GB690978A - Bright silver plating - Google Patents

Abstract

A brightening agent for addition to cyanide baths for the electrodeposition of silver consists of a reaction product of ketones of the general type RCH2.COCH3, where R stands for hydrogen, alkyl, acetonyl or carboxymethyl with carbon disulphide; the reaction product is prepared in presence of alkali metal hydroxide and is finally freed from alkali and hydrogen sulphide. Formaldehyde may be reacted with the ketones along with the carbon disulphide. The preferred ketones are acetonyl acetone, methyl ethyl ketone and acetone. Example 1 : 0.2 mols. acetonylacetone, 0.8 mols. carbon disulphide and 1.2 mols. aqueous sodium hydroxide (50 per cent) are put into a closed vessel supplied with a reflux condenser. On occasional shaking, the two-phase reaction mixture darkens, a moderate heat evolution occurs and after about 45 minutes the mixture is homogeneous, dark-red-brown and fairly warm. It is then cooled to room temperature and diluted with 400 ml. of water, 1.2 mols. of hydrochloric acid (1 : 1 by volume) are then slowly added under stirring. An almost black resinous mass is precipitated and much hydrogen sulphide escapes. The precipitate is powdered, washed with water and re-dissolved with 400 ml. of aqueous sodium hydroxide. This solution is then diluted to 500 ml. and 0.8 mols. of hydrochloric acid (1 : 1 by volume) added under stirring. The dark brown precipitate is filtered off, washed with water, and air or vacuum dried. Example 2: The product from the initial stage of example 1 is diluted to 600 ml. and instead of hydrochloric acid, slightly over 1.2 mols. of acetic acid are added. The solution is boiled down to 400 ml., neutralized with potassium hydroxide solution, diluted to 600 ml. and filtered. The dark red solution is employed as the plating bath addition. Example 3: The product from the initial stage of example 1 after the mixture has become homogeneous is cooled to room temperature and 0.1 mol (7.5 ml.) of aqueous formaldehyde (37 per cent) is added. An exothermic reaction occurs and after about 15 minutes the reaction mass is cooled, diluted and precipitated with hydrochloric acid. The precipitate in the form of a hard resinous lump is washed with water. After standing overnight it is powdered, dissolved in aqueous, precipitated with dilute hydrochloric acid, filtered, washed, redissolved in aqueous sodium hydroxide and reprecipitated with hydrochloric acid and dried to form a light brown powder. Further examples describe the preparation of products by reaction of methyl ethyl ketone with carbon disulphide and formaldehyde and by reaction of methyl ethyl ketone with carbon disulphide in the presence of dimethoxytetraglycol (CH3OCH2CH2OCH2CH2)2O.