Litmus Indicator Strips

How Does Litmus Paper Work for Acids & Bases?

What is pH Litmus Paper?

Litmus paper is the most basic way you can test if a liquid is acidic (pH<7) or basic/alkaline (pH>7). Blue litmus paper turns red in acid and red litmus turns blue in alkaline. What could be simpler? It turns out there’s more to it than you thought.

The Litmus Paper Test

Dip red litmus paper into lemon juice, nothing happens or dip blue litmus into  milk of magnesia, nothing happens. Switch papers and you will see a change. This basic litmus test should confirm whether something is an acid or a base. Sounds pretty straightforward, right? Not so fast.

Litmus Paper Colors-Red or Blue?

When we think of litmus, we think of red or blue. It turns out there is also a gray area, or purple to be more accurate. The color of litmus is only solid red below pH 4.5 and solid blue above pH 8.3.  From pH 4.5 to pH 8.3 the color goes through shades of purple.

Litmus Paper Transition Colors
The picture shows  the strip where one end was dipped in acid pH<4.5 and the other end was dipped in base > pH8.3. The purple part in the middle is pH 7 which is neutral. The bottom strip shows a central part that has no litmus in it all.
Image source: Wikipedia

You can test this further with some of the liquids listed in the chart at the bottom of our pH-Litmus paper page. You will see that some of these will give you a definitive yes or no but others do not.

How Litmus Paper Works

The litmus test uses a single chemical called 7-hydroxyphenoxazone. When it encounters an acid below pH 4.5, it is as shown below. When it is exposed to a base above pH 8.3, it loses a hydrogen atom, the white hemisphere at the bottom left.  This causes it to change to a blue color. As you go from pH 4.5 to pH 8.3 the amount hydrogen removed increases. So, the color changes from red to purple to blue.

Litmus Indicator Chemical Structure
You can explore this litmus indicator chemical structure further by clicking on the image. It will take you to our 3D Model Builder. Just enter 7-hydroxyphenoxazone & you the image will appear. You can rotate the molecule to see if from any angle.

Litmus Paper vs pH Paper

Since litmus paper has limitations, consider the alternatives. A  pH meter can do it all but is relatively expensive and not for everyone. A good choice is a pH test strip since these use more than one chemical indicator. They can give more precise answers than litmus paper. You can choose between strictly base determining pH strips, acid determining strips or ones that cover all the way from pH 1 to 14.

Explore our site for these and other test strips you can use for science experiments.  Buy now at www.indigo.com for immediate delivery.

Or, if you have any tips or tricks of your own on using litmus paper or test strips, please let us know.

Summary
Article Name
How Does Litmus Paper Work
Description
Litmus paper is the simplest way to test for acid or base in a liquid. It does have some shortcomings which you should know.
Author
Indigo Instruments

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