Chemical Reaction in a Cell
2240 SRM 1
Specific Gravity
The strength of the electrolyte is measured in points of specific gravity. For example, a solution of sulfuric
acid has a specific gravity of 1.835. Water has a specific gravity of 1.000. Electrolyte is 27 percent acid and 73
percent water and has a specific gravity of 1.275.
Chemical Reaction in a Cell
WARNING
NEVER pour water into concentrated acid.
Combining these liquids creates the rapid pro-
duction of heat which can cause the water to
boil and the acid to erupt and splash out of the
container.
ALWAYS pour concentrated acid
into water when making a dilute solution of
acid.
In a fully-charged cell, the electrolyte has a specific
gravity of 1.270 to 1.130. NEVER discharge a battery
below a specific gravity of 1.130. The battery can
have permanent damage if discharged below 1.130.
The cell generates a voltage (potential difference)
when two different types of metal are in the elec-
trolyte. The two metals in a lead-acid cell are lead
peroxide (PbO
3
) for positive plates, and sponge lead
(Pb) for negative plates.
See Figure 3.
A poten-
tial difference of approximately 2 volts per cell is
generated. The potential difference does not vary
according to the size of the cell.
Figure 3. Fully Charged Cell
During the discharge of the cell, lead peroxide and
sponge lead mix with sulfuric acid to make lead sul-
fate (PbSO
4
) on both plates. See Figure 4. This ac-
tion decreases the voltage in the cell. When the sul-
furic acid is removed from the electrolyte, the specific
gravity of the electrolyte decreases. See Figure 5.
The potential difference of a discharged cell is ap-
proximately 1.75 volts.
When a direct current is applied to a discharged cell,
the lead sulfate is changed into lead and sulfuric acid.
The lead goes to the positive plate and stays as lead
and to the negative plate and stays as lead peroxide.
See Figure 6. The concentration of sulfuric acid in
the electrolyte increases. The specific gravity of the
electrolyte increases as the concentration of sulfuric
acid increases. In a fully charged cell, the positive
plate again contains the lead peroxide and the nega-
tive plate contains the sponge lead.
Figure 4. Discharging Cell
2