New, Directly Digital Automatic Titration Apparatus G. M. Hieftje and B. M. Mandarano Department of Chemistry, Indiana University, Bloomington, Ind. 47401 A directly digital titration apparatus has been developed which is based on an entirely new concept. In this system, the titrant is delivered in the form of uniform submicroliter droplets whose production rate is controlled by an electronic digital pulse train. Titrant delivery rate can therefore be determined by measurement of the pulse frequency while the total titrant volume is related to the cumulative pulse count. I n addition, a digital control system can be used to adjust titrant delivery for end-point anticipation or for titrations involving slow reactants. Although the precision of titrant delivery is shown to be greater than many end-point detection systems, second derivative potentiometric detection has produced excellent results for some simple acid-base titration systems. Extension of the described semi-automatic device to fully automated or computer-controlled laboratory situations is considered and is simplified by the instrument’s inherent digital compatibility. Potential application of the digital titrant delivery unit to other analytical and non-analytical problems is also discussed.
THEIMPORTANCE OF TITRIMETRIC procedures to routine chemical analysis has stimulated considerable interest in the development of devices which perform these operations automatically (1-5). Innovations in end-point detection, titrant delivery systems, and in the recording of titration curves have enabled rapid titrimetric analyses with reduced operator attention and error. To further expedite these procedures, recent work in the development of titrators has focused upon digital readout and printout devices, several of which are now commercially available. These devices, in addition to providing direct printout and reduced reading error, are compatible with larger computerized control and data collection systems found in analytical laboratories in ever increasing numbers. Generally, the new digital titrators employ mechanical and control systems identical or similar to earlier designs, but are equipped with analog-to-digital or digital-to-analog converters to provide digital compatibility (6-8). These interdomain conversions (9), although necessary, unfortunately reduce accuracy and increase instrumental cost, size, and complexity. To improve this situation, it would be advantageous to eliminate the conversion steps wherever possible. One approach is to design and employ a directly digital titration device. Such a device will be described in this paper. The titration system described herein is based on an entirely new concept involving introduction of the titrant in the form of uniform, submicroliter droplets. The titrant delivery unit (1) J. J. Lingane, “Electroanalytical Chemistry,” Interscience” New York, N.Y., 1958, Chap. 8.
(2) J. B. Headridge, “Photometric Titrations,” Pergamon, New
York, N.Y., 1961. (3) A. L. Underwood, in “Advances in Analytical Chemistry and Instrumentation,” Vol. 3, C . N. Reilley, Ed., Interscience, New York, N.Y., 1964, p 31. (4) J. T. Stock, ANAL. CHEM.,42,276R (1970). (5) E. C . Toren and R. P. Buck, ibid., p 284R. (6) S . Wolf, Chem. Ztg., Chem. App., 93, 676 (1969). (7) D. Jagner, A n d . Chim. Acta, 50, 15 (1970). 43,641 (1971). (8) K. A. Mueller and M. F. Burke, ANAL.CHEM., (9) C. G. Enke, ibid., 43(1), 69A (1971). 1616
serves to completely convert the titrant solution into droplets which can be sent into a titration vessel at a rate controlled by a digital pulse train. Because of the droplet uniformity, the titrant delivery rate is proportional to and determined by the pulse frequency, while the total delivered titrant volume is related to the cumulative pulse count. This system enables extremely precise volumes of titrant to be delivered in increments of less than one microliter. In addition, titrant flow can be digitally controlled for end-point anticipation or for accommodation of slow titrant-titrate reactions (10-12). The necessary instrumentation for operation of the titrator is simple and inexpensive and because of the lack of moving parts, little maintenance is required. The instrument can be used with most equivalence point detection systems, with the detection system being the precision limiting component in most cases. In this study, a description of the titrant delivery system is given and data are presented for its application to several simple acid-base titrations using derivative potentiometric end-point detection (13). Accuracy and precision attainable with the system is discussed and the application of the device to other detection systems, different chemical samples and other, non-titration problems in chemical analysis is also considered. TITRATOR DESIGN
