[RBS] citric acid for calc removal in pump urn

[Petite Safranit]

Can anyone tell me the ratio of citric acid to water in a 5 liter pump 
urn to remove a very light covering of calc? Is a tablespoon enough, 1/4 
cup? What is a good amount to start with? If I do use too much will the 
metal start to corrode from intense acidity every week?  I want to 
prevent massive buildup of the minerals so I want to be on top of this 
but not at the expense of a new urn. Thanks and shabbat shalom.

Ilene