An Olfactory Indicator for Acid-Base Titrations A Laboratory Technique for the Visually Impaired Mark N. Flair' and William N. Setzer2 The University of Alabama in Huntsville, Huntsville. AL 35899 Whileit is impossible for the blindstudent t o be completely self-sufficient in the chemical laboratory, experimental modifications leading to greater autonomy for these individuals should be pursued. I t is desirable to develop techniques that can serve to help integrate handicapped students into chemistry and provide a more meaningful laboratory experience. A good olfactory acid-base indicator would be of great utility in increasing the participation of the visually impaired in the chemistry laboratory. Although assistance will still be required in most phases of experimental work, the ability t o perform titrations would clearly enhance the value of the laboratory for the student." The use of olfactory indicators has been described? For our specific purpose we have examined four compounds as potential olfactory indicators: eugenol, thymol, vanillin, and thiophenol.
Eugenol
Thymol
0.CQ0" OCH, Vanillin
Thiophenol
In each of the experiments, a small amount of the potential indicator was dissolved in 25 mL of 2 M NaOH. The basic solution was then titrated with 2 M HC1. The equivalence noint (endooint) in these titrations was determined based . . upon the smell of the titrated mixture. Equivalence points were later verified using phenolphthalein indicator. As olfactory indicators, thyrnol and vanillin do not provide clear endpoints in acid-base titrations. Thvmol has a pungent phenolic odor that, in our hands, seem; to linger, masking the titration endpoint. I t may also he that endpoint detection is inhibited by numbing of the sense of smell in the case of thymol. Vanillin, on the other band, is unsuitable due to its relative lack of odor a t the equivalence point, owing perhaps t o its relatively high solubility in water (vanillin, 1 mLL5 el100 mL:. thvmol. . . 1 e/1000 Thiophenol, even in minute quantities, possesses a strongIvdlsaereeableodor. but it did behaveasexoected. Although there is always the lingering smell even in basic solutions,k the equivalence point the repulsive, penetrating stench becomes overpowering. While this is chemically noteworthy, it
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Table 1. Tltratlon ot 25.00-mL Portlons of 2 M NaOH Solutlons wlth 2 M HCI Solutlons (Reproduclblllty Study) Run
Eugenol
1 2 3 4
27.57
Table 2.
Vol. HCI (mL) Phenolphthalein 27.57 27.50 27.70 27.65
27.38 27.50 27.41
Difference 0.00
0.12 0.20 0.24
Tltratlon 01 2 M NaOH Solutlon wlth 2 M HCI Solutlon 1Reverolbllltv Stud")
Voi. NaDH (mL)
Vol. HCI (mL)
Additional baselacid
cannot be seen as an alternative indicator; thiophenol is both toxic and obnoxious. Eugenol proved to be completely successful as an olfactory indicator for acid-base titrations both in terms of the obvious endpoint of the titration (the pungent fragrance of cloves bursts out of the titration mixture a t the eauivalence point) and the consistency of the results. The resilts of our titrations using both eugenol and phenolphthalein are summarized in Tables 1 and 2. The data in Table 1 show that titrations usina a eugenol indicator are as reproducible as rhose using ph&olphthalein. Note, however, chat the eugen d endpoints require ahout two drops less acid than the ohenolobthalein enduoinrs. That is. after the endooint of ihe titiation was determined by the strong smell oi cloves, nhenolohthalein indicator was added .. eivine a . oink solution and titration mnrinued until the visual endpoint (colorless) was obtained. The difference in endnuints is nerbaos due to the difference in pK, values for the indicators ( p ~ofi eugeohenolphthalein = 9.7. and pKn no1 = 10.0...DK,- of . . - of thiophenol = 6.5% The reversihility of this titration technique is illustrated by the data in Table 2. Adding additional base in 5-mL portions, and continuing the titrations still gives sharp, detectable endpoints. Two drops of eugenol are distinctly pun-
' U. A. H. undergraduate, 1988-1990.
Author to whom correspondence should be addressed. For articles on teaching and laboratory aids for the visually impaired, see: (a)Crosby, G. A. J Chern. Educ. 1981, 58, 206-208; (b) Tombaugh, D. J. Chern. Educ. 1981, 58, 222-226; (c) Smith, D. J. Chern. Educ. 1981, 58. 226-227; (d) Lunney, D.; Morrison, R. C. J. Chern. Educ. 1981.58.228-231. %be, S.:Kon, S.; Matsuo, T. Anal. Chirn. Acta 1978, 96,429-434. The Merck index. 9th ed.; Merck: Rahway. NJ. 1976. Dean, J. A. Handbook of Organic Chemistry; McGraw-Hill: New York, 1987. Volume 67
Number 9
September 1990
795
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eent in 55+ m L of liauid. Because the frazrance disanoears .. when the eugenol solution is re-basified, back-titrations are feasihle. It is suspected t h a t a small amount of eugenol (about 2 drops) would suffice for titrations of even large volumes of solution. A hlank for comparison is easily prepared using t h e desired volume of water and two drops of eugenol. It should be observed t h a t the pungent clove odor is uniformly strong even in large quantities of water. More than 10titrations were performed over t h e two studies with eugenol, and yet there appeared t o he little "numhing"of the sense of smell. It might be supposed t h a t there is a saturation point of some sort where thenose would become numb and thus temporarily "deadened" t o t h e fragrance of eugenol; this was not, however, observed. Experimental Eugenol and thiophenol were obtained from Aldrich Chemical Company and were used without further purification. Thymol was
796
Journal of Chemical Education
obtained from Fisher Scientific Company and vanillin from Eastman Chemical Company. Titrations were performed using approximately 2 M NaOH and 2 M HCI. Reoroducibilitv Studv. In each titration. two droos . of eueenol werr'dissolvrd i n 25 W ' ~ Lof NaOH sduiion. Ths mixture waa titratrd with HCI solution, using rnagneticstirrmg. Endpoints were determined by olfaction, then confirmed with phem,lphrhnlpin. 'l'he results are rompiled in Table 1. lIeoecnht1,t) Stud, .Two drops ofeugenul were dissolved in 2 X 0 ml.2 M R'aOH and titrated to t h e o l f a c t u r s m d v ~ , i n t . A n n d d ~ t ~ ~ ~ n n l 5.00 mL base was added to the mixture;eras&g the eugenol fragrance, and the mixture was again titrated to the olfactory endpoint. This process was repeated several times, for a total of five blind trials, which are summarized in Tahle 2. Thymol. Ten drops of 50% (wlv) thymol in ethanol were added to 25 mL NaOH solution; then titrated with HCI. Vanillin. Ten drops of a saturated solution of vanillin in ethanol were added to 25 mL NaOH solution; then titrated with HC1. Thiophenol. Two drops of thiophenol were added to 25 mL NaOH solution which was then titrated with HC1. ~
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