Add the soda bicarbonate, mix, and keep cooking until thick and caramel colour. It will have to be slightly thicker than custard.
Make sure your dulce de leche is still quite runny when you stop cooking it. It is like jam and it will thicken much more when it cools down. If you overcook it, you will have very sticky and chewy caramel and you will not be able to use it as a spread.