Some say the acid assists in softening the gelatin, thus making the matrix easier to ink. I haven't found that it makes any difference in the inking for me. I never use acid in my bromoil bleach. My recipe: 70 mls of 10% copper sulphate. 70 mls of 10% pot bromide. 30 mls 1% pot dichromate. Distilled water to make I liter. Constant agitation for 8 minutes. This mixture will bleach/tan about ten 8x10 matrices.