Citric acid is an organic acid that is most often found in citrus fruits. It has a very high concentration in lemons -- there can be as much as 8 percent of citric acid in the dry weight of a single lemon. The acid has a distinctive sour taste, and because of this it is added to some foods and drinks. To extract the citric acid from a lemon you'll need to add a strong acid, such as sulphuric acid.
Put on your safety gloves and safety goggles. Squeeze the juice of one lemon into a flask that can hold 2 cups of solution.
Add 1 teaspoon of calcium hydroxide solution to the 2 cup flask. Mix the solution thoroughly with the teaspoon. This will turn the solution to calcium citrate.
Fill the 4 cup flask halfway with water and ice and place the 2 cup flask inside. Allow the solution in the 2 cup flask to get cold for approximately 15 minutes.
Place a piece of a coffee filter over the top of the 5 cup flask. Slowly pour the calcium citrate that is in the 2 cup flask through the filter and into the 5 cup flask.
Add 1/4 cup of sulfuric acid solution as calcium citrate solution to the calcium citrate solution in the 5 cup flask. Stir the solution with your teaspoon. This solution is now citric acid. Though 1/4 cup of sulfuric acid should suffice, the amount of sulfuric acid you add should be about half of the calcium citrate. For example, if you have 100 grams of calcium citrate, add 50 grams of sulphuric acid.