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Anal. Chem. 1085, 57, 1112-1116
The method has advantages over the isolation procedures because it is quicker, less time-consuming, and less expensive and only a small sample is required. I t can be easily used to follow up wax oxidation reaction. If the average molecular weight of various fractions is known, their quantities can also be determined.
ACKNOWLEDGMENT The author is grateful to J. C. Petersen of the Western Research Institute, for various helpful discussions during the conduct of this work.
Registry No. Bicyclohexane-18-crown-6, 16069-36-6; n-docosane, 629-97-0. LITERATURE CITED (1) Asinger, F. “Paraffin Chemistry and Technology”, English ed.; Pergamon Press: New York, 1968; Chapter 6.
(2) Brown, R. A.; Kay M. I.; Kelliher, J. M.; Dietz, W A. Anal. Chem. 1967, 39, 1805-1811. (3) Boss, B. D.; Hazlett R. N.; Shepard, R. L. Anal. Chem. 1973, 4 5 , 2388-2392. (4) Kaffka, Karoly J.; Norris, Karl, H. Acta Aliment. Acad. Sci. Hung. 1976, 5 , 267-279. Chem. Abstr. 1977, 87, 3950s. (5) Perov, P. A.; Gerasimova, N. T. Khim. Prom-st., Ser.: Metody Anal. Kontrolya Kach. Prw‘. Khim. Prom-stl. 1979, 26-29. Chem. Abstr. 1960, 92, 103812~. (6) Petersen, J. C. Anal. Chem. 1975, 4 7 , 112-117. (7) Petersen, J. C.; Piancher, H. Anal. Chem. 1961, 5 3 , 786-789. (8) Nazir, M.; Dorrence, S.M.; Plancher, H. Pak. J. Sci. Ind. Res. 1983, 26, 202-209. Chem. Abstr. 1964, 100, 1 0 6 1 2 8 ~ . (9) Jones, R. N.; Ramsay, D. A,; Kanlr, D. S.;Dubriner, K. J. Am. Chem. SOC. 1952, 7 4 , 80-88. (10) . . Pedersen, C. J. J. Am. Chem. SOC. 1967, 69, 7017-7036.
RECEIVED for review August 27, 1984. Accepted December 26,1984. The author is grateful to the Council for International Exchange of Scholars for the award of a Fulbright-Hays Scholarship and to the Pakistan Council of Scientific and Industrial Research (PCSIR) to avail of this scholarship.
Automated Equilibrium Titrator Based on a Personal Computer Alan P. Arnold,’ Susan A. Daignault, and Dallas L. Rabenstein* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
An equllibrlum potentiometric tltrator has been developed around an IBM personal computer. The titrator allows unattended collection of hlgh-precislon potentiometric data from an lndlcating and reference electrode pair and automatlcaily adds titrant from a motorized buret under control of an Interactive BASIC program. Titrations can be performed In millivolt or pH modes, and tltrant Increments can be of constant size, or dynamically determined so as to malntain constant changes in mllilvoits or pH. These methods are sulted to obtaining hlgh-precislon equlilbrlum data for metalcomplex formatlon studies but also allow rapld and precise standardlratlon titrations. The ease of use of the tltrator and the accuracy and precision of the system have been evaiuated by determining equllibrlum constants for the formatlon of the Ni( I I)-giyclne complexes over the pH range 2-9.5. Data processlng for electrode callbratlon, solutlon standardizatlon, and refinement of acld dissoclatlon and metal-ligand complex formation constants can be conveniently performed on the IBM PC, uslng implementatlonsof standard formation constant calculation programs whlch are usually reserved for mainframe use.
