Question

What would you expect to observe if you ran a titration of an acid at a \(\mathrm{pH}\) of \(1.0\) by adding base and using bromocresol green as an indicator?

Short answer

During the titration of an acid with pH of 1.0, bromocresol green will change from yellow (initially) to green as the pH increases from 1.0 to 7.6 due to the addition of base. If more base is added and the pH increases above 7.6, the indicator will change to blue.

Step-by-step solution

Understand the properties of bromocresol green

Bromocresol green is a pH indicator that changes color depending on the pH of the solution. In an acidic solution (\(pH < 6.0\)) it is yellow, in a neutral or slightly basic solution (\(6.0 < pH < 7.6\)) it is green and in a basic solution (\(pH > 7.6\)) it is blue.

Understand the starting conditions

In the initial conditions of this titration, the solution is acidic with a pH of \(1.0\) and we are adding base. Since bases neutralize acids, we can expect the pH of the solution to rise as we add more base.

Predict the observations

As the base is added, the pH of the solution will start to increase. At \(pH = 6.0\) bromocresol green will start changing from yellow to green. It will fully turn green at \(pH = 7.6\), indicating that the solution is now becoming more basic. If more base is added and the pH gets higher than \(7.6\), bromocresol green will turn blue.

Key concepts

pH indicator

A pH indicator is a chemical that exhibits different colors when added to solutions of varying pH levels. The color change happens due to alterations in the structure of the indicator molecule at different hydrogen ion concentrations. pH indicators are crucial tools in analytical chemistry, especially in titration experiments to determine the endpoint of a chemical reaction.
In titration, the pH indicator is selected based on its transition range – the pH range over which it changes color. This range should overlap with the expected pH at the equivalence point of the titration. A well-chosen pH indicator can make the color transition sharp and easily observable, simplifying the detection of the titration endpoint.

Neutralization reaction

A neutralization reaction occurs when an acid and a base react to form water and a salt, accompanied by a change in pH towards a neutral state. This reaction is the foundation of acid-base titrations.
In the classroom, neutralization can be visualized as the process where the acidic H+ ions and basic OH- ions combine to form H2O – water, which is neutral. The pH changes observed during a titration are a result of this neutralization process. As more base is added to an acid, the concentration of H+ ions decreases, causing the pH level to rise until the equivalence point is reached, where exactly enough base has been added to neutralize the acid.

Bromocresol green

Bromocresol green is a specific type of pH indicator known for its distinct color change over its operative pH range. In an acidic environment with a pH below 6.0, it imparts a yellow color to the solution. As the pH moves towards neutrality, the indicator transitions through shades of green until it reaches a pure green at a pH of about 7.6. If the solution turns more basic, with the pH going above 7.6, the color shifts yet again, this time to a blue hue. Bromocresol green's evident and vivid color changes make it an ideal pH indicator for various analytic procedures, including the titration process described in the initial exercise.

Titration observations

During a titration, several observable changes occur that signal the progress of the reaction. Initially, if the solution being titrated is highly acidic or basic, no visible change takes place until the pH approaches the effective range of the indicator. As you add base to an acidic solution using bromocresol green, you'll first notice a gradual fade from an intense yellow towards lighter shades.
As the pH nears 6.0, a green tint starts emerging, signifying the approach toward neutrality. The moment the entire solution attains an even green color indicates a pH of 7.6, suggesting that the acid has been largely neutralized by the base. Continuing to add base past this point will turn the solution blue, providing a visual cue that the reaction has passed the equivalence point and the solution is now basic.