ferrous iron interferes to cause a false positive response to the extent of approximately 1 mg/L apparent tannic acid for each 2 mg/L ferrous iron. The interference due to up to 20 mg/L can be eliminated by dissolving one 0.05-g spoonful of sodium pyrophosphate in the sample before adding the Tanni Ver Reagent. This may result in a greenish tint to the solution if high ferrous iron levels are present with low tannin levels. But in most cases this is not a ploblem.