Recording Buret David M. Miller, Science Service Laboratories, University Sub P. O., London, Ontario, Canada the widespread use of the as a measuring device, few attempts to make it automatically record have been reported. Teitelbaum and Berg [Z. anal. Khim. 8, 152-7 (1953)j used a mercury-filled gas buret with a resistance wire traversing its axis, hut their system could not be applied to aqueous solutions. Gordon and Campbell [ANAL.CHEM. 29, 170f-8 (1957)j described a second apparatus in which the weight of the column of liquid in the buret is recorded. A third method of operating a recording buret is given hrre. A platinum wire probe is lomrrcd mechanically into the buret until i t makes electrical contact with the m e niscus of the titrating fluid, which must be electrolyte. As the meniscus drops, the contact is broken, activating an electromechanical system which further lowers the probe until contact is remade. A recording device may be linked mechanically or electrically with the movement of the probc which follows the meniscus in a stcpwise fashion. The accuracy of such a system will depend on the linearity of the relationship of tho distance the meniscus drops to the volume of titrant drained from the bnret, and the ESPITE
D buret
linearity of the linkage between the movement of the prohe and the rccorder pen. The apparatus is shown in Figure 1 and a schematic plan below section Y Y of Figure 1, in Figure 2. The probe, P, consists of a glass tube 3 mm. in outside diameter and long enough to reach the bottom of buret tube B. The bottom of P is scaled with a nonwettable material such as deKhotinsky cement, through which a piece of 30gage platinum wire passes. The diameter of the deKhotinsky plug is only slightly smaller than the internal diameter of B, so that the wire is held near the center of B. A heavy gage copper wire silversoldered to the platinum passes up through the probe and connects to the top of extension arm A . Elcctrical connection from here to the electronic relay is made through the loose wire helix, H , and shielded cable C, while a second wire, Z, sealed through the glass at the bottom of B providcs contact with the titrant. The bottom of A is attachcd to a brass weight, Ti', which is firmly held between the upright supports by a set of eight roller hearings so that only verticnl motion is possible. The 3 2 gage Chrome1 wirrs, S, which support IP, pass over pulleys at the top of the
frame and then are wound on a spindle, M , at the bottom. A ten-turn potentiometer, R, is connected directly to the spindle shaft and is rotated as the weight is raised or lowered. The spindle is threaded (16 threads per inch), so that S , by winding in the threads, is pullcd uniformly. Further, the diameter of M at the bottom of the threads is such that 10 turns lower W the full distence allowed by the frame. A flywheel, F, geared to turn four times as fast as M. slows the
I
U
Figure 2. anism
Brake and spindle mech-
id constant voltage supply Olt. "Oltl
entiometer
n
nent transformer noid
)-ma. 130-volt
VOL. 30, NO. 12, DECEMBER 1958
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initial descent of W . A toothed wheel, U (Figure 2), against which the rubber face of a soft iron bar, N , is forced by a spring, acts as a brake which may be released by energizing the solenoids, GG. The pulley, spindle, and flywheel shafts are all ball-bearing mounted to ensure smooth operation. The electrical controls, housed in the steel chassis at the bottom of the apparatus, consist of a thyratron relay and stabilized power supply as shown in Figure 3. As long as contact is maintained between 2 and P, the thyratron, VI, cannot fire. (The secondary leads from T may have t o be reversed if the tube does not cut off when first tested.) Breaking this contact removes the bias and VI can pass a current through GG. Sz may be closed to hold the brake on n-hen refilling, etc. The voltage reference tube, Ti2, maintains a constant voltage, independent of line fluctuations, across the voltage divider, Rd, P I , Pz, and R. The values shon-n are for use with a 20-mv. Brown Electronik recorder (Brown Instruments Division, XinneapolisHoneywell Regulator Co., Philadelphia] Pa.) recording from left to right. Maximum voltage is fed into the recorder when the buret is full but as it empties and R is turned, the voltage is reduced (the contact on R moves upward in Figure 3). In the resulting recording the vertical axis represents the amount of solution delivered rather than the amount in the buret a t any time. PI is adjusted by means of a screwdriver, to give a total deflection to the recorder needle for total emptying of the buret. Once the buret has been calibrated, PI should not be changed. P f , on the other hand, adjusts the zero setting of the recorder, so is provided
with a knob for adjustment after each filling. The components on which the fidelity of the recording depends and their tolerances are as follows: Buret tube B is made of precision bore Pyrex tubing, l/4 inch &0.10/, inside diameter (catalog 30. 9600, Corning Glass Works, Corning, N.Y.). The ten-turn potentiometer has a linear accuracy to = t O . l % . The 5651 voltage reference tube maintains a potential of about 87 volts with a variation of less than 0.1 volt for normal line voltage fluctuations. The theoretical accuracy of this apparatus then should be 0.3%. To test this, the buret )vas charged with 0.liV hydrochloric acid and fed through a Radiometer Titrator (Welwyn International, Inc., 3355 Edgecliff Terrace, Cleveland 11, Ohio) into a beaker containing 500 ml. of distilled water. A 0.1N sodium hydroxide solution was leaked a t constant rate from a Xariotte bottle into the beaker and the level of solution kept a t a constant height by sucking off the extra liquid through a water aspirator. The titrator was set a t pH 7.00 and a recording made of the acid consumed as a function of time over the total range of the buret. This was repeated five times, giving six lines, none of which varied from linearity by more than 0.27, of the total volume of the buret, a value which should be the lowest limit of accuracy for the apparatus.
