If you have a pH meter available, an easy way to do it is to prepare 2 solutions: 0.25 M sodium bicarbonate and 0.25 M sodium carbonate. Get a beaker, a magnetic stirrer and a stir bar. Calibrate the pH electrode with pH 7 and 10 standard buffers. Half fill the beaker with one of the solutions, start stirring, and place the pH electrode in the solution. Gradually add the other solution until the pH reads 9.4.

For related RG discussions; see: https://www.researchgate.net/post/How_can_stabilise_the_pH_of_01M_sodium_bicarbonate_solution_made_in_the_lab_to_80 https://www.researchgate.net/post/Anyone_know_how_to_prepare_2_M_of_sodium_bicarbonate_pH_100_solution_no_buffer