Phenolphthalein indicator how does it work

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. A properly selected acid-base indicator can be used to visually "indicate" the approximate pH of a sample.

An indicator is usually some weak organic acid or base dye that changes colors at definite pH values. The weak acid form HIn will have one color and the weak acid negative ion In - will have a different color. The weak acid equilibrium is:. See the graphic for more indicators, colors, and pH ranges. Magic Pitcher Demonstration:. Phenolphthalein is an indicator of acids colorless and bases pink. Sodium hydroxide is a base, and it was in the pitcher at the beginning, so when added to the phenolphthalein in beakers 2 and 4, it turned pink top half of the graphic.

The equilibrium shifts right, HIn decreases, and In - increases. As the pH increase between 8. The third beaker has only the NaOH but no phenolphthalein, so it remained colorless. The first beaker contain acetic acid and is skipped over at first. Bottom half of the graphic: When the pitcher is then poured back into beakers 2, 3, 4 it is a pink solution. In the first beaker, a strange thing happens in that the pink solution coming out of the pitcher now changes to colorless.

This happens because the first beaker contains some vinegar or acetic acid which neutralizes the NaOH, and changes the solution from basic to acidic.

When an acid is dissolved in water we get an acidic solution. When a base dissolves in water it is an alkali and makes an alkaline solution. If a solution is neither acidic nor alkaline it is neutral. Pure water is neutral, and so is paraffin. That is, at some pH. In other words the color of phenolphthalein changes perceptibly between about pH 8. Other indicators behave in essentially the same way, but for many of them both the acid and the conjugate base are colored.

The indicators listed have been selected so that their p K a values are approximately two units apart. Consequently, they offer a series of color changes spanning the whole pH range.