202 CAPILLARY TUHE COLORIMETRY 



the liquid in the small tube is forced into contact with the silver 

 chromate in the larger tube. 



8. Warm the cement, withdraw and discard the smaller tube, cut 

 away any of the larger tube to which the cement is adhering, and 

 seal the open end in a flame. 



9. Drive the silver chromate back and forth through the liquid 

 by eight successive centrifugations. Continue the last centrifugation 

 for 5 min. 



10. Make a pipette from the smaller capillary tubing and draw 

 the supernatant fluid into it. 



11. Seal the larger end of the pipette, and force the liquid into 

 this end by centrifuging for 5 min. 



12. Examine the tube under the microscope (magnification 50X) 

 to be sure the fluid is free of silver chromate particles. If not, 

 transfer to another pipette and centrifuge again. 



13. Place a new piece of the smaller tubing 10-20 cm. on the 

 microscope stage with one end in the optical field and the other con- 

 nected to the water manipulator. 



14. Introduce a 2.0 mm. column of the chromate liquid obtained 

 in step 12, and after measuring its length accurately draw it in at 

 least 2 cm. from the end of the tube. 



15. Introduce a column of the diphenylcarbazide reagent four- 

 teen times the length of the chromate fluid, draw both columns in 

 2 cm. from the end, break off the portion of the tube containing the 

 liquid, seal the ends, and place in the centrifuge with the chromate 

 fluid uppermost. 



16. Measure 4.2 ml. diphenylcarbazide reagent into each of three 

 test tubes. Start the centrifuge containing the capillary tube, and 

 as quickly as possible measure into the test tubes 0.3 ml. of each of 

 three standard chromate solns. covering the range of concentration 

 within which the unknown lies. 



17. Centrifuge the capillary tube eight times, inverting it after 

 each centrifugation. 



18. Fill a piece of capillary tubing at least 3 cm. long, having the 

 same diameter as that containing the unknown, from each test tube 

 and seal the ends with plasticine. 



19. Compare the colors of the unknown and standards on a milk 

 glass plate under Daylite electric bulbs. 



20. The result obtained gives a first approximation of the chloride 



