Equilibrium What Color Do Acids Turn Blue Litmus Paper

This litmus test uses red, blue, or purple litmus paper to determine whether a sample is acidic, basic, or neutral.

Litmus paper is pH indicator made by treating filter paper with a solution of litmus. Litmus, in turn, is mixture of dyes made from lichens that changes color in response to acids (red) or bases (blue).

Take a look at the colors litmus paper turns, how to perform and interpret the litmus test, how to make litmus paper, and how the color change works.

Litmus paper is pH paper treated with litmus dye. It can be blue, red, or purple and only changes to those colors. Sometimes people call all pH paper "litmus paper." But, this is not correct. Many indicators find use as dyes in pH paper and they can change to other colors.

Litmus Paper Colors

There are three varieties of litmus paper: purple, blue, and red. Purple litmus paper contains the dyes used in both blue and red litmus paper.

Blue litmus turns red in acid. Red litmus stays red, but it looks a bit different when wet. (image: Kanesskong, CC 4.0)

• Purple Litmus Paper: Purple at neutral pH, red below pH 4.5, blue above pH 8.3
• Blue Litmus Paper: Unchanged blue or sometimes purple at neutral pH, red below pH 4.5, blue above pH 8.3. Blue paper that doesn't change color indicates the sample is a base.
• Red Litmus Paper: Unchanged red or sometimes purple at neutral pH, red below pH 4.5, blue above pH 8.3. Red litmus paper that doesn't change color indicates the sample is an acid.

Red litmus paper actually appears more coral-colored than red when dry. It looks deeper red when wet. Blue and purple litmus paper looks slightly darker when wet, but it's not a big change. Red and blue litmus paper may appear unchanged at neutral pH, but sometimes they lean purple. Red and blue litmus paper don't necessarily indicate neutral pH.

So, purple litmus paper indicates whether a sample is acidic, neutral, or alkaline. Blue litmus paper indicates whether or not a sample is acidic. Red litmus paper indicates whether a sample is basic.

How to Perform the Litmus Test

The litmus test tells whether a liquid or gas is acidic, neutral, or basic. It indicates whether the substance is pH 4.5 or lower, between 4.5 and 8.3, or 8.3 or higher. The litmus test does not accurately gauge pH — it only tells if the sample is acidic, neutral, or basic.

Litmus Test for Liquids

1. Collect liquid sample using a clean dropper or pipette.
2. Place a drop of sample on one end of litmus paper test strip. Don't dip litmus paper into a sample because the dye spreads into it.
3. Interpreting the test result depends on the color of litmus paper. In general: red indicates acid, purple is neutral, and blue is basic or alkaline.

Litmus paper that has changed from red to blue may be used as blue litmus paper. Litmus paper that has changed from blue to red may be used as red litmus paper. Used litmus paper isn't as reliable as a fresh test strip because some residue may remain from the previous test and interfere with results. But, it works fine in most settings and is less wasteful.

Litmus Test for Gases

• Lightly dampen a litmus paper test strip with distilled water. Do not use tap water because it may not have a neutral pH.
• Expose the test strip to the gas.
• If a color change occurs, it affects the entire test strip. Neutral gases, such as nitrogen and oxygen, don't change the color of litmus paper. Acidic gases turn the paper red, while alkaline gases turn it blue.

Only test water, aqueous samples, or gases using litmus paper. Oil-based samples won't react with the indicator.

Limitations of the Litmus Test

The biggest limitation of the litmus test is that it gives a rough estimate of pH and not a numerical value. Another limitation is that litmus sometimes changes colors from reactions beside an acid-base reaction. For example, chlorine gas turns blue litmus paper irreversibly white. Basically, it bleaches the dye.

History and Preparation

The word "litmus" comes from an old Norse word for "color" or "dye." Litmus came into use around 1300 CE by Spanish alchemistry and physician Arnaldus de Villa Nova.

Dendrographa leucophaea is one of the lichens used to make litmus. As you can see, it's not red or blue. (image: Jason Hollinger, CC 3.0)

Lichens have been a source of litmus from the 16th century into modern times. Currently, the main sources of litmus are the species Roccella montagnei and Dendrographa leucophoea. Other species producing the pigment include Roccella tinctoria, Roccella fuciformis, Roccella phycopsis, Rocella pygmaea, Ochrolechia parella, Parmotrema tinctorum, Variolaria dealbata, and Parmelia sp. The lichens contain between 10 and 15 indicator compounds. Some of these are separated to act on their own, such as azolitmin and erythrotmin.

The lichens require treatment before becoming litmus. Historically, they were mixed with urine, lime, and potash. After the mixture has fermented, it's mixed with chalk or plastic of Paris to make blue powder, which can be molded into cakes. Dissolving the dye in water and treating filter paper produces litmus paper.

How the Litmus Test Works

Red litmus is a weak diprotic acid. When exposed to a base, hydrogen ions from litmus react with the base to form the blue conjugate base. The mechanism works in reverse for blue litmus. The blue conjugate base accepts protons from the acid in the test sample, producing the red litmus. At a neutral pH, the process is at equilibrium. Both the red diprotic acid and its blue conjugate base are present, blending to make purple.

Alternatives to Litmus Paper

Coating filter paper with orcein dye or azolitmin dye results in filter paper that behaves much like litmus paper. However, you can coat filter paper with any pH indicator. Universal indicator yields a rainbow of color changes that correspond to numerical pH values. Red cabbage juice is another good choice because it displays a range of colors and is much easier to find at the store than lichens or universal indicator.

References

• Beecken, H.; E-M. Gottschalk; U. v Gizycki; H. Krämer; D. Maassen; H-G. Matthies; H. Musso; C. Rathjen; Ul. Zdhorszky (2003). "Orcein and Litmus".Biotechnic & Histochemistry.78 (6): 289–302. doi:10.1080/10520290410001671362
• Musso, H.; Rathjen, C. (1959). "Orcein dyes. X. Light absorption and chromophore of litmus". Chem. Ber. 92 (3): 751–3. doi:10.1002/cber.19590920331

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Source: https://sciencenotes.org/litmus-paper-and-the-litmus-test/

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