D40.6 Cell Notation

Drawing a pictorial diagram, like the figure below, to define a voltaic cell takes a lot of time. Cell notation is an abbreviation that summarizes the important information about a voltaic cell. In cell notation, a vertical line, |, denotes a phase boundary and a double line, ||, a salt bridge. The anode electrode is written to the left, followed by the anode solution, then the salt bridge, then the cathode solution, and, finally, the cathode electrode to the right. Figure: Cell Notation below shows how the cell notation for a voltaic cell relates to various components of the cell.

Figure: Cell notation. The cell notation, Cu(s) | Cu2+(aq) || Ag+(aq) | Ag(s), is related to the diagram of the voltaic cell.

Note that spectator ions, such as NO3−, are not included in the cell notation, and if there are coefficients in a half-reaction, the coefficients are not included (that is, the coefficients of 2 in the silver half-reaction do not appear in the cell notation). When known, the initial concentrations of ions are usually included in the cell notation, so a more complete cell notation for the cell above is Cu(s) | Cu2+(aq, 1 M) || Ag+(aq, 1 M) | Ag(s).

Some redox reactions involve species that are poor conductors of electricity, such as gases or ionic solids. For such substances, an inert electrode that does not participate in the reactions is used. An example of such a voltaic cell is shown below.

This figure contains a diagram of an electrochemical cell. Two beakers are shown. Each is just over half full. The beaker on the left contains a colorless solution and is labeled “Solution of M g C l subscript 2.” The beaker on the right contains a colorless solution and is labeled “Solution of H C l.” A glass tube in the shape of an inverted U connects the two beakers at the center of the diagram. The tube contents are colorless. The ends of the tubes are beneath the surface of the solutions in the beakers and a small grey plug is present at each end of the tube. At the center of the diagram, the tube is labeled “Salt bridge. Each beaker shows a metal coils submerged in the liquid. The beaker on the left has a thin grey coiled strip that is labeled “M g coil.” The beaker on the right has a black wire that is oriented horizontally and coiled up in a spring-like appearance that is labeled “Non reactive platinum wire.” A wire extends across the top of the diagram that connects the ends of the M g strip and platinum wire just above the opening of each beaker. This wire is labeled “Conducting wire.” At the center of this wire above the two beakers near the center of the diagram is a right pointing arrow with the label “e superscript negative” at its base. Bubbles appear to be rising from the coiled platinum wire in the beaker. These bubbles are labeled “Bubbles of H subscript 2.” An arrow points down and to the right from a plus sign at the upper right region of the salt bride. An arrow points down and to the left from a negative sign at the upper left region of the salt bride. A curved arrow extends from the grey plug at the left end of the salt bridge into the surrounding solution in the left beaker. The label “Oxidation M g right pointing arrow M g superscript 2 plus plus 2 e superscript negative” appears beneath the left beaker. The label “Reduction 2 H superscript plus plus 2 e superscript negative right pointing arrow H subscript 2” appears beneath the right beaker.
Figure: Inert electrode. The oxidation of magnesium to magnesium ions occurs in the beaker on the left side in this apparatus; the reduction of hydrogen ions to hydrogen occurs in the beaker on the right. A nonreactive (inert) platinum wire conducts electrons into the right beaker.

The redox reaction involved is:

Oxidation (anode): Mg(s) ⟶ Mg2+(aq) + 2e−
Reduction (cathode): 2H+(aq) + 2e− ⟶ H2(g)
overall: Mg(s) + 2H+(aq) ⟶ Mg2+(aq) + H2(g)

This voltaic cell uses an inert platinum wire for the cathode, so the cell notation is:

Mg(s) | Mg2+(aq) || H+(aq) | H2(g) | Pt(s)

The magnesium electrode is an active electrode because it participates in the redox reaction. Inert electrodes, like the platinum electrode, do not participate in the redox reaction but must be present so that there is a complete electrical circuit. Platinum and gold are among the least reactive metals so they are good choices for inert electrodes. Graphite, also inert to many chemical reactions, is another common option.

Activity: Half-reactions and Cell Notation

Exercise: Cell Notation

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