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Posted by on Tuesday, April 5, 2011 at 3:15pm.

a)For the titration of an acid analyte with a NaOH titrant, a phenolphthalein endpoint will change from colorless to pink. If the "pink" resulted from perhaps only a half-drop(or less) of NaOH titrant and the Erlenmeyer flask is set aside on the laboratory bench, the pink color may disappear after an hour or so. Explain.

b) Explain why it is quantitatively not acceptable to titrate each of the vinegar samples with the NaOH titrant to the same dark pink endpoint.

  • Chemistry 111 - , Tuesday, April 5, 2011 at 6:23pm

    a) My experience is that it doesn't take an hour or so but the pink color disappears after a VERY short time. The reason is absorption of CO2 from the air which produces an acid.

    b) You probably should have explained a little better since we don't know what you're doing experimentally; however, I assume the question is asking why not titrate to a DARK pink end point. The reason is that the eye is more sensitive to the faintest pink visible (from colorless) versus the intensity or hue of a darker pink color.

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