[jovvi]

For one of my labs we had to use TLC to detect non-volatile acids in wine (tartaric acid etc). The eluent mixture contained water, butanol and formic acid. What is the significance of the formic acid?

The plate also had two solvent fronts, i can't figure out why this is and i don't know which one to use for my Rf calculations.

Please help

[Arctic-Nation]

I'm afraid I can't help you with your formic acid problem (actually, I hope someone will give you the answer as I've pondered the same question on similar separations on occasion), but on TLC you cannot ever have two solvent fronts, except maybe when your solvents don't mix, and then you're doing it wrong anyway. However, sometimes an impression of two solvent fronts can arise, especially when the solvent starts to evaporate. Whatever happens, you always take the highest position of the solvent as your front to calculate Rf values with.

[jovvi]

Yeah it makes sense which solvent front to use. I should have added that the eluent also contained bromocresol green, and it looked like there were two solvent fronts because there was the point where the eluent actually stopped, then after we let the plates dry in an oven (to remove the formic acid) the plates looked bluish green and this colour stopped a bit before (about 0.5 cm) the actual solvent front. Do you think maybe the indicator was separated from the rest of the eluent in the process?

[jovvi]

I looked it up and all i can think of is that formic acid is added to make the eluent more polar, which leads to a better separation of compounds. I don't really understand why but i'm sure i'll get points for trying 

[Train]

Did you use a reverse phase or normal phase TLC plate?

It seems to me than formic acid may have been added to control the protonation of your analytes.