RESULTS
Two olefin-free naphthas and one synthetic sample were analyzed. The synthetic sample was prepared by adding known amounts of pure C, and Ca paraffins to a depentanized full range platformate. Table I shows that the reproducibility of the gas chromatographic method, except for the isopentane values for naphtha A, is satisfactory for routine analysis. Analysis of the synthetic sample shows the gas chromatographic analyses to be as good as if not better than the LTFD analyses. LTFD gave results in error for Cqand Cbon the high side. This agrees with the findings of Starr and Lane
[ANAL.C H E M . 576 ~ ~ (1949)], , whostudied the test data from 70 laboratories, performing 8000 tests in a comprehensive testing of analytical methods for light hydrocarbons conducted by the Rubber Reserve Committee on Butadiene Specifications and Methods of Analysis in 1948. Gas chromatographic analysis is faster than LTFD analysis. Sample blending and instrument time totals 1 hour for gas chromatography, whereas LTFD instrument time averages 2.8 to 3.0 hours. This includes 0.1 hour for mass spectrometric analysis of the C5 and lighter fraction produced by LTFD (necessary in about 85y0 of the LTFD analyses studied). Because calculation
times for the two methods are about equal, use of the gas chromatographic method gives a net saving per analysis of approximately 2 hours. ACKNOWLEDGMENT
Thanks are due to Texaco, Inc., for publishing this information; to J. H. Shively, Texaco Research Center, for suggesting the use of ethyl chloride as an internal standard and the reverseflow technique; and to J. A. Estes for suggestions made during the investigation. DIVISION of Analytical Chemistry, 136th Meeting, ACS, Atlantic City, X. J., September 1959.
Rapid Injection and Automatic Refill Pipet H. V. Malmstadt and G. P. Hicks, Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, 111.
has been developed which operates by remote switching to deliver a n aliquot of reagent rapidly in synchronization with other events, and also automatically refills so that it does not need any attention from one aliquot to the next. It can be assembled quickly on a ring stand from a few easily available components. The pipet has proved to be extremely reliable and useful in many laboratory procedures. AFTPET
Figure 2. Schematic switch circuit
of
Manual o r remote r e l a y switch Standard female sockets for p l u g g i n g in delivery a n d refill valves, respectively y S W 1 SPDT microswitch with roller on a r m to contact cam FSW 2 Same as ySW 1 but use only normally closed contacts C1 8 microfarads, 450 V.-electrolytic condenser SE 110 V AC, 100 ma. half-wave selenium rectifier K1 DPDT 1 10 V A C r e l a y M o t o r A 2" X 2 " clutch-gear Molon motor
SI
PD, PR
DESCRIPTION OF PIPET
Figure 1 shows the basic system. One revolution is made by the cam for each injection. During the first half revolution the cam pushes the plunger of the syringe 'upward to deliver reagent through an open delivery valve. Near the top of the upward travel of the plunger the delivery valve closes and the refill valve opens. Then during the second half revolution the spring-loaded plunger follows the cam down to the start position to refill the syringe. The delivery and refill valves and motor are controlled during the cycle by two microswitches and a relay (Figure 2). The switch contacts are shown in their positions before the delivery and refill cycle is started. To start the cycle a manual or relay switch, SI, is closed, which puts a 110-volt alternating current into the circuit. As soon as the voltage is applied, a current flows through the motor and also through the coil of the delivery relay by way of relay connections 1 and 4 and the normally closed section of microswitch 1. The delivery valve is opened and the cam rotates forcing the plunger of the syringe upward and injecting the reagent. Immediately after the cam starts rotating, microswitch 2. which is held open before starting, is closed. As the
diagram
Figure 1. Diagram of delivery and refill system
cam nears the top half of the cycle, microswitch 1 is hit, which closes the delivery valve and opens the refill valve by activating relay K1. When the delivery circuit is opened, microswitch 1 closes the relay circuit and momentarily the coil of the relay is activated by way of relay contacts 1 and 4 and the normally open section of microswitch 1. Upon activation of the relay, contact 1 starts to close across contact 3 and contact 8 starts to contact 6. Opening contacts 1 and 4 breaks the 110-volt alternating current across the relay, but the condenser across the relay stores enough charge during the short activation period to discharge through the relay coil and maintain activation long enough to close contacts 1 and 3. When pins 1 and 3 make contact, the relay becomes self-locking. When pins 6 and 8 make contact, the refill valve, plugged into P E ,is opened. It is opened just before the first half of the cycle is completed. Thus, in each cycle a very small portion of reagent is pushed
back through the refill system, eliminating the danger of air or other solutions being drawn back through the injection system. Even though contacts 5 and 8 in the motor circuit are broken, the motor continues to run because microswitch 2 is closed immediately after starting. During the second half of the cycle, the spring load forces the plunger to follow the cam so that the syringe refills through the refill valve. At the bottom half of the cycle microswitch 2 is again opened and the motor stops. A small 2 2 inch inexpensive clutchgear motor (Type 26 ;\I-2.558, Molon Motors, Inc., Chicago 18, Ill.) is used. It has a high starting torque and a disengagement time of about 0.02 second for the spring-activated clutch. When the field in the coil of the motor is broken, the clutch throws the motor out of gear and the cam stops rotating. After the complete injection and refill cycle and after the motor is stopped, the 110-volt alternating current is removed VOL. 32, NO. 3, MARCH 1960
445
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 1vas 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 syrinps 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 ]vas all 1-mm. capillar>.; later a bulb, B.was introtlucctl for rvork with 1argt.r ianiplcs.. R n-as joiiicd 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 atniosmercury l ( ~ - c is l atljustctl to R1. Tl1.c 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
