We manufacture several different pH strips, including a Universal pH test paper. This is a product we have been manufacturing for a long time now, and the chemistry is pretty interesting. In our past post, For the Love of pH Indicators, we discussed that indicators can be added to a solution to determine if it is alkaline or acidic, and adding indicators to our pH test strips does the same thing. While some indicators will only change color in a narrow pH range, universal is a broad-range indicator, changing color from pH 0 to 14. Let’s take a closer look at the chemistry of universal indicator.

Universal actually contains a mixture of indicators, which is why it detects such a wide pH range. Universal is red in acidic solutions, green at neutral, and blue-purple in alkaline solutions. So, what indicators make up universal?

Let’s start at the low pH range. Methyl red is red at a pH below 5 and yellow at a pH above 5. Therefore, this gives universal the red and orange hues you see.

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Bromothymol blue is yellow at a pH below 6 and blue at a pH above 6. Since methyl red produces yellow above pH 5, and bromothymol blue produces yellow below pH 6, universal turns yellow between pH 5 and 6. Bromothymol blue also produces the blue and indigo hues you see at a higher pH.

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Thymol blue turns red below pH 2, blue above pH 8, and is yellow in between. In combination with methyl red, highly acidic solutions (pH < 2) turn universal paper a bright red color. At a neutral pH, the yellow hue from thymol blue combines with the blue hue from Bromothymol blue to create a green color. Moderately alkaline solutions (pH 8-10) turn universal a very blue color because of the combination of blue hues from bromothymol blue and thymol blue.

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Lastly, phenolphthalein turns pink above pH 8. When combined with the dark blue hues from thymol blue and bromothymol blue, it creates a purple color in very alkaline solutions (pH > 12).

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Of course, there are a few other components to our Universal pH test strips. For example, we need a solvent, such as water or ethanol to dissolve the universal indicator. We may also need a compound to adjust the pH of the solution to ensure each color is shown at the correct pH value. As some of the indicators, such as methyl red, are acidic themselves, it could affect the pH of the solution and the test strip results. We may also have to account for the properties of the paper and how that affects results.

So, although it may seem like a simple enough pH strip, there is actually quite a bit of chemistry that goes on behind the scenes to make it all work!