A pH indicator is a dye that shows how acidic or alkaline a liquid is by changing color. The surprising part is that the dye is itself a mild acid or base, so it reacts with the very liquid it is testing.1Chemistry LibreTexts. What Is a pH Indicator and How Does It Work? That one feature explains why the same idea works in a lab test strip, a glass of red cabbage juice, and even a garden hydrangea.
What pH Actually Measures
The pH scale, usually drawn from 0 to 14, tracks how many hydrogen ions a liquid contains. A hydrogen ion is the tiny charged particle that acids release. Below 7 a liquid is acidic, 7 is neutral like pure water, and above 7 it is alkaline, also called basic. Acids are substances that give away hydrogen ions, and bases are the ones that take them up.2Lab Pro Inc.. How Does PH Paper Work And What Does It Detect? Each step on the scale is a tenfold change, so pH 3 is ten times more acidic than pH 4.
Lemon juice and vinegar sit around pH 2 to 3. Pure water is about 7, and a baking soda solution lands around 8 to 9. Household bleach and drain cleaner sit near the top of the scale.
The scale runs backward from what many people expect: a lower number means more hydrogen ions. Even pure water holds a tiny number of them, which is why neutral sits at 7 rather than at zero. Adding an acid raises the count and pushes the number down, and adding a base lowers the count and pushes the number up.
Strength and concentration are different things. Strength is how completely an acid gives up its hydrogen ions, while concentration is how much acid is dissolved. A dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid. The indicator responds only to the hydrogen ions actually free in the liquid, so that is the number the color reports.
Why the Color Changes
Indicator dyes are weak acids or weak bases, meaning they only partly give up their hydrogen ions in water.3StudyPug. pH Indicators: Colorful Chemistry Tools Explained So each dye exists in two forms at once, one holding a hydrogen ion and one without it, balanced against each other.4StudyPug. pH Indicators: Colorful Chemistry Tools Explained When the liquid has many hydrogen ions, the balance tips toward the form holding one. When it has few, the balance tips the other way.5Chemistry LibreTexts. What Is a pH Indicator and How Does It Work?
The two forms have different colors. Indicator molecules contain long chains of alternating double and single bonds, and those chains absorb some wavelengths of visible light, which is why the dye looks colored.6Compound Interest. The Colours & Chemistry of pH Indicators In acid, the dye keeps its hydrogen ion and shows its original color. In alkaline liquid it loses that hydrogen ion, its electrons rearrange, and it absorbs different light, so it looks like a different color.7Compound Interest. The Colours & Chemistry of pH Indicators In some dyes, losing the hydrogen ion also lets the molecule lengthen its chain of alternating bonds, which shifts the color further.8Dyes and Pigments. Substituent effects on absorption spectra of pH indicators: an experimental and computational study of sulfonphthaleine dyes Phenolphthalein is a clear example: it is clear in its acid form and turns pink once its structure reshapes in alkaline liquid.9PubMed. Molecular structure and vibrational spectra of phenolphthalein and its dianion
Only the small amount of added dye changes color, not the liquid. It reports the liquid’s acidity without noticeably changing it, and a few drops are enough, since too much indicator can tint the result.
A drop of indicator in water holds a mix of both forms, and the color you see is the blend. In the middle of a dye’s range the two forms are present in similar amounts, so you get an in-between shade, such as purple between red and blue. Adding more hydrogen ions pushes molecules back to the hydrogen-holding form, and removing them pushes the other way, so the change is reversible.
Because the balance shifts bit by bit, the color moves through intermediate shades rather than flipping at once. Most of the visible change happens across a narrow stretch of the scale, so the color seems to switch fairly quickly there. Far outside that stretch nearly all molecules are in one form, and the color stops changing even if acidity keeps shifting.
Why Each Indicator Changes at a Different pH and How Strips Show a Rainbow
Each indicator changes color at its own specific pH range.10Lab Pro Inc.. How Does PH Paper Work And What Does It Detect? A single dye typically changes over a narrow span of about one to two pH units. Phenolphthalein, for instance, switches only in alkaline liquid around pH 8 to 10, so it can’t tell a weak acid from a strong one.
Phenolphthalein stays clear in every acidic and neutral liquid, so clear only means the liquid isn’t alkaline enough to trigger it. Litmus changes in the middle region, which is why it can tell acid from base but not how strong either one is.
The fix is to mix dyes. Universal indicator, a blend of several dyes, turns a whole rainbow of colors from acids through neutral to bases.11Science Notes and Projects. Universal Indicator Chart and Recipes Each dye changes in a different region, so as acidity shifts, one dye finishes its change just as the next begins.
The same trick works with plants: combining two kinds of plant pigment gives a smooth color gradient from pH 1 to 14.12Journal of Chemical Education. Innovative Green Chemistry: Easy and Fast Synthesis of a Universal pH Indicator from Natural Plant Extracts PH paper strips carry such dyes dried onto paper, so you dip one and match it to a chart.
On a universal indicator chart, red usually means strongly acidic, and orange and yellow mean mildly acidic. Green is close to neutral, and blue and purple mean alkaline. Compare in daylight, since indoor lighting can mislead.
Choosing a strip depends on the job. Litmus only says acid or base, red or blue. Full-range strips give an approximate number, and narrow-range strips give finer readings for pools, aquariums or soil.
