218 CUVETTE COLORIMETRY 



A diajihragm is placed in front of the cuvette to obtain a light 

 beam confined to a cross section of less than 2X2 mm. A beam of 

 this size can pass through the liquid without touching the meniscus 

 or the walls of the cuvette. The diaphragm (type A, Fig. 77) has 

 a metal disc the size of a penny through which a 1.0 to 1.4 mm. hole 

 is drilled about 1 mm. off center. Before the disc is fastened to the 

 metal sheet, it is held in the oi)ening from which the light enters 

 the cuvette and turned until the beam passes precisely in the middle 

 between the walls of the chamber when the cuvette is in place. The 

 disc is soldered at this angle to the sheet of metal (about 6X9 

 cm.) so that the hole coincides with a 3-4 mm. hole in the sheet 2.5 

 cm. from one end. The top of the sheet is bent at a right angle to 

 form a flange which lies on the top of the instrument. Wooden 

 blocks are used to raise the cuvettes so that the light beam just 

 misses the bottom of the chamber. The diaphragm is inserted and 

 removed by loosening the bolts which hold the phototube housing. 

 The carriage for the cuvettes should be oriented to bring the cuvettes 

 as near the diaphragm as jwssible. The cuvettes are numbered and 

 always set in the holder with the same orientation. 



The type B diaphragm (Fig. 77) contains a sliding strip of brass 

 with pinholes which can move in a channel cut in the sheet metal. 

 The diaphragm is inserted between the cuvette carriage and the 

 body of the instrument and the sliding strip is moved until a pinhole 

 coincides with the center of the cuvette. The stop on the strip is 

 then adjusted with a bolt so that the pinhole can be brought to the 

 same position each time. The different-sized pinholes can be brought 

 into position without disturbing the adjustment. Blocks are used 

 to raise the cells as with the type A diaphragm. By removing the 

 brass strip the instrument can be used with macrocuvettes without 

 disturbing the metal sheet. 



To obviate the effect of "play" in the cuvette carriage, the cells 

 should be moved into position from the same direction. In use, the 

 microcuvettes are left mounted in the carriage. Samples are intro- 

 duced with fine-tipped pipettes, and removed by suction with fine 

 tipped glass tubes. A macro cell may be used in the first position in 

 the carriage for the solvent or other blank solution.* 



*See Bibliography Appendix, Ref. 32. 



