from across the circuit by opening S1. This prepares the system for the nest cycle by returning all the switch contacts to the original starting positions (Figure 2 ) . The motor. relay. and microswitches can lie mounted in a small metal box with the shaft prot'ruding through one side for connection of the cam. The syringe. motor-can1 box. and valves can all be clamprd to a ring stand for rapid assembly. For convenience t x o conimercially available 110-volt alternating current solenoid unit's for pinching off gum ruhher or neoprene tubing were used as the delivery and refill valves (Part P S 4330 of SE titcator, E. H. Sargent &+ Co.. Chicago 30, Ill.). The spring around the sy:inge plunger was made liy winding a 1' 4-inch long spring with 10 turns of piano, wire 0.05 inch in diameter on a rod 2 inch in diamctcr. The cam lvas made from a Teflon rod 1' inches in diameter. h 3p 4inch slice \vas cut off and mounted with two w e n - s on a metal plzte at'tached to the motor h a f t . The plunger of the ll
5-ml. syringe travels about 0.90 em. per nil. Because 1.00-nil. delivery \vas desired for some applicat'ions, the round cam was mounted off-cenier t'o provide a linear mot'ion of about 0.95 em. during a half cyclr. LIicroswitch 1 was positioned accurately with an adjustable screw to close the delivery valve antl open t,he refill valve just' prior t o thcx maximum upnard niotion of the plunger. This fine adjustment permits the accurate delivery of a 1.00-nil. aliquot. The speed of motor u e d in all applications was about 42 r.1i.m. Therefor(,. the 1-ml. aliquot is delivered in about 0.7 second and the syringe is rcfilled in the I recently subsequent 0.7 second. ; obtained clutch-gear motor at 60 r.p.ni. n-ill provide 0.5-second delivery time. The precision and quantit>-delivered by the pipct w r c tested by titrating tlie sqiaratc aliquots of standard acid delivered by thr pipet n-ith a standard base. The quantity of solution withdran-n during refill from an accuratc buret was also nieasurcd. In both cases
t'he standard deviation n-asabout 0.009 mi. These results were obtained \vith about 10 inches of gum rubber tubing connecting the delivery and refill tip>. Results should be better tvith shorter lengths of rubber tubing, but this n-as not tested. Various sizes of cams and s y r i n p s can be interchanged easily to provide different aliquots of reagent. The syringe is hest filled initially hy removing it from its cam position and filling it' completely with the desired rcagc3nt. air 1)dhic~sare removed by turning tlic tlclivclry eiiil lip antl tapping the syringe. Then it i': conncctctl to the glass T-joint with n short section of gum ruti1it.r ! i r n('oprenc tubing. V l t h the rc4ll v a l ~ - copen. thcx roagent is forced through rlic, tilling hack to the reagent '-iott,lc iiy prcssi~ig th(> plunger. The iviiigc' is rc1io.itionetl on t h e cain anti th!. pipet op~srated several times to fill anti flush thp systtni completely. The pipct is rcmly now for reliaole and raliici ,ioli\-i~ryof separate aliquots of reaK'cdnt.
A Simple Helium Densitometer R. J.
Kokes, Chemistry Department, Johns Hopkins University, Baltimore, Md.
nieasurenients are much D more troublesome for solid materials than for liquids ( 3 ) . Often ENSITT
such measurements are made in a pycnometer with the voids filled with a fluid of known density. Complications arise, hoirever, nhen the solid material possesses sniall cracks, crevices, and micropores, n hich are not easily filled by the liquids used as space-filling fluids. I n such cases, the best procedure is to use nonadsorbable ideal gases such as helium instead of liquids and to calculate the void space in the pycnomrter by application of the ideal gas laws. Several designs have been reported (4, 6) for such helium densitometers, but these are rather cumbersome in operation and roquire rather large saniplcs. The author has recently used a densitonieter for studies of catalysts ( 2 ) that is simple in design and operation and can be uqed for small samples with an accuracy comparable to that reportrd for the older methods. EXPERIMENTAL
