Gold electroplating bath and method of making the same

An aqueous bath for the electrodeposition of gold, comprising:

• (a) about 2.0 to 16 grams per liter of auric gold ion provided by a gold compound selected from the class consisting of gold chloride, chloroauric acid, potassium gold chloride, and sodium gold chloride, and mixtures thereof;
• (b) an alkali metal cyanide selected from the class consisting of potassium cyanide, sodium cyanide, and mixtures thereof, said alkali metal cyanide being present in the ratio of at least four mols of said alkali metal cyanide for each mol of said gold ion but not in excess of about ten percent by weight of the amount necessary to provide said ratio;
• (c) about 3.7 to 75 grams per liter of a nitrate salt selected from the class consisting of potassium nitrate, lithium nitrate, sodium nitrate, magnesium nitrate, and barium nitrate, and mixtures thereof;
• (d) about 13 to 159 milliliters per liter of ethylenediamine hydrochloride (calculated as ethylenediamine);
• and
• (e) hydrochloric acid in an amount sufficient to adjust the pH of said bath to about 0.1 to 1.5.

In a method of preparing an aqueous bath for the electrodeposition of gold, the steps comprising:

• (a) preparing a first aqueous solution by dissolving in water:
• (1) a water-soluble auric gold compound in an amount sufficient to provide at least about 2 grams per liter of auric gold in said bath;
• (2) a water-soluble alkali metal cyanide in an amount sufficient to provide 100-115 percent of the stoichiometric amount, by weight thereof, required to react with said auric gold to form an alkali metal gold cyanide complex;
• and
• (3) a water-soluble nitrate salt in an amount sufficient to provide at least about 3.7 grams per liter of said salt in said bath;
• (b) separately preparing a second aqueous solution by dissolving in water:
• (1) ethylenediamine in an amount sufficient to provide at least about 13 milliliters of ethylenediamine per liter of said bath;
• and;
• (2) sufficient hydrochloric acid to provide at least the stoichiometric amount thereof required to react with said ethylenediamine to form ethylenediamine hydrochloride and to adjust the pH of said bath to not more than about 4.0;
• and
• (c) mixing said first and second solutions to provide said bath, said solutions being mixed in proportions selected to provide the specified quantities of ingredients in said bath.