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
~
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 <0.005
ppb for HF) can be prepared with this simple unit. Factors
that contribute t o high product purity are operation below the
boiling point, first described for preparation of high-purity
hydrofluoric acid by Coppola and Hughes (7) ; exclusive use of
Teflon for construction; the provision for cleaning the collecting system; the minimum of handling required; and the fact
that the still operates almost as a closed system.
RECEIVED
for review January 20, 1972. Accepted April 7,
1972.
( 2 ) W. Kwestroo and J. Visser, Aiialysr (Lorrdon),90, 297-8 (1965).
(3) W. F. Hillebrand, G. E. F. Lundell, H. A. Bright, and J. I.
Hoffman, "Applied Inorganic Analysis, with Special Reference
to the Analysis of Metals, Minerals, and Rocks," 2nd ed., John
Wiley and Sons, New York, N.Y., 1953,pp 38-9.
(4) G. R. Tilton, C. Patterson, H. Brown, M. Inghram, R. Hayden,
D. Hess, and E. Larsen, Jr., Geol. SOC.Amer. BUN.,66, 1131-48
( 1955).
(5) Leaflet No. S1-5,Quartz et Silice, 8 Rue D'Anjou, Paris 8",
France.
(6) J. M. Mattinson and A. K. Sinha, Carnegie Institution of Washington, Washington, D.C., unpublished work, 1971.
(7) P. P. Coppola and R. C. Hughes, ANAL.CHEM.,
24,768 (1952)
Modulated Power Supply for Hollow Cathode Light Sources
B. E. Holder, Robert Lim, A. S. Maddux, and G . M. Hieftje'
Lawrence Licermore Laboratory, University of CalijornialLivermore Calif.
SINGLEBEAM ATOMIC ABSORPTION spectrometers commonly
employ a mechanical, segmented-disk chopper as a means of
modulating the hollow cathode light source. The modulated
light transmitted through the atomic absorption flame is converted to a correspondingly modulated electronic signal by a
I present address, D~~~~~~~~~
of Chemistry, University of
Indiana, Bloomington, Ind. 47401
1716
photomultiplier. The signal is then amplified and demodulated by a synchronous lock-in-amplifier-detector. This is a
technique commonly employed for the discrimination of the
wanted light (hollow cathode light) and the unwanted light
(flame background). In addition t o being somewhat cumbersome and subject to mechanical wear, the mechanical light
modulator is limited in its upper frequency range to a few
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972