  We have performed $^{75}$As nuclear magnetic resonance (NMR) Knight shift
measurements on single crystals of NaFe$_{0.975}$Co$_{0.025}$As to show that
its superconductivity is a spin-paired, singlet state consistent with
predictions of the weak-coupling BCS theory. We use a spectator nucleus,
$^{23}$Na, uncoupled from the superconducting condensate, to determine the
diamagnetic magnetization and to correct for its effect on the $^{75}$As NMR
spectra. The resulting temperature dependence of the spin susceptibility
follows the Yosida function as predicted by BCS for an isotropic, single-valued
energy gap. Additionally, we have analyzed the $^{23}$Na spectra that become
significantly broadened by vortices to obtain the superconducting penetration
depth as a function of temperature with $\lambda_{ab}(0) = 5,327 \pm$
78$\,\AA$.
