  Chemical magnetometers are radical pair systems such as solutions of pyrene
and N,N-dimethylaniline (Py-DMA) that show magnetic field effects in their spin
dynamics and their fluorescence. We investigate the existence and decay of
quantum entanglement in free geminate Py-DMA radical pairs and discuss how
entanglement can be assessed in these systems. We provide an entanglement
witness and propose possible observables for experimentally estimating
entanglement in radical pair systems with isotropic hyperfine couplings. As an
application, we analyze how the field dependence of the entanglement lifetime
in Py-DMA could in principle be used for magnetometry and illustrate the
propagation of measurement errors in this approach.
