Practical investigations
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For my practical investigation, I am trying to determine the acid content of wines but have had difficulty finding the end point of titrations with NaOH. When using phenolphthalein indicator the colour change was very gradual, so I have used a pH meter to plot curves of pH/ volume of NaOH added, but how do I use these to find the end point?
Igloo replies ...
I am surprised that you have found the colour change to be gradual. Phenolphthalein normally gives sharp end-points in this titration.
In any case, using a pH meter probably gives you more accurate results, provided that you read off enough points at or near the end point to plot a good curve. If your curve has been plotted correctly you will find a point of inflexion at a place where the pH rises fairly dramatically. This represents the end point of the neutralisation.
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.
Always carry out a risk assessment and check with your teacher before starting any practical work
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 08 February 2006
