Phenol Red first showed up in the scientific record in the late 1800s. Early chemists, hungry for tools to see chemical changes by color, found its unmistakable red-to-yellow shift as pH changed. Its discovery, rooted in an era when biology and chemistry danced more closely than ever, shaped generations of lab work. I grew up seeing it sweep through my undergraduate labs. Years before specialized digital equipment, bottles of Phenol Red sat on benches, ready to reveal the secrets inside milk, blood, or cell culture bottles. Its legacy speaks to the persistence of hands-on science.
This compound goes by names like phenolsulfonphthalein and C.I. 45380, but most recognize it as that trusted pH indicator. Phenol Red signals changes from acidic to basic environments with an obvious color change—shifting from yellow at lower pH to deep pink as solutions turn basic. Labs worldwide use it to monitor health in cell cultures, check water quality, and pick up subtle chemical clues no digital readout could ever match. Its value shines not from mere analysis but from its role in learning, teaching, and hands-on experimentation.
Phenol Red appears as a pink or red crystalline powder. It’s soluble in ethanol and just enough in water for practical lab use. Its molecular formula, C19H14O5S, marks it as an organic sulfonphthalein dye. Its melting point lands near 270°C, though it breaks down before reaching a clean melt. The color change kicks in at a pH of roughly 6.8, launching the shift from yellow to red. In strong alkaline conditions, a further move toward a violet or purple hue happens. These shifts are more than chemistry—they’re language for those reading reactions by eye.
Every bottle of Phenol Red should come with labels marking purity, batch number, storage guidance, and hazard information. Most research-grade lots guarantee 98% or higher purity. Labels show the standardized CAS number—143-74-8—alongside chemical structure and other identifiers like EC Number 205-609-7. Storage instructions usually point at a dry, cool shelf, away from direct sunlight, to avoid breakdown. My own experience tells me, always check expiry and batch info, as expired or light-damaged indicator gives false readings, sometimes with costly results.
Manufacturers create Phenol Red by bringing together o-sulfobenzoic acid anhydride and phenol in concentrated sulfuric acid. Heating triggers condensation, producing the signature compound. It takes careful control of temperature and reactant purity to get the high-quality product labs demand. Quality control at this stage weeds out weak batches that might display off-color or drift out of the standard pH range. Each run’s end result gets recrystallized, washed, and dried for survey before shipping. This hands-on, production-level attention keeps surprises from creeping into analytical work.
Phenol Red doesn’t just sit in a beaker. Under the right lab setup, chemists can tweak its structure—adding or protecting groups for studies into analogues or for exploring sensitivity in new indicator ranges. Under strong oxidizers, it can break down, while reducing conditions sometimes stop its color change in its tracks. Exposing the indicator to intense UV or heat risks unplanned structural changes, so labs store it away from those extremes. Some research teams have worked up derivatives to tune exact shade shifts or to attach it onto larger polymers, each approach opening new avenues of use.
Walk through a supply catalog, and you might spot Phenol Red under names like phenol sulfonphthalein, PSP, or C.I. Acid Red 85. Each label points toward the same core compound, dovetailing with country-specific standards. Labs in one part of the world might ask for PSP, while others need the C.I. number for regulatory filings. Such synonym checks matter during audits or multinational projects, stopping crossed wires before they cause confusion or safety slips.
Every bottle carries safety data for a reason. Phenol Red powders can irritate eyes and skin. Dust invites trouble if it drifts into the nose or mouth. Good habits—gloves, lab coats, protective eyewear—remove most risk. Spills clean up with water and standard absorption pads, while powdered spills in open air should send the user for fresh air right away. Disposal walks the regulated waste path. One story I remember—an overzealous freshman dumped unused solution into the regular trash, releasing a strong dye in the sink. That cleanup lesson lingers; respect the bottle and follow the rules.
Few indicators have shaped modern biology labs like Phenol Red. Cell culture media rely on its color change to warn of pH drops sullying experiments. Water quality kits snap up its speed and visibility for field testing lakes, aquariums, or even swimming pools. Doctors in the past leaned on it for kidney function testing; a properly prepared PSP injection tracked how quickly patients’ systems cleared the dye. Alongside this, students still make their first real “scientific discovery” with test tubes containing that familiar red or yellow solution. Outside the lab, it lingers in art as a specialty colorant, its vivid hue speaking a different language.
