Paul C. Hiemenz
and Elizabeth Pteiffer California State Polytechnic College Pornona, 91768
A General Chemistry Experiment for the Blind
It is diflicult to estimate the number of blind students currently attending colleges and universities in the United States and harder still to guess how many of these students are enrolled in chemistry courses which ordinarily involve laboratory work. More to the point, it has not been possible to discover any references which suggest what arrangements are made for these blind students as far as laboratory requirements are concerned. While several works have been published (1,2) which propose science experiments for the blind, they are either written for elementary or secondary levels of education or are intended for situations in which a fair amount of equipment especially suited for the needs of the hlind is available. Wexler (2), in fact, laments that even in schools for the blind teachers often have "to make do with the standard apparatus commercially produced for science work in sighted schools." The usual college chemistry department, however, encounters blind students infrequently, and therefore must either deal with them using "standard apparatus" or else excuse them from laboratory requirements. There is some evidence that the latter is the more usual case (3). Trying to find a suitable laboratory activity for a blind student was of more than academic interest to the authors: one of whom is a hlind student (E.P.) whose curriculum requires a laboratory course in chemistry; the other (P.C.H.), a teacher who was reluctant to waive the required experimentation. To find a bma fide quantitative experiment that could be performed independently by a blind student became our goal; "playing by ear" was both figuratively and-as it turned out-literally, our method. A Method for Weighing
Most laboratory courses in general chemistry begin with instruction in the techniques of weighing. It was not difficult to develop a technique by which a sightless person could determine the weight of an object, independent of visual assistance. The triple-beam balance (Ohaus-710) is ideally suited for this. The object to be weighed is counterbalanced by positioning three riders until equilibrium is reached. The operation, therefore, consists of two parts: judging the point of balance and reading the value of the weights. Although the balance is equipped with a pointer moving against a raised scale, experience showed it is easier to judge the point of equilibrium by the feel of the pan as it moves against the base of the balance. Reading the values of the weights was trivial in the case of the "hundreds" and "tens" since the respective beams are notched for these weights. The rider which measures units and tenths of a gram moves on a relatively smooth beam, however. Its posi-
tion on the beam is easily measured using a Braille ruler and a screw on the front of the beam as a point of reference. A correction has to be subtracted from thib measurement to allow for the fact that the position of zero weight does not correspond to the reference point for the distance measurement. This measurement is facilitated if the rider is taped to the beam after the equilibrium position is located. With a little practice, the student is able to establish the weights of objects with an estimated uncertainty of *0.2 g. Condudometric Titrotion
Development of a technique for weighing opens the possibility of quantitative mixing of both solids and solutions. Thus a titration experiment is feasible, once provision is made for detection of the end point. Conductivity was the obvious method to investigate at this point. Not only does the conductivity of a solution show a discontinuity at the equivalence point, but also the use of an audio indicator to judge the null point of a bridge make this technique ideally suited for the blind. The ordinary procedure (4) in a couductometric titration requires the measurement of solution conductivity after the addition of successive increments of titrant. A plot of the data shows two linear portions which, when extrapolated until they meet, serve to indicate the point of equivalency. This procedure is unsuitable for a blind worker because it requires periodic readings of the bridge and also graphical manipuhtion of the data. Although the first obstacle may he overcome by desiguing a circuit involving decade resistance boxes on which the number of stops can be counted, a more straightforward method is available if the appropriate chemicals are used. Provided the discontinuity in conductivity is sharp enough at the equivalence point, the variation in the audio signal with variation in conductance can be used directly to detect the end point. The results to be reported below were obtained by titrating trichloroacetic acid with NaOH solution. The specific objective of the experiment was to determine the equivalent weight of the acid. I n the experiment described, the hlind student made all of the measurements without assistance, although a helper was available to assist with some manipulations. Similarly, the data were recorded by the student at the time of observation, using a Braille typewriter. A portion of "unknown" is weighed out in a tared weighing bottle. The sample is then dissolved in water, and the weight of the final solution is determined. Thus the weight percentage of unknown in the solution is known. From this stock solution aliquots of measured weight are taken, so the weight of unknown in each aliquot is known. The aliquots are placed in a Volume 49, Number 4, April 1972
/
263
