Hair Coloring 134 consequence of mixing (Formula 2). For example, polymers are added to the developer that respond to the pH increase from acid in the hydrogen peroxide formulation to alkaline in the mixed product, so the initial formulations are both liquids but form a thick gel when mixed.17Also included in the formation are other essential items such as stabilizing actives, solvents and conditioning actives, as well as aesthetic components such as perfumes, pearlizing agents, opacifiers and pigments. The developer contains the hydrogen peroxide, typically at a concentration of 6%, 9% or 12% depending on the lightening requirements of the shade. It is essential to keep the formulation at an acidic pH, ideally in a range of pH 2–5, as this is the most stable pH for hydrogen peroxide. Also essential is a stabilizing system to prevent any decomposition due to trace levels of redox active metals such as copper and iron. Even at low levels these metals can rapidly catalyze the complete decomposition of hydrogen peroxide in a very short period of time. Typically stabilizers are colloidal stabilizers such as sodium stannate (often added by the hydrogen peroxide manufacturers) and chelants such as ethylenediamine tetra-acetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA) or 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) or a combination of both systems. It is essential that any materials used in the developer formulation are stable to oxidation from hydrogen peroxide over a shelf life of ~ 3 years. The tint contains the alkalizer and the dye precursors. An essential additional ingredient is a stabilizer package of antioxidants and a chelant. The typical antioxidants added are sodium sulfite and either ascorbic or erythorbic acid, and the typical chelant used is ethylenediamine tetra-acetic acid. The antioxidants prevent color formation during manufacturing and storage of the product. Also often included in the tint is a solvent, such as propylene glycol, which helps solubilize the dye precursors. In the past few years there has been an increased use of monoethanolamine as an alternative alkalizer to ammonia. The advantage is that a wide range of shades can be achieved without