Is any complexation isotherm valid to analyse macromolecular binding of trace metal ions in environmental samples? To answer this question, a detailed study of the behaviour of some usual complexation isotherms with different underlying affinity spectra at intermediate and low coverages is performed. Metal-complexation is characterised through the behaviour of the average equilibrium function, defined as K_c ≡ [occupied sites]/([free metal][free sites]), which can be interpreted, at any metal concentration, as the average of the microscopic stability constants of the remaining free sites of the system. It is shown that, at intermediate coverages, the value of K_c depends basically on the average, μ, and standard deviation, σ, of the distribution function p(log k), the binding curve showing low sensitivity with respect to the concrete shape of the affinity spectrum. As a result, isotherms with the same values of μ and σ describe binding data of H⁺ to humic acid (HA) with similar accuracy. In contrast, at low coverages, K_c tends to the average of the microscopic stability constants of all the sites in the naked macromolecule, 〈k〉, which, for a given μ and σ, strongly depends on the shape of the isotherm. This result imposes an important constraint on the isotherms used and warns against extrapolation since isotherms with very similar behaviours at intermediate coverages, can predict very different ones for low coverages. This different behaviour is clearly evidenced by simulated voltammetric experiments, suggesting their suitability for speciation studies of trace metals in environmental samples. Normalised limiting currents of Cd/HA are used to validate isotherms suitable to describe both low and intermediate coverage.