What it is and where it comes from
E542 is made by grinding and treating the bones of animals, usually cattle. It adds calcium and phosphorus and stops powders from caking. Unless the bones come from animals slaughtered in the halal way, halal certifiers do not accept it.
Where you find it
Rare in food today. It may be found in some mineral supplements and dry mixes.
What to look for on the label
Avoid E542 unless the product carries a halal logo. Calcium phosphates made from minerals have other numbers (E341) and are halal.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.