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Xylenol orange

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Xylenol orange is an organic reagent, most commonly used as a tetrasodium salt as an indicator for metal titrations. When used for metal titrations, it will appear red in the titrand and become yellow once it reaches its endpoint. Historically, commercial preparations of it have been notoriously impure,[1] sometimes consisting of as little as 20% xylenol orange, and containing large amounts of semi-xylenol orange and iminodiacetic acid. Purities as high as 90% are now available.

Xylenol orange
IUPAC name
3,3′-Bis[N,N-bis(carboxymethyl)aminomethyl]-o-cresolsulfonephthalein tetrasodium salt
3D model (JSmol)
ECHA InfoCard 100.015.049
EC Number 216-553-8
Molar mass 672.66 g·mol−1
Melting point 195 °C (383 °F; 468 K)
200 mg/mL
R-phrases (outdated) R10, R20, R21, R22, R36, R38
S-phrases (outdated) (S1/2), S26, S27, S28, S62, S63
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oilHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroformReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorusSpecial hazards (white): no codeNFPA 704 four-colored diamond
Flash point > 93 °C (199 °F; 366 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Xylenol orange is an indicator of choice for detecting polyvalent metal ions. It (in acetate buffer, at pH = 5.81) has two absorption maxima in its absorption spectrum, at 433 nm and 573 nm.[2] In the unchelated form, the 433 nm absorption dominates. With metal chelation the absorbance at 573 nm increases. Absorbance at 573 nm can thus be used to construct linear calibration curves for the quantification of certain metal ions in solution. Metal complexes of xylenol orange have stability constants (log K ~ 5) lower than that of stronger metal chelating complexes such as those derived from DTPA (log k > 20).[2] Thus xylenol orange has been used for the selective determination of unchelated metal ion contaminant concentrations in lanthanide probes.[2]


  1. ^ Gay, Craig; Collins, James; Gebicki, Janusz M. (1999), "Determination of Iron in Solutions with the Ferric–Xylenol Orange Complex", Analytical Biochemistry, 273 (2): 143–148, doi:10.1006/abio.1999.4207
  2. ^ a b c Elshan, N.G.R. Dayan; Patek, Renata; Vagner, Josef; Mash, Eugene A. (November 2014). "Spectrophotometric determination and removal of unchelated europium ions from solutions containing Eu-diethylenetriaminepentaacetic acid chelate–peptide conjugates". Analytical Biochemistry. 464: 24–29. doi:10.1016/j.ab.2014.07.009.