[metallica00789]

Hi,
In my analytical chem lab, I standardized a 0.1 hydrochloric acid using the method coulometric titration. The components of the electrochemical cell shown below:

The cathode I used was a copper bat. Bromothymol blue (C27H28Br2O5S) was used an indicator to when the pH of the solution became basic. My questions is, after each titration, I had to scrub the copper bar (the anode) to remove a yellow/green material formed on it's surface. What was that material? Was is it copper chloride? Or the indicator reacted with the copper to form a complex on the anode surface? I was not expecting anything to precipitate on the anode since the coulometer is not a galvanic cell. It is basically a cell to perform water electrolysis.

P.S I have attached the paper that described how to build and perform the experiment.