micrograms is available from n~licroteehServices Co., Los Altos, California for approximatelv $90. It is shown in Figrw 6.
(Confin.wd on page A614)
I I
Vacuum Gauges
I n measuring pressore f r w l nlmuapherit, to 1 0 ' torr, which is the defined lowet. limit of the high vacuum range, many tvoes of eauees are used and the choice of
widely used gauges and t,heoperating pressnre range of esrlr. Although the simple U-tube mercury nmnorneter cannot 1~ considered a high vacuum gauge, i t has been included in the fieure s i l m i t is at)
mercury manometer which measures pressure absolutely. For this reason, the McLeod gauge is usually used to owlibrate the eleetronie gauges and lo test vscuumpumps. I n using a Meleod gauge, urronruus measurements eoold arise from any of the following three common sources of error: dirty glass capillaries, dirty merruly, m d the pressure of condermuble vapors. Fm calibration purposes, the inner walls rrf the gauge's capillary stems, as well as the mercury used, should be absolutely dealt. The glass gauge can be thumnghly cleaned by treat,ing with uitrie acid, rinsing s~~ceessively with xmmoninm hydroxide, distilled water and methanol, and vim~un>baking a t n 1,empemture :tpproximstely 50°C helow the glass softening point (r;~. iiO°C). O w e filled with triplydiitilled mercury, i,he find source of e r r a cat, be eliminated by inserting a culd trap to prevent condensable vapors from enteririg the gauge and producing erroneoody low read'ulgs. The eleetrurlic gauges, on t,heother I ~ B I ~ , are inherently subject t,o many otl~er sources of error including sueh eontributing factors as: electrical leakage and interference, dirty sensing elements, "gassy" gauge walls, and induced chemical der:ompusition within the gauge. I n view of the above, the advantage of using a McLeod gauge as a reference for thermal and ionization gauges is readily apparent. However, the fact that this gauge is r a t l w tedious to operate, does not read contin~ ~ o u d yand , requires a separate rough varuum far evacuating the mercury I.esn.vair, militates against. its exlusive use c m n high vamlum system. bIet,al gauges, sueh ;rs the thema1 awl some ionization gauges require the use of glass-to-metal seals for instdlation an tllr system. Some ionisation gauges, on the other hand, are made of soft glass and call for the use of graded seals for connecting to a Pyrex glass system. Mare will be said later on the types of glass-to-metal and graded seals. Figure 7 shows that n u thennal or ionisat,ion gauge will cover the entire range of pressures; t,heretore, for conveniently and continuously monitoring pressures over a. wide range and a t different points in a vacuum syst,ern, a combination of gauges is desirable. Two and three station ront,rol boxes are available for controlling one or two thermal gauges and one ionization geuge. Such combination control (Conlinwd on page A 5 l f i )
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Journal of Chemicol Education
provides continuous pressure readings to determine (1) the ranghing pressore during pump-down, (2) bhe inlet pressure of the diffusion pnmp, and (3) when it is safe to turn an the ionization gauge. For a much more thorough disoussion of pressure measuring devices, the reader is referred to article X I I of this series, Presswe Measwment by 6. T. Zenehelsky.?
Glass Seals Graded Seals. Vacuom equipment such as gauges, which often need t,o be sealed directly and permanently to a glass rnsnifold, are often constructed of non-Pyrex mat,erial thereby necessitating the use of graded seals. These seals are made from diffe~enl, glasses, starting wilh Pyrex, which each successive glass having a higher, or lower, eoeficient of exprnraion and a lower, or higher, softening point. The series ends with s. glass whielr fuses readily wit,h soft glass or qttsrt,~,as the case may be. Graded seals are available from Corning Glass Works (Corning, N. Y.) andsupplierssueh as Scientific Glass Apparatus Co. (Bloomfield, N. J.), Fisher Scientific Company (Pittsbnrgh, Pa.) and Labglass Ine. (Vineland, N. J.). Pyrsz-to-Sojt Glass. Many types of electronic equipment made of soit glass must somet,imes be joined to Pyrex glass vacuum systems. The equipment. can be attached to a vacuum system hy first buttsealing to the soft glass end of a Pyrex-tosoft glass graded seal. The other end can be either butt,-sealed 1.0 a ground glass joint or "T"-sealed directly to the manifold. This type of graded seal is rather expensive, approximately $!I, since a series of six types of glass must be joined. Pfpz-to-Vycor and Qua~lz. This seal is m,wh less expensive since only three types of glasses are used in the graded sect,ion. The hard glass end of the seal can be fused to either Vycor (96% silica) or fused qnartz (99.9% silica). If the quartz or Vyror apparatus is to be evaew ilted via a ground glass joint connection, the use of s. pyrex joint and a. graded seal is m x h less expensive than the use of a hard glass joint. For example, an 18/9 Vyeor hall joint,, a t this writing, cost $8.31 while the combined cost of n Pyrex ball joint and a I) mm. graded sealisunly $4.16. If length is a factor, these seals can be cut down to the one-inch center portion and still permit, excellent fusion between the lower expansion and the higher expansion ends of t,he seal. A quick way of determining which end of a, graded seal is made of borosilicate glass (Pyrex 7740 or Kimex Kg-33) involves the use of a. solution whose refractive index matches that of the glass (1.474). Either of the following solutions may be used: a. solution containing 16 parts methyl alcohol and 84 parts benzene or a mirtnre of 59 arts carbon tetrachloride and 41