To facilitate the use of various end-point detection systems and to enable the titrimetric determination of a number of different chemical species, the titrator was designed for maximum flexibility. Because modularity was considered to be important in enhancing this flexibility, the system was assembled from several components. These components are: the titrant delivery unit, the end-point detection system, and the digital logic circuitry used to control titrant delivery and provide readout capability. Each of these components will be discussed independently. Titrant Delivery System. The titrant delivery device, as shown in Figure 1, is based upon a principle first described by Lord RayIeigh (14) and later implemented by Lindblad and Schneider (15). With this device, the titrant solution is completely converted into uniformly sized and spaced droplets any of which can be sent into the titration vessel. To accomplish this, a liquid jet is formed by forcing a constant flow of titrant solution through a hypodermic needle. Ordinarily, a liquid jet formed in this manner would disintegrate into randomly sized and spaced droplets, in accordance with the natural disturbances within the jet. However, by artificially establishing a sufficiently intense, periodic disturbance on the jet, it can be forced to break up into uniform droplets. In this system, the artificial disturbance is launched onto the jet by vibrating the needle in a direction normal to the jet. This is (10) J. J. Lingane, ANAL.CHEM., 20, 285 (1948). (11) Ibid., p 797. (12) Ibid., 21, 407 (1949). (13) H. V. Malmstadt and E. R. Fett, ibid., 26, 1348 (1954). (14) Lord Rayleigh, Proc. London Math. SOC.,10, 4 (1879). (15) N. R. Lindblad and J. M. Schneider, Reu. Sci. Instrum., 38, 325 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9,AUGUST 1972
,Titrant (from reservolr)
Hypodermic Needle
Digital Count or Readout
-
-
Pulse Driver
+-
Digital Control System
-
. Figure 1. Schematic diagram of titrant delivery unit showing control and measurement systems
Deflection Field P l a t e s
Electrode Detector I I
\Titrant
Port T i t r o t i o n Vessel Mlcroelectrode
-Stirrer
accomplished by attaching the needle to a piezoelectric bimorph strip (Type PZT-SH, Clevite Corp., Bedford, Ohio) which is driven by a variable frequency power oscillator. In order to control the number of droplets which are delivered to the titrate solution, selected droplets must be extracted from the primary stream issuing from the hypodermic needle. This is accomplished by passing the stream of droplets through a circular charging electrode placed at the point where the droplets are breaking off from the jet. A suitable voltage applied to the charging electrode then repels charges back into the jet so that droplets breaking off from the jet at that time retain a deficiency of the charge. In this way, a single droplet or a large number of droplets can be charged by simply applying either a pulse or a constant voltage to the charging electrode. For digital control, one voltage pulse is applied to the electrode for each droplet which is to be charged. The charged droplets are then deflected by passing the entire droplet stream through a high voltage dc field. Deflected droplets pass through the titrant port while the uncharged droplets are trapped at the bottom of the titrator and drain from the enclosure to be recycled or discarded. There are several advantages of this type of titrant delivery system. Because the titrant is delivered in the form of uniform droplets, the volume of titrant is directly proportional to
the total number of droplets deflected, which is in turn equal to the number of pulses applied to the charging electrode. Therefore, the titrant volume can be directly read from a digital counter connected to the digital control system output ; titrant delivery rate can similarly be determined by digitally measuring the frequency at which pulses are applied. Alternatively, the digital output can be directed to a digital computer for computational convenience or control purposes. Another advantage of this system is the lack of moving parts. Unlike most automatic titrators, this system has only one moving part (the bimorph) and this will have a nearly infinite lifetime. All other components are electronic, so that longer service, less maintenance, and greater convenience can be expected. Routine titrations can be made especially convenient by using a titrant reservoir employing a gravity feed, thereby eliminating the time-consuming and inconvenient refilling operations required by most titrant delivery devices (e.g., motor driven burets). Alternatively, the titrant can be delivered into the titrate solution under pressure by utilizing a pressurized titrant vessel. Although this approach is less convenient, it does promote more rapid mixing of the titrant with the titrate and was, therefore, employed in this study. A third method which could be employed in titrant delivery, although not investigated in this study, is the use of a syringe driven at a constant rate.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