A sketch of the apparatus, except for the auxiliary vacuum system. is shou-n in the figurr. For all such measurements it was necessary that the helium be free of atlsorljahle impurities; tank helium 11-as purified 1,- passage through a degassed charcoal trap, CT! at liquid nitrogen tenipcratures. Heat treatment of tlic sample i n tubp T-e.g., degassing-\\-as readily accomplished 446
ANALYTICAL CHEMISTRY
and 1 niiii. in diameter, backed by a scale. XI. Prior to density measurements t'he cross-sectional arc's of this capillary 11-as accurate!?- iletcrmineci. Thus. tlie volume betn-ecn any two points, say Re antl Ra. could Iic readily computed from the diffcrt.nce in the readings on t'he meter stick. .I[,. .Ihout 50 cm. from the top of L. the left-hand leg of the E-tube, a capillary oil manometer n-as attached through SC2. One end \vag open to the atmosphere. SC3 n-as scaled to the hottom of L . which p(wnitted the. diwitonieter to be connected to the vacuum iirie or to the atmosphere via the capillary, C. Saniple tube T was conntcterl t o L Tvitli a standard-t,apcr joint. The right-hand 1c.g of L--tubc R in the original densitometi3r was all 1-mm. capillar>.; later a bulb, B.was introtlucctl for rvork with 1argt.r ianiplcs.. R n-as j o i w t l to a rcvrvoir, P. rougiilJ- tlic qiz(s of T. 11-liicli ivaq joincd t o anoth(3r capillar lr-tulie conncwterl tlirongh SC5 t o lcvcling liulh. LB. Swir thi, junction in situ by raising a sniall ovcn around ot P antl this c.apillar>- 'I i i ~ f c ~ r r n c t ~ T . (Pretrc3atmcnt of thc sample depends on the ainis of t,lie stud>- €€on-mark. R1.\vas niaclc on thc tubing. In the calibration run tlic (SniiitJ- tuhc. ever. in the tinir required for nicasurcT . i i joined to thi, ilcnsitomc~tcr. I-' ment, ea. 3 minutes. the wolution of and T arcs surrounricd by a n-atc,r bath. gas from tlie saiiiple iiiust he lws than tcm is c,vacuatetl via SC3 about 0.001 cc. STP.) n-itli SC1, - 2 . and -5 t ~ l n w i , 1IcanThe apparatus consisted of t h gas n-liiic. the hc,lium purificatioii train is purification train n-ith a hlo\v-off. BO. purqrd arid trap CT is imnic.rscd in The purified gascs could bc introduced liquid nitrogen. Then SC3 a n d - 4 int,o the densitoinct~erthrough S C l or are closetl. h(sliuni is admitted t o the the gas train could lie purged hy adjustclensitonietcr tlirough S C I , antl the ing the licliuni pressure aliovc atniosl atljustctl to R1. Tl1.c mercury l ( ~ - c is phrric and opening SCl to the air. SC1 !vas connc7ctcd to l t r i iii~-crt'ed p r c s m r of helium in tlir apparatus can 1ir wtiniatcd from tlic position of tht7 capillary L--tuIw ahout 1 nicxti,r long
n i c w u r y in BO. M~Iirn it is sup(+ atniosl)licric S C l is closed anti SC3 is opened t o C to lileed off the excess pre~sure. (Ikcause C is a very fine capillary. 1)ack-tIiffusion of air is unimportant. If C is iniiiicrsed in a licaker of water. one can readily tell when tlw pressure is atmospheric.) Tlmr pressurc is t h m clieckcd by closing SC:3. opvning S C 2 , and rccording the Iivight tlifferttnci?: if any. in the oil maiiomrtrr. Thcn SC2 is closed, -4 and -5 are o p e n d , anti the mercury is raiwil in R iint'il the height diffcrencc in L B aiitl R intlicatm that the pressurt' is again near atinosphcric. (The volumc of I' should have I t e n adjusted so that' this is near thc top of R.) Let us call this point R?. Tlion SC2 is openrd cautiously t o set' if the pressure is equal t o the initial rcading. If not', the nicrcury I t v ~ in l R is adjustcil so that tlic rtbatling on .112 is \i.itliiii 0.5 mni. of the initial reatling. If' tliere has been 110 cliangc in tcnipcrature or atmospheric prcssurv i i i tlw tiiiie rrquircd for tlic olwrvations a t R, ant1 Ry-i.c.. about 2 minutvs-tlic volume of mercury I)etn-cwi R, and R?reprcwnts the vo!umc nicasurenients can be readily csarried out in t,riplicate, because rcvvacuat'ion to a tolerablc jm'ssiirc takes o n l y ahout 3 minutes. Thc reproducihility of triplicatc runs is gcwrally licttrr tliaii =2 mm. in R?. On the tiasis of the calilirated diameter of the> capillary. this Imicans an error of aboiit -0.0015 c r . in a total volume of 8 rr. Thc alio\.c procedure i5 rcpcatcd with tlic saiiiple~ i n T and the level of the mcmxiry in R is now detcrmined to be R3. Thc tlifferrnce in volume lietwen R2 and R3 reprcscnts the rolunie of t,hc sample. arid from thc n-eight the density ?a11 lir tlrternminrtl.
Table I.
