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pH curves (titration curves)

pH curves (titration curves) - pH(TITRATION CURVES This...

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pH (TITRATION) CURVES This page describes how pH changes during various acid-base titrations. The equivalence point of a titration Sorting out some confusing terms When you carry out a simple acid-base titration, you use an indicator to tell you when you have the acid and alkali mixed in exactly the right proportions to "neutralise" each other. When the indicator changes colour, this is often described as the end point of the titration. In an ideal world, the colour change would happen when you mix the two solutions together in exactly equation proportions. That particular mixture is known as the equivalence point . For example, if you were titrating sodium hydroxide solution with hydrochloric acid, both with a concentration of 1 mol dm -3 , 25 cm 3 of sodium hydroxide solution would need exactly the same volume of the acid - because they react 1 : 1 according to the equation. In this particular instance, this would also be the neutral point of the titration, because sodium chloride solution has a pH of 7. But that isn't necessarily true of all the salts you might get formed. For example, if you titrate ammonia solution with hydrochloric acid, you would get ammonium chloride formed. The ammonium ion is slightly acidic, and so pure ammonium chloride has a slightly acidic pH. That means that at the equivalence point (where you had mixed the solutions in the correct proportions according to the equation), the solution wouldn't actually be neutral. To use the term "neutral point" in this context would be misleading. Similarly, if you titrate sodium hydroxide solution with ethanoic acid, at the equivalence point the pure sodium ethanoate formed
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has a slightly alkaline pH because the ethanoate ion is slightly basic. To summarise: The term "neutral point" is best avoided. The term "equivalence point" means that the solutions have been mixed in exactly the right proportions according to the equation. The term "end point" is where the indicator changes colour. As you will see on the page about indicators, that isn't necessarily exactly the same as the equivalence point. Note: You can find out about indicators by following this link (also available from the acid-base equilibria menu). You should read the present page first though. Simple pH curves All the following titration curves are based on both acid and alkali having a concentration of 1 mol dm -3 . In each case, you start with 25 cm 3 of one of the solutions in the flask, and the other one in a burette.
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