Question

how to neutralize colourless sodium carbonate soloution

Answer 1

If you have colorless solution of the sodium carbonate, you can put some indicator in it, phenolphtalein for example. Аfret adding, this solution becomes cororfull (crimson).Now you can neutralize this solution with some acid (HCl). In equivalent point solution becomes cororless again.

Reaction:

Na2CO3 + 2HCl = 2NaCl + H2O + CO2