Table I. Titration Re-sults with Pressuremetric End-Point Detectiono Taken, mgb Found, mg No. of detn 1.904 f 0.007 1.906 6 0.8694 f:0.0030 0.8688 6 a Ammonium ion titrated with coulometrically generated hypobromite. Ir Based on check method.
basis of this technique has been presented previously (4, 7). Briefly it involves monitoring the pressure change in a closed system due to the evolution or consumption of a gas by the reaction of analytical interest. Utilizing this pressure transducer system, accuracies and precisions of a few parts per thousand were obtained. Generally it is felt that the Pitran may offer some advantages in the monitoring of small pressure changes associated with chemical reactions of analytical interest. The system is simple, inexpensive, and provides a dc output suitable for recording. We have shown that the Pitran is capable of highly accurate and precise analytical data. Although they do not possess a multirange capability, Pitrans are (7) D. J. Curran and J. L. Driscoll, ANAL.CHEM., 38, 1746 (1966).
Re1 std dev,
z
0.4 0.4
RelbAcc
z
+o.
1 -0.1
Total gas, pmoles 53 24
available in many pressure ranges (9 models from 0.1 to 20 psid) and their relatively low cost may permit the purchase of several units depending on the range of pressures to be studied. In addition, the unit possesses the advantages of high overload capability (700% of the linear range) and very small size (TO-46 header, 0.2-inch diameter and 0.07-inch height). Finally, with a battery power supply as used in this study, the unit is capable of field operation. ACKNOWLEDGMENT
We wish to thank D. K. Roe for calling our attention to the Pitran. RECEIVED for review February 12, 1971. Accepted April 27, 1971.
System for Continuous Pumping of Solvent at Low Flow Rates Gary 0. Nelson and Robert D. Taylor Lawrence Radiation Laboratorq , University of California, Livermore, Calif. 94550
PRECISION, NONPULSING PUMPS for dispensing solvent at low flow rates (10 to 0.005 ml/min) are often required for both routine and research applications, but such pumps are not commercially available. Diaphragm, cam-actuated, and piston pumps may specify a certain flow, but their output is generally the average of peaks and valleys, which makes them unacceptable in many instances. Peristaltic, finger, and kinetic clamp pumps also exhibit surges and lapses in output flow as a result of the rollers which alternately rise from and depress a resilient tubing. Tubing deterioration and variations in system pressure are also factors which cause inconsistencies in the output flow rate. Syringe-type pumps presently seem to offer a method of continuous, low-flow solvent pumping. If liquids must be slowly added over long periods, a reciprocal syringe pump appears to be the most appropriate. Often, such pumps have a liquid flow routing control which consists of solenoidactivated clamps which alternately depress and release some type of soft tubing. Here again, tubing deteriorationthrough swelling, dissolving, and splitting when in contact with incompatible solvents-proves to be a major problem. Also, diffusion of the solvent through tubing walls can cause losses of up to 50z with many solvents. Hence, it is not uncommon to have errors of 10 to 90% between the theoretical volume (as predicted from syringe capacity and rate of plunger advance) and the actual volume delivered. This paper describes a liquid routing control which has none of the disadvantages just described and can be easily adapted to any Sage Instrument Inc. Model 220 syringe 1340
pump. All tubing, valves, and fittings can be purchased commercially and easily fabricated, The liquid comes in contact with only the Teflon (Du Pont) materials, and there is no measurable diffusion, pulsing, or deterioration with any solvent yet studied. EXPERIMENTAL
A schematic diagram of the system is shown in Figure 1. Basically, it consists of dual syringe injectors, inert tubing and routing control valves, and an air-driven valve control unit. Commercial syringe injectors normally come complete with the plunger advance and reversing switch controls. As one syringe fills from the reservoir, the other empties at a predetermined rate. The filling cycle of our pump is slightly faster than the infusion cycle; thus the reserve syringe is always filled before the delivery syringe empties. The switchover is then free from any backlash, since both syringes are actually moving forward momentarily, one of them pumping back into the reservoir just before switchover occurs. The solvent is routed through a network of 0.076-inch i.d. Teflon tubing with Kel-F female h e r fittings at both ends. The routing control system consists of two 4-way Kel-F values with male h e r fittings. All T and X connectors are Kel-F with male and female h e r fittings as required. A manually operated recycling valve allows for liquid recirculation (90' counterclockwise) during the priming process and provides for a quick shutoff during periods when solvent delivery is unnecessary. The routing control valves are attached to the pinion gear of a rack and pinion arrangement. The gear can rotate back and forth through an angle of about 45". The rack is
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
Solvent )outlet
,-Rock
and pinion gears
-Compressed
air
Figure 1. Schematic diagram of dual syringe pump using air-activated solvent routing control system
Reversing switch control Solvent reservoir
G%
4-way routing
Figure 2. Cutaway perspective view of rack and pinion gears and routing control and recycling valve assembly
Pinion gear
moved by a stud-mounted, double-acting miniature cylinder with a 3/8-in. bore. The air operation provides for rapid switching and minimal interruption in the output liquid flow rate. The reversing switch controls activate a 115-volt ac solenoid, which in turn activates a 4-way air valve. This valve routes the compressed air to the cylinder, which will initiate the switching process. Using the solenoid directly does not produce sufficient power for rapid switching. The actual arrangement of the rack and pinion gears and routing control valves is shown in Figure 2. The valve bodies are screwed into place on the sides of the supporting framework. The handles are removed from the valves and the stems are arranged 45" out of phase from one another. The stems are then secured to the pinion gear with set screws. The gear should rotate smoothly but tightly. The rack is then inserted down its Teflon slot and the stops are adjusted. This operation is tedious, but, once the control valve is set, it will operate hundreds of hours before readjustment is necessary. After the valve is set, the system should be checked for leaks under pressure. RESULTS AND DISCUSSION
