Practical investigations
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I am investigating the acidity of different vinegars. I did a titration and produced a pH against volume curve. I was wondering how I would find out the equivalence point and what does this mean?
Igloo replies
If your curve has been plotted correctly, you will find a point of inflexion at a place where the pH rises fairly dramatically. A point of inflexion is where the gradient of the curve changes its direction. In this case (assuming that volume is plotted on the x-axis and pH is on the y axis) the gradient will be increasing and increasing to a very high value, where the curve is almost perpendicular to the x-axis. Then the gradient begins to decrease. It’s at this changeover point that you need to draw a line down to the x-axis where the end point volume can be read off from the graph.
This volume represents the end point of the neutralisation, and is sometimes called the point of equivalence. This is the exact volume of alkali needed to neutralise your vinegar sample.
Assuming that you know that concentration of the alkali, you can calculate from this endpoint value the concentration of acid in the vinegar.
Risk assessment
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updated: 13 February 2006
