Dennis E. Tallman North Dakota State University Fargo. Nwth Dakota 58102
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A pH Titration Apparatus for the Blind Student
T h e student afflicted with any of a number of physical handicavs able t o adapt himself t o the teaching . is eenerallv .. latwratory so as to ol~tnina meaningful lahoratory experienw in m i t e of his disability. This adaption often occurs without suhkantial effort on t h e part o f t h e laboratory instructor. However, the student handicapped by total blindness is a t a particular disadvantage in the teaching laboratory where uisualization of chemical phenomena is the primary goal. T h e success of numerous laboratory experiments depends upon the student's ability t o perceive color changes or phase transitions, t o quantitatively dispense solids and liquids, andlor to read an instrument output from a n analog or digital meter or recorder. W e recentlv.h a d the oouortunitv t o devise a l a h .. oratory experiment for a blind studrnt enrolled in a freshman lawl analvtical couric. Since a ereat deal of ernohasis in this course is placed on the analytical applications of equilibrium chemisty, i t was natural t o attempt to design a n experiment which would provide the student with a real feel for those changes in solution composition which accompany addition of titrant in a neutralization titration. A description of such changes is eiven in the lectureldiscussion portion of the course, h u t & s t - h k d "ohservation" reinforces the hasic concepts and often stimulates additional inquiry from t h e student. Furthermore, hands-on experience with lahoratory instrumentation affords t h e student the opportunity t o witness the importance of instrument calibration and to assess the impact of real-world instrumental problems such as noise and drift. ~
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Figure 1. Schematic of circuit used to transduce the current output from the Beckman pH meter into a voltage far input to the audio-encoded Simpsan voltmeter.
Figure 2. Braille labeled buret and audio-encoded Simpson meter, components of the titration apparatus.
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Volume HCI (mi) Figure 3. Plot of dataabtained by b e blind stu6ent fortitration of 0.0598 g Na2COa with 0.0882 MHCI.
The Apparatus A Beckman Zeromatic I1 pH meter was selected for the experiment. The large push button controls at the front of this meter facilitated its operation by the unsighted student. One problem, of course, was to transduce the pH information from the visual domain to a domain readily sensed by the student. This was accomplished by removing the internal shorting strap from the recorder output and feeding the current from this output to a current-to-voltage converter and then to an inverting amplifier with adjustable gain and adjustable offset (Fig. 1).This op amp circuitry was patched together using a Heath Analog Digital Designer and Burr-Brown 3308112C op amps. The voltage output of this circuit was then sensed by means of an audioencoded Simpson multimeterl operated in the voltage mode on the 10-V scale. The Simpson meter, shown in Figure 2, produces an audio tone which is minimized in intensity when the pointer knob of the meter is adjusted so that it points to the voltage being applied to its input. The position of the pointer and the meter switch positions are braille encoded so as to he easily read by the student. We found it convenient to calibrate the instrument (vide infra) so that the lowest marking on the Simpson meter corresponded to pH 2 and the highest corresponded to pH 12. In this way, each major division on the Simpson meter scale corresponded to 1 pH unit and the pH range from 2-12 was generally adequate for most titrations. Of course, further scale expansion and a concomitant increase in precision of the reading is possible. The instrument was calibrated by the student using Beckman buffer solutions employing the two-point calibration technique. Three buffer bottles and a set of corresponding beakers were labeled in hraille to contain buffers of pH 4,7, and 10. Two of these buffers, the choice depending upon the pH range of interest, were selected for the calibration. The pH electrode, in our case a tip-protected Corning combination electrode, was immersed in the low pH buffer, the pointer on the Simpson meter was set to the buffer pH and the offset control of OA 2 (Fig. 1) was adjusted for minimum intensity of the audio tone. After rinsing, the electrode was immersed in the high pH buffer and the expansion control of OA 2 was adjusted, again for minimum intensity of the audio tone. This calibration sequence was repeated until no further adjustment of the offset and expansion
1Awilable from Srimcr tor the Blind Products. 221 Rock Hill Road. Bnln-Csnwyd,Pa. 19004. Volume 55, Number 9, Se~tember1978 / 605
controls was required. I t should be noted that adjustments of the offset and expansion controls during calibration of the Simpson meter have no effect on nH readines obtained direetlv from the Beckmsn pH meter.Thus, I f b ~ i r e dthb , Beckman p~ n&rn,uld he cal~hrated hy the instruttor prior I U fhp experiment, thrrrhy ennlhnr: the i l l strurtw tc, monitor rhc vnlldlry d t h e darad,taincd hg ihr stwJent from the Simpson meter. Titrant was dispensed from a 10-ml Metrohm piston buret, the numerical scales of which were labeled with braille encoded label tapeP (Fig. 2). The vertical ml scale on the buret waslabeled so that the top dot of each braille pattern was even with the corresponding ml marking of the scale. The thumbwheel was also labeled a t 0.1-ml intervals, with the rightmost dot in each pattern directly on top of the corresponding mark. The etched lines on the thumhwheel a t 0.01-mI intervals were easily located and counted by feel and were used to determine the hundredths place of the reading. With very little practice the student was able to dispense from and read this burette to within 0.01 ml. Discussion Sodium hydroxide and hydrochloric acid solutions of approximately 0.1 M were provided for the student. Samples to be titrated were dispensed by the laboratmy instructor. The student, however, carried out all other operations independently, including calihration of the Simpson meter, refilling the piston buret, manipulation of the eketrade and magnetic stirrer, and collection of the titration data which was recorded on a Perkins Brailler.3 The initial exoeriment oerformed hv the student was a titration the very large pH change which occurs a t the equivalence point of a strong acid-strong base titration and the data obtained was subsequently used for computing the malarity of the NaOH solution. Next, the student titrated a t 0.0598-g sample of primary standard Na2CO3 with the HCI solution. The actual date points obtained by the student are disnlaved . in ..eraohieal form in Fieure 3. Note that a rrf~llinguf the hurcr uas reqwred midway through t h rltr4ti.m ~ and u,ai awmnpliahed by the studrnr wilh nu discern~hlee t t r ~ on t thc titrat~onuara. Fnm thrvolume requ~redt