Researchers push Phenol Red into new territory by looking at ways to tweak sensitivity or pair it with digital sensors for hybrid readouts. Biomedical engineers turn to it as a model for smart sensing materials or as a dye for imaging when safer alternatives are ruled in. Teams in environmental science check its use against greener options, aiming to lower chemical footprints while keeping intuitive visibility. I’ve seen conference posters filled with possible analogues that hold onto the clean pH shift but lose toxicity. Each year’s advances build on that long scientific shadow cast decades ago, hinting that even century-old compounds can learn new tricks.
Safety researchers do not ignore Phenol Red’s possible risks. Studies show it acts as a weak estrogen mimic, prompting debate on its inclusion in all cell cultures—especially sensitive hormone research. Labs aiming for purity in hormone assays often run their experiments without it, switching to uncolored media or alternative indicators. Acute toxicity remains low on standard lab exposures, but anyone handling the raw powder on an industrial scale keeps a stack of precautions close by. Ongoing work looks at breakdown products in wastewater, checking for unexpected biological activity. For most units, short-term exposures remain safe when handled correctly, but researchers track the details as regulatory landscapes shift.
The future of Phenol Red swings on both tradition and innovation. Science classrooms will likely hold onto those familiar bottles for years to come, teaching pH and chemical change in a way no screen can quite match. On the industrial side, green chemistry and regulatory shifts could push demand for less hormonally active or less persistent alternatives. Synthetic chemists might keep finding ways to tweak the core structure for sharper color jumps or smarter pairings with digital sensors. Whether in a classic glass flask or next-generation sensor, Phenol Red’s story feels unfinished—offering lessons both for the old-school scientist and for those chasing the next breakthrough in environmental or biomedical tech.
Phenol Red looks like a simple dye in a bottle, but its task in labs and classrooms matters more than people often realize. Walk into any biology or chemistry lab, and you’ll probably spot a small vial of this stuff. It’s a pH indicator, which means it checks how acidic or alkaline a solution gets. You’ll see it change color from yellow to red, even to purple, based on the chemical action happening in a beaker or Petri dish.
Medical research and daily science classes rely on it. Anyone who’s ever grown bacteria or tried cell culture has seen Phenol Red sitting in the background, quietly showing whether things stay healthy or if something’s off. In tissue culture media, if cells start to die or a culture gets contaminated, Phenol Red changes color. That color shift can nudge lab workers to stop and check on their experiment before losing days of work or risking false results.
Swimming pools use it, too. Pool test kits use Phenol Red to make sure the water stays safe for everyone’s skin and eyes. This process saves time and prevents swimmers from stepping into unsafe water. I remember being tasked with checking pool water as a summer lifeguard, and Phenol Red made the job almost foolproof. Good water meant a healthy shade of pink. Out of range meant re-checking the chemicals before anyone dove in.
Dentists, plumbers, and aquarium owners rely on this indicator. Dental offices run it through water lines to spot dangerous microbes. Homeowners check if tap water runs too acidic and might corrode pipes. Aquarium enthusiasts use it so their finned pets don’t get sick from pH swings.
School science lessons would get a lot duller without that satisfying splash of color as students add drops to unknown solutions, letting everyone learn by watching color changes instead of squinting at complicated meters. Phenol Red lets teachers turn science into hands-on discovery instead of just lectures and numbers.
Although Phenol Red does its job, it brings up a few issues. Sometimes it can interact with solutions in a way that changes the outcome, especially in sensitive laboratory experiments. It also doesn’t work well in solutions that already have a strong color. This makes it tough when you’re dealing with samples from nature, like muddy river water.
One answer involves combining indicators or using high-tech digital pH meters. Some researchers have shifted to colorless or low-interference pH indicators for experiments that require intense precision. Others choose Phenol Red alternatives developed with less cross-reactivity, which home test kits have started adopting too.
Phenol Red has stood the test of time in science, medicine, and even at home. It’s simple to use and easy to see, making it a trusted sidekick in dozens of everyday places. Innovation continues in the field of pH testing, but this little color-shifting compound remains part of the fabric of research and routine life.
Phenol Red shows up a lot in biology labs. The bright pink powder serves as a pH indicator, showing scientists if something’s acidic or basic. You probably spotted that classic color shift in science class experiments. Hospital labs use it too, especially for cell culture. It helps researchers track pH changes, since living cells can’t handle too much acidity or alkalinity.