Indicators Hiding in Your Kitchen and Garden
The purple-red pigments in red cabbage, berries and many flowers shift between red, purple and green-blue forms as acidity changes.13Dyes and Pigments. Red cabbage anthocyanins: The influence of d-glucose acylation by hydroxycinnamic acids on their structural transformations in acidic to mildly alkaline conditions and on the resulting color Dried cabbage, hibiscus, radish and turmeric pigments give easy-to-tell colors over a broad pH range.14PubMed Central. Paper-Based Colorimetric pH Test Strip Using Bio-Derived Dyes Turmeric’s yellow turns brownish only in fairly alkaline liquid, above about pH 9.15Food Control. Color stability and pH-indicator ability of curcumin, anthocyanin and betanin containing colorants under different storage conditions for intelligent packaging development Its color also fades fast in light, losing about two-thirds in a day.16Food Control. Color stability and pH-indicator ability of curcumin, anthocyanin and betanin containing colorants under different storage conditions for intelligent packaging development So make homemade indicators fresh and keep them out of sunlight.
Red cabbage works especially well because its pigments pass through several distinct colors, so you can tell strongly acidic, mild and alkaline liquids apart by eye. The exact shades vary with the cabbage and how strong you make the extract, so compare against your own known samples rather than a fixed chart.
To make it, chop cabbage, cover it with hot water, let it steep, then strain off the purple liquid. A splash in vinegar turns it pink-red, and a splash in baking soda water turns it green or blue. Keep it in the fridge and use it within days.
You can turn this into a test of your own. Pour a little of each liquid, such as lemon juice, clear soda, tap water or baking soda solution, into separate clear cups. Add the same small amount of indicator to each so the colors compare fairly, then arrange the cups from the pinkest to the greenest to make your own pH scale.
Hydrangeas are a special case. Flower color depends on soil pH, but indirectly.17Annals of West University of Timişoara, ser. Biology. Flower color variation and detoxification mechanisms of Hydrangea sp. In acidic soil around pH 5 to 6.5 the roots take up aluminum, which binds the pigment and turns flowers blue.18Annals of West University of Timişoara, ser. Biology. Flower color variation and detoxification mechanisms of Hydrangea sp. Above about pH 6.5 aluminum stays locked in the soil and flowers come out pink.19Annals of West University of Timişoara, ser. Biology. Flower color variation and detoxification mechanisms of Hydrangea sp. Blue flowers hold roughly five times more aluminum than pink ones.20Annals of West University of Timişoara, ser. Biology. Flower color variation and detoxification mechanisms of Hydrangea sp. So the soil’s acidity matters, but it is aluminum that actually flips the color.
Acidifying the soil and adding aluminum sulfate encourages blue, while garden lime raises pH and encourages pink. Changes take a season or more, not days, and white hydrangeas won’t change color at all.
How Accurate Are pH Strips
In hospital checks of stomach fluid, strip readings tracked an electronic pH meter closely, typically reading a few tenths lower.21PubMed Central. Accuracy of pH strip testing and pH liquid testing versus standard pH meter of gastric contents in critically ill patients: a diagnostic accuracy study In a similar hospital comparison, strips and meters usually agreed within about half a pH unit but occasionally differed by up to 0.8.22PubMed Central. Practical pH Testing for Nasogastric Tube Verification: A Prospective Method‐Comparison Study of Low‐Cost Handheld Meters and Colourimetric Strips For the yes-or-no question being asked, whether the fluid was acidic below pH 5.5, the two methods mostly agreed.23PubMed Central. Practical pH Testing for Nasogastric Tube Verification: A Prospective Method‐Comparison Study of Low‐Cost Handheld Meters and Colourimetric Strips So strips are good for deciding which side of a line you are on, and less good for exact numbers.
A strip reading of 6 could really be about 5.5 or 6.5. For a rough check of a pool or fish tank, that margin is often fine. When a small shift matters, use a more precise tool.
A color very close to the cutoff deserves caution, because a half-unit error could land on the wrong side. If you get a borderline color, repeat with a fresh strip or confirm with a meter.
A few habits improve a strip reading. Read it at the time the package states, since colors keep shifting. Compare in daylight against the printed chart, and don’t touch the pad with wet or soapy fingers. Store strips sealed and dry, because moisture spoils them.
A meter is the better choice when a liquid is strongly colored or cloudy, since that hides the strip’s color. Jobs that need precision, like brewing or hydroponics, benefit from a calibrated meter, and meters need regular calibration to stay accurate.
Where pH Indicators Are Used
In titration, chemists add an acid to a base drop by drop until they neutralize, and an indicator signals the moment. The indicator must switch color right where the pH jumps sharply at that end point.24Varsity Tutors. Selecting Indicators — Select appropriate indicator based… A dye that switches too early or late would signal a point where the acid and base have not exactly balanced, giving a wrong amount. That’s why chemists pick an indicator to suit the pair of chemicals. When a strong acid meets a strong base the jump is so big that almost any indicator works.25Chemistry LibreTexts. What Is a pH Indicator and How Does It Work? Some plant extracts give end points that match synthetic indicators closely.26Next Research. Novel eco-friendly acid-base indicators from innovative plant-based sources: Water-based extraction and applications in titration
Researchers are building food packaging films with plant pigments that change color as meat spoils.27PubMed Central. Intelligent pH indicator composite film based on pectin/chitosan incorporated with black rice anthocyanins for meat freshness monitoring Spoiling food releases substances that change pH, which the film shows as a color shift. These labels are cheap and small and give shoppers a visual freshness signal.28PubMed. Recent advances in pH-sensitive indicator films based on natural colorants for smart monitoring of food freshness: a review The label supplements the use-by date and smell, and doesn’t replace safe food handling. In medicine and research, glowing indicator dyes can track acidity inside living cells.29PubMed Central. Imaging Intracellular pH in Live Cells with a Genetically-Encoded Red Fluorescent Protein Sensor
You may already use indicators. Pool and spa test kits use color drops for pH, aquarium kits work the same way, and soil test kits help gardeners choose plants. Some urine dipsticks include a pH pad.