Acid-base equilibria
Read our general notes on Risk Assessment
I have chosen to do my individual investigation of comparing the effectiveness of different methods of identifying the end-point of netralisation reactions. I am looking at different acid-base combinations and using indicators, colorimetry and conductivity to idenify the end point. I am a bit confused over how to use the graph of amount of alkali titrated against pH to find the end point, as I understand it is not the same as the pKa. Is it where the line of the graph plateaus out? Or is it the point at which the buffering capacity is overwelmed? Thanks in advance for your help!
Ulex replies
Usually the pH is plotted against the volume of alkali added (horizintal axis). However, if you are plotting volume of alkali (vertical axis) against pH (horizontal axis), the end point is marked by the horizontal part of the graph, the plateau, as you call it.
If either or both of your acid or alkali is/are strong, this plateau should be well-defined and the end point easily identified. If both acid and base are weak, the plateau may not be apparent as more than a couple of changes of slope, in which case it will be much harder to identify the end point.
The buffering capacity of the reaction mixture starts to be overwhelmed where the horizontal part of the graph starts and is complete when it finishes. In most cases the difference between these two is caused by less than one drop of added alkali.. Where there is doubt, because the horizontal part of the graph is short or not well-defined, the end point would have to be taken as mid-way between the two. Such a situation would not make for a very reliable titration using an indicator, which is why you don’t try to titrate two weak reagents together, but your other measurement methods may make it possible.
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.
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 09 March 2007
