Manufacturers use buffers as part of their dentifrice formulation to keep the pH constant. This is important for the stability and effectiveness of a dentifrice. For example, pH can be an important factor in fluoride bioavailability; fluoride is more difficult to formulate at lower pH because of greater potential for interaction with common abrasives under acidic conditions. In some countries, regulations require dentifrice to be above a certain pH. Trisodium phosphate and sodium citrate are examples of dentifrice buffers. Pyrophosphates, which are used to control calculus formation, are also very effective buffers.