Synthetic Inorganic Chemistry

190 ALKALI AND ALKALINE EARTH METALS

salts as sea water. " Rock salt" is the name applied to coarse

unpurified salt, and it contains the same impurities as sea salt

although usually in lesser amount.

COMPOSITION OF SEA SALT

NaCl

MgCl 2

MgSO 4

CaSO 4

77 76%

10 88%

4 75%

3 60%

K 2 SO 4

CaCOa

MgBr 2

2 46%

0 33%

0 22%

For most commercial purposes these impurities are not harmful.
By careful crystallization of the salt from solution, a product
sufficiently free from these impurities can be obtained to be used
as table salt. To obtain chemically pure sodium chloride, however,
more elaborate precautions must be taken. A satisfactory method
depends upon the insolubility of sodium chloride in a concentrated
solution of hydrochloric acid. A nearly saturated solution of the
rock salt is prepared, and, without removing the dirt and insoluble
matter, enough pure sodium carbonate is added to precipitate the
calcium and magnesium in the solution as carbonates. Into the
clear nitrate gaseous hydrochloric acid is then passed until the
greater part of the sodium chloride is precipitated, while the small
amounts of sulphates and of potassium salts remain in the solution.
The precipitate is drained and washed with a solution of hydro-
chloric acid until the liquid clinging to the crystals is entirely free
from sulphates.

Materials: sodium chloride (rock salt), 75 grams.

36 N sulphuric acid, 95 cc.

12 N hydrochloric acid, 50 cc.

Apparatus: 2-liter round-bottom flask with 2-hole rubber

stopper.

3-necked Woulff bottle with rubber stoppers.

65 mm. funnel.

2 thistle tubes.

delivery tube and connection; see Fig. 23.

600-cc. beaker,

tripod and wire gauze.