Sesquicarbonate form

Sodium carbonate is another widely used solution for the conservation of ferrous artefacts. This maintains the pH in the zone of passivity (pH > 9.5) and is also said to act as an anodic inhibitor. A concentration of 3 g dm 3 (0.2 M) was used in the conservation of Holland 1 in Gosport Submarine Museum. It is advisable to use the sesquicarbonate form as it is far easier to dissolve than the simple carbonate. The use of this solution is said to avoid formation of solid sodium chloride in the rust. 

The dissociation of hypochlorous acid depends upon pH and, to a much lesser extent, temperature (6). At 25°C, it is - 0% at pH 5, about 50% at pH 7.5, and - 100% at pH 10, see Figure 1. Because of the acidity formed by chlorine gas, addition of soda ash (Na2C02) or sodium sesquicarbonate (Na2C03-NaHC03) is necessary to maintain the proper pH and to replenish alkalinity. 

In humid air, there is a slow decarboxylation of NaHCOs, where as sodium sesquicarbonate Na2C03 NaHCOs 2H2O is formed. ... 

Because complexation with Ca2+ results in insoluble CaCC>3, which deposits on items being washed, carbonate alone is not an effective builder system. In contrast, tripolyphosphate forms a soluble calcium salt, preventing deposition of insoluble salts. Sodium sesquicarbonate (Na2CC>3 NaHC03 -2 0) and sodium bicarbonate (NaHCCb) have not been used in LADD formulations, except where buffering action is needed. 

The common commercial and medicinal carbonate of ammonia is a sesquicarbonate, 2 NH, + 2 HO + 3 CO,. It is formed by heating a mixture of 2 parts of sal ammoniac and 3 of dried chalk, and sublimes as a hard crystalline volatile mass, having a strong smell of ammonia. When exposed to air, it loses ammonia, and falls to a powder of bicarbonate, NH 0, CO, + HO,... 

Soda ash when stored in the dry form needs some protection from the atmosphere. It is slightly hygroscopic, and users must remember that the sodium oxide is not fully carbonated. Pickup of both CO2 and water allows reversion to trona, which is the dihydrate of the sesquicarbonate, NaHC03-Na2C03-2H20. 

Processing of trona involves recrystallization to remove impurities and calcination to convert the bicarbonate portion to carbonate. There are two versions in commercial use. The older, or sesquicarbonate, process first hot-leaches the crushed ore to extract the mineral. This is followed by clarification, evaporation, and cooling. The refined trona is then calcined to form soda ash. The newer, or monohydrate, process first roasts the crushed ore to form a crude carbonate. This is dissolved and the solution evaporated to crystallize the monohydrate, Na2C03-H20. Mild calcination then yields the anhydrous soda ash. 

Precipitating builders include sodium bicarbonate and sodium carbonate, sodium sesquicarbonate (a mixture of the two), and the borate salts. These builders provide basicity and react with calcium and magnesium ions to form the insoluble carbonates or borates. These builders are not as effective as chelating builders. After repeated washes they leave deposits of carbonates mixed with soil on the textile being cleaned. They also may decrease the water absorbency of the textile with time. 

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