Congo red is widely used as a pH indicator in analytical chemistry, particularly for detecting alkaline conditions. It changes color from blue to red when the pH shifts from acidic to basic, typically around pH 3.0 to 5.0, making it useful in titrations and acid-base experiments. It is also employed as a biological stain in microscopy, especially for detecting amyloid deposits in tissues. In histology, amyloid proteins stained with Congo red exhibit apple-green birefringence under polarized light, which is a key diagnostic feature for diseases like Alzheimer's, type 2 diabetes, and multiple myeloma. Additionally, Congo red has been used in industrial applications to detect cellulose materials and in educational labs for demonstrating dialysis and membrane diffusion due to its large molecular size. However, its use is limited in large-scale applications due to environmental concerns and toxicity.