Pt(s) | Mn2+(aq), MnO4-(aq), H+(aq) || H+(aq), IO3-(aq) | I2(s) | Pt(s)
Pt(s) | Mn2+(aq) | MnO2(s) | H+(aq) || H+(aq) || PbO2(s) | Pb2+(aq) | Pt(s)
how come the first one has comas within cell notation but not the second one,
Are you sure of the second one? It appears to have two salt bridges. || and ||
oops, the second one should be
Pt(s) | Mn2+(aq) | MnO2(s) | H+(aq) || H+(aq) | PbO2(s) | Pb2+(aq) | Pt(s)
but can you clarify the comma one
Commas separate multiple species in the same phase. The single bar is used to separate phases.
The comma within cell notation represents a phase boundary. In the first example, the comma within the notation Pt(s) | Mn2+(aq), MnO4-(aq), H+(aq) || H+(aq), IO3-(aq) | I2(s) | Pt(s) indicates that there is a phase boundary between the species Mn2+(aq), MnO4-(aq), and H+(aq) on the left side and H+(aq) and IO3-(aq) on the right side.
On the other hand, in the second example Pt(s) | Mn2+(aq) | MnO2(s) | H+(aq) || H+(aq) || PbO2(s) | Pb2+(aq) | Pt(s), there are no commas within the notation. This implies that there are no phase boundaries within this cell notation, and all the species mentioned are in the same phase.
In simple terms, the presence or absence of commas within cell notation indicates whether there is a phase boundary within the electrochemical cell.