How does an acid and base indicator work?

Answer 1

The acidity or basicity of a solution is measured by pH. pH stands for the power of 'Hydrogen.' (actually pH is defined as the negative log of the hydrogen ion activity or commonly the concentration is used since activity is difficult to define.) A substance with a pH of 0-6 is an acidic substance with 0 being the most acidic. A substance with a pH of 7 is neither acidic or basic like water (which is why it's drinkable). A substance with s pH of 8-14 are bases with the strongest base being the one with a pH of 14. An indicator changes color according to how acidic or basic a substance is. Litmus is a type of indicator which changes color in bases or acids. Blue litmus turns to red in an acid and blue litmus turns blue if in contact with a base. The dye substance in litmus turns color based on its chemical structure when more hydrogen ions are present (acidic) or when more hydroxyl ions (basic) are present. These colors for various indicators can be used for different pH ranges, with some indicators effective in the acidic pH range and some in the basic pH range. Most chemical supply catalogs have a nice list of what indicators work in the various ranges and the color changes to be expected. Color indicators are only as good as the rate of reaction that occurs to give a good end point or indication of the pH of a solution. A commercial pH meter would be the common and easy way to determine the pH of a solution. A universal indicator however, changes color according to the pH of the substance. E.g. in a strong acid with a pH of 1, a universal indicator may appear to be red Indicators are themselves weak acids or weak bases so they change their color in opposite medium at a specific range of pH.