it is. The oxidation state of the element is shown on the x axis of the Frost diagram. Oxidation states are unitless and are also scaled in positive and negative integers. Most often, the Frost diagram displays oxidation number in increasing order, but in some cases it is displayed in decreasing order. The neutral, pure element with a free energy of zero (nE° = 0) also has an oxidation state equal to zero. However, the energy of some allotropes may not be zero. The slope of the line therefore represents the standard potential between two oxidation states. In other words, the steepness of the line shows the tendency for those two reactants to react and form the lowest-energy product. There is a possibility of having either a positive or negative slope. A positive slope between two species indicates a tendency for an oxidation reaction, while a negative slope between two species indicates a tendency for reduction. For example, if the manganese in [HMnO4]− has an oxidation state of +6 and nE° = 4, and in MnO2 the oxidation state is +4 and nE° = 0, then the slope Δy/Δx is 4/2 = 2, yielding the standard potential of +2. The stability of any terms can be similarly found by this graph. Gradient The gradient of the line between any two points on a Frost diagram gives the potential for the reaction. A species that lies in a peak, above the gradient of the two points on either side, denotes a species unstable with respect to disproportionation, and a point that falls below the gradient of the line joining its two adjacent points lies in a thermodynamic sink, and is intrinsically stable. Axes The axes of the Frost diagram show (horizontally) the oxidation state of the species in question and (vertically) the electron exchange number multiplied by the voltage (nE) or the Gibbs free energy per unit of the Faraday constant, ΔG/F. Disproportionation and comproportionation In regards to electrochemical reactions, two main types of reactions can be visualized using the Frost diagram. Comproportionation is when two equivalents of an element, differing in oxidation number, combine to form a product with an intermediate oxidation number. Disproportionation is the opposite reaction, in which two equivalents of an element, identical in oxidation number, react to form two products of differing oxidation numbers. Disproportionation: 2 Mn+ → Mm+ + Mp+. Comproportionation: Mm+ + Mp+ →