HCA is a washing aid which can speed up the washing of fiber based paper after fixing. HCA is not a Hypo Eliminator. Hypo Eliminators are not recommended for archival processing of film or prints.
The active ingredient in HCA is sodium sulfite. 20 grams of sodium sulfite dissoved in a liter of water is sufficient. 5 grams of sodium bisulfite can be added to the mix, which will buffer the sodium sulfite and reduce the pH of the mixture.
Rinse the print in running water to remove the fixer from the surface of the print, then soak the print in HCA for 2-3 minutes. After the HCA treatment, wash the print for 10 to 20 minutes in frequent changes of water.
A kodak HT-2 Residual Hypo Test can be used to verify the adequacy of the washing procedure.