Barium sulfate size control

Because the regions of the alimentary tract vary widely ia pH and chemical composition, many different commercial formulations of barium sulfate are available. The final preparations of varyiag viscosity, density, and formulation stabiUty levels are controlled by the different size, shape, uniformity and concentration of barium sulfate particles and the presence of additives. The most important additives are suspending and dispersiag agents used to maintain the suspension stabiUty. Commercial preparations of barium sulfate iaclude bulk and unit-dose powders and suspensions and principal manufacturers are E-Z-EM (Westbury, New York), Lafayette-Pharmacol, Inc. (Lafayette, Indiana), and Picker International, Inc. (Cleveland, Ohio). 

Figure 5.7 shows particle size distribution of synthetic barium sulfate. The characteristic feature of these curves is their steepness which denotes a very narrow particle size distribution which was obtained by controlling the conditions of precipitation. The development of this kind of particle size distribution in a small particle sized filler allows for substantial improvement in the gloss of coatings. 

Precipitated Extenders. Precipitated extenders such as calcium carbonate (calcium carbonicum praecipitatum), barium sulfate (blanc fixe), and silica have been known for a fairly long time. When natural minerals are micronized, a minimum particle size limit is encountered. Lower particle sizes (< 1 pm) can only be obtained with considerable technical effort and expense. Extenders with finer particle sizes are produced synthetically by precipitation. The structure of one and the same extender can be modified by controlling the precipitation conditions. Particularly pure and thus bright extenders can be produced by a suitable choice of starting substances [4.12]. 

Like cements, the elemental composition is determined by XRF or AAS techniques. The XRF bead is made using lithium tetraborate at 1050°C. Sulfide content cannot be determined by XRF. Sulfite, SO3 , and sulfate, S04 , are safely analyzed by XRF. Na2C03 -I- K2CO3 fusion is carried out for Ca, Mg, Fe, and A1 analysis by AAS. Lanthanum chloride is used as a sulfate interference suppressant. Gravimetric sulfate determinations are also carried out by precipitation as barium sulfate. The Leco Carbon-Sulfur Analyzer can also be used for quality control purposes. The fluoride is determined by XRF or a pyrohydrolysis method. The measurement of particle size distribution is carried out in a manner similar to that for cements and clays. 

---