[prague441]

The pKas citric acid are pK1 = 3.1, pK2 = 4.1, pK3 = 6.4. You were tasked to prepare 1L of a 0.5 M of citrate buffer at pH 4.0. The problem is, only solid trisodium citrate is present in the lab stockroom. How will you prepare the buffer then?

[chenbeier]

In this case you cannot make a clean buffer. You have to add some other acid to convert the trisodiumsalt to Di or Monosodiumsalt so it fits to the Henderson  Hasselbalch  equation.
Probably HCl or H₂SO₄ can be used if chloride or sulfate will not disturb.