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Phenol red indicator solution changes color depending on acidity. Click for info.
A petri dish is filled with a solution of phenol red, which changes color depending on acidity. The lower pH of the solution on the left gives the substance a deep yellow color. The higher pH of the solution on the right gives the substance a reddish color. Depending on its color, the liquid absorbs more or less of the green laser light versus the blue laser light. Notice that the green light is much more visible inside the yellow liquid (top left) than it is in the red liquid (top right). In contrast, the blue light is much more visible in the red liquid (bottom right) than it is in the yellow liquid (bottom left). The more light the liquid absorbs, the more its temperature increases. By measuring these temperature changes with the optical fiber on the left (shining red light in this illustration), researchers can assess the exact color of the liquid, which tells them its pH. Image: Jennifer Lauren Lee/NIST.
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