Litmus
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Litmus is a water soluble mixture of different dyes extracted from lichens. Litmus absorbed onto filter papers is one of the oldest and most known form of pH indicator.
Contents
Composition
Litmus contains between 10 to 15 different dyes, with 7-hydroxyphenoxazone being the most important.
Properties
Litmus is a black powder, soluble in water and some organic solvents, which changes color in acidic environment to red and blue in basic solution. At pH 7 the solution is purple. Litmus, both as solution and as paper strips, is as versatile as universal indicator strips, but it offers less precise measurements of pH. Still, if you don't need to know the exact pH, litmus strips can be used instead of universal indicator strips.
Litmus paper strips come in three varieties: red, neutral (violet) and blue. Red strips are made from an acidified litmus solution, they are red by default; the inverse is true with blue litmus strips. Red and blue strips are used when you already know if you are testing for an acid or a base, and the color change is more pronounced with them (this is useful with old paper strips, which turn very pale with time). Red and blue litmus paper strips can usually be converted to neutral litmus paper strips by soaking them in water and drying.
Availability
Litmus powder and litmus paper can be purchased from chemical suppliers and online. It can be found on eBay and Amazon.
Isolation
Litmus can be isolated from certain lichens, though you'll need an appreciable amount of them to extract useful quantities of litmus.
Projects
- pH indicator
- Dye
Handling
Safety
Litmus has low toxicity and no special protection or safety measures are required when handling it. However, it can stain, so wear disposable gear.
Storage
In closed bottles.
Disposal
No special disposal is required. Discard it as you wish.
See also
- Anthocyanin, a natural dye mix similar in properties to litmus, and much easier to obtain from OTC products, which can be used instead of it.
