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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Versatile Microcomputer-Controlled Titrator A. H. B. Wu and H. V. Malmstadt" University of Illinois at Urbana-Champaign, School of Chemical Sciences, Department of Chemistry, Roger Adams Laboratory, Urbana, Illinois 6 180 1
A versatile automatic microcomputer-controlled titrator is described that is capable of handling UV-visible spectrophotometric, potentiometric, and amperometric end-point detection and acid-base, redox, compleximetric, and precipitation reactions. Titrants can be delivered either by burets operated in a continuous constant rate mode, or incremental modes, or by coulometric generation. Auxiliary pipets and burets are interfaced for dilution, buffering, and adjustments of pH or ionic strength. The instrument is made from standard laboratory modules including titrator stand, spectrophotometer, potentiostat, constant-current source, and pH meter. The characteristics of the specific modules, the types of programs used for determination of equivalence point, interactive data reduction methods, and the overall system are described.
For many years hardware-controlled automatic titrators have been used for a wide variety of titrations (1-8). They have varied in the manner and degree of automation. Some provide simple control circuitry for handling the titration operations while others combine control and data acquisition with sample-handling features and offer a higher degree of automation. In recent years, there has been considerable interest in using computers for titrations, particularly where Gran plots (9), Hofstee plots ( I O ) , or modification of these (11-20) could provide improved determination of the equivalence point in potentiometric, spectrometric, and amperometric experiments. More recently, microcomputers have greatly increased the possibilities of improved automated titrations at modest cost. Betteridge et al. (21)report the use of a microcomputer in a pH titration of an acid mixture with incremental addition of base. Equivalence points were determined by a differential plot which included a smoothing routine. Fudano (22) has applied a microprocessor titration system to the analysis of fats and oils. The analysis proceeds by a potentiometric titration where the titrant is added incrementally according to the titration signal. Earle and Fletcher (23)have developed a system for coulometric titrations and use the microcomputer for data acquisition and control of complex sample-handling procedures. Applications include multiple acid-base end points, and potentiometric redox titrations involving hypochlorite. Leggett (24)uses an 8080-based microcomputer for an automatic potentiometric titrator. Applications to the determination of the ph,s of benzoic and ascorbic acids are reported. Several commercial instruments have become available that utilize microcomputers. The Radiometer titrator (No. DTS 633 Copenhagen, Denmark) provides for a digital titration where the optimum sizes for titrant increments are calculated based on the slope of the titration curve. These points are retained in memory and are also used to calculate the end points. This system has been applied to the determination of calcium and magnesium with EDTA, and other systems (25, 26). The titrator developed by Mettler (No. SR10, Princeton, N.J. 08540) provides for potentiometric, amperometric, and colorimetric titrations. An elaborate sam0003-2700/78/0350-2090$01 OO/O
ple-handling routine has been developed which enables serial weighing, identification, transfer, and analysis of samples. The titrator also features electrode conditioning and treatment controls, operating error and diagnostic controls, thermostating controls, and components for many titration arrangements. The major purposes of this study have been twofold. First, to demonstrate that standard laboratory modules and an inexpensive microcomputer can be readily assembled into a very versatile titrator which controls titrant delivery, pretitration adjustments, end-point meaurement, equivalence point determinations, calculations, and display of results in the derived units. Second, to demonstrate with specific examples how it is easy to switch from potentiometric to amperometric to spectrophotometric end-point detection for acid-base, redox, precipitation, and compleximetric titrations by utilizing selected software programs for various types of end points. Data are presented to show excellent quality of the results for representative titrations. The characteristics of the specific modules, the types of programs used for determination of equivalence point, interactive data reduction methods, and the overall system are described.
