Indicators in Chemistry are substances that change colour when added to acidic or basic solutions. They help identify the nature of a substance quickly and safely. Hydrangeas a type of plant is a natural indicator that naturally tells the acidity or basisity of the soil. For example, if the soil is acidic, the plants blooms turn blue, purple and if the soil is neutral, and the plants bllons turn pink if the soil is basic.
The chemical substances that are used to differentiate these substances are called Chemical Indicators Or simply Indicators. Oxazine-family dyes undergo visually obvious color shifts on redox changes and serve as indicators in some biochemical fanfills dating and analytical methods. They are sensitive and useful where colorless reduced forms are needed; handle with standard lab precautions. Alizarin yellow R acts as a high-range pH indicator and complexes with certain metal ions.
A chemical called Methyl Yellow, for example, gives an alkaline solution a yellow color. When acid is gradually added, the solution remains yellow until all of the alkali has been neutralised, at which point it becomes red. Keep in mind, the color change of a pH indicator isn’t instantaneous.
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Bromothymol Blue
These tools play a crucial role in both scientific research and practical applications, helping to ensure safety and compliance across various fields. A pH less than 7 indicates an acidic solution and a pH greater than 7 indicates a basic solution. Ultimately, the pH value indicates how much H+ has dissociated from molecules within a solution. The lower the pH value, the higher concentration of H+ ions in the solution and the stronger the acid. Likewise, the higher the pH value, the lower the concentration of H+ ions in the solution and the weaker the acid.
Phenol Red
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- The chart below presents several indicators, their colors, and their color-change intervals.
- Have you ever wondered how scientists determine whether a substance is acidic, basic, or neutral without tasting it?
At some point there will be enough of the red form of the methyl orange present that the solution will begin to take on an orange tint. As you go on adding more acid, the red will eventually become so dominant that you can no longer see any yellow. A chemical indicator refers to a substance that changes its color or shows an observable change to indicate the presence, concentration, or endpoint of a chemical reaction.
Phenolphthalein, on the other hand, exhibits a color change interval that nicely brackets the abrupt change in pH occurring at the titration’s equivalence point. A sharp color change from colorless to pink will be observed within a very small volume interval around the equivalence point. The titration curves shown in the figure above illustrate the choice of a suitable indicator for specific titrations.
For example, common indicators such as phenolphthalein, methyl red, and bromothymol blue are used to indicate pH ranges of about 8 to 10, 4.5 to 6, and 6 to 7.5 accordingly. On these ranges, phenolphthalein goes from colorless to pink, methyl red goes from red to yellow, and bromothymol blue goes from yellow to blue. For universal indicators, however, the pH range is much broader and the number of color changes is much greater. See figures 1 and 2 in the introduction for visual representations. The weak acid titration curve above shows that only one of the three indicators is suitable for end point detection. Use of litmus would show a color change that begins after adding 7–8 mL of titrant and ends just before the equivalence point.
This is because the pH range isn’t well-defined (expect accuracy within 1 pH value) and color is a judgement call. It is important to be aware that an indicator does not change color abruptly at a particular pH value; instead, it actually undergoes a pH titration just like any other acid or base. As the concentration of HIn decreases and the concentration of In− increases, the color of the solution slowly changes from the characteristic color of HIn to that of In−. As we will see in Section 16, the In−/HIn ratio changes from 0.1 at a pH one unit below pKin to 10 at a pH one unit above pKin.
It shifts color in strongly alkaline solutions and can serve as a metallochromic reagent; handle as a chemical irritant and avoid release to drains. Phenol red changes from yellow to red across pH 6.8–8.4 and is common in cell culture media, water testing and aquaria. It provides a useful physiological-range indicator but can interact with some assays; non-toxic at working concentrations. Bromothymol blue changes from yellow in acid to blue in base with green near neutral (pH 6.0–7.6). Common for biological pH monitoring and demonstrations; easy-to-see midrange transition. Phenolphthalein is colorless in acid and turns pink–fuchsia above pH 8.2.
It provides qualitative pH values; paper strips often more convenient for field use. There are 41 Chemical Indicators, ranging from Alizarin S to Xylenol orange. CA Maninder Singh is a Chartered Accountant with 16+ years of practical experience and 20+ years of teaching experience.