On its own, Phenol Red doesn’t throw up as many red flags as stronger chemicals. That doesn’t mean it’s as gentle as table salt. Swallowing, inhaling, or getting it on your skin in large amounts creates problems. Animal toxicity tests put the lethal dose for rats at over 5 grams per kilogram. That’s far more than most people would ever come in contact with during normal lab work. Still, just because something isn’t extremely toxic doesn’t make it harmless.
Breathing in Phenol Red powder can irritate the nose and throat. Direct contact on bare skin can lead to redness or mild irritation. Getting a small dusting of pink powder on your fingers probably won’t send you to the hospital—but as with plenty of lab reagents, taking care matters. Consistent, tiny exposures build up over time and might lead to issues down the road, especially for anyone with sensitive skin or allergies. Chronic exposure results haven’t been well studied.
Storing or mixing Phenol Red raises questions about fire safety, too. It doesn’t usually catch fire at room temperature, but powder residue near hot equipment could present a risk. Workers can easily avoid those situations with good storage and handling habits. Upsetting the container or spilling solution on tables could stain surfaces or clothes, but not much more if handled promptly.
Folks might ask if lab workers carry danger from this powder back home. Standard hygiene—washing hands, keeping food out of the lab, using gloves—makes a big difference. Lab standards from organizations such as OSHA place Phenol Red in the “irritant” category. Eye contact or swallowing large quantities trigger trouble, so safety data sheets always recommend eye protection and proper labeling. Rinsing off right away in case of exposure brings risks down.
Disposal gets attention as well. Pouring Phenol Red solutions down the sink without dilution or neutralization leads to water pollution. Some treatments used in the dye industry found drops of Phenol Red hindered certain aquatic life. The chemical breaks down under sunlight, so outdoor exposure reduces risk for larger spills. Following hazardous waste guidelines for lab reagents helps avoid problems for people and the ecosystem.
People can make labs safer with clear training, up-to-date material safety data sheets, and proper personal protective equipment. Switches to alternative indicators could bring benefits for greener labs, but sometimes performance makes alternatives impractical. Investing in engineering controls such as fume hoods, using closed containers, and double-checking handling rules will keep risks to a minimum. Schools and research organizations have a role in setting good habits for young scientists. Respecting even minor irritants proves it’s possible to run experiments without sacrificing health or safety.
Anyone who has spent long hours at a tissue culture hood recognizes that telltale pink shade in the flask. Phenol red is more than just a splash of color: it can give you a heads-up on what’s going right—or wrong—with your cultures. Its claim to fame comes from acting as a pH indicator, signaling changes in your medium so you’re not entirely working in the dark.
I remember my first month in the lab, desperately peering at my flask, not knowing if my cells were actually growing or if my medium was about to fail. One look at the color made things clearer: bright red meant good, creamy orange meant trouble brewing. Anyone handling cells wants healthy, reliable growth, so catching problems early really helps.
Phenol red shifts color as acidity changes. Fresh, healthy medium looks reddish-pink at a pH around 7.4. Acidification, usually from cell metabolism and CO2 buildup, turns the medium orange to yellow, signaling the pH is dropping. This visual feedback becomes practical in troubleshooting. Rapid yellowing hints at contamination or overcrowding, and an odd purple tinge points to inadequate CO2 buffering.
This isn’t just an old-school hack. Accurate pH ensures cell viability, function, and reproducibility. Over-acidified media stunts cell growth, messes with cell signaling, and throws experimental results out of balance. On the flip side, medium staying deep pink or purple might reflect a faltering incubator, ineffective buffering, or media straight from the fridge, all needing attention.
Still, phenol red isn’t a solution for every scenario. Sensitive assays, especially those measuring hormones like estrogen or involving sensitive reporter genes, might get skewed by phenol red’s weak estrogenic activity. Certain stem cells and primary cells also behave better in dye-free media, reducing background and mismatched results. Many suppliers now offer phenol red-free versions, so labs can match medium to the experiment without juggling too many risks.
Tracking color changes works best with proper training. Some labs get lax, skipping visual checks during routine splits or feeds. Taking those extra seconds pays dividends. In my own work, pausing to check the shade and sniff for off odors cut down on wasted cultures and headaches. Young researchers sometimes ask: “Is the color really that important?” My answer remains yes—one small oversight can snowball into misleading data, lost time, or failed projects.
