For hand washing, just a tablespoon in a basin with hot water. The water temperature, for maximum effectiveness, must be at least 40°C as mentioned above.

Other useful natural remedies
By hand with lemon
Just soak the socks in a basin with very hot water and then add the juice of a lemon (filtered through a sieve). Then let them soak for a while and then rinse them without exaggerating with water.

More concretely, we need:

a cup of lemon juice (250 ml)
a liter of hot water
In the washing machine with vinegar
Another way to keep track of the whiteness of our socks is to use vinegar, which is often used instead of bleach for laundry. The use is elementary: pour a cup of vinegar into the washing machine tub, then proceed normally with the usual washing program.

By hand with vinegar and baking soda
But vinegar can also be used with hand laundry.

We need:

a cup of white vinegar (250 ml)
half a liter of boiling water
1 tablespoon of baking soda
A mixture of white vinegar and boiling water and baking soda is prepared and the socks are soaked in it for about 45 minutes. After this time, just add a little Marseille soap and rub vigorously.

If your problem is that white socks turn gray, you finally have the solution!