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eye on phous silica, colloidal silicon dioxide USP-NF, silica colloidal anhydrous Ph.Eur., light anhydrous silicic acid JP, and colloidal anhydrous silica BP. It has been listed under Chemical Abstracts Service number (CAS#) 7631-86-9, which identifies it as sil- ica. It is also listed under CAS# 112945-52-5 as silica, amorphous, fumed and crystalline-free. This syn- thetic material is prepared by hydro- gen-oxygen flame hydrolysis of chlorosilianes, e.g. silicon tetrachlo- ride, at 1,800°C. Upon rapid cooling from the molten reaction state, the particles collide and sinter together as aggregates. Upon further cooling, these branched, chain-like aggregates become entangled to form agglomer- ates that constitute a fine amorphous powder, typically with particles of 10 to 200 microns. This powder is char- acteristically very light and can easily produce dust. It is bluish-white, taste less, and odorless. It has a very high specific surface area: 50 to 400 square meters per gram (m 2 /g), which makes it useful as a moisture adsor- bent and anticaking agent. It is insol- uble in water but will form colloidal dispersions with a pH range of 3.5 to 5.5. It is chemically stable and inert but should be protected from mois- ture and volatile substances, as these can be readily adsorbed. Agglomerates of fumed silica are irregular in size and shape and do not pack well, leaving a considerable amount of void space between parti- cles. As a result, fumed silica takes the form of a fluffy, low-density powder with its finer agglomerates producing its characteristic dustiness. Densified or compacted grades are produced by mechanical densification which removes air and reduces the amount of dust from fine agglomerates: i.e., less than 20 microns. Den si fication does not alter the physio-chemical proper- ties such a specific surface area, but does increase the powder's tap density, making handling easier. Even though these materials have a high specific surface area, they typically have lim- ited liquid adsorption capacity. However granulated fumed silica, due to its high porosity and large specific surface area, has an increased capacity to adsorb liquids. Its spherical particle also imparts excellent flow, higher density, and results in less dust. Table 1 lists some of the common pharmaceutical-grade fumed silicas. Suppliers include Evonik Industries, Cabot, Wacker Chemie, and EMD Millipore. Precipitated silica Precipitated silica is also known as silica gel, synthetic precipitated silica, amorphous precipitated silica, syn- thetic amorphous silica, silicon diox- ide USP/NF, silica colloidal hydrate Ph.Eur, and hydrated silicon dioxide JP. CAS# 112926-00-8 has been assigned to amorphous precipitated silica. It is also listed as silica CAS# 7631-86-9. When obtained by the acidification of sodium silicate solu- tion by a mineral acid, the product is called silica gel. Precipitated silica is obtained by insolubilizing dissolved silica from a sodium silicate solution. Sulfuric acid is typically used to pro- duce precipitated silica; however, the overall pH of the mixture is kept alka- line, as the precipitation reaction is more vigorous under basic conditions. Tablets & Capsules March 2015 37 John A. McCarty Pharma Consultants excipients In this edition of the column, the second in a three-part series about using silicon dioxide (silica) as a pharmaceutical excipient, John McCarty discusses the various forms and grades of pure silica. The next installment will address the numerous pharmaceutical applications and functions of silica. As discussed in the previous col- umn, silica—in the form of quartz—is one of the most common minerals in nature and is an extremely versatile pharmaceutical excipient [1]. This article discusses the various forms, types, sources, and grades of pure sili- con di oxide. It also summarizes its chemical and physical properties, re - ac tivity, reg u latory status, pharmacol- ogy, toxicology, and numerous uses as an excipient. Silica is found naturally in several mineral forms and can be produced synthetically. Notable examples in - clude fused quartz, crystal, fumed sil- ica, silica gel and aerogels. The forms of silica most useful as pharmaceutical excipients are fumed silica and precipi- tated silica. Fumed silica is modified to increase its density by either com- paction or granulation. When fumed silica is chemically treated to make it less hydrophilic, it is referred to as hy - dro phobic colloidal silica. Manu fac - turing precipitated silica entails modi- fying the silica particles' specific surface area, size, and pore volume so it can provide a different set of func- tional characteristics. Fumed silica Fumed silica is also known as pyro genic silica, synthetic amor-

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