Moisture regulation 179 9.5.3 Calcium sulfate Certain salts, sulfate salts in particular, will take on water of hydration in fixed mole proportions. Some salts such as calcium sulfate have multiple hydration states. Anhydrous calcium sulfate can take on 2 mole of water which becomes the commonly known 'Plaster of Paris. Likewise it can take on two moles of water this is known as gypsum. Many salts then can take on water of hydration. Their utility as moisture sorbents depends on the kinetics of adsorption under the conditions of use. Some experimentation may be required to make the right choice 9.6 Irreversible adsorption Closely related to water of hydration is the addition of water to alkali metal and transition metal oxides. here water reacts with the oxide to form a compound. The reaction may be reversed but only with an input of sufficient to decompose the compound Since decompositic ars at perature well above any possible conditions of use, adsorption is considered irreversible 9.6.1 Calcium oxide The most frequently encountered example of this is calcium oxide. It is the product of high temperature decomposition of limestone, CaCO3. CaO is principally used as agricultural lime and is known as quicklime. Calcium oxide reacts with water as follows CaO+H2O→Ca(OH)2 The resulting product, calcium hydroxide, is likewise used for agricultural purposes and is known as slaked lime. To reverse this reaction requires raising the temperature to nearly 600oC. Its use as a desiccant depends on the same reaction. The great amount of energy required to reverse the reaction makes this irreversible in a practical sense. As may be inferred, the product is quite alkaline. Although it is technically a corrosive product and irritating to the skin, it is not particularly hazardous in use due to its very low solubility(about 0.06g/100cc of water) Calcium oxide is used as a desiccant principally where extremely low residual moisture is required. In a closed system it is possible to reduce moisture to a few parts per million with a suitable amount of calcium oxide. And it is a fairly efficient desiccant as well, capable of absorbing about 28% of its dry weight as water. In addition it is specific for water. Calcium oxide is used for very demanding packaging applications but, more particularly, for sealed electronic devices that are expected to last for years It must be kept in mind that calcium hydroxide is also a good carbon dioxide absorber. Therefore if calcium oxide is used as a desiccant in a carbon dioxide environment it will absorb carbon dioxide as well, releasing water in the process9.5.3 Calcium sulfate Certain salts, sulfate salts in particular, will take on water of hydration in fixed mole proportions. Some salts such as calcium sulfate have multiple hydration states. Anhydrous calcium sulfate can take on ½ mole of water which becomes the commonly known ‘Plaster of Paris’. Likewise it can take on two moles of water; this is known as gypsum. Many salts then can take on water of hydration. Their utility as moisture sorbents depends on the kinetics of adsorption under the conditions of use. Some experimentation may be required to make the right choice. 9.6 Irreversible adsorption Closely related to water of hydration is the addition of water to alkali metal and transition metal oxides. Here water reacts with the oxide to form a separate compound. The reaction may be reversed but only with an input of energy sufficient to decompose the compound. Since decomposition occurs at a tem￾perature well above any possible conditions of use, adsorption is considered irreversible. 9.6.1 Calcium oxide The most frequently encountered example of this is calcium oxide. It is the product of high temperature decomposition of limestone, CaCO3. CaO is principally used as agricultural lime and is known as quicklime. Calcium oxide reacts with water as follows: CaO ‡ H2O ! Ca(OH)2 The resulting product, calcium hydroxide, is likewise used for agricultural purposes and is known as slaked lime. To reverse this reaction requires raising the temperature to nearly 600ºC. Its use as a desiccant depends on the same reaction. The great amount of energy required to reverse the reaction makes this irreversible in a practical sense. As may be inferred, the product is quite alkaline. Although it is technically a corrosive product and irritating to the skin, it is not particularly hazardous in use due to its very low solubility (about 0.06g/100cc of water). Calcium oxide is used as a desiccant principally where extremely low residual moisture is required. In a closed system it is possible to reduce moisture to a few parts per million with a suitable amount of calcium oxide. And it is a fairly efficient desiccant as well, capable of absorbing about 28% of its dry weight as water. In addition it is specific for water. Calcium oxide is used for very demanding packaging applications but, more particularly, for sealed electronic devices that are expected to last for years. It must be kept in mind that calcium hydroxide is also a good carbon dioxide absorber. Therefore if calcium oxide is used as a desiccant in a carbon dioxide environment it will absorb carbon dioxide as well, releasing water in the process. Moisture regulation 179