Question

Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color. (a) Is the solution acidic, neutral, or basic? (b) Which of the following can you establish about the solution: (i) A minimum pH, (ii) A maximum pH, or (iii) A specific range of pH values? (c) What other indicator or indicators would you want to use to determine the pH of the solution more precisely?

Short answer

(a) The solution is acidic. (b) The pH range is between \(6\) and \(8.2\). (c) Methyl orange can help determine the pH of the solution more precisely.

Step-by-step solution

1. Determine if the solution is acidic, neutral, or basic based on the indicators' reactions.

Phenolphthalein is a colorless solution in acidic and neutral solutions. It turns pink in basic solutions with a pH range of \(8.2 - 10\). Bromthymol blue is yellow in acidic solutions (pH range of \(6 - 6.2\)), blue in basic solutions (pH range of \(7.6 - 7.8\)), and green in neutral solutions (pH range of \(6.2 - 7.6\)). Since the addition of phenolphthalein did not cause a color change, the solution is either acidic or neutral. However, the addition of bromthymol blue turned the solution yellow, which indicates an acidic solution. (a) Therefore, the solution is acidic.

2. Establish a minimum pH, maximum pH, or specific range of pH values.

Based on the bromthymol blue indicator, we know the solution is acidic with a pH range of \(6 - 6.2\). However, this range can be narrowed down since we do not observe a color change with phenolphthalein. Since phenolphthalein remains colorless in acidic and neutral solutions, the solution must have a pH value less than \(8.2\). So, we can establish the specific pH range as follows: (b) pH range is between \(6\) and \(8.2\).

3. Suggest one or more indicators to determine the pH of the solution more precisely.

To determine the pH of the solution more precisely, we need an indicator that exhibits color change over the pH range of \(6\) to \(8.2\). An appropriate indicator for this range would be: (c) Methyl orange: This indicator is red in acidic solutions with a pH range of \(3.1 - 4.4\) and yellow in neutral or slightly basic solutions with a pH range of \(4.4 - 6.2\). This can refine the pH range to a more specific value in the given range.

Key concepts

Acidic and Basic Solutions

Understanding the nature of solutions is crucial in chemistry. Solutions are characterized as acidic or basic (alkaline) based on their pH levels. Acidic solutions have pH values less than 7 and contain higher concentrations of hydrogen ions (\(H^{+}\)). Basic solutions, on the other hand, have pH values greater than 7 and feature higher concentrations of hydroxide ions (\(OH^{-}\)). A pHof 7 indicates a neutral solution, having equal concentrations of hydrogen and hydroxide ions.

Our exercise involved using pH indicators to determine the nature of an unknown solution. The lack of color change with phenolphthalein and a yellow color with bromthymol blue unequivocally identified the solution as acidic.

pH Determination

The determination of a solution's pH is a common task in chemistry and essential for a variety of scientific applications. pH is a measure of the acidity or basicity of an aqueous solution. It is expressed on a logarithmic scale that typically ranges from 0 to 14, with 0 being extremely acidic, 7 neutral, and 14 highly basic.

In the given exercise, pH determination was accomplished by interpreting the color changes after adding chemical indicators. Bromthymol blue provided initial pH information, suggesting an acidic solution, while phenolphthalein helped us limit the maximum pH value. The solution's pH range was thereby assessed indirectly through these colorimetric reactions.

Chemical Indicators

How Chemical Indicators Work

Chemical indicators are substances used to determine pH values based on color changes they undergo when added to a solution. Each indicator has a specific pH range over which it changes color, which is why different indicators are required for different pH levels.

These indicators are pivotal for experiments requiring pH estimations, such as titrations or the identification of substances. The exercise demonstration showed how different indicators respond to the same solution, aiding in identifying its pH range.

Phenolphthalein

A Popular pH Indicator

Phenolphthalein is a widely used pH indicator particularly suited for titrations and measuring basicity. It remains colorless in acidic to neutral solutions and turns varying shades of pink to deep purple as it enters basic environments, specifically within a pH range of 8.2 to 10.

In the provided exercise, phenolphthalein's lack of color change served as critical evidence that the unknown solution was not basic, narrowing down the pH possibilities.

Bromthymol Blue

Responding to Acidity and Basicity

Bromthymol blue is another pH indicator often used for measuring the carbon dioxide concentration in aqueous solutions and observing photosynthetic activity. It is yellow at pH levels below 6.0, green around the neutral pH of 7.0, and blue above pH 7.6. In this exercise, the transition to yellow upon adding bromthymol blue to the solution indicated a pH less than 6.0, thereby confirming the solution's acidity.

Methyl Orange

Narrowing Down pH Values

Methyl orange serves as a pH indicator, turning red in acidic environments (pH range 3.1 to 4.4) and yellow in neutral to slightly basic environments (pH 4.4 to 6.2). This characteristic suggested its use as a subsequent indicator in our exercise could refine the pH determination. Since we already narrowed the pH range to 6-8.2 using bromthymol blue and phenolphthalein, adding methyl orange could help further pinpoint the exact pH within this interval.