NEW MATERIAL:The 1,3-bis(2'-carboxyethyl)-imidazolium-betaine compound of formulaI(R2 is methyl, ethyl, undecyl, heptadecyl or phenyl; R4 is H or methyl).

EXAMPLE: 1,3-Bis(2-carboxyethyl)-2-methylimidazolium-betaine.

USE: Agent having rust preventing activity to silver. The agent is dissolved in a solvent and applied to the surface of silver metal. The surface of silver coated with the agent does not blacken even by leaving in air for a long period.

PROCESS: The compound of formulaIcan be prepared by heating 1mol of an imidazole compound of formula II having no substituent group at the 1-position and ≥2mol of acrylic acid in the presence of a polymerization inhibitor in a reaction vessel furnished with a reflux condenser and a stirrer at ≥40°C for a proper time interval.