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!%Ode
r p METER
Figure 2. Schemativ diagram of second derivative end-point detection system
zy: io m
OAl
WTF
OA2
O./pF I\
10 M
To Dlgltal Control System,
I\
There are several factors which must be considered during operation of the titrant delivery system. Because the titrant flow is proportional to the droplet size and because the droplet size determines the smallest volume of titrant which can be delivered, it is important that the droplets remain uniform and their size be controllable. Droplet size is determined by the size of the needle orifice, the flow rate of the titrant from the needle, and by the frequency at which droplets are formed (16). Larger or smaller droplets can be selected simply by adjusting the titrant flow or by changing capillaries. Generally, it is undesirable to control the droplet size by frequency adjustment since droplet formation becomes unstable at certain frequencies. Thus fixed frequency operation is preferable (17). Also, because constant titrant flow is important for instrumental simplicity and stability, changing capillaries is the best method of adjusting droplet size. Because particulate matter present in the titrant solution can conceivably clog the capillary or otherwise affect droplet formation, a millipore filter (Millipore Corporation, Bedford, Mass.) has been placed in the titrant feed line just before the needle. This precaution ensures a stable titrant flow to the needle to produce a continuous, stable stream of droplets which can be maintained for extended periods of time. Derivative End-Point Detection System. A potentiometric derivative end-point detection system (13) was employed in this study. This choice was arbitrary and represents only one of a number of possible approaches which could be used. The purpose of this investigation is not to evaluate the derivative technique; this has been done elsewhere (1, 13). Rather, the approach which has been taken with this technique will, hopefully, be helpful in applying the titrant delivery device to other end-point detection schemes. Potentiometric measurements of the titrate solution were made continuously using a glass microelectrode (Sargent Model S30070-10) and a pH meter. Standardization of the pH meter was made unnecessary because the derivative technique was employed. The recorder output of the pH meter provided the input to the second derivative circuitry, as shown in Figure 2. To reduce noise input to the derivative operational amplifiers, OA2 and OA3, a gain of ten operational amplifier, OAl, with a one-second time constant served as a preliminary filter. In addition, OA2 and OA3 have been provided with damping capacitors and resistors to reduce the (16) G . M. Hieftje and H. V. Malmstadt, ANAL.CHEM., 40, 1860 ( 1968). (17) J. M. Schneider, Ph.D. Thesis, University of Illinois, Urbana, Ill., 1964. 1618
0
-
4 F 100K
sensitivity of the second derivative to random variations in stirring and electrode response. Noise reduction in the derivative technique is especially important (13), and this is but one of several approaches which can be used to minimize it. The filtered second derivative of the titration curve, available at the output of OA3 is then used to control the logic circuitry which determines the droplet introduction rate. Logic Circuitry for Titrant Delivery Control. The logic circuitry used to control and measure the droplet titrant delivery rate is shown in Figure 3. A high voltage pulse driver using a 2N3439 transistor supplies a 300-volt pulse to the charging electrode whenever a droplet is to be charged and deflected into the titration vessel. The amount of charge picked up by the droplets depends on the duration and time of application of the pulse. These parameters have been made adjustable by means of the delay monostable, M1, and the pulse width monostable, M2. For this study, the pulse delay and width was variable from 0.1-1 msec. A single trigger pulse applied to the input of M1 thus produces one deflected titrant droplet. In this circuit design, it was desired to incorporate a provision for equivalence