In the late 19th century, chemists chased reliable tools for measuring acidity. Cresol Purple entered the stage during a dynamic era in chemical research, showing up in scientific papers and manuals as the hunt for robust indicators took off. Once William Ramsey and other foundational figures set the stage for pH measurement, indicator dyes like Cresol Purple made a difference in labs. It became popular with researchers and teachers, thanks to the consistency and clear color change it brought to the table. Even as digital methods have taken over much of analytical chemistry, many still remember that familiar purple tinge cropping up in titrations and pool water tests.
Cresol Purple carries the formula C21H18O5S and goes by names such as o-Cresolsulfonphthalein and Crystal Purple. It usually appears as a fine powder, showing off a faint purple or reddish hue, depending on the light and purity. Most chemists recognize its unique transition between yellow and violet, a range that lends itself to many uses, especially in classrooms and field kits. Every bottle carries a promise: mix with water, and this indicator responds quickly to acid and base—shifting colors with a reliability unmatched in home aquariums, swimming pools, and even clinical tests.
A closer look under the microscope reveals a finely crystalline solid. Its melting point clocks in around 228–232°C, but breakdown happens fast above those temperatures. In water, Cresol Purple barely dissolves unless an alkaline agent steps in. Once it hits a basic solution, it dissolves readily, giving off that signature vibrant purple. Its color switch digs deep into the pH scale—yellow in acidic solutions (pH below 7.6), murky gray at the tipping point, and vivid purple once things get truly basic (above pH 8.8). This broad range supports a ton of experimental setups, especially when students and scientists need quick, visual confirmation of acidity shifts.
Manufacturers usually supply Cresol Purple in laboratory-grade plastic or amber glass containers, often in 25-gram or 100-gram sizes. The label states the CAS number (5758-62-9), chemical purity (commonly >98%), recommended storage temperatures (well below 25°C), and emergency contact details, in keeping with global compliance rules. Labels warn about inhalation, skin contact, and accidental ingestion, following standardized hazard statements. The risk symbols and suggested PPE (personal protective equipment) make it clear: gloves, goggles, and good ventilation matter. Each supply batch passes through rigorous purity checks, ensuring the product does its job reliably in high-precision lab work.
Producing Cresol Purple isn't simple—success starts with pure o-cresol and phthalic anhydride. Chemists combine these ingredients, then add concentrated sulfuric acid to spark sulfonation, producing a sulfonated phthalein derivative. Careful purification follows, washing away byproducts, drying, and finally, crystallizing the target compound. Each of these steps calls for precise timing, temperature control, and scrupulous safety processes. Mishandling even a small batch can set off unwanted side reactions or degrade the quality. Having helped on small-scale syntheses, I remember the vinegar-sharp tang of acetic acid and the patience needed during slow crystallization. The trick is maintaining low temperatures—too much heat ruins the entire run.
Cresol Purple stands out for its reactivity at its phenolic and sulfonic groups. Exposing it to strong acids or bases flips the molecule’s configuration, swapping between protonated and deprotonated forms, and this shift powers its color-changing magic. Under UV light, some users report mild photo-bleaching, driving organic chemists to explore ways to stabilize its structure. Modifications, such as metal chelation or sulfonic acid esterification, can push its color properties around. Advanced labs continue to tweak the molecule, aiming for tailored pH sensitivity or improved shelf life. These efforts offer hope for specialty uses, especially in sensitive diagnostic kits or high-throughput testing workflows.
Across catalogs and paperwork, Cresol Purple masquerades under a handful of aliases. You'll find it as Cresol Red S, o-Cresolsulfonphthalein, C.I. 86508, and Crystal Purple. Suppliers sometimes tack on catalog numbers or branded trade names, but the chemical core remains unchanged. Having sorted through lab inventories, I’ve seen the confusion these synonyms stir up—double-checking the molecular structure saves hassle and avoids costly mix-ups in experiment prep.
Diving into safety routines, Cresol Purple sits in the mild-to-moderate category for laboratory hazards. In dry powder form, it can irritate airways, eyes, and skin, so most guidelines call for use inside a chemical hood. Eye protection and lab gloves are non-negotiable—splashes leave long-lasting stains. Spills turn workbenches purple, but more worrying is the risk of inhalation, which stings the throat and nose. The MSDS further outlines first aid measures, disposal guidelines (avoid dumping down the drain), and fire risks. Labs adopting strict standard operating procedures, spill kits, and clear waste containers have seen fewer accidents. Years in research hammered home the point: you don’t skip safety, even for a dye as familiar as this one.
