A mold formed reaction container includes a lower portion having four sides forming an open top parallelepiped structure and an upper portion in the shape of a threaded hollow cylinder over which a cap may be screwed. The lower portion is inserted in a square opening of an automatic chemical analyzer so that two of the sides thereof are in a radiant energy path. The opening of the analyzer has two slot indentions removed from two opposite sides thereof and the lower portion of the container has two corresponding key extensions therefrom to insure the proper two sides of the container are placed in the energy path. Further, one of the sides of the container not having a key extension has a label on a portion thereof and that portion intersects and blocks a second radiant energy path through the opening in such a manner that the insertion of the container can be detected.
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