ANALYTICAL CHEMISTRY
2036 mond point is exactly opposite the lower side of the meniscus. AS the upper part of block C is just below the meniscus, the bottom of the meniscus appears dark, and can therefore be aligned with the diamond poirlt with great accuracy. test tube is then rotated once or more, while being pressed gently into the V-shape groove and onto the stage. This gives a very fine mark all around the tube. The reproducibility in actual use was determined for 20 tubes, calibrated to contain 100 ~ 1 . Each tube was first weighed dry to 0.02 mg., then placed in a holder on a rack and pinion stage, and filled t o the mark wit'h water from a vertically clamped capillary pipet, The addition of fluid from the pipet was controlled by means of a 1.0-nil. hypodermic syringe, connected with the pipet through plastic capillary tubing. Observing the tube through a magnifying glass facilitated the setting of the meniscus. The tube was weighed again, and the volume of water contained in the tllbe w a ~calciilated from the difference in \\-eight.
The average volume was 99.03 @I., with a standard deviation of 0.72 pl. The constriction pipet,, used to add t,he water prior to marking the tubes, had a calibrated volume of 99.0 pl. .is the inner diameter of these tubes was 4.0 mm., t,his reproducibilitJmeans that the meniscus can be set to nithin =kO.O5S mni. from the mark. Parallax can easil!. be avoided, as the mai~liextends completely around the tulle.
the small elevated tip of the calomel electrode (Figure 1,A). The wells are connected a t the bottom by a narrow channel drilled in from one end only. The portion of the channel not connecting the wells is plugged with a cemented-in rod of Plesiglas. The sample chamber is cleared and smoothed by flushing with Plexiglas solvent. Dimensions for this particular vessel may be obtained from Figure 2. The scale near the left edge of the vessel (Figure 2 4 ) indicatrs, from bottom to top, the levels reached in the wells by 0.25-, 0.50-, 0.75, and 1-ml. samples, respectively. Khen electrodes are inserted to the bottoms of the wells, the level of each of these various sized samples is raised and more electrode surface is bathed by a given sample than is indicated by the scale. Hundreds of check pH determinations with the indicated electrodes have shown that this vessel does not change the accuracy of measurement. For greatest accuracy the sample chamber and electrodes should be dry before adding sample. Also, rather serious eirors may be incurred in the measurement of thin lajers of poorly buffered solutions due to the solubilit? of the gl:1ss.
Plastic Vessel for pH Measurements of Small Samples Kenneth M. Richter, Department of Anatomy, University of Oklahoma School of Medicine, Oklahoma City, Okla.
ONE
principle which seems to have been followed in designing electrodes for p H and electromotive force measurements has been to make electrodes fit the amount of sample avai1at)Ie. Thus, electrodes are listed commercially for large samples, for samples from 3 to 5 ml., for 1-drop samples, anti foi those even smaller. A \
I \
B
Figure 2.
Orthographic projection of plastic pH vessel
This type of vessel has been used for 4 years to make p H measurements on supernatants of 2 ml. and less from roller tube cultures, as well as in the routine preparation of tissue culture media, buffer solutions. and the like. A plastic cell of small capacity has also been suggested by Dietz [Dietz, V. H., Science 108, 338 (1948)] but its shape doos not conform t o that of the electrodes as does the vessel described here. ACKNOWLEDGMENT
Figure 1. Plastic pH vessel A , B. Wells, I 2-inch diamA has b u t t i i n i eter. shoulder. C. Connecting channel D. Cemented plug
A vessel has been fabricated in 11hich the sample chamber closely reflects the size and shape of standard Beckman electrode tips, and which can accommodate samples as small as 0 25 ml The vessel illustrated (Figure 1) is designed for use with the No 4990-80 and S o 4970 Beckman standard electrodes measuring x 51/4 inches. It consists of a block of Plexiapproximatrlv 7 glas, 1 X 1 5 x 2 inches, in nhich two closely spaced wells One well has a shoulder a t the l / 2 inch in diameter are drilled. bottom to conform to and at the same time prevent damage to
The author wishes to thank H. A. Shoemaker, Pharmacology Department, for preparing the vessels illustrated, and Ernest Hiser, Medical Art Department, for preparing the illustrations. This work was supported by grants-in-aid from the Helen Hay Whitney Foundation.
Ultraviolet Scanner-Camera for Paper Chromatography Norman A. Drake, William J. Haines,' Raymond E. Knauff,z and Eldon D. Nielson, The Upjohn Co., Kalamazao, Mich.
R I E F descriptions of scanner-cameras for the visual and photographic detection of ultraviolet-absorbing steroids on paper chromatograms have been published from this laboratory previously (1, 2 ) . Details are presented here for the construction 1 2
Present address, Armour Laboratories, Chicago, Ill. Present addrpss, G. D. Searle and Co., Chicago. Ill.