conductivity cell (Smgent-29870) equipped with a magnetic stirrer for titration with NaOH solution. The electrodes were attached to a conductivity bridge [Industrial Instruments, Inc. Model RC 16B2) whose output, which normally activates an eye-type null detector, was passed through an amplifier (CenCo 80576) to a loudspeaker. A buret containing NaOH, weighed when full, was used to add the base until a minimum in sound intensity was perceived, indicating the end point. The amount of base added was determined by re-weighing the buret. Detection of the end point was improved by use of a "blank" in the following manner. The electrodes were immersed in a solution titrated to the phenolphthalein end point, then the amplifier gain control was adjusted to a barely audible signal. The electrodes were then replaced in the unknown solution and the titration was carried out immediately. Having a specific sound to aim toward and reestablishing it for each titration, made it easier to identify the end point. The equivalence point determined in this manner was always within about *0.4 ml of the end point as judged by the color change of the phenolphthalein added to the same solutions. I t should be emphasized that the blind student evaluates the end point completely independently by this procedure, the presence of the phenolphthalein is for the benefit of the helper who must then resist the temptation to coach from the visual appearances of the solution. An anecdote of an actual experience may help convince the skeptical. I n one titration, during which the helper was occupied elsewhere, the blind worker decided the end point had been overrun, since the sound intensity had passed through a minimum. The solution still appeared colorless so the helper meddled by suggesting that another drop of base be added. Although the change in sound intensity was virtually imperceptible to the helper, the student insisted that it was getting louder. Addition of an "extra" drop of phenolphthalein produced a pink solution! The moral of the story is obvious: to the blind variations in sound perception may be used as reliable indications of the progress of a chemical reaction. Results
I n the present experiment, three different stock solutions of unknown were prepared and a total of eleven titration runs were performed. It was of interest to see whether repeated determinations would lead to any improvement in either the precision or accuracy of the results. This was not the case. Once the basic technique was developed, all subsequent runs were within the ranges cited above. The practice obtained in the repetition did enable the run to be completed more rapidly, however. The table summarizes the results of this study. I n calculating the results of the experiment, the weight of titrant was converted to a volume, using 1.02 g/ml as the density of the standardized 0.4917 N NaOH solution (5). Thc equivalent weight is then calculated using the following expression
Consideration of the accumulation of random errom
in this experiment is noteworthy. Each of the weights in the expression above involves two weighings, so the expected uncertainty in a weight is about Zt0.4 g; the end point determination introduces another Zt0.4 g uncertainty in the titrant weight. Considering typical sample sizes, random error theory predicts a scatter of results within about 6.5y0 of the true value, i.e., 163 .L11. Only one of the determinations fell outside of this range while the average deviation was 4.1%. The average value of the equivalent weight in the various runs is within about 0.5% of the true value. Conclusions
An experiment of this kind must surely rank as a "classical" general chemistry experiment, a t least when done with the usual visual indicator. With fairly common laboratory equipment, a modification can be devised which a blind student can complete virtually unassisted. Although some extra effort is involved in setting up this type of experiment, the opportunityafforded infrequently to most instructors-to share in the student's accomplishments compensates for this. The student's reactions are exactly those which pedagogical theory says a laboratory should accomplish. First, the development of new skills generates enthusiasm; and second, concepts learned in lecture (e.g., normality, equivalency, neutralization, etc.) become more real when one actually works with them in a concrete experimental situation. Lastly, some suggestions might be made for further adaptations of experiments to the needs of the blind. The work reported here reveals how one experiment which is usually performed in general chemistry can be modified for use by the blind. The techniques described above can obviously be extended by combining with other procedures. For example, an experimental study of the variation of pH with volume of added titrant could be developed by devising a method for measuring pH. Removal of the protective cover from the face of a pH meter and adding raised markers (e.g., a length of wire glued or taped to the dial) a t occasional intervals could be read in much the same manner as a Braille watch. In fact, an extended pointer could be read against an enlarged scale written in Braille with the same accuracy as the parent meter. Other experimental procedures of a totally different nature can also be developed. For example, interpretation of spectra might be taught in the following manner. The pen motion of a recording spectrophotometer can be fol-
Summow of Exoerimental Results in Acid-Base Titrotions
Percent Unknown Vol Equivalent wt of unknown W t aliquot in aliquot 0.4917 IV in solution (p) (p) NaOH (m1) unknown (g)
lowed by the gentle touch of a hand to locate the position of prominent peaks which could then be marked with a stylus on the chart paper. Marking the peak locations of a known standard on the same chart would ~ e r m i tsome degree of qualitative identification from paper to pursue these ideas further.
Literature Cited (1)
F ~ K E B ,
chsrles
W. H., FDLHER, M., M~eohniquea ~ i t h~ c T ~ O ~ S Sspringfield, . I I I ~ ~ O ~1968. S.
~
~
~
i
b
~
(2) WEXLZR. A., "Experiment~lScienoe for the Blind," Pergamon Presm New York. 1961, p. i.(d. bibliography). (3) SCOW. R.A,. " T ~ S~ ~ k ofi ~~ ~ .i "M~U:. d R ~ sape ~F O U ~ ~ ~~S ~ ~~ O ~ I . .
..
(5) "International Critical Tables." 3, 79 (1928).
Volume 49, Number 4, April 1972
/
265
~
