One of the most common acids in the world, citric acid is responsible for giving fruits like lemons and limes their characteristic tartness. Pure citric acid is available as a crystalline powder at gourmet cooking stores. In addition to giving any food item a citrus tang, citric acid crystals can be dissolved to make a citric acid solution, which you can use as a cleaning agent or to develop photos. Making the solution is relatively straightforward, although you'll need to take care when preparing and storing, as the solution is conducive to fungal growth under certain conditions.
Purchase at least 1 lb. of citric acid crystals, sometimes sold as "Sour Salt," from your local gourmet food store. Boil 1 or 2 pints of distilled water for every pound of citric acid you plan to dissolve, depending on how strong or weak you want your solution.
Place the citric acid crystals in a non-metal pot. Remove your boiling water from heat and tip its container slightly to slowly introduce it to the acid crystals, stirring with a non-metal spoon while doing so. Filter the solution using filter paper (a coffee filter will do) to remove any non-dissolved solids.
Store your solution in an airtight container. According to an excerpt from "A Treatise on Beverages" by Charles Herman Sulz, citric acid solution is prone to developing fungus if exposed or otherwise left vulnerable to air or contaminants.