The mutual salts solubility in the title system at 323 K was determined using the isothermal solution saturation method. The results were used to plot the solubility isotherm in the oblique projection on a plane according to Janecke's method. A detailed knowledge of the isotherm course is essential for designing a new concept based on the utilization of the after-filtration liquor from the filtration of NaHCO3 solids in Solvay's process.