Cresol Purple reaches across several fields, including environmental monitoring, college chemistry instruction, and biochemistry. Pool maintenance kits depend on it for quick pH checks. Clinical labs employ it when automated pH meters are unavailable or impractical. In cell culture labs, Cresol Purple appears in test kits, guiding scientists through critical pH-dependent protocols. Soil scientists, field biologists, and even oceanographers pack it in their travel kits; its stability and straightforward results shine outside the controlled lab. In my experience, having a dropper bottle of Cresol Purple beats fiddling with digital meters in the rain or by a muddy creek.
Today’s R&D teams take classic indicators like Cresol Purple and chase incremental improvements. Advances in indicator formulation focus on tighter pH ranges, reduced background staining, and improved compatibility with multiplexed assays. Some researchers bundle Cresol Purple with other dyes, targeting faster, sharper response times or reduced photobleaching. Others look beyond pH, exploring how this molecule anchors to membranes or acts as a tracer for microfluidics. Journals keep reporting on new uses for legacy dyes—digital imaging, environmental biosensors, and even art restoration. These real-world pushes don’t always generate headlines, but small gains often multiply across entire workflows.
Toxicological studies peg Cresol Purple as relatively safe at low concentrations, especially in buffered solutions. Animal lab trials show low acute oral and dermal toxicity, with most risk coming from long-term, high-dose exposure. Chronic inhalation or skin contact can provoke allergic reactions or dermatitis in susceptible users. Regulatory bodies like OSHA and the EU’s ECHA provide occupational exposure limits, and ongoing studies keep refining these numbers. Given environmental concerns about phenolic compounds, wastewater from dye use gets strict scrutiny, routed for professional disposal instead of municipal drains. Training and good laboratory practice minimize risks. My own training drilled home the rule: always assume even “harmless” chemicals leave an impact unless proven otherwise.
Cresol Purple faces new possibilities in the age of smart sensors and digital diagnostics. Collaborations between chemistry and engineering teams could amplify its value as part of lab-on-chip devices, wearable sensors, or high-resolution imaging platforms. Ongoing refinement of its molecular backbone could yield variants for more precise biological assays or robust industrial monitoring tools. The original blend of reliability, cost-effectiveness, and color clarity ensures Cresol Purple won’t fade from shelves soon. I’ve watched old-school indicators like this get a modern boost as researchers seek rugged, field-ready solutions—and Cresol Purple meets that demand, promising both tradition and innovation in one reliable bottle.
Cresol Purple takes me back to middle school science labs, with bubbling beakers and strips of litmus paper. I didn’t know it by name back then, but its purpose sticks around wherever there’s a need to measure acidity. As a pH indicator, Cresol Purple helps reveal whether a solution is acidic or alkaline. Drop it into a sample and the purple fades out if acid creeps in. This isn’t just a “science fair only” compound. It carries real value in labs where accuracy matters.
Beyond classrooms, Cresol Purple finds work in medical diagnostics. Blood gas tests require careful readings—small shifts in pH can point to respiratory trouble or metabolic imbalances. Health professionals need tools they can count on. Studies published in journals like Clinical Chemistry highlight how pH indicators like Cresol Purple allow technicians to spot slight changes during these vital checks. Years ago, a friend’s newborn spent time in the hospital for breathing concerns. Nurses paid close attention to blood chemistry panels that owe some of their accuracy to indicators like this one. The peace of mind those numbers brought us never felt abstract or academic.
Fish enthusiasts and environmental scientists use Cresol Purple to monitor water quality. Freshwater and saltwater tanks demand healthy chemical balance—fish can’t survive in water that’s too acidic or too basic. At home, hobbyists turn to test solutions containing Cresol Purple to keep reef tanks thriving. In the wild, researchers performing water analysis on rivers, lakes, or ocean samples also rely on visual cues from this indicator to catch sudden contaminations or seasonal shifts. It’s a small step with large consequences, especially when communities count on those water sources.
Cresol Purple doesn’t just signal changes. It guides chemists who need to control pH for complex experiments. Synthesis reactions rarely cooperate when the environment grows too acidic or too alkaline. Simple, reliable colors from a well-understood indicator help chemists steer clear of wasted time and ruined batches. Its clear, distinct shifts mean less time squinting at tiny color charts and more time pushing forward.
Science shouldn’t block people with high costs or unnecessary hazards. Cresol Purple itself carries some health risks—lab safety instructions always stress using gloves and avoiding skin or eye contact. Many parents have kids experimenting with homemade volcanoes or egg-shell coloring at home. They deserve information on which substances pose less risk. It’s good to see safety data sheets for these chemicals become easier to find, encouraging better protections for hobbyists and families.
