CITRIC ACID a very widely distributed acid, present in most com mon fruits, such as gooseberries, currants, lemons, citrons, cherries and many others. It is also a constituent of the sap of many plants. It was first separated and distinguished by Scheele in 1784. It is generally prepared from the dark treacle-like concentrated lemon juice imported from Sicily, Bergamot juice from Calabria or lime juice from Montserrat and Dominica. The process consists of filtering and neutralizing with chalk and quicklime, by which citrate of calcium is precipitated. This, by decomposition with a slight excess of sulphuric acid, gives the acid, which is then purified by repeated crystalliza tion. The theoretical yield is from 51 to 64 ounces of commercial citric acid per gallon of juice, but this is seldom reached in practice. Several improvements have of late been intro duced, both in preparing the crude lemon juice for exportation and in the subsequent purifica tion and crystallization of the acid. As found in commerce citric acid is sometimes adulterated with tartaric acid which is readily detected by the brown coloration in the presence of potas sium dichromate. Citric acid is white when pure; it crystallizes in two forms, one belong ing to the trimetric system is the common form, and in it the acid contains one molecule of water, as indicated in the formula given above.
The other form contains half the quantity of water. The ordinary crystals effloresce in the air in a warm room. Citric acid has a pleasant sour taste. It dissolves very readily in water and is soluble in alcohol, but insoluble in ether. When heated it undergoes decomposition and yields aconitic, itaconic and citraconic acids, along with other products. It is acted on by nitric and sulphuric acids and by other re agents, yielding a variety of decompositions and derivatives.
Citric acid combines with the metals, form ing citrates. They are crystalline salts and many of them are soluble in water. Crude citric acid is largely employed by the calico printer as a resist and as a discharge.
Citric acid and the citrates are used in medi cine as cooling drinks, as antidotes to alkalis and for the purpose of rendering the urine alka line, thus overcoming abnormal acidity. Cit rates of magnesium are used as laxatives and cathartics.