Universily
John T. Stock of Connecticut Storrs
An Apparatus for Demonstrating Automatic Potentiometric Titration
O n r gv11w:11111ct11odof (i~!twtiug:I poIrntion~cLrkr ~ point ~ d is by opposing the ditration cell m ~ fby a fixed r ~ n fequal to that drvdoprd by thc cell a t the end point. The nrt enrf then brromrs zero a t the imd point and, on slight ovrrti tratiou, reappears with ~.hitngcof sign. A pol~vity-smsitivc rontrollcr can drtcrt tho i k n ~ g of e sign and stop the thtbion. If both speed rind prwision :~rcd r s i l ~ d sonlr . for111of end point na~tiriptioninnst he providrd. llnpid titration can theu owur unt,il nrnr the end poiut, hut the. pmress can hr finishni slowly to minin~ize"overshoot." Autiripat ion r i u ~br ;~rhievndhy lot,ating thr i~~dicnt,or clectrodo nrar the poiut of cntry of thr titrmt ( I ) . A more flexible n m ~ ~ ~ g c r nisr ntot n ~ : ~ llimited :, \vhi.hirh triggers
I l ~ c"fast" valve. ('losurc of this valvr causes S2 lo switch to its lower position. Drop-out of I,, during the ~nomentarypower rut caused by switching operations is prevented by large capacitor C:!, which provides a "hold" of a few tenths of a second. Residcs restoring Imwr to LP,the switching of S2 cr~tsout the "anticipate" emf and causes the sign at the transistorized relay input to reverse again. Drop-out of Ill and (a fraction of a second later) of Lz occurs. The few final drops of titrant are added slowly, then I,, closes again. This t,ime power flows to La, triggering thc "slow" valve :md lighting the "titration completed" lamp PJ. The arrangement shown in Figurc 2 suits a titratiou system emf that decrcascs as the end point is approached. Systems that exhibit an increasing emf (e.g., the titration of iron(I1) with ceric or dichromate solulion) are handled by placing a constant emf of about 1 v i r r opposition to the system. This emf can be supplied by a Weston standard cell or a mercury "dry" cell. Thc net emf applied to the control unit then decreases as the titration proceeds. Alternatively, a 4-pole switch that reverses the input polarity, changes the "anticipat,e2'sequence, and alters the meter connections can be huilt into the control unit (7). Both increasing-emf :md decreasing-emf titrations can then be handled without the aid of an auxiliary cell. Although the control unit uwks nicely whcn a pH~nelertype saturated calomel electrode of moderately high resistance is used as a reference, it cannot operate unaided with systems of really high resistance. Titra(.ions involving the glass electrode, and some that irrvolve nonaqueous solutions or microelectrodes, arc, examples of such systems. Even these become amemble if a suitable pH meter is used as an impedancematching device. This method is attractive because the progress of the titration is continuously indicated by the pH meter. .\ rrrv satisfactory input can be takcn
:u:ross an 800-ohnr resistor that shorts t l ~ o"auLomalic teinperaturc compensator" outlet of a Leeds and Northrup Type 7664 pH meter. This signal, which appears when the compensator switch is moved from "manual" to "automatic," is about 400 mv when the pH meter rcading is near zero. Increasing the pH rnetcr reading causes the voltage t,o fall, to become zero a t a reading of 8.16, and thcn to increase again with rcversal of sign. Titrations, such as of acids with alkalies, that do not require passage of the pH meter needle over the "8.16" linc (or its equivalent on the millivolts scale) can be ~ w r i c dout if the pH meter outlet is connect,cd with correct polarity. However, meter M no longrr provides a direct indication of the end-point setting. The zero control of the pH meter is used to bring its rcading temporarily to the desired end point value. Then the reading of M is slowlv irrcrcasrd rmtil response is ohtained. Tcmpcrat~~re compensation is of course lost whcn the pH meter is used in this manner. However, the small pH error (about 0.2 units in the present, experimmts) is unimportant in routine titrations that arc c:trried out a t nvcrage room temperature. This apparatus was developed wiLh the partial support of the National Science Foundation's progmm for t,hcdesign of sciencr tmrhing cquipmcnt. Literature Cited (1) LINGANE, J. J., Anal. C'hem., 2 0 , 2 8 5 (19.18). (2) STOCK, J. T.,Lab. Prarl., 7, 349 (1958). (:() STOCK, J. T.,AND FILL,I\I. A., 11%&8l, 85, 6lIY (l!4(illl. (1) STOCK, J. T., AXD FI~J.,31. A,, J. CHEM.EDUC.,30, 2% (1953). (5) HILL,B. J., Radio Eleclronica, 29, (i),86 (I!t58). ( 6 ) KOLTHOFF, I. iH., AND LAITINEN, H. A,, "pH and Elaetro Titrabioni," John Wiley &Sorts, Irtr., New York, 1941, p. 91. l i ) STOCK, J. T.,An&rI, 87,YO8 (1062).
Volume
43, Number 8,
August
1966 ,' 427