Glass-to-Metal Seals These seals are ext,remely useful for connerling lecture bottles, theremocouple (Continued on page A5181
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Journal of Chemicol Education
ganges, copper tubing, etc. to glass manifolds or t o ground glass joints. Cunversely, they can also he used for attaching glass parts to metal lines or equipment. Ilirect sesls of Pyrex glass tu copper tnhing are available from Lnhg1:~ss Ina. and Scientific Glass Apparahus Company. A highly versatile seal is the Kovar seal whieh is a from Kovar, an alloy of iron, nickel, and cobalt, tu a matching glass. The Kovar can be braaed or soft soldered to many other metals. These seals cost about half as much as the direct copper seals and are available from practically every supplier of scientific glassware. For more infarmatiun on the uses and composition of Kavar metal, the user should contact Stupnkoff Cerninir h hIanufaeturing Company, Latrohe, Pemsylvania. Glass to metal seals consisting of tongsten imbedded a t one end of n Pyrex tube are also available from Scientific Glass Apparatus Company. A tungsten lead forms a vacuum seal t,hrorrgh the glass with a short multistrand copper wire nu the outside for contact t o electrical eirruits. These seals can be used for rep i r i n g automatic Toepler pumps, making gas discharge tubes, etc. One other useful vacuum rormertion worth mentioning is a metal belluws-n one piece expansible and collapsible tohe having deeply folded or corrugated sidewalls. These connections are indispensi~ ble when connecting heavy or v i b r a h g equipment to a glass vacuum system. The bellows act as a. shork ahsvrher and absorh the vibration before it reaches the glass system. In making bhis connectio~l
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to the vnucinrr iyslcm, ii Kovar metnl-loglass seal is USUPIII~used. hIeLd bellows are available from Hobertshnw-Fdtou Contruls Cu. (Knoxville, Tenn.).
Break Seals and "Seal-Offskir" I t iri often iwesssry to i u t r ~ d u c ~ma1 terial into n v:lrnrun line or to remove it from the system without losing vacuum or enposing the materid lo ail.. Uutll operalions e m be perfrnmed t.lrra~ghthe use c,i freak s e a l and "sea-c,Hskiu." A break seal is a tube with a18 easily b~.eekahlepartition, snch 8.: u drawu point, or a thin hobble. A nail or metal weight is e ~ d o s e di n the upper seetian uf the break seal which is then attached to the vacuum manifold and evltc~~ated.The seal is broken by raising the enclosed weight with an external rnitgnet and allowing it to drop. A "seal-ofski" i~ simply a thick wall constriction which facilitates flame sealing under vac,unm. Removal oT eondensnhle or sublimable material from the vacuum system involves condensing it into a. closed lube equipped with a "aealoffski" and Hame sealing at the romtriclion with a n air/oxygeti hand torch. Pressure monitoring is recommended during seal-off in order 11, detect glnss pressure rises due to improper Hame sealing or due to averhent,ed grease joints above the "seal-offski." Break seals are available frunz mosl of (.he laborato~y-supplyhonseu and manufactures of chemical glassware. The cost per break seal varies from approximately $1 to V2, dependiug on the tnhing diameter. "Seal-ohkis" are rmt com-
mercially available rrnd must t11erefot.e be made b y the user.
Manifolds Any length of tubing having several litteral ootlets for t,appinginto t,hevacuruu liue, can be called s manifold. Figure X depicts a simple n~sniioldfor use as a n~ixingor t,ransfer line in organic and radiation chemistry experiments. Each of the outdets shown on the diagram ia med to bring out n point,. The first outlet a t the left is for use with reaction Husks. Note t h a t the connection is made a t t,hr top of the mmifold to eliminate a direct, path to the vacuum line in rase of b u m p ing. The next outlet is a socket joint and is used primarily for evacuating heavy or bulky devices such as a sublimato~.. The hall and socket joint removes the danger of breakage through lateral strains. The third connection, to a break seal, illustrates t,he logic of using the outer part of ground glass joints on manifold cwrneetions. For example, accidental overheating of the constriction would cause the joint lubricant to flow outside the tube instead of inside where it can contaminate the sample. This problem can be rirrumvented by using greaseless joints, Teflon sleeves or water-cooled joints. The mercury manometer is attached s t I he lower end of the slopping manifold to collect any mercury, whieh might get into the manifold. The isolating stopcock at the left should have a 10 or 15 mm born while the pendant stopcocks should be &her 2 or 4 n m bore. The glass tubing used for the manifold should h e t~pproximately 15-25 mm. I t should never be less than the bore of the isolating valve.