or two such determinations are sufficient. When alkali is used as the titrant, nitrogen gas or carbon dioxide-free air may be passed through the side arm, K , to prevent carbon dioxide diffusing into the solution. For weak electrolytes, such as acetic acid, for which the resistance of the solution is too high to operate the relay because of the distance between P and 2,both contacts may be brought in through P , so that they are about 1 mm. apart. Contact of the probe with the solution will then bridge this gap. The buret described has been in operation almost continuously for a year and has given a minimum of trouble, because of its simplicity of operation. A previous model operating with a motor was more complex, and, having a fixed speed, unable t o follow the wide varieties of titration rates it was called upon to record. The only disadvantage to the system is the stepwise motion of the recorder, which is determined mainly by the surface tension of the titrant-Le., the ease with which the meniscus pulls away from the probe. Because the platinum tip is relatively fine, these steps do not exceed 0.2% of full scale, and under optimum conditions they are less than the thickness of the pen line. ACKNOWLEDGMENT
Because the recording is linear, calibration is simplified. The easiest method of calibration is to fill the buret with standard acid and titrate a measured volume of standard alkali. One
The author wishes t o thank R. A. Latimer and B. L. Huston for their assistance in the development of this apparatus.
Direct Print Photographic Paper for Rapid Analysis of Paper Chromatograms Containing UltravioletAbsorbing Materials
.
Frank M. Ganis, Department of Biochemistry, University of Rochester School of Medicine and Dentistry, Rochester, N. Y. LTHOUGH
various
photographic
A methods have been utilized for ob-
taining permanent records of the movement of materials on paper chromatograms, the photographic films or prints of the paper chromatograms must be processed by development, fixation, and drying before being useful. When there are large numbers of chromatograms to be analyzed, or immediate reproduction of a chromatogram is desirable, the processing procedure takes more time and effort than the actual photography of the chromatogram. The method described offers the investigator an immediate print of a chromatogram without the delay or necessity of chemical processing. It has been used successfully in this laboratory for routine analysis of large numbers of chromatograms containing 2068
ANALYTICAL CHEMISTRY
quantities of ultraviolet-absorbing materials, particularly steroids, in the order of 5 y or more per square centimeter of paper. METHOD A N D DISCUSSION
The ultraviolet scanner-camera and the techniques employed to obtain prints of chromatograms are similar to those described by Haines (“Recent Progress in Hormone Research,” Vol. 7, pp. 255-305, Academic Press, KenYork, 1952). The following modifications were found essential. Over a thoroughly dried chromatogram is placed a sheet of Kodak Linagraph direct print single weight photographic paper which is exposed for 15 seconds t o 1 minute, depending upon the concentration of the chromatographed material. A 15-watt ultra-
violet lamp (General Electric germicidal lamp, G 15T8) is mounted approximately 25 em. from the filter glass. The photographic paper must be held in close contact 71 ith the chromatogram by a pressure plate cover. After exposure, the paper is removed and the latent image is developed by holding the emulsion side of the paper 18 inches from a 30-watt fluorescent bulb until a real image of a desirnble contrast is obtained. The development process is thus observed and overexposure compensated for by shorter exposure to the developing lamp. An underexposed print may be darkened by longer exposure to the developing lamp. The method allows for immediate observation of the development process and judgment of errors and correction of these errors, or for repetition of the printing process.
