For example, the transport numbers of hydrochloric acid (HCl(aq)) may be determined by electrolysis between a cadmium anode and an Ag-AgCl cathode. The anode reaction is Cd → Cd + 2 e so that a cadmium chloride (CdCl 2) solution is formed near the anode and moves toward the cathode during the experiment. An acid-base indicator such as bromophenol blue is added to make visible the boundary between the acidic HCl solution and the near-neutral CdCl 2 solution. The boundary tends to remain sharp since the leading solution HCl has a higher conductivity that the indicator solution CdCl 2, and therefore a lower electric field to carry the same current. If a more mobile H ion diffuses into the CdCl 2 solution, it will rapidly be accelerated back to the boundary by the higher electric field; if a less mobile Cd ion diffuses into the HCl solution it will decelerate in the lower electric field and return to the CdCl 2 solution. Also the apparatus is constructed with the anode below the cathode, so that the denser CdCl 2 solution forms at the bottom.