Why phenolphthalein is used as indicator in acid base titration

If we add base, we shift the equilibrium towards the yellow form. This behavior is completely analogous to the action of buffers. If most is present as HIn, then we see the color of the HIn molecule: red for methyl orange. For methyl orange, we can rearrange the equation for K a and write:. The above expression describing the indicator equilibrium can be rearranged:. The last formula is the same as the Henderson-Hasselbalch equation, which can be used to describe the equilibrium of indicators.

No change in color is visible for any further increase in the hydronium ion concentration decrease in pH. The pH range between 3. Many different substances can be used as indicators, depending on the particular reaction to be monitored. In all cases, though, a good indicator must have the following properties:. Red cabbage juice contains a mixture of substances whose color depends on the pH. Synthetic indicators have been developed that meet these criteria and cover virtually the entire pH range.

In addition, some indicators such as thymol blue are polyprotic acids or bases, which change color twice at widely separated pH values. It is important to be aware that an indicator does not change color abruptly at a particular pH value; instead, it actually undergoes a pH titration just like any other acid or base.

Thus most indicators change color over a pH range of about two pH units. The truth is that there is delocalisation over the entire structure, and no simple picture will show it properly.

Don't worry about this exact structure - it is just to show a real case where the colour of a compound is drastically changed by the presence or absence of a hydrogen ion. You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different. You should be able to work out for yourself why the colour changes when you add an acid or an alkali.

The explanation is identical to the litmus case - all that differs are the colours. Note: If you have problems with this, it is because you don't really understand Le Chatelier's Principle. Sort it out! In the methyl orange case, the half-way stage where the mixture of red and yellow produces an orange colour happens at pH 3.

This will be explored further down this page. In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink.

The half-way stage happens at pH 9. Since a mixture of pink and colourless is simply a paler pink, this is difficult to detect with any accuracy! Note: If you are interested in understanding the reason for the colour changes in methyl orange and phenolphthalein, they are discussed on a page in the analysis section of the site about UV-visible spectroscopy. This is quite difficult stuff, and if you are coming at this from scratch you will have to explore at least one other page before you can make sense of what is on that page.

There is a link to help you to do that. Don't start this lightly! 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. We will call it K ind to stress that we are talking about the indicator. Note: If this doesn't mean anything to you, then you won't be able to understand any of what follows without first reading the page on weak acids.

Use the BACK button on your browser to return to this page. Think of what happens half-way through the colour change. At this point the concentrations of the acid and its ion are equal. In that case, they will cancel out of the K ind expression. You can use this to work out what the pH is at this half-way point.

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:.

Indicators don't change colour sharply at one particular pH given by their pK ind. Instead, they change over a narrow range of pH.

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