= Crystal violet
\b[Crystal violet] is an organic <chloride> salt containing the singly charged <crystal violet cation> and one chloride <counterion>. Its formula unit is $\mathrm{C_{25}H_{30}N_3^+\,Cl^-}$, conventionally combined as $\mathrm{C_{25}H_{30}ClN_3}$. ChEBI records the disconnected ionic structure and zero net charge for the complete salt.<Crystal violet in ChEBI>[CHEBI:41688, definition, formula, SMILES and has-part relation]{ref}
The neutral formula unit should not be confused with an uncharged dye molecule. The organic <cation> carries charge $+1$ and the chloride <counterion> carries $-1$. One millimole of ideal anhydrous salt therefore represents one millimole of organic cations and one millimole of chloride ions. With rounded <molar mass> $407.99\,\mathrm{g\,mol^{-1}}$, that formula-unit amount corresponds to about $407.99\,\mathrm{mg}$.
As a stain, crystal violet can provide a readout different from tetrazolium reduction. One primary study used crystal violet staining as its cell-number comparison and found that this readout fell while the <MTT> <formazan> signal rose in kaempferol-treated cultures.<Interference of plant extracts, phytoestrogens and antioxidants with the MTT tetrazolium assay by Regina Bruggisser and others>[Abstract, crystal violet staining compared with MTT]{ref} The disagreement motivated controls on the chemical origin of the tetrazolium signal.
A <crystal violet biomass response> still needs its own measurement model. For an original idealization, let each retained cell contribute average stain amount $q$, with $N$ retained cells. After extraction into a matched fixed volume, a linear optical model gives
$$
A-A_0=K Nq.
$$
Halving $N$ halves net <absorbance> only if $q$ and the optical factor $K$ remain constant. Doubling stain retention per cell while halving the retained cell count leaves this modeled signal unchanged. Changes in <cell adhesion>, material retained during washing, extraction or optical conditions can alter the interpretation.
Crystal violet staining and the <MTT assay> thus interrogate different aspects of a preparation. Agreement can support a model within validated conditions; the dye identity itself does not make a staining signal a universal count of living cells.
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