point anticipation and subsequent reduction of the titrant delivery rate to minimize end-point overshoot. This task is simplified by the use of second derivative end-point detection. Upon initiation of the titration, and before the end point is reached, the second derivative of the titration curve will be zero, or at a level below that of the reference voltage of comparator C1. The output of C1 will thus be zero, so that the output of gate G 3 will be in a “1” state, opening G4. Also, upon inversion by 11, gate G1 will be opened so that pulses from the square wave input will be passed through G1 and G4 and trigger the pulse driving system. Because the square wave input is obtained from a source synchronized with the bimorph driving signal, one 300-volt pulse will be delivered for each cyclic bimorph vibration, or for each droplet formed, Therefore, when the second derivative is first zero, all droplets will be charged and sent into the titration vessel. As the end point is approached, the second derivative of the titration curve becomes positive (or negative, depending on the shape of the titration curve). This causes comparator C1 to change states although comparator C2 remains unchanged, because C2 triggers on the opposite polarity. The C1 output, now being a “1,” opens gate G3, and, after inversion by 11, closes G1. The output of G1, being positive, then opens G4 so that pulses from the decade scaler can trigger the dropletcharging pulse network. The decade scaler can divide the frequency of the square wave by any power of 10 so that the
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
TO Dlgltal PB
0
Counter
10K
+300V
Second Derivative-1
input
mi
-
Figure 3. Logic circuitry for endpoint anticipation and automatic termination of titration
-m2
k
Square Wave
Input
titrator sends every (1On)th droplet into the titration vessel. For the chemical systems used in this study, the solution reactions were sufficiently rapid to permit the use of division by a single decade with negligible end-point overshoot. When the equivalence point is reached, the second derivative of the titration curve rapidly changes polarity so that both comparators C1 and C2 change states. The output of C2, which was previously in a “1” state, becomes zero to reset the R-S flip-flop formed from G5 and G6. This closes gate G7 to prevent any further pulses from being applied to the charging electrode and therefore terminates the titration automatically. To initiate another titration, pushbutton PB is set to zero to reset gates G5 and G6, open G7, and once more permit pulses to pass. Because comparators C1 and C2 have returned to their initial states, all gates are in the proper polarity for titration so that no further resetting is necessary. This circuitry has been found to be reliable and to provide adequate discrimination against noise with proper adjustment of the reference levels of comparators C1 and C2. End-point anticipation is provided, termination is automatic, and repeated titrations require only the introduction of an additional aliquot into the titration vessel and a logic level (or switch) application to gate G5. These operations can, of course, be conveniently performed automatically. Operation of the Titrator. For routine or multiple determinations, parameters such as the noise filtering time constant and comparator reference level settings can be optimized, although in this study it was possible to use the same settings throughout. It is important that the charging pulse width and delay, monostable M1 and M2, be adjusted when the titrant delivery system is initially set up. Once set, however, these parameters need not be changed as long as the titrant flow rate and droplet production frequency remain fixed. The generation frequency must also be adjusted initially, as it depends on the size of the hypodermic needle. In this study, a 23-gauge needle with a flow rate of approximately 10 ml/min generates titrant droplets very stably over a frequency range of =t150 Hz centered at 450 Hz. To perform a titration, the electronic system is first turned on for a brief (10 min) warm-up period. The titrant flow is then established at the desired level, in our system, by pres-
Table I. Instrumental Components of Automatic Titration System Combination microelectrode Sargent Model S30070-10 Leeds & Northrup Model 7401 pH meter Digital counter/frequency Hewlett-Packard 5211A meter Sine/square wave generator Model EUW-27, Heath Co., Benton Harbor, Mich. Model EUW-19 Operational AmPower amplifier plifier, Heath Company Model EUW-15, Heath Company 300 VDC power supply Model 203, Pacific Photometric In-2000 VDC power supply struments, Berkeley, Calif.