The collection of high-precision potentiometric equilibrium titration data is essential for accurate determination of metal-ligand formation constants. This is particularly true for ternary M-L-L’-H systems, in which the complexity of the solution composition may require collection of hundreds, perhaps thousands, of titration points on solutions of varying metal-ligand ratio. In addition to the automated titration equipment which is available commercially from several manufacturers, many designs for automated titration apparatus have been described in the last decade, and these fall into two main categories: Present address: Department of Chemistry, University of Tasmania, Box 252C, G.P.O., Hobart, Australia 7001. 0003-2700/85/0357-1112$01.50/0
(i) Dedicated, hardware oriented titrators which may be microprocessor controlled (1,2). These are suited to repetitive standardization or simple equilibrium titrations but do not have the flexibility provided by high-level language interactions between operator and instrument. Many commercial titrators fall into this category. (ii) Titrators designed around mini- (3-6) or microcomputers (7-13)in which the interactions between operator and instrument are, to varying degrees, more “user-oriented”. Microcomputers used in these previous studies, while adequate to the tasks of slow data collection required in equilibrium titrations, suffer from limited memory, which precludes all but the simplest types of data analysis without data transfer to a more powerful computer. While minicomputers are generally better suited to the data processing requirements, their use in potentiometric data collection is an inefficient use of a relatively expensive instrumentation resource. Moreover, it appears that costly development of interface hardware has been an integral part of all of the previous studies. The relatively recent proliferation of low-cost microcomputers with large (up to 1 Mbyte) memory capacity has provided thirdparty vendors an opportunity to supply low-cost data-acquisition boards which are bus-compatible with several of these computers and are more than adequate for sophisticated control applications. We describe here the design and implementation of a titrator which uses an IBM personal computer (IBM PC) together with an inexpensive, commerically available data acquisition board, to interface to a research grade pH meter and commercially available motorized buret. The titrator is designed for reliable, unattended collection of precise volume, pH data pairs under equilibrium conditions. Data processing is performed on the IBM PC for solution standardizations, for electrode calibration, and for obtaining formation constants of metal-ligand complexes, using FORTRAN programs such as ACBA (14),MINIQUAD (15-13, and KINET (18), which are usually reserved for mainframe use. The performance of the titrator has been evaluated for both end point and equilibrium titrations. The performance for equilibrium titrations was 0 1985 American Chemical Society
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evaluated with the Ni(II)-glycinate-proton system, which has been the subject of a detailed interlaboratory study (19).
EXPERIMENTAL SECTION pH Measurement. Philips GAT130 low-resistance glass (0-14 pH) and Philips R44/2-SD/l inverted glass-sleeve, doublejunction, saturated calomel reference electrodes were used with a research grade Orion 701A pH meter. The outer liquid-junction solution of the reference electrode was 1M NaC1. The electrodes were supported in an airtight double-wall cell of capacity 15-100 mL, with a design similar to that of Perrin and Sayce (20). The electrode assembly was thermostated over its entire length to minimize thermal gradients in the electrodes and provided stable potentials over long periods. To check the stability of the electrode system, millivolt data were collected automatically overnight from a magnetically stirred NBS pH 4 phthalate buffer with the TITRATE program. Fifteen-second averages of 30 readings, taken every 2 min, showed a drift of less than 0.1 mV/h over a 24-h period, with no excursion from an hourly mean value of more than 0.1 mV. Preparation and Standardization of Solutions. Doubly distilled, deionized water (resistance >5 x loe Q cm) was used throughout. All solutions except the titrant were prepared in 0.99 M (Na+)Cl-and 0.01 M HC1, hereafter referred to as “solvent”. Sodium hydroxide titrant was prepared by dilution of a saturated solution and was found by equilibrium potentiometric titration of primary standard grade potassium hydrogen phthalate (Fisher) under “dynamic” conditions with potential increments of 10 mV to have a concentration of 0.8754 f 0.0005 M. The titrant was stored in a polyethylene reservoir of the Mettler DVll autoburet and was delivered to the titration solution through a Pederson 5-pL micropipet, the tip of which was placed in the solution. Drift in pH due to diffusion from the tip is negligible. The acid concentration of the solvent was determined to be 0.09431 f 0.00007 mol kg-’ by titration of gravimetrically dispensed aliquots with the NaOH titrant. From these titrations, the electrode pair was calibrated as a proton concentration probe at 25 “C, using the relationship
