Litmus Indicator Strips

How Litmus Paper Works for Acids & Bases

What is 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.

Litmus Paper and Test Strip Bundles.
Litmus paper/pH test strip bundles. Save 10-15% over buying separately . See below for more details.

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.

 

You can find out for yourself using these combination packs of pH & litmus test strips. We have put together 3 different bundles you can try. All include a vial each of 100 blue and 100 red litmus papers.

  1. pH 1-14. Compare the results of litmus paper to wide range pH 1-14strips.
  2. pH 4.5-10. Compare the results of litmus paper to narrow range pH4.5-10 strips which give results just above and below neutral pH 7.
  3. pH 1-14 & pH 4.5-10. Compare results on all of them as you go from strong acid to strong base.

You can see examples of materials, many of them found around the home, that cover low to high pH at the bottom of this page where it lists the pH of Things.

Litmus Paper and Test Strip Bundles.

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

About Stephan Logan

VP Sales & Marketing at Indigo Instruments. Previously sold hi tech science instruments to researchers. B.Sc. & M.Sc. from McGill University.

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