To prepare a 10 mM citrate buffer solution at pH 6.0, dissolve 2.1 g of citric acid (C6H8O7) and 2.94 g of sodium citrate (C6H5O7Na3·2H2O) in 1 liter of distilled water. Adjust the pH to 6.0 using either citric acid or sodium citrate. Filter the solution to remove any particulate matter and store it at room temperature.