Synthetic Inorganic Chemistry

CHEMICALLY PURE SODIUM CHLORIDE 189

and the anode arm into the other beaker. Rinse out the cathode

arm with three portions of 3 to 5 cc. of distilled water, adding the

rinsings to the beaker. Add a few drops of phenolphthalein.

Now carefully remove the anode from the coulometer, and set

it in a test tube full of distilled water. Be careful not to bruise it.

Then rinse it with alcohol and, after it has dried, weigh it. Return

the CUSO4 solution to a bottle marked " Used CUSO4 Solution."

Titrate the cathode solution with the standard acid prepared

in Experiment 6, and thus determine the number of equivalents

of NaOH produced in the cathode compartment.

From the loss of weight of the copper anode determine the num-

ber of faradays of electricity that were used in the cell. The ratio

of the number of equivalents of NaOH to the number of faradays

gives the current efficiency.

QUESTIONS

1. One coulomb per second is the unit of electric current, and
   is called the " ampere." From the loss of weight of the anode
   and the elapsed time, calculate the current that flowed through
   the cell on the assumption that it was uniform during that time.


2. At the beginning of the electrolysis what ions are transferred
   through the cotton plug of the U-tube? As the electrolysis
   progresses and an OH~ ion concentration is built up in the cathode
   compartment, how is the picture of the ion transference altered?


3. If Na+ ions are practically the only positive ions which move
   up to the cathode, how do you explain that H+ rather than Na+
   ions are discharged at the cathode surface? Describe some form
   of cell in which sodium metal is actually liberated at the cathode.
   What is the essential difference between the latter cell and the
   cell used in this experiment?


4. If the current efficiency of the hydrogen discharge is 100
   per cent, calculate from the copper coulometer data the volume of
   hydrogen escaping.


5. If a silver coulometer had also been inserted in the circuit,
   what weight of silver would have been deposited on the cathode?

PREPARATION 18
CHEMICALLY PURE SODIUM CHLORIDE FROM ROCK SALT, NaCl
All deposits of salt in the earth are a result of the evaporation
either of sea water or of water containing approximately the same