THE SENSES AS INDICATORS IN VOLUMETRIC ANALYSIS 1. P. PHILLIPS University of Louisville, LouisviIle, Kentucky
Ir; THE modern chemical world of instmmental indicators and automatic titrators the only one of man's senses still considered important is sight, and even this is recommended mainly for the relatively undemanding task of reading a meter. Although the ability of the eye to perceive small changes in color is certainly not exceptional as compared t o photoelectric devices, the ability to detect light of very low intensity is. It is estimated (1) that only 70 light quanta, corresponding t o an energy of about 3x10-l7 joule, can produce the sensation of light for an observer. The only indicator applications that have been proposed to take advantage of this most sensitive characteristic of vision are chemiluminescent titrations in which the solution gives off light at the end point. The titration of acids with bases using luminol (with oxidants) as indicator is the most successful of these methods, giving accuracies of the order of 1 part per thousand (B,3,4). The perception of odor is apparently capable of equally extraordinary sensitivity. One part of ethyl nlercaptan in fifty billion parts of air can be detected by odor (51, and the existence of compounds (6) certified by expert sulfur chemists to be infinitely worse than ethyl mercaptan would seem to extend the limit of identification t o much lower values. The use of sodium butvrate. sodium sulfide. ~vridine. and ouinoline as indfcators for the titratidn'if strong base; with acids has been explored recently (7), and accuracies of 0.2% have been claimed for several of these smellometric indicators. More powerful substances could probably be similarly employed. Although T. W. Richards, a Nobel prize winner in chemistry, long ago suggested that the taste of strong acid could be used as an indicator (81, this has recently been subjected t o further study. According to this
VOLUME 35. NO. 1, JANUARY, 1958
recent work (7) accuracies of no better than 1%or 2y0 can be attained in acid-base titrations based on taste changes. The well-known slippery feel of sodium hydroxide solutions has been made the basis of an instrumental titration method (9). A thread lubricated by the solution titrated is rotated on a wooden wheel; when the thread is no longer lubricated, due to the consumption of all the base by titration, friction causes the wheel t o jerk. Titrations t o an auditory end point do not appear to have been developed as yet. However, a number of possibilities suggest themselves and an explosometric end point may become a reality. It is known, for example, that one drop of 0.1 N sodium hypochlorite mixed with one drop of 0.1 N sodium azide, if slightly acidified, produces enough nitrogen trichloride t o be exploded by a burning splint held over the flask (10). (Larger quantities produce explosions of fearful violence.) The application to acid base titrations is obvious, although the experiment would perhaps require remote-handling equipment for safety. LITERATURE CITED (. I .) ZWORYKIN. V. K.. AND E.
G.RAMBERO. "Photoeleotricity," John Wiley & sons, Ino., 1949, p. 466. F., AND R. B. KURTZ, Anal. Chem.,23, 339 (1951). (2) KENNY, F., AND R. B. KURTZ,Anal. Chem.,23, 382 (1951). (3) KENNY, F., AND R. B. KURTZ,Anal. Chem.,24, 1218 (1952). (4) KENNY, (5) FISCHER, E., AND F. PENZOLDT, Ann., 239, 131 (1887). (6)FROMM, E., AND E. BA-NN, Ber., 22, 2594 (1889). E., AND J. A. KROGH, Aeta Chem. S c a d . , (7) RANCKE-MADSEN, 10, 495 (1956). T. W., Am. Chem.J . Baltimore, 20, 121 (1889). (8) RICHARDS, R. E. D.. J . Soe. Chem.Ind.. 59. 216 (1950). (9) CLARK. (ioj ~ a s c a ; F., ~ , BW., 41, 4194 (1908).
