The potassium iodate assay

Gallotannins, can be detected quantitatively with the potassium iodate assay. This assay was first described by described Haslam (1965), and is based on the reaction of potassium iodate (KIO3) with galloyl esters, which will form a red intermediate and ultimately a yellow compound. The concentration of the red intermediate can be measured spectrophoto-metrically at 550 nm. The reaction was initially performed by adding 1.5 mL of a saturated potassium iodate solution to 3.5 mL of tannin, followed by a 40-min. incubation at 0°C for (Haslam, 1965). Since the red intermediate turns yellow over time, it is important to be consistent in terms of the time and temperature of the reaction. Bate-Smith (1977) recommended performing the reaction at 15°C until a maximum absorbance was reached (regardless of the time). The formation of the precipitate - resulting from the absence of free hydroxyl groups on the pentagalloyl parent residue - can be avoided by changing the solvent of the reaction. Aqueous acetone (20% (v/v) or methanol above 10% (v/v) generally resulted in less precipitation than just water. Since most standard curves for this assay are made with tannic acid, the concentration of hydrolysable tannins is expressed as tannic acid equivalents (TAE).

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