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Autoxidation is oxidation of a substance by molecular oxygen, commonly proceeding through a self-propagating free radical chain in organic materials. The name does not mean that initiation is unnecessary. Light, existing peroxides or other radical sources can start a chain, and changes in initiation can alter the observed rate.

A basic hydrocarbon propagation scheme writes the substrate as $\mathrm{RH}$:A peroxyl radical is converted into a hydroperoxide while generating a new carbon-centered radical. The two-step cycle consumes substrate and oxygen but regenerates a chain carrier. Radical–radical reactions can terminate chains; hydroperoxide decomposition can create further initiating species.

An original limiting kinetic model assumes abundant oxygen, approximately constant substrate concentration and termination dominated by pairs of peroxyl radicals. If initiation supplies radicals at rate $R_i$ and termination removes them at $2k_t{P}^2$, the steady radical concentration isQuadrupling $R_i$ then doubles the propagation rate rather than quadrupling it. This scaling follows from the stated balance; it need not hold when oxygen delivery, changing substrate or other termination pathways control the experiment.

A motivating modern apparatus is the inhibited autoxidation assay. A primary method co-oxidizes a hydrocarbon substrate with a BODIPY-based signal carrier and monitors progress with a visible-light spectrophotometer. Comparing inhibited and uninhibited traces tests radical trapping while distinguishing reaction rate from trapping capacity.^[ref]

A reduced signal slope can reflect chemical inhibition or inadequate oxygen supply, so controls for oxygen-limited autoxidation, probe response and matched initiation matter. The chain mechanism helps explain why excluding oxygen, intercepting radicals and changing peroxide decomposition can affect oxidation differently. An arbitrary antioxidant-assay signal alone does not establish product identity or a biological benefit.

Autoxidation lag standard addition estimates an initiation rate or an equivalent background trapping capacity by adding known amounts of a reference radical-trapping antioxidant to a matched sample matrix. It uses a series of autoxidation inhibition periods, rather than treating one delay as a chemical concentration.

Under constant effective initiation rate $R_i$, fast quantitative trapping and unchanged reference stoichiometry $n_s$, let $K_b$ be the initial background capacity and $C_s$ the added reference concentration. Capacity balance givesThus the linear regression intercept and slope have different roles. The concentration $a/b$ is an equivalent amount of the chosen reference; it does not identify the background chemical species.

For an original series, $C_s=0,4,8\mu \mathrm{M}$ gives delays $30,70,110\mathrm{s}$. Then $a=30\mathrm{s}$ and $b=10\mathrm{s}\mu {\mathrm{M}}^{-1}$. With $n_s=2$, the model gives $R_i=0.20\mu \mathrm{M}{\mathrm{s}}^{-1}$ and $K_b=6\mu \mathrm{M}$ of trapping capacity, equivalent to $3\mu \mathrm{M}$ of that reference. Using only the largest addition and dividing $n_s{C}_s$ by its full delay would give about $0.145\mu \mathrm{M}{\mathrm{s}}^{-1}$, because it incorrectly includes the background delay in the reference-only balance.

At first order, if $n_s$ is known, $u(R_i)/R_i\approx u(b)/b$. Inference of $K_b=n_{s}a/b$ also involves the fitted intercept-slope covariance; treating fitted parameters as independent can misstate the measurement uncertainty. Replicates at several concentrations test linearity and estimate the delay variability.

The chemical observation must still be defined. A primary spectrophotometric method monitors a BODIPY-based signal carrier co-autoxidizing with a hydrocarbon and distinguishes trapping kinetics from stoichiometry.^[ref] The regression above is a stated capacity model, not an assertion that this paper used this particular numerical series. Keep solvent addition, initiation, temperature, oxygen supply and breakpoint rule matched. Curvature can reveal altered chemistry, oxygen-limited autoxidation or time-varying initiation; it should not be hidden by forcing a straight line.