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Raw materials: Acids, salts, and other excipients Carbonate salt is a key material in effervescent formu- las because it's the source of CO 2 . The most commonly used carbonate salts are sodium carbonate (soda ash) and sodium bicarbonate (baking soda). See Table 1. Sodium carbonate has a lower percentage of CO 2 than sodium bicarbonate, and since it requires 2 moles of acid per mole of the salt, it's slightly more difficult to neutral- ize. That makes products formulated with sodium car- bonate a bit more stable than those containing only sodium bicarbonate. Even so, ingestible products typi- cally don't use sodium carbonate because the appropriate grades (USP, NF, and FCC) are not easy to find and can be expensive. Non-ingestible products, however, such as cleansers, often include it. Because sodium bicarbonate has a higher proportion of CO 2 than sodium carbonate and can easily break down and release water, products formulated with it tend to react/dissolve more quickly and be less stable than products that use sodium carbonate. Some manufacturers have developed proprietary processes that improve the stability of products that use sodium bicarbonate. Potassium, magnesium, and calcium carbonate salts can also be used. The other key component in effervescent composi- tions is the acid (Table 2). Citric acid is the most com- monly used. It's inexpensive, easily available, and highly soluble and it has good neutralizing power. It's available in several grades, which differ by particle size distribu- tion. The choice of grade hinges on compression and product-uniformity considerations. Fruit acids such as malic acid and tartaric acid are also suitable. They are more expensive than citric acid, but they often moderate citric acid's sharp flavor. Fumaric acid is used occasionally and, even though it's only diva- lent, it's actually a more efficient neutralizer than citric acid on a weight-to-weight basis. Fumaric acid, however, is much less soluble than citric acid and thus reacts more slowly. That's likely why products containing fumaric acid tend to be more stable than those that use citric acid. As with the carbonate salts, the acid you choose depends on how you want the product to perform and how you plan to manufacture it. The ratio of acid-to-car- bonate will also have an impact on the product. In gen- eral, the higher the acid-to-carbonate ratio, the faster the reaction. A higher proportion of acid will also ensure that the carbonate is completely reacted. At minimum, the acid must stoichiometrically balance the carbonate. Otherwise, some of it will remain unreacted and settle at the bottom of the dissolving vessel. A 1-to-1 ratio of acid to total carbonate (by weight) is common. However, highly reactive, highly soluble combinations may have ratios as high as 10-to-1. When formulating ingestible products, flavor must also be considered. If there is a high proportion of acid, the product could become too tart. Conversely, too much carbonate could make it too salty. The goal is to balance the ratio in combination with the flavoring and sweeten- ing ingredients to make the product appeal to consumers. If the product is a tablet, the acid-to-carbonate ratio affects how fast it dissolves. As the acid level increases, dissolution time generally decreases due to increased reactivity. The drawback to faster dissolution is a decrease in stability, so you'll need to strike a balance. Like all tablets, effervescent tablets require lubricants so they eject smoothly from the press. The most com- mon tablet lubricants, however—magnesium stearate and stearic acid—are of limited use. That's because virtually all effervescent tablets are dissolved in water before con- sumption and stearates are insoluble in water. Thus, they tend to float on the surface, creating an unattractive "oil slick." Sometimes this can be countered using emulsifiers, such as lecithin or dioctyl sodium sulfosuccinate. An alternative to adding stearate lubricants to the blend is to Effervescent products must be manufactured in a highly dehumidified environment to prevent them from picking up unwanted moisture. Table 1 Two common carbonate salts Table 2 Common acids in effervescent forms Tablets & Capsules July 2016 13 Na 2 CO 3 NaHCO 3 Molecular weight 106 84 Equivalent of acid to neutralize 2 1 CO 2 (%) 41.5 52.4 Citric Fumaric Malic Tartaric Molecular weight 192.1 116.1 134.1 150.1 Moles of acidity 3 2 2 2 Equivalent weight 64.05 58.05 67.05 75.05 Solubility (%) 68.6 1.1 55.8 >100