surizing the titrant vessel with air from a regulated compressed gas cylinder. Because the droplet generation frequency and other electronic parameters were previously set, all that is necessary for automatic titration are the conventional steps of pipetting a sample into a suitable reaction vessel ( e . g . , a 100ml beaker), inserting the glass combination microelectrode, establishing stirring, and pushing the start button, PB (cf. Figure 3). The titration then proceeds automatically, with the droplet delivery rate being reduced by a factor of 10 near the end point, where delivery is halted automatically. A digital counter connected to the pulse driving circuit (cf. Figure 3) records the cumulative number of droplets delivered during the titration. The analysis can then be completed by suitable titrimetric calculations. Although these operations are being performed manually with the present system, it is evident that operator time and error could be reduced by full automation or computer control, both of which would be simplified by the directly digital control and output of this titrator. EXPERIMENTAL
The titration assembly (Figure 1) was formed from sheet metal with a Plexiglas faceplate to facilitate viewing. Titrant droplets were directed into a 100-ml beaker which served as a titration vessel and was provided with magnetic stirring. Most instrumental components are listed in Table I ; other
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Table 11. Determination of Precision of Droplet Production by the Titrant Delivery Unit No. of drops Wt of HzO Calculated Sample delivered delivered drops/ml 1 2 3 4 5 6 7 8 9 10
15,007 14,943 14,978 14,994 14,957 14,984 15,013 15,040 14,962 14,960
5.1940 5.1690 5,1826 5,1890 5.1745 5.1785 5.1853 5,1930 5.1644 5.1663
Relative standard deviation
2,890 2,891 2,890 2,895 2,891 2,894 2,895 2,896 2,897 2,896
= 0.097 %
Table 111. Precision of Strong AcidStrong Base Titration Using the Digital Titration System NaOH No. of aliquot, drops of Calculated NaOH Runs Sample ml HC1 titrant concentration,C, 1 2
1 2 1 2 3 4 5 6 7 8 9 10
50 50 20 20 20 20 20 20 20 20 20 20
72,177 72,200 29,106 28,914 29,932 29,148 29,241 29,186 29,163 28,998 29,359 29,301
Relative standard deviation 3
1 2 3 4 5 6 7 8 9 10
10 10 10 10 10 10 10 10 10 10
0.009976 0.009979 0.01006 0.00999 0.01oO0 0.01007 0.01010 0.01008 0.01008 0.01002 0.01014 0.01012
= 0.50%
14,556 14,647 14,509 14,592 14,681 14,713 14,701 14,816 14,838 14,885
0.01006 0.01012 0.01003 0.01008 0.01015 0.01017 0.01016 0.01024 0.01025 0.01029
Relative standard deviation = 0.88 % Run 1 was obtained using aliquots titrated 1 hour apart. Run 2 was performed using fresh aliquots of base after discarding the previously titrated solution. Run 3 was made by adding each successive aliquot to the previous solution. a
electronics were breadboarded on a commercial unit (Model EU-801A, Heath Co., Benton Harbor, Mich.). All reagents used were certified standards or were prepared by dilution from stock solutions made from the reagent grade chemicals. RESULTS
To illustrate the use of the new digital titration apparatus, two simple acid-base chemical systems were studied. However, because the end-point detection system rather than the titrant delivery unit is expected to limit precision, a preliminary investigation of the attainable precision was performed. Readout considerations will serve to indicate some advantages and disadvantages of the approach. Readout and Calculation of Concentration. In a conventional titration, the concentration of an unknown species, C, is calculated from the well-known relationship 1620
V U
where CK and V K are the concentration and volume of the standard titrant species, respectively, and Vu is the volume of the unknown solution which has been titrated. This relationship must be modified for use in this study, because a number of volume increments (droplets) is determined rather than the titrant volume itself. For this modification, the following equation can be employed
