In a previous article1 a method was described whereby the adsorption of rose bengal B (tetrachlortetraiodofluorescein) on proteins could easily be determined in a quantitative way. This dye stuff attains its maximum rose color at pH 4.8 or above, but it behaves as an indicator dye and is decolorized on adding acid; dilute aqueous solutions are practically colorless below pH 2.2. If proteins are present the dye will combine with them in the form of its colored salt, and on bringing the acidity of the solution to pH 2.2, that portion of the dye in combination with the proteins retains its color, while the free dye is decolorized. The color that remains on acidifying the solution therefore represents the adsorbed dye and it may be easily estimated by colorimetric comparison with a standard solution of rose bengal of known strength.
It has been shown that this is a colloidal reaction
ROSENTHAL SM, ACKMAN FD. THE COMBINING POWER OF PROTEINS WITH ROSE BENGALII. APPLICATION AS A QUANTITATIVE TEST ON THE SPINAL FLUID. Arch Intern Med (Chic). 1926;38(4):527-535. doi:10.1001/archinte.1926.00120280119008