= Formazan
A \b[formazan] belongs to the family defined by the parent connectivity $\mathrm{H_2N{-}N{=}CH{-}N{=}NH}$ and derivatives formed by substitution at carbon or nitrogen. The chain contains four <nitrogen> atoms and one central <carbon> atom. A particular substituted formazan is a chemical compound; the family name does not identify one universal dye.<Formazans in the IUPAC Gold Book>[Entry F02486; underlying terminology recommendation, p. 1336]{ref}
The linked nitrogen–carbon framework provides a <conjugated system>. Substitution and chemical conditions affect its optical behavior, so a measured <formazan optical response> should identify the derivative and its state. The strongly colored product obtained by reducing <MTT> is a specific example, not the definition of every member of the class.
In the manufacturer's MTT test principle, yellow <tetrazolium salt> is reduced to purple formazan crystals, then the product is dissolved for optical measurement.<Cell Proliferation Kit I MTT instructions by Roche>[Section 3.1 and Figure 4, printed p. 9]{ref} Product formation, crystal recovery and final dissolved-product <absorbance> are different stages; incomplete dissolution or optical scattering can change their relationship.
For an original dissolved-product model, the <Beer-Lambert law> gives
$$
A-A_0=\epsilon\ell\frac{n_F}{V},
$$
where $n_F$ is product amount, $V$ solution volume, $\ell$ optical path length and $\epsilon$ the appropriate <molar absorption coefficient>. At fixed $\ell$, doubling $V$ while keeping $n_F$ unchanged halves the ideal net absorbance. Doubling the amount and the volume together preserves concentration and therefore preserves that ideal signal.
The equation requires a suitable solution measurement regime and a specified product. A tetrazolium-derived color can arise from cellular or noncellular <reduction>; identifying a formazan product does not by itself identify its electron source. This is why <tetrazolium-reduction interference> matters when formazan is used as a readout of <cell viability>.
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