True litmus paper is made by soaking absorbent paper strips in a solution extracted from specific lichens, then drying them. The process is straightforward in concept but depends heavily on the quality of your indicator solution and the paper you use. Since real litmus lichens are hard to source, most people making pH indicator strips at home use red cabbage or similar plant-based dyes, which work on the same principle and are surprisingly effective.
What Makes Litmus Paper Work
Litmus gets its color-changing ability from a compound called azolitmin, derived from a natural acid found in lichens. This compound has a molecular structure that physically rearranges when it encounters acids or bases, shifting the wavelengths of light it absorbs. In acidic solutions (below about pH 4.5), the paper turns red. In basic solutions (above about pH 8.3), it turns blue. Between those values, colors can appear as various shades of purple, which is why litmus works best as a simple acid-or-base test rather than a precise pH measurement tool.
The traditional source organisms are lichens in the species Roccella tinctorum and Lecanora tartarea, historically harvested in coastal regions and processed in the Netherlands. Unless you’re a lichenologist with access to these species, you’ll likely use a plant-based alternative for your indicator dye.
Making Indicator Paper With Red Cabbage
Red cabbage is the most accessible and reliable substitute for lichen-based litmus. The leaves contain anthocyanins, pigments that change color across a wide pH range. Unlike true litmus, which only distinguishes acid from base, cabbage anthocyanins produce a whole spectrum: red in strong acids, purple around neutral, green in mild bases, and yellow in strong bases. This actually makes cabbage-based strips more informative than commercial litmus paper for many purposes.
Preparing the Indicator Solution
Tear several leaves of red cabbage into small pieces and place them in a zip-closing plastic bag. Add about a cup of water, press out as much air as possible, and seal the bag. Squeeze and mash the leaves through the bag for two to three minutes until the water turns a deep blue-purple. Pour the liquid into a cup and discard the solids. This liquid is your indicator solution.
You can also make the solution by simmering chopped cabbage in water on the stove for 20 to 30 minutes, which extracts more pigment and produces a more concentrated solution. A stronger solution means more vivid color changes on your finished paper. Let it cool completely before using it.
Choosing the Right Paper
The paper you use matters more than most people expect. You need something absorbent enough to soak up the dye evenly but sturdy enough not to fall apart when wet. Coffee filters work well for home projects. They’re made of cellulose fibers with a consistent, porous texture that absorbs dye uniformly and dries without warping too badly.
For better results, look for qualitative filter paper from a science supply store. Research on paper-based pH strips has found that a cellulose-cotton blend (roughly 50/50) provides good dye uptake and consistent color response. Printer paper and cardstock are poor choices because their coatings repel liquid and produce blotchy, unreliable strips.
Soaking and Drying the Strips
Cut your filter paper or coffee filters into strips roughly half an inch wide and two to three inches long before soaking. Submerge the strips completely in your cabbage indicator solution and let them sit for at least five minutes. Longer soaking, up to 30 minutes, produces strips with deeper color saturation that are easier to read.
Remove the strips and lay them flat on a clean surface or hang them on a line to air dry. Don’t use a hair dryer or oven, as heat can break down anthocyanins and weaken the color response. Drying takes a few hours at room temperature. The strips will look pale blue-purple when dry. They’re ready to use once they’re no longer damp to the touch.
Using Your Homemade Strips
To test a substance, dip one end of a dry strip into the liquid for a second or two, then pull it out and watch the color change. Compare the result to a reference chart you’ve made by testing known substances: white vinegar for acid (should turn pink or red), baking soda dissolved in water for a base (should turn green or blue-green), and plain water for neutral (should stay purple or shift only slightly).
Your homemade strips won’t give you a precise pH number. Commercial narrow-range pH papers are calibrated against certified buffer solutions and can resolve differences as small as 0.5 pH units. Homemade strips are useful for determining whether something is acidic, basic, or roughly neutral, which is all most home and classroom experiments require.
Other Plants That Work as Indicators
Red cabbage is the most popular option, but many deeply pigmented fruits and flowers contain anthocyanins that respond to pH changes. Blueberries, blackberries, red onion skins, and hibiscus flowers all produce workable indicator solutions using the same soak-or-simmer method. Turmeric is another option, though it works differently: it stays yellow in acidic and neutral solutions but turns reddish-brown in bases.
The intensity and range of color changes vary by plant. Red cabbage remains the gold standard for home indicator strips because its anthocyanin profile produces the widest and most visually distinct range of colors across the pH scale.
Storing Your Strips
Homemade indicator strips degrade over time. The two biggest enemies are moisture and air exposure. Paper naturally releases small amounts of acidic compounds as it ages, and oxygen drives oxidative breakdown of both the cellulose fibers and the dye molecules embedded in them. Research on paper degradation has shown that filter paper can shift by several pH units over weeks of exposure to volatile compounds in the air, which would obviously ruin an indicator strip’s reliability.
Store dried strips in a sealed plastic bag or airtight container, ideally with as little air inside as possible. Keep them away from direct sunlight, which accelerates anthocyanin breakdown. A cool, dark drawer or cabinet is ideal. Under these conditions, homemade strips typically remain usable for several weeks to a couple of months. If the strips have faded noticeably or no longer change color when dipped in vinegar, it’s time to make a fresh batch.
Leftover indicator solution can be stored in a sealed jar in the refrigerator for about a week before it starts losing potency. This lets you make new strips without starting from scratch with more cabbage.