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 “black and white”.
Litmus Paper-Red or Blue?
When we think of litmus paper, the colors red and blue 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 the top strip in the image below.
You can test this for yourself further with some of the liquids in the chart below. You will see that litmus paper can only give a definitive yes or no for acid/base for some of them.
pH of Things Around the House
You can demonstrate the properties of a litmus test test using the liquids listed below. You will see for yourself that litmus paper test can only confirm the most acidic or basic ones but not the ones closer to neutral pH 7
Litmus Paper Tests
An interesting feature of litmus is that the color change is reversible. Drop lemon juice onto the milk of magnesia spot and milk of magnesia onto the lemon juice spot and see what happens.
Try this with different liquids. What happens if you make a spot with a weak base & then drop a strong acid on it. Or, a strong base & then a weak acid? There are all sorts of possibilities.
If you have an artistic streak, you can evoke your inner Jackson Pollock and create weird and wonderful patterns like those below on the 8×10″ (200x250mm) sheets of litmus paper.
Litmus Test Chemistry
So why does litmus behave this way? It’s because of 7-hydroxyphenoxazone. When exposed to acids below pH 4.5, the molecule looks like the image below and this gives litmus paper its red color. As the pH of the test solution moves towards alkaline, one of the hydrogen atoms (white hemispheres) starts to break off (disassociate). In the picture below, it is the hydrogen at the bottom left.
As the pH approaches neutrality (pH 7) more and more of the indicator molecules lose this hydrogen atom. At pH7, half the litmus molecules will still have this hydrogen in place and the other half won’t. This mixture 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 and the litmus indicator is now blue.
Litmus Paper vs pH Strips
Since litmus paper gives indeterminate results around neutral pH 7 one has to consider alternative tests in this range. A pH meter is one option but this is relatively expensive and not for recommended for science fair or homeschooling use.
pH test strips use different chemical indicators. Some have just one indicator that reacts in a narrow range such as pH 3-6. Other test strips use several indicators which can work with a wider range of pH levels. You can even choose between strictly base pH 7-14 strips or acid pH 0-7 strips. Or, use ones that cover a very wide range of acid & base such as the pH 1 to 14 or just the ones on either side of neutral like the pH 4.5-10 test strips.
You can do interesting experiments with these litmus paper and pH strip combination packs; see them list in the 3rd of pH/litmus paper.
There are also pH test strips for some foods and beverages such as: