2230
Anal. Chem. 1980, 52, 2239-2240
Improving the Precision and Accuracy of Incremental Potentiometric Titrations Terry W. Glass and David J. Leggett” University of Houston, Department of Chemistry, Houston, Texas 77004
The recent automation of pH-metric titrations for the study of complex equilibrium systems has relieved the tedium of data collection and improved the precision and accuracy of the gathered data ( I , 2). In titrations requiring incremental addition of titrant, it has been necessary to leave the titrant delivery tube immersed in the solution during the complete titration, which often lasts for 3-4 h. Since i t is common practice to use concentrated titrants with dilute solutions of ligand and metal ions, diffusion of titrant will occur from the tip of the delivery tube. The effects of diffusion are readily discernible from the processed data, especially when in weakly buffered regions of the titration. Attempts to minimize diffusion by adjusting the vertical position of the needle tip with respect to the buret valve and by using a fine bore (0.3 mm) Teflon needle were only partially successful in reducing the problem. When these titrations are conducted manually, the problem is simply overcome by positioning the tip of the delivery tube 2-3 m m above the surface of the solution being titrated. After a n addition of titrant the needle is briefly immersed in the solution thereby removing any drops of titrant adhering to the delivery tube. The unit reported here permits, under microcomputer control, the immersion and subsequent removal of the needle tip from the solution being titrated.
i
f
KEYBOARD AND BURET C O N T R O L
Flgure 1. Circuit diagram to control
the reversible
motor.
GUIDE POSTS
,I,
Y.2
EXPERIMENTAL SECTION A Sargent Model C automatic constant rate buret, with the piston and glassware removed, provided the source of a reversible motor. Two TTL compatible relays (Potter and Brumfield, EOT 1DB42), one 4050 Hex noninverting buffer and TTL driver, one 74121 monostable multivibrator, single, not retriggerable, and one 7400 Quad 2-input NAND gate comprise the components of the circuit. Values of capacitors and resistors are shown on the circuit diagram, Figure 1. A diagram of the immersion unit is shown in Figure 2. The microcomputer’s power supply is used to supply +5 V to the circuit. Initiation of this unit is achieved by tapping from pin 14 of the keyboard and buret control programmable 1/0 port (Intel 8255) used in the microcomputer-controlled potentiometric titration system (ref l, Figure 2). The delivery of the titrant, from an Abul3 autoburet, is controlled by a low-high-low pulse. The titrant is delivered to the solution when the pin voltage is high. ‘Thus, negative-edge triggering is necessary for this unit to be activated at the end of a titrant delivery. A negative-edge-going pulse from the microcomputer activates the down relay, and the aluminum block, controlling the Teflon needle, is lowered until it strikes the switch SW1, turning the down relay off and the up relay on. The block is raised until it strikes the switch SW2 which breaks the circuit to the up relay, thus completing the cycle. The unit now waits to receive another “end of titrant delivery” pulse from the microcomputer. The time for completion of one cycle depends on the distance of travel of the needle. This is controlled by the position of the microswitches SW1 and SW2, which are clamped on the guide posts and may be repositioned. Since the microcomputer uses CMOS chips having low current sourcing capabilities, the 4050B is used as an interface between the 8255 and the unit described here. (For reliable operation of CMOS chips all unused input pins must be connected to ground ( 3 ) ) .In this situation, the negative edge of the output pulse of the 4050B triggers the 74121 monostable, causing the voltage output at pin 1 to go low, for a short period of time. The pulse width generated depends on the capacitor and resistor used between pins 10, l l , and 14 and is not critical for this application. 0003-2700/80/0352-2239$01 .OO/O
VESSEL
Figure 2. Immersion unit.
The output of the 74121 is connected to the 7400 (Quad 2-Input NAND gate) TTL chip. The two NANID gates located between pins 1 and 6 are cross coupled forming an R. S. flip-flop. The set-reset flip-flop hasJwo stable states with two outputs, a Q output at pin 6 and a Q output at pin 4. The memory device of the R. S . flip-flop is used to drive the relays either on or off, depending on the input values at pins 1 and 5. The remaining two NAND gates between pins 8 and 13 me c r w coupled to make a conventional debougcing circuit for the microswitch SW1. The outputs Q and Q are connected to two relays which control the reversible motor of the Sargent buret. These relays replace the original control switch on the buret. The Sargent buret, long retired, provided the reversible motor and other necessary hardware for the unit. However, any reversible motor would suffice. Attached to the motor’s shaft is a piece of 1/4-20 all-thread rod. A small aluminum block is threaded to this rod and two guide posts control the movement of the block. Thus, any rotation of the motor either forward or in reverse will result in the block being moved either up or down the lead screw. A rod is attached to the block and is positioned above the titration vessel. A plastic block connected to the opposite end of this rod holds the Teflon needle in position. Consequently, the up and down motion of the aluminum block is transferred to the Teflon needle. RESULTS AND DISCUSSION T h e unit has been in operation, without failure, for 6 months. In every titration it has been found that reliable data 0 1980 American Chemical Society
2240
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
data. The precise effect of the unit is seen in Figure 3. The first plateau, with the unit disabled, shows the effects of diffusion in the positive slope of 0.015 pH units/min. T h e second plateau, with the unit enabled, it virtually horizontal with a positive slope of less than 0.002 pH units/min. These data are taken from a titration of tartaric acid, 0.002 M, with 1.010 M potassium hydroxide.
0 002 pH unitr/mln
~
LITERATURE CITED (1) Leggett, D. J. Anal. Chem. 1978, 50, 718-722. (2) Guevremont, R.; Rabenstein, D. L. Can. J. Chem. 1979, 57, 466-471. (3) Lancaster, D. “CMOS Cookbook”; H. W. Sarns 8 Co.: Indianapolis, IN, 1979; p 31.
0
2
4
6
8
Time ( m i n l
Figure 3. Electrode response with (lower plateau) and without (upper plateau) titrant delivery tip in solution.
RECEIVED for review June 16,1980. Accepted August 18,1980.
can be gathered from weakly buffered regions and there are no discernible diffusion effects to be seen when processing the
Financial support for this work was provided by The Robert A. Welch Foundation (Grant E-755) and is gratefully acknowledged.
CORRECTION Pseudopolarographic D e t e r m i n a t i o n of Metal Complex Stability C o n s t a n t s i n D i l u t e S o l u t i o n by R a p i d S c a n Anodic S t r i p p i n g V o l t a m m e t r y In this article by Steven D. Brown and Bruce R. Kowalski (Anal. Chem. 1979,51, 2133-2139) the term kf, in eq 5 and 8 should read k f h . Equation 10 should read
The last term in eq 11 should read t/21. The last line of eq 1 2 should be
ML,-,”+
+L
ML,”+
In the experimental section (p 2135, right-hand column), the sentence “The cell (27) was cleaned thoroughly with 5 N citric acid...” should read “The cell (27) was cleaned thoroughly with 5 N nitric acid...”.
