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eye on • Reduce picking and sticking of tablets • Improve the mechanical proper - ties of tablets by increasing hard - ness and reducing friability • Improve oral bioavailability of poorl y soluble actives • Increase the bulk and tap den - sities of powders • Improve disintegration, and • Prepare moisture-absorbing dry granulations. Silica-based compounds Silica is a mineral of the general composition silicon dioxide, or SiO 2 . In addition to its many uses, it is known by many names, including silicic acid, fumed silica, colloidal silica, amorphous silicon dioxide, hydrophobic colloid silica, precipi - tated silica, and silica gel, among others. There are also compounds and complexes of silicon dioxide, e.g. magnesium aluminum silicate, magnesium trisilicate and magnesium silicate. Calcium silicate is another silica useful as a tablet and capsule excipient. Hydrated alumina silica, a naturally occurring mineral (e.g., kaolin), is used as an anti-diarrheal. It was also a traditional excipient but that use is archaic. More interesting are calcium silicate and the various compounds and complexes of silica metal oxides. Calcium silicate Calcium silicate—Chemical Ab - stracts Service number (CAS#) 1344- 95-2—occurs naturally, but the excipient-grade material is usually manufactured by reacting calcium oxide and silica in various ratios. Also known as calcium orthosi licate, cal - cium hydrosilicate, and calcium metasilicate, calcium silicate's general chemical formula is CaSiO 3 or Ca 2 SiO 4 . It is a white to off-white free-flowing powder that may be crystalline or amorphous. It occurs in various hydrated states. The USP-NF defines it is a compound of silicon dioxide and calcium oxide in a ratio of 0.5-to-20. It typically has a low bulk density and a large capacity to adsorb water, which makes it useful as an anti-caking agent. It is also used as an antacid. It is insoluble in water, and as an aqueous suspension, it has an alkaline pH ranging from 8.4 to 10.2. It is chemically stable but should be protected from moisture. Calcium silicate is non-toxic and is approved by the World Health Organization as a safe food additive. Inhalation of the dust particles may cause respiratory irritation, and when handled in large quantities it can irritate the eyes and skin. Gloves, eye protection, a respirator, and protec - tive clothing should be used when handling. It carries a generally recognized as safe (GRAS) listing and is included in the FDA's Inactive Ingredient Database (IID) for use in oral solid dosage forms. The main application of calcium silicate is as an anti-caking agent. Indeed, it has one the highest water- and oil-adsorption capabilities of any inorganic material, adsorbing liquids in quantities as high as four times its own weight. This makes it useful as a Tablets & Capsules January 2015 37 John A. McCarty McCarty Pharma Consultants excipients Silica-based excipients perform a variety of functions and are available from a number of sources and in a variety of types, grades, and uses. This column sum - marizes the chemical and physical properties, reactivity, regulatory status, pharmacology, toxicology, and ap - plications of calcium silicate and metal oxide silicas. A future column will address pure silica and the various forms and functions of silicon dioxide compositions. Silica, one of nature's most com - mon materials, is estimated to form more than 60 percent of the earth's crust. Many sands, clays, and rocks are largely composed of small silica crystals. Silica-based materials are also extremely versatile pharma - ceutical excipients. In semi-solid formulations, they are used to • Form transparent gels • Prevent syneresis in ointments • Thicken water-in-oil emulsions • Improve homogeneity of actives in suspensions, and • Increase the softening point of suppositories without changing the melting point. They have also been used in solid dosage forms to • Improve powder flow • Adsorb moisture • Stabilize pigment suspensions in tablet coating systems • Adsorb liquid ingredients, such as flavors