How does pH paper work?
A pH paper is impregnated with a chemical (or series of chemicals) whose chromatophore is a different color at different pH concentrations.
Litmus paper is just one of many different types of pH papers, although according to the article linked, litmus was used for the first time about 1300 AD by Spanish alchemist Arnaldus de Villa Nova, and the chemical structure responsible for the color of litmus paper (7-Hydroxy-1H-phenoxazin-1-one) is found naturally in a number of blue lichen, typically present in a number of closely related compounds known as Orcein and isolated from the lichen Roccella tinctoria.
Middle English litmose, of Scandinavian origin; akin to Old Norse litmosi herbs used in dyeing, from litr color (akin to Old English wlite brightness, appearance) + mosi moss; akin to Old English mōs moss (Definition of Litmus- Merriam Webster Dictionary)
The range of colors found in most pH paper usually depend on the presence of multiple indicator chemicals/ In the case of pH paper, chemicals such as thymol blue, methyl red, bromothymol blue, and phenolphthalein each have a different color depending on what the pH of the solution is.
At very low pH, for example, thymol blue is a reddish color, because the chemical form of the thymol blue compound is completely protonated.
At higher pH, the chemical loses a proton and forms a ionic compound, whose color is closer to yellow. As the pH gets even higher, the compound loses a second proton and forms a compound whose color is blue.
When you have compounds on the paper whose color is both bluish and yellowish, this mixture of colors is perceived as green. So by having a series of dyes whose color is different at different pH ranges, you get a nice perceived spectrum of colors across the entire range of pH.
Litmus literally means “the color of moss”, although its usage as a simple test for distinguishing acids and bases (blue for base, pink for acid) has solidified the term in a number of related phrases. Anytime something can be determined true or false with a simple test that indicates a clear color difference is said to have a “litmus” test, whether or not it is related to the color of moss or is measuring acids or bases. It should be noted that the exact pH at which a particular indicator color makes its transition from one color to the other depends on a number of factors, and can be influenced by the presence of other chemicals which can shift the color, such as isopropanol (rubbing alcohol).
This can very clearly be seen with the indicator bromothymol blue, whose protonated versus unprotonated forms are in different concentrations at different pH levels.
at pH 2, the bromothymol blue molecules are all fully protonated and the color of the solution is a very bright yellow. At pH 8, the bromothymol blue molecules are fully deprotonated and the color is a very bright blue. However, over the range of pH concentrations depicted inbetween, you are directly perceiving that some of the bromothymol blue molecules are protonated, and others are deprotonated, because the equilibrium for the reversible reaction is only pushed to an intermediate point. In fact, you can directly measure the concentration of each of the distinct forms by measuring the absorption of light at different wavelengths, even though the color appears to be in a continuum or spectrum including yellow, green, and blue, there is no “green” bromothymol blue molecule form.
The protonated form of bromothymol blue has its peak absorption at 427 nm thus transmitting yellow light in acidic solutions, and the deprotonated form has its peak absorption at 602 nm thus transmitting blue light in more basic solutions. Bromothymol blue — Wikipedia
A litmus test does not respond to the presence or absence of protons, hydronium ions, or hydroxide ions- rather it is undergoing a chemical change whose equilibrium is influenced by the presence or absence of a significant concentration of these types of ions. In almost all circumstances, the color change requires the presence of a solvent in order to change color, and blue litmus paper or red litmus paper is colored such because it is dried in the presence of chemicals (typically buffers) which hold the compound in the desired chemical form.
answer originally published on Quora: How does a pH paper detect acid or base? What is the difference between pH paper and litmus paper?
matt harbowy is a scientist, activist, and data management expert. He is one of the founders of the non-profit Counter Culture Labs, working to bring fairness and egalitarian ideals to people interested in learning about science and biotechnology. He is also a top writer on the question and answer site, Quora.
