206 CAPILLARY TUBE COLORIMETRY 



PROCEDURE 



1. As in the procedure for chloride (page 201), mount a 15 cm. 

 length of capillary tubing (0.35 mm. inside diameter, and check the 

 outside diameter with a stage micrometer — it should be just 0.5 mm.) 

 on the microscope stage and fix a 15 cm. steel rule beside it so that 

 the zero on the rule is opposite the 35 mark on the micrometer scale. 



2. To one end of the tubing attach a water manipulator and place 

 the other end over the 30 or 40 mark of the stage micrometer so that 

 a 0.2-0.4 fA. sample will be in the center of the optical field. 



3. Introduce a 2-4 mm. column of sample and draw it into the 

 tube just far enough to give a fully curved meniscus. Measure the 

 length of the sample column and pull it in about 5 mm. 



4. Depending on the size of the sample, introduce rapidly from 

 a rather coarse capillary pipette, just filled with freshly filtered re- 

 agent, a 30-40 mm. column of zinc uranium acetate reagent. Meas- 

 urement of the reagent column need not be precise but do not move 

 the column back and forth, since evaporation of the liquid will give 

 high results. Draw the column in about 10 mm. 



5. Cut off the portion of the tube containing the liquids, seal both 

 ends in a microflame without heating the liquids, centrifuge and in- 

 vert the tube ten times, allow to stand at room temperature for 10 

 min., and then centrifuge and invert five times more. During the 10 

 min. intervals examine the filter under the microscope and repack 

 it gently. 



6. Examine the empty end of the precipitation capillary to make 

 sure no crystals have remained there. Cut off the end of the tube 

 containing the precipitate and insert it into the filter tube so that 

 the open end is about 6 mm. above the filter mat. Seal the two tubes 

 together with DeKhotinsky cement as shown in Figure 76, taking 

 care to avoid heating the reagent. Insert the tubes in a small hole in 

 a rubber stopper and fit into a test tube as in A of Figure 76. 



7. Lower the assembly into a centrifuge cup by means of rubber- 

 tipped forceps, and spin rapidly for about 15 sec. Examine the capil- 

 lary and filter; usually a little liquid is found above the filter. Cut 

 off the sealed end of the capillary without disturbing the assembly, 

 centrifuge again for about 6 sec, and again inspect. No fluid should 

 appear above the mat. Centrifuge for 2 min. more to insure com- 

 plete draining. 



