Litmus Paper Test Sheets

How Does Litmus Paper Work?

What is a Litmus Test?

A litmus test originally meant a simple method to determine whether something was acidic or basic. It then morphed into an expression meaning a test that gives a simple yes or no answer.   It turns out that a real litmus test isn’t always so clear cut.  Find out more below.

The Litmus Paper Test

Dip red litmus paper into lemon juice, nothing happens.  Dip a strip of blue litmus paper into  milk of magnesia, nothing happens. Switch the color of papers, repeat and now you will see a change. This would seem to illustrate that the litmus test readily confirms whether something is an acid or a base. Sounds pretty straightforward, right? Not so fast.

Litmus Acid Base Comparison
2 large sheets of litmus, 1 red & 1 blue are compared. The on the right is red litmus sprayed with an alkaline solution. The one the left is red litmus sprayed with acid.

Litmus Paper Colors-Red or Blue?

When we think of litmus paper colors,  red and blue immediately come to mind. 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 as you can see in strip at top in the image below.

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

You can test this for yourself further with some of the liquids we list in a 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 answer but others do not.

Litmus Paper Test for Acids and Bases

The litmus test uses a single chemical called 7-hydroxyphenoxazone. For acid solutions that are below pH 4.5, the molecule is exactly as it appears in the image below and this gives litmus paper its red color.  When the pH of the solution starts to change towards alkaline,  one the hydrogen atoms (white hemispheres) start to disassociate. In the picture below, it is the hydrogen at the bottom left.

As the pH approaches neutrality (7.0)  half the litmus molecules will still have this hydrogen in place and the other half won’t. This mixture is what produces the shades of purple in the mid range. Once the solution goes to alkaline pH 8.3 or higher, all these hydrogens will have disassociated the litmus indicator be a shade of blue.

Litmus Indicator Chemical Structure
This single molecule is responsible for indicating both acid and base. The presence of acid causes litmus to turn red while the presence of base (alkali) turns litmus blue.

Litmus Paper vs pH Paper

Since litmus paper gives inexact results around neutral pH 7 one has to  consider the alternatives if this range is important. A  pH meter is one option but is relatively expensive and not for everyone.

The logical is a pH test strip designed for a specific pH range. Some of these ranges are very narrow and while others are broader since they use more than one chemical indicator. By using different combinations of indicators you can choose between strictly base pH strips or acid pH strips or ones that cover both. Some acid+base strips are wide range such as the pH 1 to 14 while others are narrow range and cover pH 4.5-10.

You can see examples of materials, many of them found around the home, that cover low to high pH. See our list: The pH of Things.

Litmus Paper and Test Strip Bundles.
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. 

“Specialty pH Test Strips”

Of course, you might have more specific things you need to test for pH. Four narrow range pH test strips are useful for specialty food and beverage people.

  1. pH 3-6 acid test strips are good for sushi rice and salsa.
  2. pH 2.8-4.4 strips, designed for wine making, are also good for kombucha tea.
  3. Homebrewers of beer will want the pH 4.8-6.2 acid test strips.
  4. At the base end of the scale are pH 10-13 alkaline strips for maple syrup makers.
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.

2 thoughts on “How Does Litmus Paper Work?

    1. Good question. Just wet the paper/strip, preferably with deonized or freshly distilled water & press against the object. If acid or base is present, it should cause a color change.

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