The surface of the Earth is covered in 70% water and approximately 98% of the total water is contained within the oceans, inaccessible as potable water. The solubilising properties of water help to keep polar molecules in solution and the non-polar molecules are driven to adopt structures that avoid contact with water. The solubility of a protein is not solely dependent on its association with water; the presence and concentration of anions, cations and the pH of the surrounding medium are critical to maintaining a protein in solution. Water can also ionise to form negatively charged hydroxyl and positively charged hydronium ions. This property underpins the pH scale, the concept of acids/bases and the concept of buffers. Water as a solvent has many unique properties, most of which can be attributed to its structure. The Henderson–Hasselbalch equation can be used to determine the quantitative aspects of buffers.