Baking Soda for whitening teeth?

Hey
I've Heard that baking soda whitens teeth but.....?
How long does it take for results to show?
and what are the specific instructions?
What about Lemon?
Does Brushing with Baking soda really whiten teeth?
Thanks
peace
Update: and for how long should i hold the baking soda+toothpste in my mouth?

THANKS
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