Acid-base indicators
Acid-base indicators are substances that change color based on the acidity or alkalinity of a solution, providing a visual representation of pH levels. They are crucial in various scientific fields, particularly in forensic science, where they help identify unknown substances and detect contamination in solutions. The acidity or alkalinity is measured on a pH scale that ranges from 0 to 14, with lower values indicating acidic solutions and higher values indicating basic solutions. Common acid-base indicators include litmus paper, phenolphthalein, and methyl orange, each with specific pH ranges for color change. For instance, blue litmus paper will turn red in acidic conditions (pH 4.5 or below), while red litmus paper will turn blue in basic conditions (pH 8.2 or above).
These indicators are often used in titration, a technique that allows chemists to quantify the amounts of acid or base in a solution by adding a reagent until the indicator changes color. While acid-base indicators are effective for initial analyses, they are generally seen as less precise compared to more advanced analytical techniques. Overall, acid-base indicators serve as accessible tools for understanding and measuring the chemical properties of various solutions in both laboratory and field settings.
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Acid-base indicators
DEFINITION: Substances that show the acidity or alkalinity of solutions within a narrow range.
SIGNIFICANCE: Among the tools forensic scientists use to identify unknown substances are acid-base indicators, also known as pH indicators. Such indicators can also enable scientists to detect the presence of contaminating chemicals in solutions, and their use in the analysis of human tissues can provide clues to the cause of death.
The acidity or alkalinity of a substance is indicated by its pH, which is a measure of the concentration of hydrogen ions (H+) in a solution. The pH scale is logarithmic and ranges from 0 to 14. The lower the pH, the more acidic the solution, and the higher the pH, the more alkaline, or basic, the solution; pH 7.0 is neutral and is the pH of pure water.
Acid-base indicators are organic dyes that change color depending on the concentration of hydrogen ions present in a solution. The change does not become visible at a precise point; rather, it happens within a fairly narrow pH range. Many different acid-base indicators are available, and they change colors within different pH ranges. For example, phenolphthalein is colorless at a pH of 8.2 but turns red at a pH of 10. Methyl orange is red at a pH of 3.2 but turns yellow at a pH of 4.4.
The most common acid-base indicator is litmus paper. It comes in two forms: red and blue. When dipped into a solution, blue litmus paper turns red if the pH of the solution is 4.5 or below, indicating the solution is acidic. If the pH of the solution is 8.2 or above, blue litmus paper remains its original blue color. Conversely, red litmus paper remains red when dipped into an acidic solution but turns blue when dipped into a basic solution.
Most often, acid-base indicators are used with a technique called titration. Titration allows analytical chemists to make quantitative determinations of how much acid or alkaline material is in a solution. In the titration of an acid solution, a known quantity of base is added until the correct acid-base indicator changes color. The chemist then measures how much base was used and can calculate how much acid is in the solution. The procedure is reversed with a basic solution.
When investigating an unknown substance, such as a confiscated drug, a forensic technician may dissolve a small amount of the substance in water and then test its pH. Conversely, if the substance has been identified and the pH of that substance in pure form is known, the technician may dissolve a small amount of the substance in water to see if the pH varies from the known pH. If it does, this suggests that the substance is contaminated with another chemical.
Acid-base indicators are useful but crude analytical tools. To complete most chemical analyses, forensic scientists usually need to employ more precise analytical tools.
Bibliography
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