Why EDTA is secondary standard?

EDTA (ethylenediaminetetraacetic acid) is considered a secondary standard because it is not typically used as a primary standard due to its complexation properties and the difficulty in accurately determining its purity. Instead, it is standardized against a primary standard, such as calcium carbonate or sodium carbonate, which can be measured with high accuracy. Once standardized, EDTA can then be used reliably in titrations to determine concentrations of metal ions in solution. Its ability to form stable complexes with various metal ions makes it particularly useful in analytical chemistry.