CITRIC ACID (Formula, + H 0 ; molecular weight, 2m).—An acid which occurs in lemons, oranges, cranberries, and also with malic acid in gooseberries, whortleberries, rasp berries, etc. It also occurs in many plants and roots. It is obtained by allowing the juice to undergo an incipient fermentation, fil tered, and neutralized by boiling with chalk and lime. The precipitated cal cium citrate is then decomposed by an equivalent quantity of sulphuric acid.
Citric acid is a tetratomic tribasic acid, usually crystallizing in a white transparent form, soluble in water, and having an acid taste. Citric acid is frequently adulterated. The presence of sulphuric acid can be detected by adding a large excess of hydrochloric acid, followed by barium chloride. A white precipitate will occur. To detect tartaric acid, take a strong solution and mix it with an excess of caustic soda and permanganate of potassium suffi cient to color a light pink. If the mix ture be boiled it will become brown should tartaric acid be present.
It acts as a powerful restrainer of the action of the developer. It is also useful in keeping the shadows clear when inten sitying a negative. Mixed with sulphate of iron it be comes use ful in clear ing yellow stains from negatives. In positive printing it is largely used. If added to the nitrate solution for sensitizing it assists in the preser vation of the paper, and also in giv ing more brilliancy to the tone. In gelatino-chloride of silver positive paper large quan tities are added, and it assists in keeping the paper in good condition for many months.
CLEANINO.—Cleanliness is perhaps more essential in photo graphic operations than in anything else. Cleaning is an art only arrived at by practice. A skillful operator will clean a glass in a few seconds. while another, unused to the art, will not have produced