INSTRUMENTATION We have found that most routine titrations can be performed accurately and precisely using a buret that is operated in a continuous mode. Slight overshoot errors can be readily corrected by blank-correction calculations. This simplifies the measurement process without sacrificing precision and accuracy. Linear and quadratic extrapolation routines have been developed in the determination of some of these end points. More difficult end points utilizing incremental titrant delivery and Gran plots are also possible with this system. Coulometric generation of titrant is very useful when applicable and this, too, has been used with this system. At present, titration programs are stored on paper tape and can be loaded into the computer memory prior to a specific type of titration in seconds. In the routine mode, the program automatically selects the measurement parameters necessary for that particular titration. In the investigative mode, measurement parameters can be selected by the operator prior to and during the course of the titration. Interactive data reduction routines have been built into several end-point detection programs to allow for computational interaction. Titrator Stand/End-Point Transducers. The titrator stand used is a part of the spectro-electro titrator (No. S-29700, Sargent-Welch Scientific Co., Skokie, Ill. 60076). It was originally built to be capable of automatic titration with spectrophotometric or potentiometric end-point detection. However, for spectrophotometric titrations, the stand has been completely changed to incorporate a commercial spectrometer (series EU-700 GCA/McPherson, Acton, Mass. 01720). The use of a stable UV/visible light source, monochromator, and photomultiplier provides a much larger spectral range including the ultraviolet region, better wavelength isolation, low stray light, and greater sensitivity for spectrophotometric end-point detection. P M tubes that are sensitive to specific spectral regions have been used for a further increase in sensitivity (No. R166, 1P28, and R4840, RCA Corp., Somerville, N.J. 08876). A special optical rail has been built to C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
-l . .......:.
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........................................ ............ ....................................... . . . . .
...."lt""'"'."'''"''".'
I
I . . . -
INTERFACE
-#
Ti t r a t i o n Signals
Auxrl I a r y Potentiai
Electrometer
+Signal d n t rol 4 Interface
'
Potent I ostat On/
off
-I
Auxiliary Buret
AUX
Y
Titrant
L
Current
Encoder Pulses
Buret
kliv
c
\
L
L
'L
3 Const C u r r
- 1"
Electrode A r r plif ier
Current
Generator
Sour
*. T i t rator Stand
W
Stirrer
\
t
Mono
PM' Currmt (chuge)
A-
Potential
,
.
Figure 1. Block diagram of automated titrator
align all of these modules including the titration stand so that the spectrometer is easily assembled. Potentiometric titrations are performed by using the titrator stand in its original form. The electrodes for potentiometric titrations are connected to an amplifier whose output signals are sent directly to the computer. Amperometric titrations are performed using a simple system (27) that is based on operational amplifiers. Computer System. The microcomputer used is the ADD8080 system developed by Avery and Lovse (28). This system consists of components housed on functional modular cards designed for easy rearrangement and expansion. The microcomputer requires the use of the microprocessor unit board (MPU) where the Intel 8080A microprocessor resides (Intel Corp., Santa Clara, Calif. 95051). In addition, the following peripheral cards are needed for the titrator: 16K of random-access memory (RAM) (No. T M S 4045, Texas Instruments, Dallas, Texas 75222) organized on two 8K boards, 2K of programmable read-only memory (PROM) (No. 2708, Intel Corp.), a serial 1 / 0 board where the USART resides (No. 8251, Intel Corp.) for serial communication via a teletype or CRT, one &bit parallel DAC (DAC-08, Precision Monolithics Inc., Santa Clara, Calif. 95050) port for output via an 8 bit 1 / 0 port (No. 8212, Intel Corp.), and 8 peripheral 1/0 ports residing on 3 parallel 1 / 0 cards each containing a
programmable peripheral interface chip (Yo. 8251, Intel Corp.). These 8 1 / 0 ports provide for data acquisition, titration sequencing decisions, control functions, device selection, and communication through an optical paper tape reader (OAE OP80-A, Oliver Audio Engineering, No. Hollywood, Calif. 916105). All titration programs were written in Altair BASIC (MITS, Albuquerque, N.M. 87108) and are stored in paper tape form. The onboard BASIC interpreter as well as user programs are loaded into RAM through the tape reader. BASIC occupies 6K of RAM, thus 10K is available for titration and data reduction programs, and memory storage. The PROM is programmed to contain the 8080 system design kit monitor (SKD-80, Intel Corp.). It enables examination and deposition of memory programs and registers, and allows for machine-level language programming. Other PROM programs enable reading and punching of user programs in a number of formats. A block diagram of the titration system is shown in Figure 1. Cells and Components. Various types of cells and components were found to be optimal for certain titration arrangements. For spectrophotometric titrations in the visible region, 30-, SO-, and 100-mL glass beakers were used. For titrations in the UV, special 30- and 50-mL cells were made from 42 mm i.d. quartz tubing sealed at the bottom. For potentiometric and amperometric titrations, disposable 30-
