Practical investigations
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Hi. I am doing my advanced higher chemistry project on finding the best procedure to do a weak acid/weak alkali titration. Could you please tell me how I would know if my results were correct. The solutions I am using are:
Calcium hydroxide
Barium hydroxide
Ammonia
Citric acid
Ethanoic acid
and finally Oxalic acid.
Ulex replies
First of all, you have a very nice selection of weak acids, mono-, di- and tri-protic, but your selection of bases is not so appropriate. Of your three, only ammonia is weak. The other two, though limited in solubility, are both strong, which is to say that in solution ionisation is effectively complete. In fact it is quite difficult to find common, accessible weak alkalis. You might consider amines but they are rather unpleasant to work with.
I presume you are going to try a number of ways of detecting the end point. The only ways I can think of to check the accuracy of your results are as follows.
1 Use solutions of exactly known concentrations. This is possible with citric acid and oxalic acid because, being solids, you can weigh them. Ammonia cannot be weighed so you would have to determine its concentration using titration against a strong acid. This would involve end-point determination using an indicator, which you may feel defeats the object of the investigation.
2 Use a pH meter to plot the titration curves. This has the merit that the result does not depend on one single measurement. You can check the accuracy of the pH meter by using buffer solutions made up from buffer tablets.
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updated: 28 January 2007
