m-CRESOL PURPLE

m-Cresol purple (metacresol purple, CAS 2303-01-7, C21H18O5S, molecular weight 382.43) is a sulphonephthalein pH indicator supplied as a green-to-brown powder. It has two useful ranges — pH 1.2–2.8 (red to yellow) and pH 7.4–9.0 (yellow to violet) — and is the indicator of choice for spectrophotometric pH measurement of seawater and ocean-acidification studies because its absorbance ratio can be read precisely (λmax 578–584 nm in 0.1 N NaOH, absorptivity 800–900). RXSOL supplies it in 5 g, 25 g and 100 g packs alongside the other indicators (bromothymol blue, phenol red, thymol blue, methyl red, nitrazine yellow and more).