Rubidium chloride reacts with sulfuric acid to rubidium hydrogen sulfate. Rubidium and caesium are often used in atomic clocks. Incompatible elements commonly include potassium, barium, caesium, and rubidium. Cesium and rubidium cations also form clusters with helium. Steric hindrance forbids the existence of rubidium or caesium nitride. The sodium, potassium, rubidium, caesium, and ammonium phosphates are all water-soluble. Potassium, rubidium and caesium are also incompatible elements due to their large ionic radii. Later rubidium and cesium beam clocks were used. These included the production of pharmaceutical iron compounds, chemically pure hydrofluoric acid, rubidium and caesium. Caesium has physical and chemical properties similar to those of rubidium and potassium. Several methods are available for separating potassium, rubidium, and caesium. The biological behaviour of caesium is similar to that of potassium and rubidium. It is one of the major sources of the rare alkali metals rubidium and caesium. Rubidium and caesium borides have not even been characterised. They contain large amounts of rubidium, strontium and barium.