Grapefruit cannot be used in place of lemons

You would think that it’s possible to use grapefruit juice when lemon juice is called for but you unnervingly find your fridge to be a lemon-free zone. I mean really, who wouldn’t?
For the record, I’m putting it out there. Spinach, fetta and grapefruit juice; don’t go. Parsley & grapefruit juice; don’t go. Gin & freaking grapefruit [...]