What is the pH range for the Alizarin Red S staining solution?

• A. 3.0-3.5
• B. 4.1-4.3
• C. 5.5-5.9
• D. 6.8-7.2

Answer: (B)

Alizarin Red S staining for mineralized tissues relies on the dye forming a calcium-dye complex that is stable in a weakly acidic environment. The staining solution is used at about pH 4.1 to 4.3, which promotes binding of calcium in hydroxyapatite to the dye and produces the characteristic red staining of mineralized areas. If the pH is much higher, binding becomes less specific and background staining can increase; if the pH is too low, the dye may not interact effectively with calcium, reducing signal. So, the pH range around 4.1–4.3 is the correct window.