Automatic Titrator for Digital Recording of Potentiometric Titration Curves J. J. Kankare, P. 0. Kosonen, and P. 0. Vihera D e p a r t m e n t of
Chemistry. University of Turku, 20500 Turku 50, Finland
Several methods have been employed for recording potentiometric titration curves. The oldest, most cumbersome, but perhaps most accurate method is, of course, manual point by point measurement. The second stage in development is recording the titration curve on a strip chart recorder. This method is fairly rapid, but low reading precision makes it only semiquantitative. Several instruments for automatic digital recording of both the titrant consumption and potential have been previously devised (2-7), some of them being commercially available. Some of the titration methods are based on the computer control of the process (3-5). For our studies concerning mostly solution equilibria, the digital recorder of titration curves should fulfill the following conditions: The reading accuracy of potential must be a t least kO.1 mV; it has been reported that even accuracy of *0.01 mV is possible in glass electrode measurements ( 8 ) . The reading precision of titrant consumption must be 0.01%. Because the titrator is to be used for studying different equilibrium reactions, it is desirable that the equilibration times between the titrant delivery and potential readout can be adjusted in rather wide ranges. It is not desirable that an expensive computer is tied for controlling this kind of‘very simple process. To our knowledge, none of the commercial equipments fulfills all these conditions. Two of the “hard-wired’’ titrators ( 2 , 6) seem to be accurate enough, but their construction is somewhat unattractive because of the rather expensive commercial building blocks (titrant delivery unit, programmers, scanner, clock etc.). Also the total volume of the commercial buret unit used in both constructions is 20 ml, which is far too large for our purposes. Fortunately, by utilizing the modern integrated circuit technology, it is rather easy to construct an inexpensive apparatus which fulfills these conditions. Our approach to this problem is discussed in the following. Potential Measurement. Potential is conveniently measured with a digital voltmeter. In glass electrode measurements, it should be noted that if a 4I/$-digit precision is required, input impedance of the DVM should be a t least 1 O I 2 ohms. We have used a Radiometer (Copenhagen, Denmark) PHM 52 pH meter which provides the required precision and input impedance. In those cases when a jyz-digit precision was required, we have used a Data Precision (Wakefield, Mass.) Model 2530 digital voltmeter. The input impedance of this DVM is only lo9 ohms and thus an impedance converter was needed. This was constructed by using an Analog Devices operational amplifier 311K connected as a voltage follower. The amplifier was encapsulated into a waterproof metal case and immersed into a thermostated waterbath for reducing the temperature-induced drift of the input offset voltage. Titrant Delivery System. Two methods for accurate dispensing of small volumes of liquids are apt for easy automation. One is the conventional syringe buret method, ( 1 ) D Jagner.Ana/. Chim. Acta. 50, 15 (1970).
( 2 ) A. Johansson and L. Pehrsson, Analyst (London).95, 652 (1970) (3) T. Anfalt and D. Jagner, Anal. Chim. Acta. 5 7 , 177 (1971j (4) K. A. Mueller and M . F. Burke, Ana/. Chem.. 4 3 , 641 (1971). (51 S. Gobom and J. Kovacs, Chemica Scripta, 2, 103 (1972) (6) 0. Ginstrup, Chem. Instrum.. 4 , 141 (1973). (7) G M . Hieftjeand 8. M Mandarano,Ana/, Chem., 44, 1616 (1972) ( 8 ) R P Henry, J. E. Prue, F J C Rossotti and R . J Whewell. Chem. Cornmun.. 1971, 868
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u .............. ..............
