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What it is. This bromophenol blue indicator is a ready-to-use 0.04% (w/v) aqueous solution of bromophenol blue sodium salt, offered in 100 mL and 1 pint sizes. It gives a clear yellow-to-blue color change in the acidic range and doubles as a familiar tracking dye in gel electrophoresis.
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The bromophenol blue pH range. Bromophenol blue is yellow at pH 3.0 and below and blue to blue-violet at pH 4.6 and above, with a green midpoint. Its transition sits lower than bromocresol green's, making it one of the few common indicators for endpoints in the pH 3 to 4.5 range.
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Chemical identity. Bromophenol blue (3',3'',5',5''-tetrabromophenolsulfonphthalein, free-acid CAS 115-39-9) is a sulfonephthalein dye. Ready-made aqueous solutions use its sodium salt, which dissolves readily in water. In its blue, deprotonated form the dye absorbs most strongly near 589 nm.
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Two sizes for different workloads. The 100 mL size (BB4040-100) suits occasional titrations, gel loading buffer preparation, and teaching labs. The 1 pint size (ZB1418-16) supports labs that use the indicator daily or prepare larger batches of loading buffer.
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Titration uses. Bromophenol blue marks endpoints for titrations that finish in the acidic range, such as weak bases titrated with strong acids. A few drops of the 0.04% solution give a sharp, reversible color change without enough dye to affect the result.
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Electrophoresis tracking dye. Bromophenol blue carries a negative charge at neutral pH, so it migrates with nucleic acids and proteins in agarose and polyacrylamide gels. Added to loading buffers, it lets you see sample loading and follow how far a run has progressed, which is why it is a standard component of DNA and protein loading dyes.
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Reading the endpoint. The change from yellow through green to blue is easiest to judge against a white background with good lighting. Comparing the flask with a reference solution at the target pH helps when the endpoint falls in the green midpoint of the transition.
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Why a ready-to-use solution. At 0.04%, the solution is prepared to the standard working strength for sulfonephthalein indicators. Buying it ready-made avoids weighing milligram amounts of dye and gives a consistent concentration from lot to lot.
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Choosing between related indicators. For endpoints slightly higher, compare Lab Pro's Bromocresol Green Indicator, 0.1% (w/v), which changes between pH 3.8 and 5.4, and Bromocresol Purple Indicator, 0.04% (w/v), which changes between pH 5.2 and 6.8.
Technical Specifications
| Product |
Bromophenol blue indicator solution |
| Active Ingredient |
Bromophenol blue sodium salt |
| CAS Numbers |
115-39-9 (bromophenol blue, free acid); 7732-18-5 (water); sodium salt CAS per SDS |
| Synonyms |
3',3'',5',5''-Tetrabromophenolsulfonphthalein, BPB |
| Concentration |
0.04% (w/v), 0.4 g/L in water |
| Transition Range |
pH 3.0 (yellow) to pH 4.6 (blue to blue-violet) |
| Absorbance Maximum (basic form) |
About 589 nm |
| Solvent |
Water |
| Package Sizes |
100 mL; 1 pint |
Available Variants
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BB4040-100: 100 mL size for occasional titrations, teaching labs, and loading buffer preparation.
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ZB1418-16 (BB4040-500): 1 pint size for daily use and larger loading buffer batches.
Applications / Common Uses
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Acidic-range titrations: Marks endpoints between pH 3.0 and 4.6.
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DNA loading buffers: Tracks sample loading and migration in agarose gels.
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Protein gel loading buffers: Shows the dye front in polyacrylamide gel electrophoresis.
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Teaching labs: Demonstrates indicator behavior in strongly acidic solutions.
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Quick acidity checks: Shows whether a sample is above or below about pH 4.
Storage, Handling & Shipping
Store tightly closed at room temperature, away from direct light and strong oxidizers, which can fade the dye. The dilute aqueous solution carries low hazard, but the dye stains skin, gel trays, and benches, so wear gloves and eye protection. Dispense only what you need for each run to keep the stock solution free of acids, bases, and nucleases.
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Incompatible materials: Strong oxidizers.
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Personal protection: Safety glasses and nitrile gloves.
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Spill response: Wipe up with absorbent material and rinse the area with water.
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First aid: Eyes: rinse with water for several minutes. Skin: wash with soap and water.
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