Phenolphthalein indicator


4 December 2002

My quest for the perfect titration photograph resulted in this image. I would have liked to get Ivan's face showing through the flask, but that just gives me something to shoot for next semester.

This is a titration of 25-ml of Heinz Distilled White Vinegar (5% acidity) with a sodium hydroxide solution that the students standardized during their lab last week. The indicator is, obviously, phenolphthalein, which is magenta-colored in bases, like NaOH, and colorless in acids such as vinegar.

I particularly like this photo because it shows the titration as it appears after the addition of one of the final few drops prior to the "end-point," when the entire solution would turn pink. What you see here is one drop of NaOH (pink) dispering through a solution that is primarily acidic (i.e., vinegar, also known as acetic acid). This drop eventually cleared up completely. After a drop or two more, the solution turned a nice, pale shade of pink: a perfect end-point.

The object of this titration is the determination of the precise concentration of the vinegar. Last week, the students found the precise concentraation of their sodium hydroxide solution by titrating it into a solution containing a measured mass of potassium hydrogen phthalate (an acid in solid form that is typically used for this purpose).


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