Calibrafia: Once the volume of t,he manifold is known, s. simple application of Bayle's Law enables the operator to det,ermine the smaunt af material transferred from one flask t o another in a vaenmn line. The manifold volume is easily determined by attaching a bulb of known volume to the manifold and evacoating the entire system. Dry air is admitted into the manifold volume and the pressure is recorded. The stopcock hetween the bulb and the manifold is opened and this reduced pressure is also recorded. According to Boyle's Law, the ratio of the initial pressure to the find pressure is equal t,o the ratio of the total volume to the manifold volume. Manipulation of this simple equality yields the desired volume of the manifold. Degassing: This term should not he confused with outgassing, which is the spontnneans evolution of gas from a mi+ t,erial in a. vacuum. Degassing is defined as the deliberate removal of gas from a material hy the use of heat under high vacuom. In the case of glass manifolds, degassiug is accomplished by applying a brush-type flame to the glass walls while pumping. Care should he taken in keeping the flame temperature below the softening point of the glass (820°C) in order to prevent the glass from collapsing under the strain of the pressure differential across the wall. Degassing helps t,o produce a higher vacuum by driving off m y ahsorbed vapors which may have previously condensed on the internal walls a t higher pressures or lower temperatures. Leak Detectia: A convenient met,hod of determining whether a real leak is present in a. large vacuum system is t,hrough the use of the cut-off test, sometimes called the isolation test, or rate-ofrise test. This technique merely involves isolating different parts of the vacuum s y ~ t e mfrom the pump and observing the rate of pressure rise. This simple test, also gives the operator an idea of the magnitude of the leak. Undoubtedly the simplest and quickest. way of finding leaks in an all-glass system is with the nse of a hizh freauencv " dis charge coil, also called a Tesla coil. Fit,, the system is evacuated with a roughing pump to a pressure between 10 tom and lo-' torr. I n this pressure range, a hlue glow is produced when the spark coil contacts the glass. With the tip of the T e s l ~coil against the glass, the vacuum system is carefully explored. If and when the spark probe comes within a centimeter of the pinhole or hairline crack, the sparks converge into a bright beam which "points" to the leak. Tesla coils are avail~
~~~
- .
Cold Traps A cold trap can be defined as a vessel designed to contain a refrigerant and used in a vacuum system to "trap" vapors by condeming them on the vessel's cold interior surface. B y design, cold traps create an impedance in the vacuum system thereby obstructing the flow of gas and reducing the pumping speed of the am-eondensahlea. On the other hand, (Continued a page A522)
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when pumping condensable vapors, a cold trap contributes it awn pumping speed hy removing the condensables from t,he system and thus increasing the mean free path of the gas being pumped. Various types of traps are shown in Figure 9. Traps ( a ) , (b), and ( c ) have the same basic desien and are usuallv used fmm oil-eontuminat,ing vapors.
Traps
IJnlike the three trrtps above it i n the figure, it does not require the use of an exi.ernsl Dewar flask since the coolant container is insulated by the vacuwn line. The condensate can be removed or transferred l.hrough the tube at, the bottom of I he trap. Of all the traps shown, (e) u t h s the least resistance to flow and is therefore usually used where high condurtawe is reqttired throughout the vacuum line.
U
Figure 8. A Vacuum System Manifold.Illustr~fing [from left to right] 0 line for connection l o n reaction Rask, a socket joint connection. a break seal, ond mercury monometer connection%
(a) and (bi are eauiooed with removable
ait,hout emptying the mercury well. Trap ( d ) is a less efficient trap, since il has less impedance built into i t ; however, it is ideally suited for dynamic: pmrping and deseration of conde~wshle fluids.
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Trap ( f ) is sonletimes called a condensnlion trap and is most after, used ait,h a t least two other U-traps connected in series to form a condensation train. By proper selection of cooling baths, these trains ran he used to separate efficiently mixtures oi volatile gasses hy fractional condensation ( 4 )(5). ( P o , ~ t i n s e don page 21524)
When evacuating a. vacuum system that has been previously opened to the atmosphere, care should he taken uot to cool the traps too soon in order to eliminate the subsequent possibility of virtual leaks. A virtual leak is one which causes a n exponential increase of pressure with time .when separated from the pumping system. A real leak, on the other hand, causes x ront,iniwus presswe rise when
131 Figure 9.
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Journal of Chemical Education
Figure 10 shows six different types of stopcocks dong with a few words about the uses of each. The last four conventional valves are list,ed in increasing order of their ability to maintain a high vacuum. Stopcock ( c ) is "open" at the top and bottom of the plug while ( f ) has a merr w y seal s t the bop and a vacuum cup a t
IcI
Sever01 Derignr of Cold Traps
isolated from t,he pumps. The primary source of virtualleaks is water vapor in the atmosphere and the easiest way of keepiug it from condensing in t,he vacuum t,rsps iq to keep the traps warm while pumping on thesystem. In this respect, the permanent use of trap (a) in a. vacuum sy8tem is not recornmonded since the inner dip tnhe cannot be cleaned and desorbed wilhont removing the entire trap from t,he vacuum line. Vacuum traps can be obtained from ~ n , vlaboratory glassrwe supplier.
A524
High Vacuum Stopcocks
