Crystal violet

Crystal violet is an organic chloride salt containing the singly charged crystal violet cation and one chloride counterion. Its formula unit is ${\mathrm{C}}_{25}{\mathrm{H}}_{30}{\mathrm{N}}_3^{+}\mathrm{C}{\mathrm{l}}^{-}$, conventionally combined as ${\mathrm{C}}_{25}{\mathrm{H}}_{30}\mathrm{Cl}{\mathrm{N}}_3$. ChEBI records the disconnected ionic structure and zero net charge for the complete salt.^[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 $407.99\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$, 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.^[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 givesHalving $N$ 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.