The point at which a substance (such as a colloid or protein) has zero net electric charge. Usually such substances are positively or negatively charged, depending on whether hydrogen ions or hydroxyl ions are predominantly absorbed. At the isoelectric point the net charge on the substance is zero, as positive and negative ions are absorbed equally. The substance has its minimum conductivity at its isoelectric point and therefore coagulates best at this point. In the case of hydrophilic substances, in which the surrounding water prevents coagulation, the isoelectric point is at the minimum of stability. The isoelectric point is characterized by the value of the pH at that point. Above the isoelectric pH level the substance acts as a base and below this level it acts as an acid. For example, at the isoelectric point the pH of gelatin is 4.7. Proteins precipitate most readily at their isoelectric points; this property can be utilized to separate mixtures of proteins or amino acids.