Performance curves for both the unmodified and the modified syringe pump are shown in Figure 3. The output
Time
- min
Figure 3. Output flow data comparing unmodified tubing clamp apparatus with modified air-activated routing control system Dashed lines represent theoretical delivery rates for toluene (upper) and CHJ (lower). Air-actuated valves are compared to tubing clamps or silicon rubber tubing (l/l~-in. wall, l/l& i.d.) for toluene and to surgical gum latex tubing ('/I6 in.-wall, 1/,6-in,i.d.) for methyl iodide
A N A L Y T I C A L C H E M I S T R Y , VOL. 43, NO. 10, A U G U S T 1971
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liquid flow rates for toluene and methyl iodide are shown as examples. The dashed lines indicate the theoretical delivery rate predicted from measurements of syringe capacity and rate of plunger advance. Note that for toluene, using the unmodified system of tubing clamps on silicone rubber, the actual output flow builds up gradually but never exceeds 80% of the theoretical output. The balance is lost by diffusion of the liquid through the tubing walls. The large periodic negative spikes are caused when the infusion cycle changes from one syringe to the other; the system pressure first drops but is slowly built up as the syringe moves forward. However, if the inert, air-activated valve switching system is substituted, the actual and theoretical outputs are practically coincident, and the actual output is almost pulse-free during the infusion syringe changeover. The case of methyl iodide is more dramatic and illustrates the problems encountered with gross incompatibility between solvent and tubing. Using the unmodified system of tubing clamps on surgical gum latex, the output flow rate again approaches about 80% theoretical output, but as time progresses liquid not only is lost by diffusion but actually begins to leak through fissures in the tubing wall. The air-activated valves, however, again provide dependable, pulse-free injection rates.
Chlorinated hydrocarbons, alcohols, ketones, and aromatic materials have all been pumped with the air-activated valve system, and the amount of solvent actually delivered has been consistently within *0.6% of the theoretical injection rate. By actually sweeping out the valve volume during syringe changeover, rather than depressing the tubing with solenoid-activated clamps, an accurate and practically pulse-free liquid displacement can be achieved for almost any solvent or solution. ACKNOWLEDGMENT
The authors gratefully acknowledge the efforts of Bernell J. Bequette for his work in valve machining and assembly and James E. Dixon for his skillful electronic modifications.
RECEIVED for review February 8, 1971. Accepted May 3, 1971. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Atomic Energy Commission to the exclusion of others that may be suitable. Work performed under the auspices of the U S . Atomic Energy Commission.
Reliable Low Voltage Signal Generator for Cyclic Voletammetry Russell H. Bull Department of Chemistry, St. Louis University, St. Louis Mo. 63156
Geoffrey C . Bull Department of Electrical Engineering, Christian Brothers College, Memphis, Tenn. 38104
IN ELECTROCHEMICAL investigations conducted with laboratory constructed devices capable of generating voltage or current programs over a wide range of frequencies and amplitudes, each instrument must incorporate the dual features of potentiostatic or amperostatic control of the electrolysis cell and a reliable signal generator for flexible signal shaping at the working electrode. Described here is a functionally reliable, compact, low voltage signal generator easily adapted to any potentiostat design the experimenter may require to fit the needs of a particular cell configuration
(0. GENERAL REQUIREMENTS
Whether single sweep, repetitive scan, or cyclic experiments are to be conducted, certain controls must be present in the signal generator for ease in interpreting the results. It is necessary that the signal produce a truly linear relationship between voltage and time and that the frequency of the sweep [i.e., scan rate = frequency X (anodic voltage limitcathodic voltage limit)] be independent of the amplitude, i.e., limits, of the sweep. It is often necessary to vary the initial voltage level of the working electrode, i.e., the electrode potential may need to be at some nonzero (either positive or negative) potential to prevent the formation of some spurious material. Further, it may be necessary to stop (1) W. M. Schwarz and I. Shain, ANAL.CHEM., 35, 1770 (1963).
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the experiment at either of its voltage limits for a period of time long enough to reset frequency and voltage limits and then resume cyclic operation, without disturbing the “holding” program. Additional flexibility may be found by the incorporation of a method for shifting a fixed voltage amplitude over a wide voltage range. Thus a “reference level” voltage should be present and adjustable without changing either the amplitude or frequency of the resultant triangular output. As an experiment requires the frequency to increase beyond the limits of manual control over the number of cycles generated, a device should be used that allows the user to generate a single cycle at any preset frequency and amplitude. All of these capabilities have been incorporated into the design of the following signal generator. CIRCUIT DESCRIPTION
Circuits for the generation of nonsinusoidal or discontinuous signals invariably employ a multivibrator (either electronic or mechanical) circuit to produce a linear function of various frequencies and amplitudes. The present signal generator is concerned with the formation of a symmetric triangle which may be easily produced by the electronic integration of a square wave. The following instrument uses an operational amplifier as a sensing device which essentially compares the instantaneous output voltage of an operational amplifier/integrator circuit with a reference
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
