Question

Congo Red in neutral, water solution has a \(\lambda_{\max }\) in the visible spectrum at \(4970 \AA\). In acid solution it shows absorption maxima at 6470 and \(5895 \AA\). (a) What colors are likely to be observed for neutral and acidic Congo Red solutions? (b) How could addition of acid to a neutral Congo Red solution cause the absorption maximum to shift to longer wavelengths? (c) What would you expect to happen to the color in very strong acid solution?

Short answer

In a neutral Congo Red solution with \(\lambda_{\max} = 4970 \AA\), the observed color is likely to be Orange. In acidic Congo Red solution with absorption maxima at \(6470 \AA\) and \(5895 \AA\), the observed color will be a combination of Blue and Indigo. The shift in absorption maximum to longer wavelengths upon the addition of acid can be attributed to structural changes in the compound due to protonation or deprotonation of its functional groups, which in turn affect electronic transitions. Predicting the exact color of Congo Red in a very strong acid solution is difficult without additional information about its structural properties or experimental data.

Step-by-step solution

Determine the colors corresponding to the given wavelengths

According to the visible light spectrum, colors are associated with specific wavelength ranges. To determine the color corresponding to the given wavelengths, we can compare them with the visible light spectrum ranges: • 4000-4800 Å corresponds to Violet • 4800-4900 Å corresponds to Indigo • 4900-5600 Å corresponds to Blue • 5600-6000 Å corresponds to Green • 6000-6250 Å corresponds to Yellow • 6250-7000 Å corresponds to Orange • 7000-8000 Å corresponds to Red So, in a neutral Congo Red solution with \(\lambda_{\max} = 4970 \AA\), the corresponding color is Blue. In acidic Congo Red solution with absorption maxima at \(6470 \AA\) and \(5895 \AA\), the corresponding colors are Orange and Yellow.

Determine the observed colors in the solutions

To determine the observed colors in the solutions, we need to consider the complementary colors of the absorbed light. The complementary colors can be found using the color wheel concept: • Complementary color of Blue is Orange • Complementary color of Yellow is Indigo • Complementary color of Orange is Blue Based on the complementary colors, we can answer part (a) of the exercise: (a) The likely observed color for neutral Congo Red solution is Orange, while for acidic Congo Red solution will be a combination of Blue and Indigo.

Explain the shift in absorption maximum

In part (b), we are asked to explain how the addition of acid to a neutral Congo Red solution causes the absorption maximum to shift to longer wavelengths. The absorption wavelength (and thus the color) of a compound depends on its chemical structure and electronic transitions. When the pH changes (due to the addition of acid), it can lead to protonation or deprotonation of the compound functional groups, resulting in structural changes. These structural changes can affect the electronic transitions and cause the absorption maximum to shift to longer wavelengths.

Predict the color in very strong acid solution

In part (c), we need to predict the color of Congo Red in a very strong acid solution. As we previously established, the color changes as a result of structural changes due to protonation or deprotonation in response to pH. As we keep increasing the acid concentration, it is expected that the compound will keep undergoing structural changes. However, it is difficult to predict the exact color in a very strong acid solution without additional information about the compound's structural properties or experimental data.

Key concepts

Visible Light Spectrum

When we talk about the visible light spectrum, we are referring to the range of electromagnetic wavelengths that are detectable by the human eye. This spectrum includes wavelengths approximately from 400 to 700 nanometers (nm), with each range corresponding to a different color. Visible light is a tiny part of the electromagnetic spectrum, which includes other types of waves like radio waves, infrared radiation, ultraviolet light, X-rays, and gamma rays.

The colors we perceive are associated with specific wavelength ranges. For instance, violet light falls in the range of 400-450 nm, indigo is around 450-475 nm, blue ranges from 475-495 nm, green between 495-570 nm, yellow from 570-590 nm, orange between 590-620 nm, and red spans from 620 nm up to about 700 nm. An interesting takeaway is that when a substance absorbs light at a specific wavelength within this range, our eyes perceive the complementary color of the absorbed light.

Absorption Maximum

The term 'absorption maximum' refers to the wavelength at which a compound absorbs the highest amount of light. Molecules can absorb energy in the form of light; this absorbed energy causes electronic transitions within the molecule. In the context of Congo Red, the absorption maxima represent the specific wavelengths within the visible light spectrum where the absorption is at its peak. Understanding this concept is crucial because the wavelength of maximum absorption can give us insight into the compound's electronic structure and contribute to our perception of its color.

Color Change in Acid

When an acid-base indicator like Congo Red is dissolved in a solution, its color can change with variations in pH due to the interaction with acids or bases. This color change is tied to the protonation or deprotonation of the indicator molecule. When an indicator molecule gains or loses a proton (H+), its electronic structure is altered, often resulting in a change in the light absorption properties. Consequently, this change is observed as a shift in the absorption maximum, and therefore a color change, because the molecule now absorbs light differently within the visible spectrum.

Protonation and Deprotonation

Protonation and deprotonation are processes whereby a molecule gains or loses a proton, respectively. This gain or loss can significantly influence a molecule's properties, especially its ability to absorb light. In the context of pH indicators such as Congo Red, these processes lead to changes in the molecule's electronic structure. This can result in different absorption behaviors and is responsible for the indicator's color change in response to different pH levels. Since pH measures the concentration of hydrogen ions in a solution, indicators can 'sense' these changes and respond visually.

Complementary Colors

Complementary colors are pairs of colors that, when combined, cancel each other out by producing a grayscale color like white or black. They are located opposite each other on a color wheel, an illustrative organization of color hues around a circle. For example, the complementary color of blue is orange, and the complementary color of green is red. This concept is essential in understanding color changes of pH indicators. When Congo Red absorbs light in the blue range, the color that the solution appears is orange, the complementary color to blue.

Electronic Transitions

Electronic transitions are changes in the energy levels of the electrons in a molecule when they absorb or emit light. These transitions take place between different orbitals in an atom or molecule, predominantly in the valence shell where the electrons are involved in chemical bonding. Visible light induces electronic transitions in the molecules of colored compounds, and each different transition corresponds to a different absorption of light. The energy and wavelength of the light absorbed are directly related to the structure of the molecule. When it comes to indicators like Congo Red, small changes in molecular structure due to changes in pH can cause significant shifts in the wavelengths of light absorbed, hence altering the observed color.