Clear communication about everyday tools like Cresol Purple builds trust. Community water boards, aquarists, and students all benefit from knowing not just the “how,” but the “why” these indicators matter. Researchers continue to investigate new, safer alternatives so fewer people face chemical hazards. More accessible science, safer products, clearer explanations—this keeps the next generation of scientists interested and the rest of us a little bit safer.
Cresol Purple often appears on the shelves of school labs or university prep rooms. It gives off an eye-catching violet hue and gets plenty of use as a pH indicator. Plenty of chemistry students remember adding it to their flasks and watching the colors shift. It looks tame compared to acids and bases that fizz and burn, but is it safe to use with bare hands, or could carelessness cause trouble down the line?
Cresol Purple’s real name is o-Cresolsulfonphthalein, and its molecular structure has a couple of methyl groups and a sulfonate. Pure, dry indicator powder drifts into the air quickly and sticks to moist surfaces–skin included. Some folks might think a splash of colored powder seems innocent, but the chemical has known toxic effects. Acute exposure can bring irritation to the eyes, respiratory tract, and skin. Even mild symptoms like an itchy throat deserve attention in the lab.
One of the underappreciated risks involves chronic exposure. Regular contact, even if small, could stack up over time. No one working in a teaching lab signs up for red, itchy hands or nosebleeds that trace back to cutting corners. According to study results listed in databases like the European Chemicals Agency and Sigma-Aldrich, cresol-based dyes may enter the body through skin contact. Once in the system, phenolic compounds stress the liver and kidneys. Nobody wants that lingering in the back of their mind during an experiment.
Schools and universities use the dye for basic titration lessons. Sometimes hobby chemists order it for home science projects, where the urge to count on casual gloves or skip goggles can feel real. I remember a classmate who scoffed at those reminders–until a dusting ended up in his eye, and the rest of the day became a blur of rinsing and sitting out. The worst damage came not from the chemical but from believing a “benign” indicator couldn’t do harm.
Safety doesn’t mean wrapping yourself in plastic or banning indicators from your toolkit. It boils down to habits. Goggles and gloves aren’t optional in a world where chemicals drift and splash. Use tools like spatulas for transfer and avoid a bare-handed feel test. Clean up spills as soon as they happen and label containers clearly. There’s still fun and curiosity in experimenting with color changes, but basic safety shields your skin and lungs from unnecessary risk.
Dispose of solutions and powders by following local guidelines. Pouring down the drain or mixing indicators with regular trash tempts fate. Most labs now maintain MSDS (Material Safety Data Sheets) close by–keep them handy. Information on long-term harm sometimes sits buried under jargon, but any warning about skin or inhalation risk should stand out.
Real safety comes from paying attention. It means getting into the routine of working clean, covering skin, and teaching anyone new to a lab that even the most vivid indicators deserve respect. Cresol Purple has a place in science, but nobody should trade health for colorful experiments. Give the process the respect it earns, and the risk drops fast.
Cresol Purple sounds fancy, but making a working solution doesn’t need to feel intimidating. This stuff plays a serious role in everything from checking water pH to advanced biology labs. All sorts of students, researchers, and even aquarium owners use it. It’s like litmus paper, but with more personality. Any time you prepare Cresol Purple, you want accuracy, safety, and a clear plan—especially since your results can rely on it.
People often pick Cresol Purple as a powder. Don’t just trust the label—make sure it reads “indicator grade” and states purity. One time in undergraduate chemistry, I saw someone mix up a batch using an old, unmarked jar, and their measurements were useless. The sample turned out to be contaminated. Shortcuts early on mean unreliable results.
Water right out of the tap looks clean but still contains minerals and small particles. These interfere with sensitive solutions. For science projects, mistake-free readings start with distilled or deionized water. I learned that one the hard way: a chemistry club experiment once failed because of unnoticed city water additives. Reliable results demand good habits.
A digital scale makes life easiest. Check instructions for the amount of Cresol Purple to mix—usually they suggest 0.1g in 100 mL for stock. Measure as close as possible. A small error at the start can mean you throw out a whole batch later. Pour the powder into a beaker, not a flask—the wider opening helps with mixing.
Add a little water to the beaker to start, and swirl the powder to prevent it sticking to sides. Add the rest of the water. Glass rods work better than spoons for stirring. Dissolving sometimes takes a few minutes. Warm the mixture slightly if the powder clumps, but don’t boil. Before you finish, check for undissolved granules at the bottom.
