The stepper coil, K2, is in series with niicroswitch S5b, which closes shortly after the start of rotation of the stopcock. Each time the stopcock starts a cycle the armature of the stepping relay moves one position. Therefore if the 5 position is dialed on S6, the pipet will recycle to deliver 5 aliquots before terminating automatically. The pipet is immediately ready to deliver the exact number of aliquots preset on the dial. DELIVERY DATA
Typical data for delivery of single and multiple aliquots using a cam cut to deliver 0.9 ml. per cycle are shown
in Table I. These data were obtained by weighing water delivered into a rveighing bottle and show reproducibility of delivery within 0.002 ml. for both single and multiple deliveries. The pipet has been in use for more than a year and the reproducibility has remained the same. During this time the absolute volume delivered per stroke has changed by a few thousandths of a milliliter. This would be expected because of slight changes in cam and syringe. For all applications in our laboratories the delivery of a constant volume for a few days or a week with
errors not exceeding those in Table I has been the only requirement. The actual volume delivered can be checked within a couple of minutes every few days b y weighing aliquots. If i t is necessary to deliver a specific volume, such as 1.000 f 0.001 ml.. over a long period, i t would be necessary to provide a calibration adjustment for the cam. ACKNOWLEDGMENT
The authors thank Verle Kalters for valuable suggestions and construction of the unit.
Versatility of Dynamic Sorption Method for Routine Measurements on Solids K. V. Wise and
E. H.
Lee, Hydrocarbons Division, Monsanto Chemical Co., Texas City, Tex.
HE dynamic rorption method has been used primarily for rapid surface area determinations of solids (9, 11, 12); and in this connection, the reported uses h a w been limitpd to the higher range of specific surface areas, about 1 to 1300 sq. meters per gram. By using krypton, n e have extended the dynamic surface area technique to the measurement of evtremely low surface areas (about 6 sq. em.). We have alqo used the dynamic sorption method for determining chemisorption characteristics of catalysts. Modification of the surfave nrea technique for pore volume niewurciiients has also been described (12). Thc major adan tag^ of the dynamic method over the itatic method are simplicity, speed. and greater versatdity. In general. the dynamic method in\ olver the nieasurement of sorbed gasei by a cooled (or heated) solid from a floning mixture of adsorpt;on and inert carrier gasps If the solid sample is interposd betn een tivo arms of a Wheatstone bridge arrangement of a thermal cwnductirity ccll, tlic amount of sorbed gas can bp easily dcterniined by ccmparing the bridge unbalance due t o sorption to tlic unbalances obtained from ii,jwting k n m n volumes of the adsorption gas downstream from the sample. To niinimize any errors emanating from nonlincw rcsponse-concentration relationships, the calibration volumes should be close to the observed sorption volumes. This work involred the use of a PerkinElmcr-Shell Sorptometer (Modcl 212, Perkin-Elmer Corp., Korwalk, Conn.) n i t h grade A helium as the carrier gas. As little as 10 p.p in.of krypton or 50 p . p m of nitrogen in helium could be detected when a 12-volt battery was the current source.
Table I.
ildsorbent Activated carbon, Pittsburgh BPL Activated alumina
adsorbate Nitrogen
Red Chromosorb
Kitrogen
Pumice Platinum gauze Glass tubing
Specific Surface Areas
Nitrogen
Nitrogen Krypton Krypton Krypton
SURFACE AREA MEASUREMENTS
As shown in Table I, the entire range of specific surfacc. areas n as reliably determined by the dynamic sorption method. Both the nitrogen and krypton sorption measurements w r e made a t liquid nitrogen tempwatures. using reduced pressures of the adsorbates bctween 0.05 and 0.35. Isotherms and specific surface arcas were calculated according to the equations advanced by the RET theory ( 2 ) . The values 4 38 and 5.52 vere used for the areas (in square meters) corered by 1 nil. (STP) of nitrogen and krypton, respectively (7, 23). Adsorbate saturation pressures were determined from vapor pressure-t~,mperature relationships (8, 10); the temperature of the liquid nitrogen bath was determined n i t h a nitrogen thermometer. For the krypton measurements, a mixture of 0 570 krypton in helium n-as used as the adsorption gas. All samples were outgassed in a stream of helium a t 250'
Method Dynamic Static Dynamic Static Dynamic Static Dynamic Dynamic Dynamic Dynamic
Surface Area, sq. M . / G . 1372 1360 218 195 4.6
4.8 0.41 0.40 49 sq. cm./g.
