Why phenolphthalein is used in acid base titration

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. We have stated that a good indicator should have a pKin value that is close to the expected pH at the equivalence point. For a strong acid—strong base titration, the choice of the indicator is not especially critical due to the very large change in pH that occurs around the equivalence point.

This figure shows plots of pH versus volume of base added for the titration of At some point during the movement of the position of equilibrium, the concentrations of the two colours will become equal. The colour you see will be a mixture of the two.

The reason for the inverted commas around "neutral" is that there is no reason why the two concentrations should become equal at pH 7. As you will see below, that isn't true for other indicators. 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. That's the obvious place for it to go. Not so! In fact, the hydrogen ion attaches to one of the nitrogens in the nitrogen-nitrogen double bond to give a structure which might be drawn like this:.

Note: You may find other structures for this with different arrangements of the bonds although always with the hydrogen attached to that same nitrogen. 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. These methods range from the use of litmus paper, indicator paper, specifically designed electrodes, and the use of colored molecules in solution.

Other than the electrodes, all of the methods are visual and rely on some fundamental changes that occur in a molecule when the pH of its environment changes. In general, a molecule that changes color with the pH of the environment it is in can be used as an indicator. In this reaction, adding acid shifts the indicator equilibrium to the left. Conversely, adding a base shifts the indicator equilibrium to the right. In the case of the indicator methyl orange, the HIn is colored red and the ionized In — form is yellow.

Methyl orange : The molecule methyl orange is commonly used as an indicator in acid-base equilibrium reactions. In base form, on the left in the figure, the color is yellow. Adding a proton yields the structure on the right, colored red. Note that this color change occurs over the pH range from approximately The eye is sensitive to color changes over a range of concentration ratios of approximately or over two pH units. Below pH 2. Because of the subjective choice determination of color, pH indicators are susceptible to imprecise readings.

For applications requiring precise measurement of pH, a pH meter is frequently used. Sometimes a blend of different indicators is used to achieve several smooth color changes over a wide range of pH values. These commercial indicators e. Indicators usually exhibit intermediate colors at pH values inside a specific transition range.

For example, phenol red exhibits an orange color between pH 6. The transition range may shift slightly depending on the concentration of the indicator in the solution and on the temperature at which it is used.

Common acid-base indicators : Common indicators for pH indication or titration endpoints is given, with high, low, and transition pH colors. When viewed on the pH scale itself, the color transitions as determined by their transition ranges becomes clearer and the context of the indicator sensitivity over ranges of pH is laid out more informatively. The correct answer is C. In the titration of a weak acid with a strong base, the conjugate base of the weak acid will make the pH at the equivalence point greater than 7.

Therefore, you would want an indicator to change in that pH range. Both methyl orange and bromocresol green change color in an acidic pH range, while phenolphtalein changes in a basic pH. Privacy Policy. Skip to main content. Acid-Base Equilibria. Search for:. Learning Objectives Calculate the concentration of an unknown strong acid given the amount of base necessary to titrate it.

Key Takeaways Key Points An acid — base titration is used to determine the unknown concentration of an acid or base by neutralizing it with an acid or base of known concentration. Neutralization is the reaction between an acid and a base, producing a salt and a neutralized base.

A strong acid yields a weak conjugate base A — , so a strong acid is also described as an acid whose conjugate base is a much weaker base than water. Common examples of strong bases are the hydroxides of alkali metals and alkaline earth metals, such as NaOH and Ca OH 2. Very strong bases are even able to deprotonate very weakly acidic C—H groups in the absence of water. Example: What is the unknown concentration of a Learning Objectives Distinguish a weak acid-strong base titration from other types of titrations.

Key Takeaways Key Points In an acid — base titration, the titration curve reflects the strengths of the corresponding acid and base. If one reagent is a weak acid or base and the other is a strong acid or base, the titration curve is irregular, and the pH shifts less with small additions of titrant near the equivalence point. Acid-base titrations depend on the neutralization between an acid and a base when mixed in solution. Key Terms stoichiometry : The study and calculation of quantitative measurable relationships of the reactants and products in chemical reactions chemical equations.