In Equation 2, NK is the number of titrant droplets which have been delivered to the equivalence point, D is the volume of one droplet, and CK, C,, and Vu are as defined above. From Equation 2 it is apparent that the droplet volume must be known and constant if this technique is to work. When this condition is met, the product D x C, can be determined experimentally for a given titrant by using a standard titrate solution. Alternately, though less conveniently, D can be determined by the same calibration procedure or through the use of a number of different techniques which are available (18, 19). For convenience and accuracy, it is suggested that the standard calibration technique be used to determine the droplet volume, D. Once determined, this value will be applicable to all other titrants, provided the experimental conditions remain constant and the physical characteristics of the solution (Le.,viscosity) do not change appreciably. Precision of Droplet Size and Titrant Delivery. The importance of the droplet volume in determining the precision of the digital titrant delivery unit makes a study of the reproducibility of droplet size imperative. To perform this investigation, droplets of distilled water were charged and deflected into a preweighed weighing bottle. The weight gain of the bottle was then divided by the total number of droplets which had been deflected to obtain the volume per droplet in cma. Although this method is adequate for a study of precision, it must be pointed out that it would be less satisfactory than the calibration technique discussed above for the determination of absolute droplet volume. Variations and uncertainty in the density of the liquid would introduce added complications into the measurement. Table I1 shows the results of a IO-sample run of distilled water using this procedure. The relative error is seen to be less than 0.1%. This reveals the precision to which droplets can be introduced and the reproducibility of droplet size, This also determines the attainable accuracy and precision for any method utilizing this system. Strong Acid-Strong Base Titration. The stability of the titrator was examined by titrating 0.01N NaOH with 0.02N HCl in two series of determinations separated in time by one hour. Using 50-ml aliquots of the base, the results shown in Table I11 were obtained. Also shown in Table I11 are the results of two series of 10 determinations obtained using two different titrate addition methods. In the first method, successive 20-ml aliquots of base were titrated and the titrated solution was discarded after each run. In the second set of determinations, 10-ml aliquots of base were added directly to the existing titrate after automatic termination of each titration. The excellent results from this latter set of analyses indicate the responsiveness of the titrator even for increasingly dilute solutions. The precision achieved indicates the control ~~
~
(18) K. R. May, J . Sci. Oistrum., 27, 128 (1950). (19) J. A. Browning in “Literature of the Combustion of Petroleum,” Ad~arz.Chem. Ser., 20, 143-8 (1958).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972
~~
over titrant delivery and the absence of overshoot which this device provides. These latter observations suggest the application of this device as a high precision reagent addition tool. Weak Acid-Strong Base Titration. Table IV shows the results which were obtained using the digital titrator in the determination of 0.01N NaOH with 0.0175N acetic acid. Little difference can be seen between these data and those obtained with a strong acid titrant. Again, two series of determinations were performed, the first involving repetitive titration and discarding of the titrant while the second employed the successive addition and titration of aliquots of base in the previously titrated solution. As before, this is a fast reaction so that the precision obtained is expected to be and is comparable to that obtained in the case of the strong acid-strong base. DISCUSSION
The results presented in the preceding section confirm the end-point detection system as the precision-limiting component in the present titrator configuration. As successive aliquots of base are added to the previously titrated solutions, the resulting titration is seen to deviate more from the mean value because of increasing dilution of the titrate and the consequent reduction in response of the electrode system. This effect is, of course, not observed with either chemical system when the titrated solution is discarded after each determination. Also, the precision of titrant delivery has been shown to be more precise than that of the titration system as a whole. This can be rectified, of course, by adoption or development of a more sensitive or accurate end-point detection technique such as the high precision method of Lingane ( I ) . Of course, the approach described in this paper can be applied to a great number of other end-point detection systems and titration monitoring devices as well, such as amperometric, photometric, and many others, It is the purpose of this study merely to point out the utility of the