Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

(^116) OXY-HALOGEN ACIDS.
current density should be the same at the anode as at the cathode
and about 15 amperes per 100 sq. cm. of electrode surface (con-
sidering only one side of the electrodes, since it is chiefly between
the inside surfaces that the current passes). The current strength
holds fairly constant and does not need to be regulated much by
changing the resistance. The minimum potential necessary for
the electrolysis of a normal sodium chloride solution is 2.3 volts,
but for obtaining the desired current density at least 6 volts will
be required. The temperature of the electrolyte should not be
allowed to exceed 20°. Follow the extent of the hypochlorite
formation by an analysis every 10 to 20 minutes. For this pur-
pose, remove 15 c.c. of the electrolyte with a pipette, let it stand in
a beaker a short time until the gas bubbles have escaped, and then
pipette 10 c.c. of it into a solution of potassium iodide which is
slightly acid with hydrochloric acid. Titrate the iodine set free
with 0.1-normal thiosulphate solution, and from the amount
required calculate the entire amount of hypochlorite present in
the whole solution. To determine the current yield,
1
find the total
number of ampere-seconds used at the time of taking the samples,
then calculate from this the theoretical yield, on the basis that
96,540 amperes flowing for one second would set free one equivalent
each of sodium and of chlorine and thus produce one-half mol. of
NaClO. The observed data and the calculated values can be
arranged advantageously in a table as follows:
Time
in Min-
utes.
0
10
20
40
Volume
in c.c.
500
500
500
485
Temper-
ature.

10

11

13

18

Amperes.

6.

6.6

6.3

6.2

Theoretical

Yield in

Grams.

0

2.91

2.89

Ha 2 S 2 O 3

Used.

0

12.6

' 19.8

Actual

Yield in

Grams.

0

2.34

1.30

Current

Yield.

0

80.4%

45.0%

Continue the electrolysis until the current yield falls below
30%, which may take about an hour, and calculate, from the last

titration showing a yield better than 30%, the number of grams

of "active" chlorine in a liter of the solution (7 to 9 g.).

B. Repeat the above experiment, using again 500 c.c. of
3-normal sodium chloride as the electrolyte, but adding to it 2.5 g.

(^1) Compare the data in No. 14.