E,,,
= Eo + 59.159 log [h]
by an IBM PC implementation of the FORTRAN program ACBA (14),which allows simultaneous refinement of any parameter for a general acid-base titration. Initial estimates for the solvent acid concentration, electrode E O , and pK, were simultaneously refined. For consistency with the previous nickel-glycine studies, we have used the above approach rather than using electrode calibration algorithms which account for increasing liquid junction potentials at low pH and with variable slopes of the type
Emeter = Eo + s log [h]+ j,[h] Aliquots of glycine hydrochloride solution (Eastman) and solvent were dispensed gravimetrically from small polyethylene wash bottles and were titrated with NaOH over the range 1.6 < pH < 11.5. The ligand concentration, acid association constants, electrode calibrationparameter, E O , and pK, were simultaneously refined by ACBA on individual sets of titration data, collected over a period of 6 weeks. The electrode calibration parameter, E O , was evaluated each time the electrodes were removed from solution, in order to account for changes in liquid-junction potential and other, less well-defined potential changes in the electrode chain. A ligand concentration of 0.022 94 i 0.000 04 mol kg-’ of solution was obtained, which corresponds to a purity of 100.0 i 0.2%. Weighted mean values of 9.621 i 0.004 and 12.064 f 0.032 for the acid association constants of glycine and 13.693 f 0.003 for pKwcobtained from these data fall well within the ranges found from the previous interlaboratory study: 9.653 (range 0.041), 12.083 (range 0.1041, and 13.693 (range 0.06), respectively (19). These values were fixed in subsequent MINIQUAD treatments of nickel-glycine data. Figure 1 shows half of the experimental points from two titrations used to obtain glycine pK,s and the titration curves calculated with the mean values of the equilibrium constants. A stock solution of NiC12.6H20 (Aldrich) in solvent was standardized by gravimetric titration with EDTA in ammoniacal solution, using murexide or pyrocatechol violet as the indicator
10
8
I n 6
4
0.2
0.4
0.6
0.8
1.0
1.2
Volume,mL
Flgure 1. Potentiometric titration curves for two concentrations of glycine in acid solvent. Half of the experimental points are shown; the solid curves are the theoretical titration curves calculated with the constants obtained from the data.
(21) and was found to have a nickel concentration of 0.015 19 k 0.00002 mol kg-l. The solution was stored in acid-washed borosilicate glass. IBM PC Configuration. The memory of a first generation IBM PC with 64 kbyte RAM was extended to 320 kbyte with a Quadboard multifunction board (Quadram Corp., Norcross, GA). The Fortran compiler (Microsoft version 3-13),running under PC-DOS 1.10, utilized the hardware floating point support provided with an Intel iAPX8087 fast numeric processor chip (IBM PC 8087 Math Coprocessor,part 1501002). The use of two floppy disk drives (320k) to store programs and data separately is convenient but not essential. Twenty-four lines of the 8255 parallel interface on a PC-Mate Lab Tender data acquisition board (Tecmar, Inc., Cleveland, OH) were programmed to read the BCD output of an Orion 701A pH meter. One channel of the AM9513 timer chip on the board was programmed to send trains of TTL pulses (10000 = 1 mL) to the trigger inputs of a Mettler DVll motorized buret. Four more timer channels are available for additional burets or other devices if required. It was necessary to isolate the digital i/o lines on the acquisition board from the BCD output of the Orion meter to prevent loading of the meter circuitry. A small box containing 12 dual optoisolators (MCT6) was inserted in the cable connecting the BCD output from the pH meter to the parallel interface. A 5-V supply line for the optoisolators was taken from the meter. (Details are available from the authors.) This optical isolation box was the only hardware modification in the titration system and should not be necessary with pH meters having galvanically isolated BCD outputs or with RS232C outputs which may be connected directly to serial i/o ports of the IBM PC. Additional A/D and D/A converters on the Lab Tender may be useful for temperature monitoring or magnetic stirrer control but have not been used in this work. The Control Program, TITRATE. TITRATE is written in interpreted IBM BASIC to take advantage of IBM PC features such as programmable function keys, and sound capabilitiesto provide prompts and warnings to the user during data input and collection phases of the titration. A flow chart of the program is shown in Figure 2. A listing is available from the authors on request. The control parameter input routine prompts the user to enter the required parameters to control the titration. Typical inter-
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY I985 C0"tlOl PBmmeIer
2.5
Input
2.0
and pH meter
& Read pH meter 30 llmei
-E
1
\
I"(
1.5 In' '0 f
1.0
0.5
0
1.22
1.24
1.26
Volurne,rnL
Derivative curve in the end-point region of the titration of -0.045 M HNO, with -0.2 M KOH in 0.3 M KNO,.