Most challenges with pH and phenol red come down to technique and environment. Handling media under the hood, keeping lids closed, and controlling exposure outside the incubator all prevent unnecessary shifts. Investing in regular incubator maintenance and good recordkeeping makes detective work after something goes wrong less painful. In shared labs, clear labeling and communication keep everyone on the same page—not just science, but a bit of culture in both senses.
Phenol red seems small, but it helps keep cell culture honest. Its practical role grows alongside airtight protocols and scientific vigilance. Paying attention not only strengthens experiments, but teaches everyone that simple, careful observation remains a staple for quality, reproducible science.
Life in the lab rarely feels simple, but some tools pull more weight than others. Phenol red appears as just another bottle on the shelf, but it teaches more lessons than its unassuming look suggests. Watch a high school student swirl a test tube with phenol red. The color shift brings chemistry to life right in their hands. I’ve watched the surprise hit their faces—suddenly, invisible pH values reveal themselves in real time, transforming science from textbook theory into direct experience.
This indicator gives quick answers without much fuss. Most scientists can tell the importance of measuring pH, from monitoring pool safety to testing blood cultures. Small mistakes in pH spell trouble—food spoils faster, enzymes misbehave, our bodies start to signal distress. So picking the right range can mean the difference between good science and wasted effort.
The pH range for phenol red falls between 6.8 and 8.4. In this narrow window, phenol red flashes a series of colors. Anything below 6.8 shows yellow, often alarming in that unmistakable way of an old bruise. At neutral or just above, orange and then slowly red emerges. Push above 8.4, and the red shifts toward pink or even fuchsia, like a flower too wild for its own garden. This visual scale gives anyone peering into a flask fast feedback—much faster than fiddling with electrodes or strips.
Why is that two-unit range important? Consider everyday examples. Blood lands at about 7.4 on the scale. Most tissue cultures in research also like hanging around here. Even swimming pools stay in the same window, where chlorine kills germs without stinging swimmers’ eyes. Phenol red connects these worlds by quickly revealing if something’s off. If yellow creeps in, that’s acid creeping up. If the solution turns hot pink, alkalinity takes over.
I remember swapping stories with a cell culture tech in a hospital research wing. She swore everyone relied on phenol red’s color shifts for fast checks—red for steady cultures, yellow if bacteria crashed the party. Nobody trusts just the indicator, but it sure points out trouble before the numbers come in. The same rule applies across food, pool water, or even fish tanks. My old neighbor once saved an entire aquarium batch watching phenol red fade toward yellow after adding new decorations treated with acid.
Efficiency matters, especially when budgets run tight. Glass electrodes work great, but pricey calibrations and downtime slow the process. Phenol red doesn’t mind a busy lab. Small bottles last ages, shelf stable and friendly to anyone with working eyes. Yet phenol red does not do it all—you can’t measure strong acids or bases with any accuracy, since the color stops changing out of range. For needs beyond 6.8–8.4, grab a different indicator or use a pH meter.
No one gets far in science without double checking. Even phenol red’s interpretation depends on clear lighting and healthy eyesight. Some labs struggle with subtle shifts in color, especially for those with color vision problems. Digital tools help, but costs and complexity may put them out of reach. For classrooms, clinics, and field testers, phenol red delivers an honest, useful answer—straight from bottle to beaker.
Anyone who’s handled Phenol Red in a laboratory setting knows it’s more than just a colored chemical; it’s a linchpin for cell culture, pH testing, and water quality work. A lot goes wrong if phenol red degrades. You look for reliable readings and stable solutions. That only happens if storage isn’t left to guesswork.
I’ve found that tossing phenol red anywhere on a crowded shelf brings predictable headaches. Light exposure starts breaking it down—a problem I didn’t fully believe until a batch on a sun-facing bench lost its punch, making pH checks drift far from the truth. Sticking to amber bottles and keeping them away from direct light preserves potency longer than any warning label lets on.
Temperature swings in the lab can sneak up and cause headaches. High heat? Phenol red oxidizes and wrecks results. Cool, steady temperature beats fancy refrigeration every time for longer storage. Between 15 and 25°C keeps it stable enough for six months or more. Freezing? That’s a hard no. Ice crystals mess with the buffer systems and stir up new sources of contamination. I’ve seen frozen, thawed phenol red solutions go cloudy, changing how they react in routine assays.
