<P> Usually, the color change is not instantaneous at the pK value, but a pH range exists where a mixture of colors is present . This pH range varies between indicators, but as a rule of thumb, it falls between the pK value plus or minus one . This assumes that solutions retain their color as long as at least 10% of the other species persists . For example, if the concentration of the conjugate base is 10 times greater than the concentration of the acid, their ratio is 10: 1, and consequently the pH is pK + 1 . Conversely, if a 10-fold excess of the acid occurs with respect to the base, the ratio is 1: 10 and the pH is pK − 1 . </P> <P> For optimal accuracy, the color difference between the two species should be as clear as possible, and the narrower the pH range of the color change the better . In some indicators, such as phenolphthalein, one of the species is colorless, whereas in other indicators, such as methyl red, both species confer a color . While pH indicators work efficiently at their designated pH range, they are usually destroyed at the extreme ends of the pH scale due to undesired side reactions . </P> <P> pH indicators are frequently employed in titrations in analytical chemistry and biology to determine the extent of a chemical reaction . Because of the subjective choice (determination) of color, pH indicators are susceptible to imprecise readings . For applications requiring precise measurement of pH, a pH meter is frequently used . Sometimes, a blend of different indicators is used to achieve several smooth color changes over a wide range of pH values . These commercial indicators (e.g., universal indicator and Hydrion papers) are used when only rough knowledge of pH is necessary . </P> <P> Tabulated below are several common laboratory pH indicators . Indicators usually exhibit intermediate colors at pH values inside the listed transition range . For example, phenol red exhibits an orange color between pH 6.8 and pH 8.4 . The transition range may shift slightly depending on the concentration of the indicator in the solution and on the temperature at which it is used . The figure on the right shows indicators with their operation range and color changes . </P>

When do you use indicators and when do you use a ph meter
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