Citrate Buffer is prepared from citric acid and sodium citrate and functions as a weak acid buffer system for maintaining acidic pH conditions. Citric acid is a triprotic acid with three dissociation constants (pKa values of approximately 3.13, 4.76, and 6.40), enabling effective buffering within the pH range of about 3.0 to 6.2. This buffering system is widely used in biochemical and histological workflows where controlled acidic conditions are required.

A common application of citrate buffer is in heat-induced epitope retrieval (HIER) procedures used for formalin-fixed paraffin-embedded (FFPE) tissue samples. During aldehyde fixation, protein crosslinks may form that alter the conformation of antigenic sites and reduce accessibility for antibodies or nucleic acid probes. Heating tissue sections in citrate buffer solutions, typically in the range of 0.01–0.1 M, is widely used to help reduce fixation-induced crosslinks and restore antigen accessibility, improving downstream detection in immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) assays.