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Table I. Summary of Control Bits for Titrator signal input interface (Figure 2a) BIT 0 current (spectrometric, amperometric) or voltage (potentiometric) input selection BIT 1 primary or auxiliary titration signal input selection BIT 2 analog differentiation (Type I ) or direct voltage input (Types 11-IV) selection device control interface (Figure 2 b ) BIT 3 primary analyte electrode activation BIT 4 titration stirrer activation BIT 5 coulometric generator activation BIT 6 auxiliary buret activation BIT 7 auxiliary electrometer activation analyte buret control interface (Figure 2c) BIT 8 counter reset BIT 9 latch activation buret mode selection BIT 1 0 BIT 11 00 off 0 1 delivery 1 0 refill 11 rapid delivery
and 50-mL polystyrene beakers (American Hospital Supply Corp., McGaw Park, Ill. 60085) were used. The critical step of titration stirring so as to provide rapid mixing was accomplished by motor-driven rotating stirring rods (No. S-76669, Sargent-Welch Scientific Co.). Various blade sizes from AA to B were used according to the size of titration cells. Titrants delivered by burets pass to the cell through delivery tips (No. S-29702) designed for the specific titrator stand t h a t was used. One or two tips, carefully positioned next to the stirrer, were used depending on the titration application. Platinum flag electrodes were used in coulometric titrations. The counter electrode was isolated from the system by a glass isolation tube with a porous frit a t the bottom of the compartment. For potentiometric titrations, the indicator electrodes used were the ring-type platinum-rhodium and the silver/silver chloride electrodes. Reference electrodes were the saturated calomel and the carbon electrodes. A combination glass/calomel electrode (No. 51209 Graphics Control Corp., Buffalo, N.Y.14240) was used as the indicator/reference pair during pH titrations. Two identical platinum wire electrodes were used for amperometric titrations. Signal Control Interface. A number of different titration arrangements can be selected from software using 12 output bits of information generated from the signal control 1 / 0 device of the ADD8080. These 12 bits control analog switches and triacs (29) that select the various titrimetric circuits and devices desired. Figure 2 shows the relationship of these output bits with the switches and the various devices they control, and Table I summarizes these operations for the titrator. A special note concerning the important process of titration stirring is to be made. Bit 4 allows for independent titration stirring by activating the motor that drives the stirring rod. Prior to the activation of the titrant delivery, the operator can either wait for the titration reaction cell to reach an equilibrium with this stirring process or it can be controlled by software. Programs can be set for either a specified wait time or a specified precision of the incoming titration signal. Bits 6 and 7 control the selection of an auxiliary buret and electrometer should one be desired. A pretitration for the control of solution pH is a very practical example of when these would be needed. The buret would contain buffering material and the glass/calomel pair would serve as the auxiliary electrodes. The delivery of the analyte
titrant requires bits 8-11 of the computer to perform all of the necessary operators as described in the next section. Delivery of T i t r a n t s . Coulometric generation of titrant was accomplished using an unmodified coulometric generator (Model IV, Sargent-Welch Scientific Co.). Motor-driven constant rate burets (No. S-11120-52,Sargent-Welch Scientific Co.) were used to deliver titrants in noncoulometric titrations. These burets were operated in either the 1 or 5 mL/min modes. The burets were modified for automatic control. An encoder wheel with black and white painted edges was mounted on the drive shaft of the motor enabling pulse production from an optical isolator (No. H13B1 General Electric Co., Syracuse, N.Y. 13201) positioned next to the encoder wheel. This isolator sends a pulse on each darkto-light transition of all edges of the encoder and provides 40 pulses per revolution of the motor shaft. This produces a resolution of 0.25 fiL/pulse in the delivery mode. The pulses are sent to a 16-bit counter and its count is fed to the computer. Control is through bits 8 and 9 of the first control 1 / 0 as shown in Figure 2c which allows for precise counter control. For a titration, the counter is first reset prior to the delivery of titrant. Then, during the titration, the program can sample the contents of the counter without disturbing the titrant flow. The counter continuously tracks the buret delivery during the titration while the computer fetches samplings at regular intervals. Each sampling is deposited into memory and consists of a value corresponding to the amount of titrant delivered a t the time of the sampling and a value corresponding to the titration signal for that sampling. The three major buret operations of delivery, refill, and fast delivery have been placed under computer control through the use of mechanical relays. Bits 10 and 11 of the fast control 1/0 enable any of these operations. In normal or fast delivery operations, the contents of the buret are delivered to the cell. In the refill mode, the buret must be connected to the titrant reservoir. Consequently, a solenoid valve (series 1-17-900, General Valve Corp., East Hanover, N.J., 07936) has been installed