Humidity creeps into open containers quicker than most realize. A desiccator isn’t just a suggestion—it’s practically policy in any serious workspace. I keep packets of dry silica gel in storage drawers. Ever since I stopped ignoring those, reagent shelf life shot up. Humidity clumps the powder, making it tough to get a clean measurement and undermining calculations.
Clean scoops and no double-dipping. Once, by rushing through setup, a colleague reused a spatula and cross-contaminated a whole jar. Irritation aside, it means possible bacterial growth, mold, or subtle chemical changes. Every time the jar opens, airborne dust and skin flakes find a way in, so minimizing uncapped time makes a difference.
Every bottle gets dated on opening. I don’t keep bulk containers at the bench, either. Splitting into smaller vials gives me flexibility—with most of the supply safe in cool, dry storage, and only a week’s worth handy for daily use. That way, a single spill or accident never ruins the entire batch.
Many suppliers hand out basic storage advice, scattered across technical sheets. For most labs, real-world experience shapes careful routines. Colleagues tend to favor tight lids, dry places, and darkness for good reasons: the protocol keeps results predictable. The science backs up what veterans know from trial and error.
Keeping phenol red potent means respecting its quirks as much as its uses. Avoiding careless shortcuts with storage habits saves time, money, and a lot of rework. Simple rules—store it dry, cool, and out of the light—deliver confidence in the accuracy of your experiments, day in and out.
| Names | |
| Preferred IUPAC name | Phenolsulfonphthalein |
| Other names |
Phenolsulfonphthalein Phenol Red sodium salt C.I. 45350 PSP Sodium phenolsulfonphthalein |
| Pronunciation | /ˈfiː.nɒl rɛd/ |
| Identifiers | |
| CAS Number | 143-74-8 |
| Beilstein Reference | 1325958 |
| ChEBI | CHEBI:31991 |
| ChEMBL | CHEMBL1377 |
| ChemSpider | 12008 |
| DrugBank | DB11047 |
| ECHA InfoCard | 100.039.110 |
| EC Number | 200-001-8 |
| Gmelin Reference | 5867 |
| KEGG | C05618 |
| MeSH | D010634 |
| PubChem CID | 4766 |
| RTECS number | SX8145000 |
| UNII | LF5GTA2R2J |
| UN number | UN2821 |
| Properties | |
| Chemical formula | C19H14O5S |
| Molar mass | 354.38 g/mol |
| Appearance | Red to reddish-violet crystalline powder |
| Odor | Odorless |
| Density | 1.31 g/cm³ |
| Solubility in water | Soluble |
| log P | 1.2 |
| Vapor pressure | <0.01 mmHg (20°C) |
| Acidity (pKa) | 7.9 |
| Basicity (pKb) | pKb = 8.0 |
| Magnetic susceptibility (χ) | -64.0e-6 cm³/mol |
| Refractive index (nD) | 1.326 |
| Viscosity | Viscous liquid |
| Dipole moment | 6.74 D |
| Thermochemistry | |
| Std molar entropy (S⦵298) | 278.0 J·mol⁻¹·K⁻¹ |
| Std enthalpy of formation (ΔfH⦵298) | -900.8 kJ/mol |
| Pharmacology | |
| ATC code | V04CH02 |
| Hazards | |
| Main hazards | Harmful if swallowed. Causes serious eye irritation. Causes skin irritation. May cause respiratory irritation. |
| GHS labelling | GHS07, GHS08 |
| Pictograms | GHS05,GHS07 |
| Signal word | Warning |
| Hazard statements | H302 + H312 + H332: Harmful if swallowed, in contact with skin or if inhaled. |
| Precautionary statements | Wash hands thoroughly after handling. IF ON SKIN: Wash with plenty of water. If skin irritation occurs: Get medical advice/attention. Take off contaminated clothing and wash it before reuse. |
| NFPA 704 (fire diamond) | 2-0-0 |
| Flash point | 79 °C |
| Lethal dose or concentration | Lethal Dose (LD50) Oral Rat: 826 mg/kg |
| LD50 (median dose) | LD50 (median dose): Oral, rat: 5000 mg/kg |
| NIOSH | SD2450000 |
| PEL (Permissible) | 50 mg/m³ |
| REL (Recommended) | 10 mg/L |
| IDLH (Immediate danger) | No IDLH established. |
| Related compounds | |
| Related compounds |
Phenolsulfonphthalein Cresol Red Thymol Blue Bromothymol Blue Methyl Red Methyl Orange |