Snnmple S i shot i :C~Q
Density Determinations
Ilensity, C+./Cc. Lit. Detd. 8.90 8.93 1 , 989b 1,980c 1.06Yd
1, 9 8 2
1 .9iGe Hg 13.536 13.525 KC1 dried at 140' C. T7alue reported (5') for fused KC1 :it Q
20° c
c.
1-alura obtained on same sample on
different days xhen ambient conditions w r e different.
Value reported ( 5 ) for KC1 at 22" C. after drying in vacuo at 160" C. e Different samplc from same batch as C.
For investigations on catalj sts it is oft?:: dcsirahle to obtain densities with both helium and mercury as the displacing fluid. The difference in these t n o can lie used t o calculate rore volume ( I ) . Although tlie apparatus n a s not actually uscd for this I)urpose, it swms clear that rntwury densities can he obtained by using T as a pycnometer. T can be filled after evacuation by raising LB, so that thc mercury floir s o w r the top of R into T . Results of helium d.nsit! dcterminations for s c v r a l materials are shon n in Table I. On the basis of the detrrniinations of mercury and nickel the accuracy ould appear to be n ithin 0.37,. The case for potassium chloride is not so clear, but w e n in this case the results appear good to about +0,5%. The error in density measurements for any solid depends on the amount of free space in the sample tube; hence,
for a fised-volume pycnomctcr, the error increases n i t h decreasing bulk density. Variations in atmospheric pressure and temperature from run to run are unimportant and errors from this source can be expected only for variations nithin a given run. The results nith potassium chloride bear this out. The same sample examined on diffuent da! s uiider different ambient conditions gave results in agreement t o 1 part in 1000. Check runs n i t h different samplc s agreed to nithin about 2 parts in 1000. This ap~iaratusappears to offu :I promising method for studying c1iangc.s in dcnsity n i t h pretri~itnient. If T and P are the same tcmperaturc, the rcading error will be independent of snniple size; hcncc, 111 principle, volume changes nith, say, hvat treatment on a sample having a volumc of 100 cc. can be detcrmined to 10 1i.p.m. Such accuracy nil1 be attainable if the baronictric pressure is constant during the time required for t h t nieasiircnicnts and the level In the oil manomcter is adjusted to 0.1 nim. LITERATURE CITED
(1) .inderson, It. B., Hall, W.K., Hofer, L. J. E., J . Am. Chem. Sac. 70, 2465 (1948). ( 2 ) Kokes, R. J., Emmett, P. H.?Ibsd., 81,5032 (1959). (:i) Reilley, J., Rae, JV. S., "Physicochemical Methoc
Chem. 166. 257 (1957). (6) Smith, R. C., Jr., Howard, H. C., Ind. Eng. Chem. 34,438 (1912). \
I
A Heater for Gas Chromatographic Columns John N. Roper, Jr., Research Laboratories, Tennessee Eastman Co., Division of Eastman Kodak Co., Kingsport, Tenn.
channeling ivould occur. The column has no nrapped-on heating clement or insulation, so it is easily replaccd or repacked. The heating rate, because of the low lieat capacity of the furnacc, is almost as, fast as if the heating cleiiieiit were n.rapped 011 the column. This makes the apparatus easily adxptahle to the programmed temperaturc-risc technique [Dal Sogare, S..I h i n e t t . C.E,, *Ix.~L. CHEJI.30, 1157 1 1 9 S ) ] saiid it is simple to go quickly froin one opcr:tting temperature to anothcr. The temperature staliilitj- i,s wrprising1)- good, K i t h a minimum of manual control of variablc-wltage transformer scttings. tlie olicrating temlmxture vniies 110 niore than 2" or 3" C. in 9 hours: :inti no cliarige greater than 1' C. 1)cr hour hns Iiecii ohservcd. This is not stnhir enougli for qwntita-
tive peak-height tletcrminations, but it is sufficient for qu:ilitatiT-c work when the method of acldiiig a known compound for peak identification is used or when peak arca is measured for quantitative determinations. CONSTRUCTION OF FURNACE
Thc furnace, niaile from stainless st'eel tuhing. is shon.n in Figure 1. The open ends of t!iP t'ulic, arc. closi'd with dainlcss steel split circl(+ attachcd by s c r e w through overlapping mrs. A . Onc of the ~ n piccc~s d on the l o n - c ~half of the furnacc is flanged for attachment to the dctcctor 1mitc.r litis 1)y slicet metal s c r r m , B . The picccs attached to the uppcr half an' slottrd t o acconimodatc the tuhiiig going to and from the column, so t,hnt the furnace may be opened for rapid cooling n-hilc the column is in placi'. A slottcd, arlj ustahlc suDport,ing nicinlirr. C. is (3rd
VOL. 32, NO. 3, MARCH 1960
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