to automatically perform this function. Bits 10 and 11are also connected to a decoder so that when the refill mode is selected, the valve automatically switches to the refill position. When refill operations are complete, the valve switches back to the normal operating position. D a t a Acquisition Interface. A number of methods can be devised to perform the conversion of signals in the analog domain to the digital. We have chosen an integrating-type voltage-to-frequency converter for performing this function for the titrator. This method allows for either fast conversions for certain titrations where the conversion rate governing the resolution is of primary importance, or for slower conversions where the signal-to-noise ratio is improved by the integration. A block diagram of the circuit used for this conversion is shown in Figure 3. The voltage from the signal-control interface is first fed into a voltage-to-frequency converter (CA-8400 Intech Incorporated, Santa Clara, Calif. 95050). The V-to-F converter accepts only positive voltage signals so that the absolute-value circuit is necessary. The frequency produced is then sent t o a 16-bit counter whose output is fed into the data 1/0 of the computer. The latch and reset for the counter are controlled by separated monostables which are triggered by the time base. When the measurement time is over, the first monostable triggers for the latch, informs the awaiting 1 / 0 that the data are available for input, and then the 2nd monostable resets the counters for the next measurement period shortly after the information has been transferred to the computer. The length of integration for each point is controlled by the selected time-base duration. A crystal oscillator and appropriate scalers enable variable time bases to be set. Typical time bases used in our titrations vary but
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
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C BIT
8 16 Bit Coilnter
r5V
$00
BIT9
ii 120 V K
BIT
IO
Buret 2 m , r
+r-
Buret
i> 'ell
To P l m c r
BIT I I
Figure 2. Signal control interface. (a) Signal input interface, (b) device control interface, (c) analyte buret control interface
generally allow between 2 t o 50 points/s to be taken. End-Point Detection/Equivalence Point Determination. Because of the many different types of titration curves
it is necessary t o use several different techniques for determining the equivalence points. End points have been classified in this report into 4 types and are schematically shown in
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Table 11. End-Point Detection Schemes and Typical Examples classification end-point program I analog 1st derivative digital mathematical extrapolation of least squares fit I1
typical examples spectrometric, with indicators; potentiometric spectrometric, one titration component' absorbs; amperometric, of some irreversible systems I11 digital mathematical extrapolation of least squares fit spectrometric, more than one component absorbs; amperometric, some reversible systems with 1 polarized electrode IV special programs and interface spectrometric, of multiple components; amperometric, of some reversible systems specialized with 2 polarized electrodes (dead stop end points end point) ' Titration component is defined as either titrant, reactant, product of reactant, or an added indicator.
, I I
Figure 3.
I1
Data acquisition interface
Figure 4. Table I1 summarizes the types of end-point detection schemes used and typical examples are given. These titration curves have been reviewed by a number of investigators (7, 8, 30, 32). We have found that the most effective method of data reduction for end points in type I is the 1st derivative approach where the peak maximum of the first derivative corresponds to the end point ( 2 ) . The differentiated signal is converted to a digital value that is sent into the microcomputer. The program inputs successive values at a rate of 20 ms/pt. yielding a resolution between points of 0.33 pL. While keeping track of how much titrant has been delivered, the software continuously checks to see if the differential input value has exceeded some threshold value that indicates the beginning of the end-point region. When such a threshold is reached, the program inputs and stores the next 200 points which comprise the end-point region information including sufficient overshoot of the end point. After the end-point region, the titrant delivery is automatically stopped and the computer routine searches over the stored 200 points for the maximum signal, determines how much titrant was delivered for that maximum, and calculates the equivalence point based on this. It was found that direct digital differentiation did not provide precise results for broader sigmoid curves typical for titrations of analytes of low concentration. A simple solution was to use analog filtering and differentiation circuit that is shown in Figure 2a. Digital filtering and differentiation are being investigated in our laboratory. End points of type I1 and 111 have been previously handled in two ways: either by taking a derivative of the curve thus producing a sigmoid curve and processed as with a Type I end point, or by graphical extrapolation of linear portions of the curve to a point of intersection. The first method will produce poorly defined sigmoid curves if the original titration curve exhibits curvature or nonlinearity, or if the slopes of the two linear segments do not differ by much. In addition, analog
MI - LIEOblVALE hCE OF TlTRA*.I
Figure 4.
