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 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.
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 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.
“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.