Aims. The existence of an anti-correlation between the Equivalent Width (EW) of the neutral narrow core of the iron K alpha emission line and the 2 - 10 keV luminosity ( the so-called "X-ray Baldwin" or "Iwasawa-Taniguchi" effect) has been debated in the last years. We aim at testing this claim on the largest catalogue of radio quiet AGN high-quality X-ray spectra ever published. Methods. The final sample comprises 157 objects. We search for a relation of the iron line EW not only with the X-ray luminosity, but also with the Black Hole mass, the Eddington ratio and the cosmological distance. The data presented here were analyzed homogeneously, all spectra are from the same instrument and with high Signal-to-Noise Ratio. Results. A linear censored fit on the EW versus 2 - 10 keV luminosity is highly significant and yields log(EWFe) = (1.73 +/- 0.03) + (- 0.17 +/- 0.03) log(L-X,L- 44), where EWFe is the EW of the neutral iron K alpha line in eV and L-X,L- 44 is the 2 - 10 keV X-ray luminosity in units of 10(44) erg s(-1). The anti-correlation with the Eddington ratio is also very significant, while no dependence of the iron EW on the BH mass is apparent.