approach. Several advantages of the digital titration device are immediately apparent. The system has a directly digital output and is controlled by a digital signal, so that interfacing with small laboratory computers or hardware controllers is greatly simplified. Also, the inaccuracies inherent in analog-todigital and digital-to-analog procedures are avoided. Control over titrant delivery is also greater than that possible with other systems such as those employing stepping motors. In this system, the titrant increment which is delivered (i.e., a single droplet) can be chosen merely by proper selection of the size of the titrant ddivery orifice (the hypodermic needle in this case). The size of the droplets and the force with which they can be sent into the titrate promote rapid mixing for a further advantage. In addition, because large reservoirs or recycling syringe-driven systems can be easily utilized with this system, automation of a routine determination procedure is further simplified. Finally, the device has the advantage of simplicity. Although commercial components, power supplies, etc., were employed in this work, instrumental cost could be reduced and maintained at a rather low level by use of special purpose or OEM components. The circuitry is simple and reliable, essentially no mechanical wear exists, and instrumental size is small, all of which contribute to convenience and a long service life under repetitive analyses. Two limitations to the use of this titrator are recognized and must be pointed out. First, strong bases should not be used as titrants with this system because of the likelihood of reaction between the titrant and atmospheric carbon dioxide as
Table IV. Precision of Weak Acid-Strong Base Titration Using Acetic Acid and Sodium Hydroxide NaOH No. of drops Calculated NaOH aliquot, of acetic acid RunQ Sample ml titrant concentration, C, 1
1 2 3 4 5 6 7 8 9 10
20 20 20 20 20 20 20 20 20 20
33,060 32,912 32,816 32,971 33,113 33,187 33,096 32,918 33,070 33,227
0.01000 0,00995 0.00992 0.00997 0.01001 0.01004 0.01001 0.00995 0.01000 0.01005
Relative standard deviation = 0 . 4 0 % 2
16,668 16,694 16,812 4 16,914 5 16,841 6 16,768 7 16,896 8 16,903 9 16,918 10 16,972 Relative standard deviation = 0.60z 1
2 3
10 10 10 10 10 10 10 10 10 10
0.01008 0.01009 0,01017 0.01023 0.01018 0.01014 0.01022 0.01022 0.01023 0.01026
Run 1 was obtained using fresh aliquots of base after discarding the previously titrated solution. Run 2 was made by adding each successive aliquot to the previous solution, the titrant droplets pass through the air toward the titration vessel. Also, strong reducing agents can be expected to produce some error because of oxidation during transit to the titration vessel. Although these difficulties can be eliminated through such techniques as shielding the titrant droplets with a stream of nitrogen, it is important in conventional titrations to note their existence to prevent unnecessary error. Generally, if a titration involves a strong base or reducing agent, it is good practice to employ the corresponding titration partner (i.e., acid or oxidizing agent) as the titrant and thereby minimize any error. The titrant delivery unit employed in this study has a great number of other possible uses, many of which are being examined in this laboratory. A logical extension of this work is the development of a p H stat for use in chemical and biochemical kinetic analyses (20, 21). The use of the system as an enzyme or reagent dispenser for very small or very precise volumes is also obvious and is being considered by one manufacturer (Contact Princeton Applied Research Corporation, Princeton, N. J.). A new area of research, electrospray mass spectroscopy (22) would also benefit by utilizing a directed monodisperse aerosol generator of the type employed in this titrator. Although a large variety of additional applications of the device could be cited or proposed, these few examples will serve to illustrate its potential as a tool in chemical analysis and characterization. RECEIVED for review December 13, 1971. Accepted March 30, 1972. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1972. Supported in part by National Science Foundation Grant NSF 24531, (20) J. B. Neilands and M. D. Cannon, ANAL. CHEM.,27, 29 (1955). (21) H. V. Malrnstadt and E. H. Piepmeier, ibid., 37, 34 (1965). (22) M. Dole, H. L. Cox, Jr., and J. Gieniec, Abstracts, 162nd National Meeting of the American Chemical Society, Washington, D.C., September 1971, No. Anal. 44.
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