Chemical amounts (A2)
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I titrated molar HCl aginst the same strength fixed volumes of sodium hydroxide, potassium hydroxide, sodium hydrogen carbonate and potassium hydrogen carbonate for my coursework. I expected the two hydroxides as strong bases to have the highest volume of acid to reach the end point. I found however that the potassium hydroxide had the lowest and the other three were pretty close. I did three readings for each alkali and they all were in a reasonable range so I can't make sense of this. I used methyl orange indicator. Can you help? 260308
Ulex replies......
If you used the same concentrations of the bases, you were titrating with a strong acid and you used methyl orange as indicator, I think the titres should all have been the same! Any variation in the values is simply a measure of the lack of accuracy in the concentrations of your solutions.
Titration volume is not a matter of strength; weak bases, just like strong ones, react completely with strong acids. The fact that weak bases have a smaller concentration of hydroxide ions is irrelevant because when those hydroxide ions are used up by the acid, more are produced to take their place and this continues until all of the weak base has been used up. In the cases of the weak bases you used, the acid reacts directly with the hydrogencarbonate ions, in all probability, so even this argument may not apply.
There is a subtle point, however. Methyl orange changes colour over a pH range which is just on the acid side of neutral. Fortunately, the pH change at the end point of all these titrations straddles the range over which methyl orange changes colour. If you had used phenolphalein as indicator, the situation would have been different.
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updated: 28 March 2008
