Allowable Variations in Apparatus 1
Type 316 stainless steel is very resistant to corrosion but that does not mean it is totally immune to attack in corrosive media, particularly dilute acid. A pure gold plating up to 2.5µm thickness is therefore allowed by the USP for use in acid media or where the active drug reacts with the plain steel basket. If gold is used, the shaft as well as the basket should be coated. The economics may or may not justify the extra expense, however, as uncoated baskets deteriorate from contact with acid they become less rugged and are more likely to distort during assembly on the drive rod. The only remedy then becomes replacement baskets. Other coatings are available and have been used where equivalence can be shown, these include PTFE and Titanium.
The USP Method 1 requires a 40-mesh screen unless otherwise specified in the monograph. Other meshes can be investigated to solve individual problems. One of the most criticised characteristics of the basket method involves the clogging of the mesh openings by particles or excipients or the random release of particles through the mesh to the bottom of the flask. Baskets with a wide variety of mesh openings are commercially available and if the deaggregated particle size is consistent, a variation of mesh size may solve some difficult dissolution problems.