Remember that the equivalence point of a titration is where you have mixed the two substances in exactly equation proportions. You obviously need to choose an indicator which changes colour as close as possible to that equivalence point. That varies from titration to titration. The next diagram shows the pH curve for adding a strong acid to a strong base. Superimposed on it are the pH ranges for methyl orange and phenolphthalein.
However, the graph is so steep at that point that there will be virtually no difference in the volume of acid added whichever indicator you choose. However, it would make sense to titrate to the best possible colour with each indicator. If you use phenolphthalein, you would titrate until it just becomes colourless at pH 8. On the other hand, using methyl orange, you would titrate until there is the very first trace of orange in the solution.
If the solution becomes red, you are getting further from the equivalence point. This time it is obvious that phenolphthalein would be completely useless. However, methyl orange starts to change from yellow towards orange very close to the equivalence point.
This time, the methyl orange is hopeless! However, the phenolphthalein changes colour exactly where you want it to. The curve is for a case where the acid and base are both equally weak - for example, ethanoic acid and ammonia solution.
In other cases, the equivalence point will be at some other pH. You can see that neither indicator is any use. Phenolphthalein will have finished changing well before the equivalence point, and methyl orange falls off the graph altogether. It may be possible to find an indicator which starts to change or finishes changing at the equivalence point, but because the pH of the equivalence point will be different from case to case, you can't generalise.
On the whole, you would never titrate a weak acid and a weak base in the presence of an indicator. This is an interesting special case. If you use phenolphthalein or methyl orange, both will give a valid titration result - but the value with phenolphthalein will be exactly half the methyl orange one. It so happens that the phenolphthalein has finished its colour change at exactly the pH of the equivalence point of the first half of the reaction in which sodium hydrogencarbonate is produced.
The methyl orange changes colour at exactly the pH of the equivalence point of the second stage of the reaction. If this is the first set of questions you have done, please read the introductory page before you start. How simple indicators work Indicators as weak acids Litmus Litmus is a weak acid. The un-ionised litmus is red, whereas the ion is blue.
Adding hydroxide ions: Adding hydrogen ions: If the concentrations of HLit and Lit - are equal: At some point during the movement of the position of equilibrium, the concentrations of the two colours will become equal.
Methyl orange Methyl orange is one of the indicators commonly used in titrations. In an alkaline solution, methyl orange is yellow and the structure is: Now, you might think that when you add an acid, the hydrogen ion would be picked up by the negatively charged oxygen.
Phenolphthalein Phenolphthalein is another commonly used indicator for titrations, and is another weak acid. The pH range of indicators The importance of pK ind Think about a general indicator, HInd - where "Ind" is all the rest of the indicator apart from the hydrogen ion which is given away: Because this is just like any other weak acid, you can write an expression for K a for it.
If you re-arrange the last equation so that the hydrogen ion concentration is on the left-hand side, and then convert to pH and pK ind , you get: That means that the end point for the indicator depends entirely on what its pK ind value is. For the indicators we've looked at above, these are: indicator pK ind litmus. At what pH does phenolphthalein change color? Why does phenolphthalein show pink Colour? Do acids change the color of indicators? What Colour is phenolphthalein in neutral? Which of the following turns phenolphthalein pink?
Which salt will turn phenolphthalein pink? In which 0. Does NaNO3 turn phenolphthalein pink? Why does the pink color disappear in a titration? Why is phenolphthalein a good indicator? Why will the phenolphthalein eventually fade? How long does phenolphthalein stay in your system? Why does phenolphthalein turn pink in sodium hydroxide?
Why does the pink Colour disappear if the flask is swirled for 2 or 3 minutes? Does the rate of fading of phenolphthalein depend on its concentration? Why does CO2 make the solution you blew into change color from pink to clear?
Why do acid base indicators change color? What is it called when the indicator changes color? What are the 3 indicators? What are the indicators of human wellbeing? How can health impact wellbeing? Why is wellbeing important? How do you promote health and wellbeing? Consequently, they offer a series of color changes spanning the whole pH range.
If a careful choice of both colors and p K a is made, it is possible to mix several indicators and obtain a universal indicator which changes color continuously over a very wide pH range.
With such a mixture it is possible to find the approximate pH of any solution within this range. So-called pH paper, as seen below, is impregnated with one or several indicators. When a strip of this paper is immersed in a solution, its pH can be judged from the resulting color. What indicator, from those listed in the table, would you use to determine the approximate pH of the following solutions:.
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