For use as a pH indicator, set your Cresol Purple solution to the right pH range—often about 7.6. This calls for tiny amounts of sodium hydroxide or hydrochloric acid. Don’t guess; grab a calibrated pH meter or reliable strips. Sloppy pH ruins color changes later. Add acid or base drop by drop with constant stirring. I’ve seen frantic adjustments end up outside of the target range, wasting time and resources.
Write the date and concentration right on the bottle. I’ve lost track of how many times good work got wasted because mystery bottles collected on a shelf. Dark, cool places slow down any light or chemical breakdown. If you see color fade or sediment appear, make a new batch. Don’t risk using a solution with questionable reliability.
Cresol Purple might stain tables and skin. Goggles, gloves, and careful measuring keep accidents in check. A clean workstation matters; a friend once knocked over a half-mixed solution, leaving a purple stain that lasted months. Dispose of old solutions following local lab safety rules. Smart prep, clear notes, and a tidy bench prevent headaches later.
Cresol Purple isn’t flashy, but it has carved out an important spot in the world of chemical indicators. Most chemistry teachers remember their first time watching it swirl from yellow to purple—an impressive way to drive home the power of acids and bases. The pH range for Cresol Purple sits typically between 7.6 and 9.2. Below 7.6, the solution glows yellow. Once you push past 9.2, it fades into a soft purple. Outside this window, it stops giving you clear clues.
I spent many afternoons in the lab myself, testing pool water and freshwater samples. You know an indicator’s worth when the result matters—say, making sure drinking water isn’t too harsh or too gentle on the pipes. For Cresol Purple, the shift right around neutral pH gives you a quick check on water safety or the progress of a titration in teaching labs. Let’s face it: not everyone has access to fancy digital meters. Sometimes a reliable color shift makes all the difference.
Cresol Purple gets the nod in classrooms, water treatment plants, and hobbyist setups. It’s stable, lasts long on the shelf, and kids can use it safely for most classroom experiments. That pH range—7.6 to 9.2—lands in a sweet spot. You won’t get as much out of it in strong acids, of course. If you need to catch pH dips under 7, there are better choices. But for catching the move from weak base to weak acid, this dye shines.
Lab techs use Cresol Purple to keep aquariums healthy. They monitor small changes as fish waste tips pH into dangerous territory. Yeast and bacteria thrive best in certain ranges too, and bread bakers and kombucha brewers learn to spot trouble using simple indicators.
With all this attention on water and food safety, there’s real peace of mind knowing such an accessible tool exists. It’s not about having the “best” indicator, but finding one that works reliably when you need results fast.
As simple as making a color change sound, chemical indicators still call for respect. Cresol Purple is mostly safe in small doses, but like many dyes, misuse can add undetected toxins to water systems. I’ve learned from working with local environmental groups that cleaning up even small spills can challenge wastewater plants. Teaching responsible use—and disposal—remains crucial. Kids and adults alike should use small amounts and follow disposal recommendations instead of pouring leftover solutions down the drain.
Laboratories could take the lead by training students in ways to minimize waste. Reusing solution when possible, and neutralizing before disposal, helps protect waterways. Research into greener alternatives goes on, but for now, knowledge and good habits carry the most weight.
Some argue that digital pH meters have made indicators like Cresol Purple obsolete. In certain scenarios, that holds up—precision is hard to beat. Still, there’s something to using your eyes, seeing that honest shift in color with your own hands. In my experience, students connect to chemistry more deeply through these simple demonstrations. With good labeling, a clear understanding of the indicator’s properties, and some real-life practice, you create not just better chemists, but more mindful citizens.
Cresol Purple works at the heart of many labs. As a pH indicator, people rely on it to be predictable and stable. Anyone who’s handled this compound a few times remembers the first rule: don’t leave the bottle sitting out on the bench. Sunlight, heat, and moisture mess with its structure. Too much of any of those and the color readings start shifting for good. That’s not something a scientist wants to discover after hours of prep or an expensive round of tests.
I’ve seen analysts grab the jar, leave the cap loose, and run off for a minute. Minutes stack up and humidity sneaks in. Cresol Purple draws in atmospheric water and starts to clump. That doesn’t seem like a disaster, but little bits of contamination ruin the next measurement. Keeping the cap closed keeps the powder dry—no shade of purple can warn you if the sample’s been spoiled.