A
0
C
Figure 1. Nozzle c o n s t r u c t i o n ( A ) and tip dipping m e c h a n i s m (6, C) of the titrator (not d r a w n in the s a m e scale) a. Agla syringe, b . Teflon (Du Pont) nozzle, c. Brass nut, d . Silicone septum, e. Teflon (Du Pont) capillary tubing, f . Teflon (Du Pont) guiding tube, g. Sliding glass tube, h. Steel wire, i. Wheel turned by an electric motor, j , Microswitch, k . Titration vessel
which in most cases gives a rather high precision. The usual sources of error are a leak between the plunger and barrel, a nonuniform inside diameter of the barrel, and inaccurate measurement of the position of the plunger. The other method for dispensing liquids is the very recent digital droplet counter (7), which is very attractive because of the nearly complete lack of moving parts and its inherent digital compatibility. However, in the present form, the precision of the instrument is not sufficient for our purposes, and we chose the syringe buret method. An allglass microsyringe Agla (Burroughs Wellcome & Co) was used in all titrations. The total volume of the syringe is 0.5 ml and a 0.5-mm displacement of the plunger corresponds to 10 pl. The micrometer was turned by a stepping motor (48 pulses per revolution, Philips) connected via a gear-box (125:3, Philips). This combination gives 0.005 pl per pulse. The nozzle of the syringe was connected to the titration vessel via a 0.6-mm i.d. Teflon (Du Pont) capillary tubing. The nozzle construction is shown in Figure 1A. The dead volume in the syringe, nozzle, and capillary tubing is ca 200 11. The end of the tubing was drawn t o a narrow tip. In most automatic titrators, the end of the capillary tubing which delivers the titrant is held below the liquid level during the titration. In our case, the aliquots and also the total volume of the titrant are so small that diffusion from the capillary tip may become a marked source of‘ error, especially in those cases where the equilibration time between the titrant delivery and potential readout is long. Hence, it was decided t o construct a device which holds the tip above the liquid level and dips it into the solution only for a short time immediately after the titrant delivery. This was achieved by using a small electric motor and a flexible mechanical connection as shown in Figure 1. The capillary tip is put into motion by a steel wire h moving in a Teflon (Du Pont) guiding tube f The
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period
(5)
I
r
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. -
Figure 2.
DIGITAL pH-METER
,]I
i j
Block diagram of the digital titrator
steel wire is fastened to a wheel i which can make one complete revolution per each pulse received by the motor. It should be noted that the microburet has no facility for automatic refilling. The reason is that the authors have never seen a completely leak-proof three-way stopcock. In this case, when working at microliter level, the leaks that normally are left unnoticed, become increasingly important. However, the syringe can be rather conveniently refilled through the delivery tubing. Logic Circuitry. The titrator was intended for studying some complexation and precipitation equilibria where the slow rate of the reactions prevents manual titrations.
BCO OUTPUTS INTtRFPCt
Thus, it was necessary that the titration rate could be controlled in wide ranges. In certain commercial instruments, timing is arranged by RC-circuits. This method is unreliable in case of long time delays. Hence, all the relevant time delays were generated using a digital clock. The master clock frequency is 100 Hz obtained by full wave rectification and squaring the 50 Hz mains frequency. Operation of the instrument is best understood by referring to the block diagram in Figure 2. The clock frequency is fed to a five-decade master counter which divides it by suitable numbers and distributes into three presetable four-bit binary down-counters. Binary numbers from 0 to 15 can be loaded into the counters. The first counter 2etermines the length of the total period between the titrant additions. The pulse frequency it receives from the master counter can be selected to be 0.1, 0.01, or 0.001 Hz. Thus, the period can be set from 10 sec to 15000 sec. The second counter determines the stirring time which can be adjusted by similar steps. The titrant increment is set by the third counter. The pulse frequencies which can be selected from the master counter are 0.5 and 0.05 Hz. Because the clock frequency is 100 Hz and 200 pulses correspond to 1 pl, the increments which can be selected are 1 to 15 pl and 10 to 150 pl. The fourth preset counter counts load pulses of the other counters and thus controls the total number of titrant additions. It is preceded by a divideby-ten counter so that this number can be set from 10 to 150. In the beginning of the titration, the reset line is in a high state and all the counters are reset to zero. At this moment, the push-buttons giving in binary form the total period, stirring time, titrant addition, and the number of periods can be pushed down. When the start button is pushed, this binary information is loaded into the preset
IO PRINTIR
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INDICATES +5V INDICATES OY
Figure 3. Schematic diagram of the logic circuitry of the titrator. All resistors are given in kilohms and capacitors in picofarads unless otherwise noted. P1 to P4-SN 7 4 1 9 3 (Texas Instruments), D 1 to D10-SN 7 4 9 0 ( T I ) , FF-SN 7473 ( T I ) , N A N D gates-SN 7400 ( T I ) , M S 1 and MS2-SN 74121 ( T I ) A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 9 , A U G U S T 1 9 7 4
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Table I. D e t e r m i n a t i o n of Precision of Titrant Delivery U n i t b y Weighing Water Aliquots" Sample
Volumeh of H20 delivered, pl
49.81 49.80 3 49.90 4 49.81 5 49.86 6 49.80 7 49.86 8 49.80 9 49.83 10 49.80 Mean 49.83 Relative standard deviation 0.07y0
Table 11. Precision of Strong Acid-Strong Base T i t r a t i o n Using the Digital Titrator Sample
W t of KH(IOI)I, mg
Consumption of I N KOH, p l
1 2
103.72
101 .12
267.06 260.12 255 .04 222 .40 259.08 281.81
1 2
a Panel setting of the titrant increment was 50 density of water a t 26.8 OC, 0.99657 gram m1-I.
