soda

the technical name of the sodium salts of carbonic acid (H₂C0₃). The different types of soda include calcined soda (Na₂Có₃, anhydrous sodium carbonate, soda ash), baking soda (NaHC0₃, sodium bicarbonate), and soda crystals (Na₂CO₃-10H₂O, Na₂C0₃-7H₂0, Na₂C0₃-H₂0). Caustic soda is the common technical and household name for sodium hydroxide (NaOH).

Soda has been known since antiquity. The Egyptians used soda occurring in nature (in lake water) as a detergent and also in the melting of glass. Until the 18th century, the word “alkali” referred to both sodium carbonate and potassium carbonate. In 1736 the French scientist H.-L. Duhamel-Dumonceau was the first to differentiate the two compounds; sodium carbonate was called soda (after the plant Salsola soda, from whose ash the salt was obtained), and potassium carbonate was called potash.

In nature, soda is encountered in the minerals trona (Na₂C0₃-NaHC0₃-2H₂0), natron (Na₂CO₃10H₂O, also known as natrit), and thermonatrite (Na₂C0₃-H₂0). Soda crystallizes in the monoclinic system, forms white granular or powdery masses, and quickly loses water in the air. The hardness on Mohs’ scale varies from 2.5 (trona) to 1.5; density varies from 2,110 kg/m³(trona) to 1,420 kg/m³ (natron). In the USSR, soda lakes are found in Transbaikalia and Western Siberia; Lake Natron in Tanzania and Searles Lake in California are well known soda lakes. Mineral deposits of soda (trona) of industrial significance are found in the Eocene stratum of the Green River formation in Wyoming. In addition to trona, many minerals that had previously been considered rare are found in this sedimentary stratum. Among them is dawsonite [NaAlC0₃(OH)₂], which is considered a source of soda and alumina. In the United States, naturally occurring soda supplies 40 percent of the country’s demand. Soda is not extracted in the USSR owing to the absence of large deposits.

Until the early 19th century, the principal source of soda was the ash of certain marine algae and coastal plants.

Anhydrous Na₂C0₃ is a colorless crystalline powder with a density of 2.53 g/cm³ at 20°C and a melting point of 853°C. It readily dissolves in water, to a concentration of 17.7 percent (by weight) at 20”C and 31.3 percent at 100°C. It forms the mono-clinic crystal hydrate Na₂CO₃10H₂O, which is stable up to 32.017°C, and the orthorhombic crystal hydrate Na₂C0₃-7H₂0, which converts to orthorhombic Na₂C0₃-H₂0 at 35.27°C. At 112.5°C and 1.27 atmospheres, Na₂C0₃-H₂0 is converted to Na₂C0₃. Aqueous solutions of Na₂C0₃ are strongly alkaline as a result of hydrolysis.

The first industrial method for the production of soda was invented in the years 1787–89 by N. Leblanc, and production of soda by the Leblanc process was begun in France in 1791. With this process, rock salt (NaCl), under the action of concentrated H₂S0₄, is converted into sodium sulfate:

2NaCl + H₂S0₄ = Na₂S0₄ + 2HC1

A mixture of sodium sulfate and ground limestone and coal is then roasted in a fired furnace to about 1000°C. The following reactions occur at this stage:

Na₂S0₄ + 2C = Na₂S + 2C0 ₂

Na₂S + CaC0₃ = Na₂C0₃ + CaS

Soda is extracted from the cooled melt with water. CaS is removed from the solution of soda, the solution is evaporated, and the crude soda is purified by recrystallization. The Leblanc process yields soda in the form of the decahydrate Na₂CO₃-10H₂O, which contains approximately 62.5 percent water. Thus, calcining to red heat is required to dehydrate the soda obtained (hence the name “calcined soda”). The by-products of this process are HO (initially released into the air but later subjected to absorption by water to yield technical hydrochloric acid) and CaS (which forms great heaps).

These disadvantages were eliminated by E. Solvay’s ammonia-soda process, which Solvay patented in 1861. The first factory producing soda by the Solvay process began operation in 1863 in Belgium. The Solvay process is based on the following reactions, which proceed in aqueous solutions:

2NH₃ + H₂0 + C0₂ = (NH₄)₂C0 ₃

(NH₄)₂C0₃ + H₂0 + C0₂ = 2NH₄HC0₃

Ammonium bicarbonate (NH₄HC0₃) reacts with a solution of

NaCl:

NaCl + NH₄HC0₃ = NH₄C1 + NaHC0₃

The precipitate NaHC0₃ is filtered off and by heating to approximately 140°-160°C is converted to Na₂C0₃ (calcined soda):

2NaHC0₃ = Na₂C0₃ + C0₂ + H₂0

The C0₂ liberated in the reaction is fed back into the production cycle. To regenerate NH₃, the mother liquor, containing (NH₄)₂C0₃, NH₄HC0₃, and NH₄Cl, is heated to 80°C, with the result that ammonium carbonate and ammonium bicarbonate are decomposed:

(NH₄)₂C0₃ = 2NH₃ + H₂0 + C0₂

NH₄HC0₃ = NH₃ + H₂0 + C0₂

The solution containing NH₄C1 is heated with milk of lime to liberate ammonia:

2NH₄CI + Ca(OH)₂ = CaCl₂ + 2H₂0 + 2NH₃

The ammonia is fed back into the production cycle. All the reactions occur at low temperatures. The sole waste product is the solution of CaCl₂, which, however, has some practical use. The Solvay process yields anhydrous soda of very high purity. In light of these advantages, the Solvay process replaced the Leblanc process in the late 19th and early 20th centuries. In Russia, the Ber-ezniki factory (1883) was the first plant to produce soda by the Solvay process.

NaHC0₃ is a white crystalline powder with a density of 2.16–2.22 g/cm³. Upon heating to approximately 50°C, the liberation of C0₂ begins, and at 100°-150°C, the substance undergoes complete decomposition, yielding Na₂C0₃. Aqueous solutions of NaHC0₃ are slightly alkaline. This salt was first described in 1801 by the German pharmacist B. Rose. In industry, NaHC0₃ is obtained by passing C0₂ under pressure through a saturated solution of Na₂C0₃ at 75°C:

Na₂C0₃ + C0₂ + H₂0 = 2NaHC0₃

Soda is one of the most important products of the chemical industry. It is used in large quantities in the glass, soap, paper, and dye industries; it is also used for softening water for steam boilers. Na₂C0₃ is the starting material in the production of NaOH, Na₂B₄0₇, and Na₂HP0₄. NaHC0₃ is used in the production of soft drinks, baked goods, and confectionery. In medicine, sodium bicarbonate, in tablet form, powders, and solutions, is used internally for neutralizing excess hydrochloric acid in gastric juice, for example, with gastritis. Externally, it is used for gargling and for washing the skin after contact with acids. With some diseases, solutions of NaHC0₃ are introduced intravenously. The salt is a component of many pharmaceutical agents.

In 1975, the USSR produced 4.7 million tons of calcined soda and 2.4 million tons of caustic soda.