Catechol-Metol

I'm experimenting with a Catechol-Metol film developer and trying to find the right times for APX 100 and Efke films. The recipe is from the 30's and the results so far seems good. This is the recipe:

Warm water 750 ml Metol 9g Catechol 9g Sod. Sulfite 60g Sod. Carbonate 60g Cold water to 1000 ml

Dilute 1+9 or 1+19.

Does anyone know how the catechol and metol work together?

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