BACTERIOLOGICAL GRADE AGAR, GELATIN, AND PEPTONES tj7 



samples, omitting agar. Divide total number of counted particles by 1.5 to obtain 

 count per gram. 



GELATIN^ 



''Gelatin is a product obtained by the partial hydrolysis of collagen derived from 

 the skin, white connective tissue, and bones of animals. Gelatin derived from an 

 acid-treated precursor exhibits an isoelectric point between pH 7 and pH 9, known as 

 Type A, while Gelatin derived from an alkali-treated precursor has an isoelectric 

 point between pH 4.7 and pH 5, known as Type B. 



"Description. Gelatin occurs in sheets, flakes, or shreds, or as a coarse to fine 

 powder. It is faintly yellow or amber in color, the color varying in depth according to 

 the particle size. It has a slight, characteristic bouillon-like odor. It is stable in air 

 when dry, but is subject to microbic decomposition when moist or in solution. 



''Solubility. Gelatin is insoluble in cold water, but swells and softens when 

 immersed in it, gradually absorbing from 5 to 10 times its own weight of water. It is 

 soluble in hot water, in acetic acid, and in a hot mixture of glycerin and water. It is 

 insoluble in alcohol, in chloroform, in ether, and in fixed and volatile oils. 



"Identification A. To a solution of Gelatin (1 in 100) add trinitrophenol T.S. or a 

 solution of potassium dichromate (1 in 15) previously mixed with about one-fourth its 

 volume of diluted hydrochloric acid: a yellow precipitate is formed. 



"Identification B. To a solution of Gelatin (1 in 5000) add tannic acid T.S.: 

 turbidity is produced. 



"Residue on ignition, page 912 — Incinerate 5.0 Gm. of Gelatin without the use of 

 sulfuric acid: the weight of the residue does not exceed 100 mg. (2 per cent). Save the 

 residue. 



"Odor and water — insoluble substances — A hot solution of Gelatin (1 in 40) is free 

 from any disagreeable odor, and when viewed in a layer 2 cm. thick is only slightly 

 opalescent. 



"SuZ^fe— Dissolve 20 Gm. of Gelatin in 150 ml. of hot water in a flask having a 

 round bottom and a long neck, add 5 ml. of phosphoric acid and 1 Gm. of sodium 

 bicarbonate, and at once connect the flask with a condenser. Distil 50 ml., receiving 

 the distillate under the surface of 50 ml. of 0.1 N iodine. Acidify the distillate with a 

 few drops of hydrochloric acid, add 2 ml. of barium chloride T.S., and heat on a steam 

 bath until the liquid is nearly colorless. The precipitate of barium sulfate, if any, 

 when filtered, washed, and ignited, weighs not more than 3 mg., corresponding to not 

 more than 40 parts per million of sulfur dioxide, correction being made for any sulfate 

 which may be present in 50 ml. of the 0.1 iV iodine. 



"Arsenic — Heat 15 Gm. of Gelatin with 60 ml. of dilute, arsenic-free hydrochloric 

 acid (1 in 4) in a covered flask until all insoluble matter is flocculated and the Gelatin 

 dissolved. Add an excess of bromine T.S. (about 15 ml.), and heat until the excess 

 bromine is expelled. Neutralize with ammonia T.S., add 1.5 Gm. of sodium phos- 

 phate, and allow to cool. Add a slight excess (about 30 ml.) of magnesia mixture 

 T.S., allow to stand for 1 hour, filter, and wash with five 10-ml. portions of ammonia 

 T.S. diluted with 3 volumes of water. Drain the precipitate well, and dissolve it in 

 dilute hydrochloric acid (1 in 4) to make exactly 50 ml. Subject 5 ml. of this solution 

 to the test for Arsenic, page 803 : the stain, if any, does not exceed in length or intensity 

 of color that produced in a test made with similar quantities of the same reagents and 

 1.5 ml. of the standard arsenic test solution (1 part per million). 



1 Quoted (with permission) from U.S.P. XV, pp. 305-306. 



