What are the advantages of using citric acid instead of vinegar when canning tomatoes?

Citric acid doesn't add any flavor to the food, like vinegar does. It's also concentrated, so it goes a long way.

How much citric acid should I use when substituting it for lemon juice?

Half a teaspoon of citric acid powder is equal to 2 tablespoons of vinegar or lemon juice.

Why does a recipe call for using citric acid when canning jellies?

Recipes call for using citric acid because it's a natural preservative that raises the acidity level in food. This prevents mold and hinders the growth of potential bacteria.