pl.
3 4 5 6
* Calculated from the
counters and the potential with no added titrant is printed out. At the same time, pulses are fed to the control circuit of the stepping motor via open gates 1 and 3 (Figure 3) and monostable MS1. The clock frequency is divided by twenty in decade counters D1 and D2 and fed to a four-decade counter D7-DlO whose parallel BCD output gives the total titrant delivery to the printer. The titrant delivery stops when the down-counter P3 reaches zero and the borrow pulse generated by P3 reverses the state of latch 11-12. The same borrow pulse generates a 1-sec pulse in the monostable 13-14 and activates the relay which controls the tip dipping motor. Stirring is also activated by the start pulse and stops when counter P2 reaches zero. When counter PI reaches zero, the total period has been completed, the printer receives a print command, the state of latch 11-12 is reversed, the preset counters P1, P2, and P3 receive a load pulse from monostable MS2, and a new period begins. Titration is completed when counter P4 reaches zero and the borrow state from P4 closes gate 1. The instrument also has a facility for coulometric generation of titrant. Flip-flop FF and gate 2 generate a pulse whose length in seconds is twice the setting of preset counter P3. This pulse can be used for activating an external constant current generator. The schematic diagram of the printer interface has not been included, because it is specific for the printer. The interface allows the simultaneous printout of titrant volume and pH on a Kienzle D24 printer (Kienzle Apparate GmbH, W. Germany).
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Calculated KOH concentration, M
0 ,99607 0.99701 99.10 0.99655 86.38 0.99613 100.68 0.99666 109.55 0 ,99699 Mean 0.99657 Relative standard deviation 0 , 0 4 1%
Precision of Titrant Delivery. The precision of the titrant delivery unit was tested by weighing successive 50-yl aliquots of distilled, deionized, and freshly boiled water in a small narrow-necked weighing bottle. Table I shows the results of this procedure. The difference between the preset value 50 yl and the mean 49.83 y l is rather large, though it has no analytical importance. On the other hand, the relative standard deviation is 0.07% which is sufficient for many purposes, but not for the study of titration errors which is one of our goals. The relative standard deviation was strongly dependent on the amount of lubricant used for the Agla syringe. The best results were obtained with very thin layers of stopcock grease. Construction of a better greaseless syringe buret is in progress in our laboratory. Another test of the instrument was performed by titrating accurately weighed amounts of potassium biiodate with 1N KOH. The results of these titrations are shown in Table II. The equivalence points were determined by using Gran's method (9) with least squares refinement. The relative standard deviation from six determinations was 0.04% which is somewhat better than the deviation obtained by weighing water aliquots. This may be due to the fact that the samples of potassium biiodate were nearly equal, and thus the plunger of the syringe was nearly in the same position a t the equivalence points. The component cost of the titrator described is about $500 excluding the printer and pH meter.
ACKNOWLEDGMENT We are greatly indebted to U. Lindstrom for his technical assistance. Received for review September 26, 1973. Accepted March 18, 1974. (9) G Gran. Analyst (London). 77, 661 (1952)
A U G U S T 1974
