Determining tannin content (Löwenthal method)

[B West]

Hi everyone,

I have carried out a determination of tannins in a solution using the Löwenthal method and everything went to plan. I am now trying to understand what was going on chemically. Why does the solution go from blue to yellow?

Any help would be fantastic!

Thank you,

Ben 

[Alessandro Ruffin]

Hi Ben,

I am not an expert. 

I can tell you the little I know:

- The indigo-carmine acts as a redox indicator and turns yellow because it reacts with the permanganate;

- In presence of tannins the permanganate reacts first with the tannins and then with the indigo carmine; 

- The blank tells you the amount of permanganate that is needed to react with the indicator and so you remove it from the titration getting the amount of permanganate that reacted with the tannins;

- The equivalence between 1 ml of N/40 permanganate solution and 1 g of tannin is an arbitrary standard to which many experimental results are referred to and is well in line with results of the Folin method as Andrew shows in his website.

I hope this helps

Best regards

Alessandro