Neutron coincidence measurements of plutonium samples with uncertainties <0.5% could reduce the amount of costly destructive analysis required for nuclear material accountancy in plutonium handling plants. The ratio of (, n) emission to spontaneous fission neutron emission, , of plutonium samples is important to the interpretation of neutron coincidence measurements. When the “known alpha” analysis method is used, an error on the value propagates to approximately the same percentage error on the measured plutonium mass. Molality data of Charrin and the SOURCES code have been used to update the calculation of for both pure plutonium nitrate solutions and plutonium/uranyl nitrate solutions of different concentrations and acidity. This paper gives equations for the density of the solution as a function of heavy metal concentration and for the weight factors that can be used in the analysis of neutron coincidence measurements.