What is calcium bicarbonate deacidification? _Industrial additives

Calcium bicarbonate deacidification method is widely used in paper repair, but many restoration workers are confused about its deacidification principle, especially the fact that the pH value of the calcium bicarbonate deacidification solution itself is acidic. This article attempts to explain the basic principles and related issues of this deacidification method.

The main reactions involved in the calcium bicarbonate deacidification method

What is calcium bicarbonate deacidification? Why does calcium carbonate act as a buffer?

Calcium carbonate is commonly found in rocks and soil and exists in three crystal forms: calcite (trigonal crystal system), aragonite (rhombic crystal system), and vaterite (hexagonal crystal system). In the papermaking industry, calcium carbonate is often used to increase the whiteness of paper. In addition, as an acid-base buffer, it can also delay paper aging. This effect is mainly due to the fact that calcium carbonate can consume hydrogen ions through reaction (1) and reaction (2). Therefore, calcium carbonate is also used as an alkali storage substance to prevent paper acidification. However, since calcium carbonate is consumed during the reaction, if the paper continues to be affected by acidic factors (such as acidic pollutants, carboxylic acids formed by carbonyl oxidation, etc.), its action time as a buffer is also limited.

What is calcium bicarbonate deacidification? Reactions during deacidification of calcium bicarbonate

Calcium bicarbonate is an amphoteric compound, which can consume hydrogen ions through reaction (2) to exhibit the characteristics of an alkaline substance, and can also provide hydrogen ions through reaction (3) to exhibit the characteristics of an acidic substance.

When acidic paper is soaked in a calcium bicarbonate solution, the calcium bicarbonate reacts with unstable hydrogen ions in the paper (2), thereby removing them. At the same time, calcium bicarbonate forms calcium carbonate through reaction (3) and reaction (4), and calcium carbonate remains in the paper as an alkali storage substance. The calcium carbonate precipitate generated by the above reaction is mainly calcite. Among the three crystal forms of calcium carbonate, calcite is relatively stable.

What is calcium bicarbonate deacidification? Deacidification using chemical equilibrium

It should be noted that the above four reactions are all reversible, that is, depending on the initial concentration of the reactants, forward (=>) or reverse (<=) reactions may occur. For example, in a solution with a lower pH value, the concentration of hydrogen ions is higher, and reaction (1) is mainly forward, so the solubility of calcium carbonate is higher in acidic solutions. On the contrary, when the pH value of the solution is high, calcium carbonate cannot be decomposed because there are too few hydrogen ions. If calcium ions and bicarbonate ions are present at the same time, reaction (1) proceeds in the reverse direction and calcium carbonate is rapidly generated.

The calcium bicarbonate deacidification method consists in controlling the reaction directions of the above four reactions of mica according to the needs of different stages of deacidification. When preparing the deacidification solution, the forward reaction of reaction (1) is used to dissolve calcium carbonate in an acidic medium to form a calcium bicarbonate solution. In order to avoid the introduction of other substances, the acidic medium uses carbon dioxide and water, that is, the reverse reaction of reaction (2) is used to dissolve gaseous carbon dioxide in pure water to form hydrogen ions and bicarbonate ions.

What is calcium bicarbonate deacidification? How to prepare calcium bicarbonate deacidification solution?

To prepare the calcium bicarbonate deacidification solution, dissolve approximately 1 g of calcium carbonate solid in 1 L of carbon dioxide saturated aqueous solution.

Under carbonic acid conditions in the laboratory, a carbon dioxide-saturated aqueous solution can be prepared by passing carbon dioxide into the water, but this method is not convenient and easy to master. Under the conditions of the repair room, sparkling water with dissolved carbon dioxide can be used instead. When the amount of deacidification is not large, commercially available bottled sparkling water is sufficient. In this case, you should choose a brand with high bicarbonate content and low other chemical components. Another method is to use a household sparkling water machine, coupled with a filtered water system, which not only allows you to prepare large amounts of solution, but also allows you to better control the chemical composition of the solution.

Sparkling water is best kept refrigerated before use and avoid shaking to minimize the release of carbon dioxide gas when opened. After adding calcium carbonate, carbon dioxide bubbles will be generated. The bottle cap should be tightened immediately. After leaving it overnight, most of the calcium carbonate powder will dissolve.

The degree of solubility of calcium carbonate depends on the concentration of the carbon dioxide solution. Usually the dissolution is not sufficient, and some powder will remain at the bottom. This often confuses restorers because it can easily call into question the quality of the deacidification solution. If the calcium carbonate is not completely dissolved, does that mean the water used in the first place contained calcium? Or is there not enough carbon dioxide dissolved in the water? To evaluate whether the prepared deacidification solution is effective, it is recommended to take the supernatant and test whether the concentration of dissolved calcium carbonate exceeds 0.8 g/L (800 ppm). This test can be performed using the Calcium Hardness Titrets® Kit.

When stored airtight, the prepared solution can be stored for a long time. Tilt the bottle to obtain the supernatant liquid, which is an effective deacidification solution. This clear liquid should be used immediately after being obtained. Its pH value is between 5.5 and 6.

What is calcium bicarbonate deacidification? How to deacidify acidic solutions?

Soak the paper to be deacidified (usually with a pH value between 3 and 4) in the calcium bicarbonate solution for 10 to 20 minutes. When it is just taken out of the solution, the pH value of the paper is still acidic. The pH value will not rise until it is dried (generally 6 to 8, depending on the specific conditions of the paper).

pH during drying�The rise is because carbon dioxide is released into the air and the concentration of dissolved carbon dioxide in water decreases, causing reaction (2) to be dominated by the forward reaction, with hydrogen ions being consumed by bicarbonate; because hydrogen ions are consumed, reaction (1) is dominated by the reverse reaction Mainly, generate calcium carbonate. Therefore, part of the bicarbonate is converted into carbon dioxide, and the other part becomes calcium carbonate precipitate. These two reactions can occur simultaneously because bicarbonate ions are formed from carbon dioxide and calcium carbonate, and there is an excess of calcium ions.

What is calcium bicarbonate deacidification? Advantages of calcium bicarbonate deacidification method

Calcium bicarbonate is the mildest of all deacidification methods. The pH value of the paper gradually rises from acidic to weakly alkaline, and it avoids contact between the paper and strong alkaline substances. This is essentially different from those deacidification methods that use strongly alkaline solutions (pH>9). This is especially important when the paper contains pH-sensitive writing or drawing media. For example, some synthetic dyes used in seals will change color in different pH environments. For another example, for documents written with iron tannate ink, strong alkaline solutions will greatly increase the risk of ink fading. We are not advocating deacidification of iron tannate ink manuscripts here (as water-based processing can cause other side effects). However, if an aqueous solution is to be deacidified on an iron tannate ink manuscript, calcium bicarbonate solution is clearly superior to other strongly alkaline hydroxide solutions.