Apparatus 2, commonly known as the paddle method, was originally developed by Poole (1969) and was refined by scientists at the FDA for Drug Analysis in St Louis. The specifications for Apparatus 2 are identical with those for Apparatus 1 except that the paddle is substituted for the rotating basket.
Some of the dimensions and tolerances for the paddle are critical if consistent results are to be obtained from flask to flask. The USP specifies that the paddle must rotate smoothly without significant wobble. The arc of the paddle blade creates considerable flow and wobble has the effect of increasing the angular velocity at the paddle tips in a manner that couples with the fluid much more significantly than would a comparable wobble in the basket. The contours of the paddle blade must not include any sharp edges — at the tips for instance — that could produce turbulent instead of laminar flow patterns. The USP constrains wobble and vertical alignment with the axis of the vessel to within ±2.0mm.
The USP suggests that paddles ‘may’ be coated with polyfluorocarbon and most commercial paddles are accordingly coated. Such coating serves two purposes: it prevents corrosion and the introduction of unwanted ions into the media and it seals the joint where the blade is attached to the shaft, thus preventing the accumulation of traces of contaminants.
The coating process varies widely from supplier to supplier and if it is not done properly, the coating will begin to strip and flake after little use. The proper coating process costs about three times as much as cheaper methods but it will ensure against peeling or flaking. The proper process, however, requires considerable heat, thereby distorting the straightness of the shaft. The paddle shafts must therefore be hand-straightened after coating.
Because of the precise geometry required for the repeatability of the paddle method, the stirring paddle has been specified as a stainless steel device rather than a glass one with a detachable blade, largely because glass cannot be manufactured to such close cost specifications without incurring excessive cost.
One might be tempted to view the paddles as a crude stirring device similar to other agitators common to the laboratory but to think of the paddle used in dissolution Apparatus 2 in this fashion would be erroneous. It is a precisely defined, closed-tolerance instrument and must be carefully selected, constantly inspected and handled with the care used in working with any delicate instrument.