Types of titration curves
differentiation introduces much noise. Graphical extrapolation tends to be tedious and is subject to operator bias. In order to produce a more precise and unambiguous end-point assignment, we have developed mathematical routines for each curve of Types I1 and 111. Such an extrapolation is also known as the tangent method and has been described previously (32). Not all chemical systems allow for tangental extrapolation and thus more complex numerical routines have been developed by other investigators (19,331. To illustrate the technique, consider the curve in Figure 4-11 labeled "*". Before the end point, the absorbance of the solution (assuming a spectrometric titration) does not change. A horizontal line can be obtained that characterizes this region of the titration curve. After the end point, the absorbance increases with increasing titrant addition because of titrant absorption. A linear least squares fit can be made to describe this portion of the curve. But because dilution tends to reduce the linearity of this region, we have chosen to select a quadratic least squares fit of the points obtained in this region. The end point is then determined by the program by finding the mathematical intersection of the horizontal and quadratic lines
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
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Table 111. Results of Test Titrations type
a
end-point transducer system
titrand
titrant
acid-base
spectrophotometric ( I )
HOAc
1.0 M NaOH
precipitation
potentiometric ( I )
NaCl
0.1 M AgNO
compleximetric
spectrophotometric ( I V )
2.0 mM EDTA
redox
amperometric ( I V )
0.1 M KMnO,
redox
potentiometric ( I )
0.1 M NaOCl
redox
spectrophotometric (11)
0.05 M NaOCl
redox
coulometricispectrometric (11)
In umol.
(NII,),SO,
In me Ni250 mL.
describing the titration curve before and after the endpoint. Points are integrated a t 0.4-s intervals. This longer integration time permits the entire curve to be retained in memory, since there are fewer points. The program determines whether a significant change in slope has occurred between points signifying an end-point region. When one is detected, 20 points are taken both before and after the end point. Because the titration usually exhibits curvature at the end point, points in this region are not used in the mathematical routines. T h e exact number of points removed depends on the degree of curvature and must be determined during the development of a procedure. Figure 4-11 shows possible regions used for the mathematical extrapolation. For these titrations, interactive data reduction procedures can be used. Frazier (34, 35) has developed interactive titrimetric procedures that allow the operator to choose the most Nernstian region of potentiometric titrations. McCullough and Meites (36) use point-rejection routines to discard points in multiparametric curve-fitting programs for segmented titration curves. For titrations in this system, since the digital curve is available immediately after the end of the titration, the operator can change measurement parameters and calculate a new end point if he suspects the validity of the original calculation. Nonhomogeneous mixing may cause noisy points and may require the operator to remove these and recalculate the end point. Entirely different measurement regions can be chosen for the routine since the data are stored and are available for inspection. U'e have found that most titrations do not require removal of points within a measurement region. and thus point rejection routines are not used to simplify programming. Hard copy of the titration points is available
coulometric generated OBr-
Taken, mmol 0.2568 0.7553 1.069 1.357 1.925 0.0625
Found, mmol RSD.% 0.2563 0.32 0.21 0.7575 1.066 0.10 0.19 1.352 0.14 1.920 0.32 0.0628
0.1000
0.1000
0.17
0.1700 0.2000 0.2500 0.99 7 9' 1.497 1.996 2.744 0.2006 0.4117 0.6417 1.053 13.01b 30.06 38.75 52.44 64.64 74.66 7.14a 14.28 35.70 49.98
0.1697 0.2006 0.2502 0.9964 1.500 1.994 2.735 0.1989 0.4099 0.6358 1.050 13.08 30.21 38.62 52.41 64.63 74.16 7.22 14.24 35.74 49.99
0.08 0.15 0.12 0.36 0.37 0.27 0.24 0.56 0.52 0.43 0.26 0.59 0.37 0.32 0.20 0.16 0.09 0.35 0.23 0.25 0.18
1.790a 3.580 5.370 7.159 8.949
1.780
3.590 5.376 7.156 8.956
0.39 0.45 0.21 0.09 0.13
__-
if the operator desires, for data reduction a t a later time. End points in the last cla5sification require special programs to handle titrations not belonging to Types 1-111. T h e dead-stop titration of Figure 4-IV provides for the very simple end-point detection of monitoring for the theoretical zero current level. For redox couples that are not completely reversible. a program searching for the minimum current is employed. In the chemical example given in the next section, the titration of H202with KMnO,, the flow of titrant is shut off and the equivalence point calculated after the input current signal has been reduced to
