Pure Sodium Chloride from Rock Salt

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 

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 filtrate 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 hydrochloric acid until the liquid clinging to the crystals is entirely free from sulphates. 

Materials sodium chloride (rock salt), 75 grams. 

Apparatus 2-liter round-bottom flask with 2-hole rubber stopper. 

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Hot brine solution is held in an open pan approximately 4-6 m wide, 45-60 m long, and 60 cm deep at 96°C. Flat, pure sodium chloride crystals form on the surface and fall to the bottom. The crystals are raked to a centrifuge, separated from the brine, and dried. A purity of 99.98% is obtained. Grainer salt dissolves more readily and is preferred in some applications, such as the butter and cheese industries. It is more expensive because of energy use for the hot brine. Its cost can be as much as six times that of rock salt and 20 times that of brine. 

Davy discovered several elements in this way, beginning in 1807. After releasing purified potassium metal from potassium hydroxide, it took him only a year to produce magnesium, strontium, barium, and calcium. Fewer than 30 elements had been isolated by 1800, but by 1850, more than 50 were known. Most of these new elements were isolated using electrolysis. Figure 17.2 shows the modern commercial electrolysis of molten rock salt. Rock salt is sodium chloride, NaCl. In this process, pure sodium metal and chlorine gas are produced. 

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