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ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
Microcomputer Controlled Potentiometric Titration System for Equilibrium Studies D. J. Leggett University of Toronto, Erindale College, Mississauga, Ontario, Canada L5L 1C6
A microcomputer controlled automatic potentiometric titration system has been developed for solution equilibria studies. It is programmed, using Programmable Read Only Memory (FROM) to deliver an optimized volume of base that will correspond to uniform changes in electrode potential throughout the entire titration. Data are output via a thermal printer and a hardware option is available for recording the same data on magnetic tape using a cassette recorder. The choice of data processing algorithm employed to elucidate all equilibrium parameters is left to the user. Trial titrations demonstrate that the system can produce data that lead to very precise and accurate equilibrium constants. The pK,s of ascorbic and benzoic acids have been redetermined to be 4.00, and 3.99,, respectively in 0.100 M KCI at 25.0 'C. The hardware has been designed to permit the conversion of this system to a multiple end-point titrator simply by replacing the existing PROMS.
T h e determination of stability constants and therefore the elucidation of complex solution equilibria is commonly performed by potentiometric titration. Although many other techniques are available, it has been found that potentiometry is capable of yielding rnore definitive answers to a wide range of equilibrium problems ( I ) . T h e normal procedure involves the measurement of hydrogen and/or metal ion concentrations. by means of a suitable concentration probe(s), together with the volume of titrant added to produce that concentration. Frequently this is performed with a manually operated microsyringe, research quality p H meter or electrometer, and a suitable titration vessel ( I , 2 ) . Although this approach does lead. with suitable precautions, to precise data. it can be arduous to perform on a daily basis, particularly if the probe is slow t o reach equilibrium. Automation of this type of potentiometric titration system has been described by Ginstrup ( 3 ) and Gobom and Kovacs ( 4 ) . T h e systetn developed by Ginstrup is not based around a computer, but consists of a series of electronic modules. The functioning of the system may be described as a rigid cycle program with some key parameters that may be adjusted before commencing the titration. The titrator has no inherent facilities to decide whether equilibrium is attained. Gobom and Kovacs 14) have developed a potentiostatic type titrator for solution equilibria studies, built around a Henlett-Packard HP2114A computer. Coulometric generation or mechanical delivery of titrant are available as options. Several automated end-point titrators are commercially available that have the capability to record the full course of a potentiometric titration. T h e common disadvantage with these digital titrators is that, in order to achieve this, the researcher is purchasing a system having several features unnecessary in this context. T h e Radiometer Company (Radiometer A/S. Copenhagen, Denmark) has produced a system based on work by Christiansen et al. ( 5 ) controlled by a microcomputer. Although this system is reasonably suited to solution equilibria studies, it is nearly impossible to change the software owing to the use of read-only-memories (ROM) 0003-2700/78/0350-0718$01 .OO/O
Table I. Instrument Components of Titration System Digital p H meter
Radiometer Model PHM64 Accwracy .0.002 pH; & 0 . 2 mV, 0.05% of reading. Radiometer Model A B U l 3 Autoburctte Type B230, Capacity 0.25 mL. Accuracy + 0.25 p LL2;.'L%o Electronic counter with BCD output. Date1 Systems, Inc. Model DFP-7. Intel 8080A microprocessw . Intel chips, RAMS and PROhTS, SD IC-80-B. +
Automatic buret
Digital panel printer Microcomputer
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that cannot easily be reprogrammed 'This puhlication reports the design nf a potentinmetric titration system controlled h> a iriicrocoinpiiter T h e sriftwarc~ has heen designed to produce n permanent iernrd nf the total volume o f titrant deli\ ered and tlie cnrresporiding pH meter reading T h e data may he processed bv conventional algnrithms tn jieltf overall ~ t a h i l i t \ tnnstanrs for metal ion protoq. ligand interactions. XLlinor cliaiiges in the -oftu:ire. made possible through the use of prograrnmahle r e a d - m l \ memories iPROhl) permit this
