An Automatic Titrator for Demonstrating Indicator Color Change
John T. Stock University of Connecticut Storrs, Connecticut 06268
Two high-sensitivity relays and a latching relay are used in a device for demonstrating the behavior of an acid-base indicator upon alternate addition of acid and alkali.' Figure 1 shows the circuit of a simplified version that has a single low-cost relay. Transistors Q2and Q3 are in a Schmitt trigger arrangement that is fed by isolating transistor Q1. The maximum current passed by the transistors is only ahout 20 mA, so that they can be energized by penlight cells. An alternative is an inexpensive transistorized power supply such as Lafayette 99T5068.'
Figure 1.
Cimuit of titrator.
Fixed r r b m n are half-watt.
R1. 14. Rz, Rp, RLO, R, .RI
8 2 0 ohm 3.3K 5 0 0 ohm (LEVEL) 2 0 ohm
Re
4 7 0 ohm
RT
220 ohm 5.1K 1 5 ohm (RANGE) 1 0 ohm
Ui Rn Ru
G, O.lpF
G,1OOlrF Dl,diode.
1 N 3 4 or similar Q,, Q1, Q b 2 N 3 3 9 1 A (General Electric) K, Relay, Potter & Brumflold 6 V. t v ~ oRSSD I , 2, i n d i 3, 4, titrant valve powen 5, 6, ACID volvs 7 , 8 , ALKALI valve
Like the earlier device,' the present version is operated by the signal obtained from the "automatic temperature compensator" outlet of a Leeds & Northrnp pH meter. This outlet is connected so that binding post 2 becomes decreasingly positive as the pH falls. Suppose that the contacts of relay K are in the position shown, so that the solution containing bromothymol blue or other indicator is being titrated with acid. Eventually, transistor Qz cuts off sharply a t a potential that is set by LEVEL control Rs. This turns on transistor Qs, energizes K, and changes the titrant from acid to alkali. The pH and the potential of post 2 begin to rise to a limit that is set by RANGE control Rll. The relay then drops out, the titrant changes back to acid, and the cycle repeats. STOCK, J. T., J. CEEM.EDUC., 38,633 (1961). Lafayette Radio Electronics, 111 Jericho Turnpike, Syosset, N.Y. 11791.
Before starting automatic titration, approximate settings of R3 and Rll are made while rotating the zeroing control of the pH meter, so that the meter pointer moves slowly hack and forth over the desired pH span. A second titrator, made with "three for a dollar" 2N3392 transistors, was almost as sensitive as the one based on 2N3391A devices. Figure 2 shows the circuit of a simple line-operated adaptor for driving the titrator by the signal from the "recorder" outlet of any suitable direct-reading pH meter. The 8-transistor PA 238 integrated-circuit operational amplifier retails a t $3.30. Because this adaptor inverts the signal, input from the pH meter is connected so that post B becomes increasingly positive as the pH falls. The pH meter reading is then adjusted to give exactly zero recorder output. (This reading, obtainable from the maker's leaflet, is not always 7.00. The Leeds & Northrup older type 7664 meter has zero output a t a pH reading of 8.16). BIAS control R14 is then used to bring the output of the adaptor to zero. Because of mismatch of supply voltages and of resistors Rlz and Ria, it may he necessary to permanently shunt one of these resistors with a high 0.5 - (approximately -. M) resistor. Controls R1, (FEEDBACK, set at approximately 0.25 M ,) and R,. can be baseboard mounted within the ;diptor case. -'~lternatively, RANGE control RII and the 10-ohm resistor Rlz in the titr+tor can he replaced by a &ohm fixed resistor. The width of the pH span over which titration occurs is then adjusted by the FEEDBACK control. This apparatus was developed with the partial s u p port of the National Science Foundation's program for the design of science teaching equipment. ~
I
-
~~
I
Rm Rls, RM,
33K 5 0 0 ohm [BIAS)
Rm, RII,
3 9 0 ohm 1 M (FEEDBACK]
Ru, R19, 4 7 0 ohm Ca. 0.001 pF CI, 4 7 pF Cs, Cs, 5 0 0 pF DI,Ds,1 N 4 0 0 1 (Motorold Z,, Zn, rener diode, 6V, VR6A (Sarker Tamion) F. fuse. 2 5 0 mA. slow blow 1; 1 2 . 6 . ~tranrforrnor (Triad F-25x1 IC, PA238 (General Electrid A, 8, input 1 m titrator post 1 2*, to titrator post 2
*,
1
1
Figure 2.
Circuit of adaptor foruse with recorder outlet.
Fixed resiston
are half-watt.
Volume 47, Number 4, April 1970 / 31 1
