CITRIC ACID. This acid is gradually formed in lemons and other frAits, by the oxygen of the air acting on the mucilages which are found in the fruits in their less matured condition ; just as tannin is converted into gallic avid by the same means. It is precipitated by chalk as citrate of lime, and liberated afterwards by a stronger acid. A gallon of lemon juice gives 8 ounces of citric acid, and at times more. A drachm of lemon juice is, therefore, equivalent to about 3 grains of the acid, and, when lemon juice is substituted for the vegetable acids, this proportion should be remembered. As it contains more oxygen in proportion to its carbon and hydrogen than acetic acid it is a much stronger acid, but weaker than tartaric acid. Anhydrous citric acid, such as it exists in citrate of silver is H5 Ci. = 165, but the ordinary crystals contain 4 atoms of water, and are = 201. Other crystals with more and less water may be formed. The usual crystals are soluble in less than their weight of water, and are soluble in alcohol, but not in ether. The dilute aqueous solution, like gallic and other acids, soon becomes mouldy. It unites with three atoms of base to form citrates : sometimes the place of one or two of these atoms of base is supplied by water. It cannot exist without an atom of water,
and when , this water is taken from it, it is resolved into aconitic acid and hydrated citric or other compounds, or even into oxalic and acetic acids. It is used as acetic acid in photography, and also as a test of iodate of potassa in iodide of potassium, and to form citrate of soda with the bicarbonate of that alkali for the first preparation of printing papers. Papers soaked in a solution of one part citric acid to 10 water will have the particles of metal which are often so annoying, removed as soluble ammonio-citrates if they are afterwards put into dilute ammonia of 20 parts water to one part liquid ammonia. This acid is a weak deoxidizer : boiled with a solution of chloride of gold it removes the chlorine without giving off gas ; chlorocarbonic acid is probably formed, for citric acid may be regarded as a compound of oxalic and acetic acids, and water ; and oxalic acid 02 Os, as a compound of carbonic acid C 02, and carbonic oxide C 0 ; and carbonic oxide in the light unites with chlorine to form chlorocarbonic acid.