A von Hamos geometry based wavelength dispersive spectrometer combined with an in situ reactor cell has been developed to measure non-resonant sulfur K alpha emission for the in situ speciation of low concentrations of sulfur. The spectrometer operates at 15 cm focusing radius, is equipped with a curved Si(111) crystal and a position sensitive detector, and is capable of achieving an energy resolution of 0.56 eV at 2.3 keV. We present the details of the spectrometer and dedicated sample-cell design to study chemical reactions in situ. The spectrometer capabilities are exemplified by an in situ study of sulfur speciation during H2S poisoning of SiO2 supported Ru nanoparticles performing CO methanation.