According to Pomona's Universal Pectin, citric acid can be substituted with lemon juice. Two teaspoons of citric acid is equal to 4 teaspoons of lemon juice.
The New England Cheesemaking Supply Company describes citric acid as a flavoring agent and preservative used in juices, soft drinks and seltzers. Citric acid is also referred to as sour salt and is commonly used in a powder form. WhatsYourTemperature.com says citric acid is naturally found in fruits, such as lemons, limes, oranges and tangerines. Citric acid can also be used in cleaning products, the chemical process of developing photographs and in artificial enamel formation and regeneration.