would fall from the maximum $5 volts to 0 volts in 0.5 second. Figure 3 also shows the ramp mode and its relationship to the digital up-down count. As a design feature, the trapezoidal waveform represented in Figure 3 is divided by digital gating into four modes, see Figure 4. Each mode gives a different, independent waveform-Le., mode 1 gives a ramp (hold = 0), mode 3 gives a triangle (hold = 0). The minimum hold (hold = 0) is in the order of 23 nanoseconds and is due to the switching (propagation delay) time inherent in the digital components. This minimum hold time is insignificant compared to the variable 1-millisecond to 100-second period of the rise and fall sections of the waveform. The rise time of all the waveforms is limited by the slew rate of the digital-to-analog converter-ca. 50 microseconds (Computer Products, DA 435 E). The pulse mode, Figure 1, consists of two back-to-back R C delay monostable multivibrators (8). As the 10-bit counter and the digital-to-analog converter are already wired together, the addition of two one-shot multivibrators through the appropriate gating makes a pulse generator available. The pulse width and interval between pulses are independently adjustable from 1 millisecond to 100 seconds. Gross frequency adjustments are obtained from decade valued capacitors (Le., 0.033,O.l to 3000, 10,000pF) with fine control through 50 kohm potentiometers. A complete circuit schematic and parts list are available by writing to the authors.
MODE I Adjust Rise Time hold :0
c lI
4
MODE 2 Adjusl Riae /Hold Time
MODE 3 Adjurl Rise / Fall Time Hold Time * 0
MODE 4
Figure 4. Various modes in ramp mode
of the output potential. To illustrate the operation and function of the three oscillators, consider the shaping of the trapezoidal waveform represented in Figure 3. The input “CONTROL” of the Up/Down Counter is enabled (Le., UP COUNT), input “STORE” is disabled, the OSC. 1 (set to 2048 Hz) is switched in. The output potential would rise from 0 volts to the maximum + 5 volts in 2.0 seconds. Next, input “STORE” is enabled, the OSC. 2 (set to 1024 Hz) is switched in. The output potential would remain constant for 1.0 seconds. Finally, input “STORE” is disabled, input “CONTROL,” is disabled (ix., DOWN COUNT) and OSC. 3 (set to 512 Hz) is switched in. The output potential
ACKNOWLEDGMENT
We thank E. F. Guignon for his technical assistance. RECEIVED for review August 13, 1971. Accepted March 17, 1972. The support of this research by the University of Connecticut Research Foundation is gratefully acknowledged. Monostable Multivibrator,” Bulletin No. 68, Stewart Warner Corporation, Microcircuits Division, Sunnyvale, Calif.
(8) “54/74121
Preparation of Hydrofluoric, Hydrochloric, and Nitric Acids at Ultralow Lead Levels James M. Mattinson Geophysical Laboratory, Carnegie Institution of Washington, 2801 Upton St., N . W., Washington, D.C. 20008
TATSUMOTO ( I ) recently described a new system for the preparation of ultrapure hydrofluoric acid (Pb = 0.08 ppb). Unfortunately, Tatsumoto’s system is rather complicated and requires close attention for safe and successful operation. This paper describes a distillation unit that is simple, is not restricted to hydrofluoric acid, and, most important, produces hydrofluoric acid containing less lead by one to two orders of magnitude than hydrofluoric acid prepared by the Tatsumoto system. DESCRIPTION AND OPERATION The still consists of two 1000-ml FEP Teflon (Du Pont) bottles connected at right angles by a threaded TFE Teflon (Du Pont) block (Figure 1). A 300-watt heat lamp supplies heat for slow subboiling evaporation from the feed bottle, and the vapor condenses in the water-cooled collecting bottle. The feed bottle might also be heated with a heating sleeve or heating tape. One advantage of the lamp, particularly when used in conjunction with carefully placed sheets of aluminum foil, is that heat may be concentrated on the upper part of the
__________
(1) M. Tatsumoto, ANAL.CHEM., 41,2088-9 (1969).
feed bottle, above the liquid level. This speeds distillation by reducing condensation in the feed bottle but without exceeding the boiling point of the liquid. Prior to use, the bottles and connecting block are given a thorough preliminary cleaning. About 100 ml of acid is then added to the feed bottle, the still is assembled, and the collecting system is given a find cleaning by operating the still in the cleaning position (Figure 2 4 for several days. This operation provides continuous refluxing of the collecting system by ultrapure acid. The feed bottle is then removed, emptied, filled about three-quarters full with acid, and replaced on the still, which is now operated in the collecting position (Figure 2B). The following precautions should be observed : the still should be operated with the collecting bottle unscrewed a fraction of a turn to permit escape of excess vapor, thus preventing pressure build-up; the distance between the heat lamp and the feed bottle should be adjusted so that no bubbles form in the acid. Bursting of bubbles produces tiny droplets, which may enter the collecting bottle and reduce the purity of the distillate. When most of the acid has been distilled, the collecting bottle is unscrewed, capped, and used for dispensing acid directly. A fresh batch can be started immediately with a second collecting bottle.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
1715
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tI.5
4
~~
Table I. Pb Concentrations in Acids in Parts Per Billion grams Pb per gram acid)
L6
Threads per inch
Figure 1. TFE Teflon connecting block. Fits 32-oz Nalgene, narrow-mouth FEP Teflon bottles (screw cap size 30-430)
I
I I Collecting
Heat lamp
1
bottle
1
A Feed bottle TFE Teflon
Figure 2. Two-bottle Teflon still: ( A ) cleaning position, ( B )collecting position RATE OF DISTILLATION Rate of distillation is determined by a number of factors, including the efficiency of the collecting bottle as a condenser and the amount of condensation in the feed bottle. When these factors are optimized by using an ice-water bath t o cool the collecting bottle and by minimizing condensation in the feed bottle (see above), 600 to 700 ml of HCI or HF can be distilled in 3 t o 4 days, and the same amount of concentrated HNO, in 5 to 6 days. Use of room-temperature water to cool the collecting bottle extends distillation times by a factor of two or three.
6.2N 70z Method 48% HF HCl "03 Two-bottle Teflon stillQ 0.002 0.0015 0.023 (this paper) O.OC5 0.049 Tatsumoto system ( I ) 0.08 ( I ) Isothermal distillation (2) 0 . 2 ( I , 6) Distillation in platinum (3) 0.2-1.0 ( I ) Passage of filtered HF gas into H20 ( 4 ) 0.2-1 .o ( 1 ) Subboiling distillation. in quartz (5) 0.12 (6) 0.18 (6) Starting materials: reagent-grade hydrofluoric acid, singly distilled 6.2N hydrochloric acid, and reagent-grade nitric acid. RESULTS AND DISCUSSION Acids prepared by this method were analyzed for lead by isotope dilution. About 100 ml of acid plus a small amount of enriched *aPb tracer was evaporated in a Teflon beaker under a laminar flow of ultraclean air for each determination. N o corrections were made for lead introduced during evaporation or preparation for the mass spectrometer; thus the figures quoted are maximum values. The results are summarized in Table I. Data for acids prepared by other methods are listed for comparison. The results show that acids of exceptional purity (Pb
