A) describe what type of molecule would function as an acid-base indicator? B) d

Question

A) describe what type of molecule would function as an acid-base indicator? B) do all acid-base indicators undergo a color change at the same pH? explain reasoning.

Explanation / Answer

Acid - Base Indicators: The most common method to get an idea about the pH of solution is to use an acid base indicator. An indicator is a large organic molecule that works somewhat like a " color dye". Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid - base indicators , which do respond to a change in the hydrogen ion concentration. Most of the indicators are themselves weak acids. The most common indicator is found on "litmus" paper. It is red below pH 4.5 and blue above pH 8.2. Color Blue Litmus Red Litmus Acid turns red stays same Base stays same turns blue Other commercial pH papers are able to give colors for every main pH unit. Universal Indicator, which is a solution of a mixture of indicators is able to also provide a full range of colors for the pH scale. A variety of indicators change color at various pH levels. A properly selected acid-base indicator can be used to visually "indicate" the approximate pH of a sample. An indicator is usually some weak organic acid or base dye that changes colors at definite pH values. The weak acid form (HIn) will have one color and the weak acid negative ion (In-) will have a different color. The weak acid equilibrium is: HIn --> H+ + In- For phenolphthalein: pH 8.2 = colorless; pH 10 = red For bromophenol blue: pH 3 = yellow; pH 4.6 = blue An acid-base indicator is an indicator that will change color over a specific pH range. Indicators are commonly used in titration experiments to detect the endpoint, such as in the determination of acetic acid in vinegar lab performed in Chem184. The reason that the indicator undergoes a color change has a straightforward chemical explanation. pH indicators are themselves weak acids that have a molecular structure that can absorb light of different energy depending on whether the molecule is protonated or deprotonated. Different indicators have different structures and the molecular formulas of indicator molecules can be quite complex. For simplicity, lets just refer to the protonated indicator as HIn. The HIn species can lose the proton to form In- according to the equilibrium reaction: 1) HIn H+ + In- The equilibrium constant for this disassociation is written: 2) Ka = [H+] [In-] / [HIn] This expression can be rewritten in the form, 3) pH = pKa + log [In-]/[HIn] known as the Henderson-Hasselbalch equation. This equation illustrates how the amount of the HIn and In- forms of the indicator change with pH. The change in color of the solution containing the indicator comes from the fact that the species, HIn and In-, have different spectral properties and so absorb light of different wavelengths. In today's lab, the indicator phenol red will be used. The HIn form of phenol red is yellow, while the In- form is red. When the indicator is in a solution of low pH, the major form of the indicator is HIn, thus the human eye sees this solution as the color of the Hin (yellow). As the pH is increased by adding base, the OH- of the base neutralizes the H+ and more In- is formed according to LeChâtelier's principle. As the In- becomes the predominate species, the eye sees the solution color as the color of the In- (red). The color the human eye detects really depends upon the relative amounts of the two forms, or the [In-]/[HIn] ratio. A spectrophotometer is much more sensitive to changes in color than is the human eye. In this experiment the spectrophotometer can simultaneously measure both the acid and base forms of the phenol red indicator since these two species absorb light at different wavelengths.

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