Figure 3.
Figure 2.
Flow chart of the control program
TITRATE.
See text for
details. actions are as follows: Is the titration in mV or pH? Are volume increments, AV, to be constant (if so, to what value) or dynamically controlled to maintain constant mV or pH increments (if so, what should the increments ApH or h V be)? What is the initial buret reading (usually zero, but this provides for some prior manual titrant addition)? What is the maximum allowable pH or mV change per minute, slope,,, for equilibrium to have been considered attained? All of these parameters may have been previously stored on a disk file from psevious titrations and can be recalled at this stage. When all titration parameters are entered, TITRATE asks whether the data will be processed by our implementation of a recent version (17) of MINIQUAD (15, 16) or in another way. If MINIQUAD is chosen, processing parameters such as the number of formation constants, their initial estimates, concentrations of titrant, etc. are requested. Again, these may have been stored in the parameter file from previous titrations. If MINIQUAD processing is required, the collected titration data and processing parameters will automatically be written to a disk data file in the standard MINIQUAD format. If another method of processing is required, only volume and pH data pairs are written to the data file to be manipulated later. We have used the rigorously weighted nonlinear least-squares program KINET (18) on the IBM PC, to refine estimates of titrant concentration and electrode Eo from standardization titrations, but find that ACBA is more convenient for stanqardization and ligand titrations. The algorithm in ACBA appears, however, to be much less efficient than that of KINET or MINIQUAD. Typical run times on the IBM Pc for four to five parameter refinements of 60-100 points used to obtain the glycine acid association constants fall in the range 5-30 min (depending on hitid estimates) with ACBA, several times longer than MINIQUAD run times with the more complex Ni(I1)-glycine-proton equilibrium systems (see below). After parameter entry is complete, the program halts to allow the user to make final solution adjustmenb etc. and then begins the data acquisition on request. The Orion pH meter is read when its "data ready" signal is h'igh. Typically 30 meter readings are collected, twice per second, and analyzed by linear regression to
determine the trend and the standard error of the regression. When the change in meter readings over this period falls significantly (twice the standard error of the slope) below the user requested maximum (e.g., slope,, = 0.001 pH or 0.1 mV/min), the system is considered to be at equilibrium. In unattended operations, it is possible that the desired equilibrium may never be attained under certain circumstances,e.g., in the case of a noisy electrode if the equilibrium criterion is unrealistic, or near unbuffered equivalence points. To prevent the program from reading a drifting or fluctuating meter indefinitely, a maximum delay parameter is included, after which time the next titrant increment is necessarily added. In the dynamic titration mode, the number of volume pulses necessary to provide the required potential increment is calculated by the hyperbolic extrapolation algorithm of Smit and Smit (22). This predictive algorithm fits the titration curve to the previous three titration points and thus cannot be used to calculate the first two titrant additions. Thus for dynamic titrations, the user is prompted to enter a value for the first titrant increment during the parameter input phase of TITRATE. A second titrant increment to give the required change in potential is calculated by assuming a linear slope of the titration curve at the beginning.
RESULTS AND DISCUSSION End-Point Titrations. The performance of the dynamic titration mode of TITRATE for end point titrations was evaluated by titrating small aliquots (5-7 mL) of ionic strength The medium (0.3 M KNOB)containing -0.045 M "Os. H N 0 3 solution was introduced gravimetrically into one compartment of an H-shaped cell. The two compartments were separated by a small porous junction and contained the indicating glass electrode and titration solution, and reference electrode and 0.3 M KNOBliquid-junction solution, respectively. Figure 3 shows the derivative curve (dE/dV) in the region of the end points from one of several dynamic titrations with 0.2 M KOH, prepared from concentrated volumetric solution (J. T. Baker, C0.270 K2C03) and diluted with deaerated, doubly distilled deionized water. Each titration was performed with potential increments of 10 mV, with 120 s maximum delay times between titrant additions. Equilibrium (
