OCTOBER, 1946
Changing Phenolphthalein from Colorless to Pink by Adding Acids LEON MeCULLOCH ff
Westinghouse Research Laboratories, East Pittsburgh, Pennsylvania
color of phenolphthalein in strong soluTHEtlons . pinkof sodium hydroxide fades and the solution becomes colorless, the change being reversible. If the solution is largely diluted, the pink color reappears after a time-rapidly on heating. It may not be known that the reversal from colorless to pink can take place immediately by adding a few drops of strong acid to the dilute colorless alkalme solution. This unusual reaction could be demonstrated before an audience to excite curiosity and might even lead some student to inquire into the molecular rearrangements invo1ved.l .Prepare a solution of sodium hydroxide and water in
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IThe rearrangements in pheqalphthale+ are discussed by R. C. F u s o ~AND H. R. SNYDER m "Orpanlc Chemlstw," John Wiky and Sons, Inc., 1942, pp. 4723, and by C. A. PETERS AND B. C. RBDMON, J. CHEM. EDUC.17, 525-9 (1940).
equal parts, add phenolphthalein in alcohol, and let stand until the pink solution becomes colorless. Dilute to about ten times its volume and i t remains colorless still. Into a portion of the colorless alkaline solution in a test tube, add a few drops of hydrochloric, nitric, dilute sulfuric, or phosphoric acid. The alkaline solution immediately turns pink. Excess acid, of course, makes i t colorless as usual in acid solutions. The change may be produced also. by formic acid but not by acetic. Neutral salts are without effect. The strong acids seem to act as catalysts of the colorproducing reaction in the phenolphthalein. The slight temverature rise caused by the neutralization of the acid by the base does not account for the accelerated reversal, ~~~h higher temperatures are to produce the change in similarly short periods of time.
