lumps of marble (CaCO3) are sometimes added to cultures containing fermentable substances like glucose, sucrose or lactose, to maintain the pH unchanged, that is, to neutralize the acid produced by fermentation. otherwise the organisms in the culture would quickly cease growth or die. colloidal substances such as charcoal, precipitates of phosphates, kaolin, etc., which adsorb hydrogen or hydroxyl ions directly, are also active buffers and are used in biological systems.