Most chemicals handle light poorly, and Cresol Purple offers no exception. Storing the bottle in the closest cabinet doesn’t work if the lights stay on all night. Over time, direct and even indirect light deteriorates sensitive powders and ruins the color response you count on. I always shove it deep inside a cupboard reserved for light-sensitive supplies, because a single failed titration speaks louder than any memo.
Many labs sit on the cooler side, but temperature swings can creep up. Every label says ‘store at room temperature’, but that assumes a steady climate. Unheated storage rooms or windows create cold spots; radiators and sunny areas lead to spikes. Both encourage chemical breakdown. I keep Cresol Purple in a spot with real temperature control—usually alongside other indicators—so I dodge headaches from unpredictable results.
Glass amber vials come standard for a reason. Amber glass blocks most UV rays. Transferring to a clear container only increases light damage. I learned that lesson after a batch of clear test tubes left the indicator pale and useless. The supplier's jar offers some added insurance—original seals signal whether someone tampered or swapped the powder, too.
Labs juggle many chemicals, so clear labeling saves more time than any quick-fix system. Every time I open new Cresol Purple, I date the jar. Expired or aging powder starts failing color checks, making troubleshooting a nightmare. Rotating stocks and pitching old inventory keeps experiments honest.
Shortcuts sometimes tempt busy techs, but best practices around storing Cresol Purple actually shape the quality of the work. Failing to protect chemicals shows up later, muddling results and wasting resources. Good habits—like dry storage, proper containers, temperature awareness, and strict labeling—come from experience, but sharing those lessons saves others a lot of frustration.
| Names | |
| Preferred IUPAC name | 3,3-bis(4-hydroxy-2-methylphenyl)-2,1λ⁶-benzoxathiole-1,1(3H)-dione |
| Other names |
Cresol Red o-Cresolsulfonphthalein C.I. 75205 |
| Pronunciation | /ˈkriːsɒl ˈpɜːpl/ |
| Identifiers | |
| CAS Number | 2303-01-7 |
| 3D model (JSmol) | `/model=csid,18436` |
| Beilstein Reference | 626943 |
| ChEBI | CHEBI:86148 |
| ChEMBL | CHEMBL2103837 |
| ChemSpider | 8656 |
| DrugBank | DB14040 |
| ECHA InfoCard | 100.028.723 |
| EC Number | 230-026-9 |
| Gmelin Reference | 81112 |
| KEGG | C04809 |
| MeSH | D003434 |
| PubChem CID | 12326 |
| RTECS number | GO3145000 |
| UNII | UJ2G645EEG |
| UN number | UN1993 |
| CompTox Dashboard (EPA) | DTXSID6020287 |
| Properties | |
| Chemical formula | C21H18O5S |
| Molar mass | 406.483 g/mol |
| Appearance | Dark violet to brown crystalline powder |
| Odor | Odorless |
| Density | 1.31 g/cm³ |
| Solubility in water | slightly soluble |
| log P | 1.94 |
| Vapor pressure | Vapor pressure: <0.01 hPa (20°C) |
| Acidity (pKa) | 7.9 |
| Basicity (pKb) | 7.5 |
| Magnetic susceptibility (χ) | -81.0e-6 cm^3/mol |
| Refractive index (nD) | 1.551 |
| Viscosity | Viscosity: 1.14 cP (20°C) |
| Dipole moment | 7.63 D |
| Thermochemistry | |
| Std molar entropy (S⦵298) | 355.8 J·mol⁻¹·K⁻¹ |
| Pharmacology | |
| ATC code | V03AB37 |
| Hazards | |
| Main hazards | Harmful if swallowed. Causes serious eye irritation. Causes skin irritation. |
| GHS labelling | GHS02, GHS07 |
| Pictograms | GHS07,GHS05 |
| Signal word | Warning |
| Hazard statements | H302: Harmful if swallowed. H315: Causes skin irritation. H318: Causes serious eye damage. |
| Precautionary statements | Precautionary statements: "P264, P280, P305+P351+P338, P337+P313 |
| NFPA 704 (fire diamond) | 2-1-1 |
| Flash point | 87.8 °C |
| Lethal dose or concentration | LD50 oral rat 5200 mg/kg |
| LD50 (median dose) | LD50 (median dose): Oral rat LD50 = 826 mg/kg |
| NIOSH | SN8525000 |
| PEL (Permissible) | PEL (Permissible Exposure Limit) for Cresol Purple: Not established |
| REL (Recommended) | 0.02 – 0.05 g/L |
| Related compounds | |
| Related compounds |
Bromocresol purple Cresol red Cresol blue Phenol red |
