  We consider the spin-1/2 antiferromagnetic Heisenberg chain in a staggered
magnetic field describing materials such as copper benzoate and copper
pyrimidine dinitrate. Using the density-matrix renormalization group for
transfer matrices (TMRG) we calculate the magnetization of these materials at
finite temperature and arbitrary magnetic field. These results are in excellent
agreement with experimental data, allowing for a determination of the
inhomogeneity parameter c of copper benzoate (c = 0.043) and copper pyrimidine
dinitrate (c= 0.11). The TMRG approach can be applied to rather low
temperatures yielding singular field and temperature dependences of
susceptibilities.
