SODIUM 207 



8. Set the assembly in a wooden block. Dip the end of a clean 

 microfimnel (about 15 /A. capacity) such as pictured at the top of C 

 in Figure 76 into the magnesium uranium acetate soln. Fill the funnel 

 completely by pinching and releasing the attached rubber tubing. 

 Wipe off the outside, and slip the rubber tubing over the end of the 

 small capillary. Centrifuge for 6 sec, take off the funnel, and re- 

 centrifuge for 30 sec. Now repeat the process with two funnel fillings 

 of alcohol and two of ether. After evaporation of the ether, clean, 

 dry, yellow crystals should be seen on the filter mat. 



9. Transfer the stopper and capillaries to a larger tube calibrated 

 to contain exactly 6 ml. (C, Fig. 76). Introduce a little redistilled 

 water into the small capillary, remove the funnel, and centrifuge for 

 a few sec. 



10. Cut off the capillary about 1 cm. above the DeKhotinsky 

 cement, and proceed as before after attaching the funnel, filling the 

 funnel with water six to seven times. This should dissolve the pre- 

 cipitate and transfer the liquid completely into the tube which is 

 then filled with water to the 6 ml. mark. 



11. From a 0.1 or 0.2 ml. Mohr pipette drawn out to a fine tip, 

 add 0.1 ml. diphenylthiocarbazone soln. to each unknown tube, and 

 also to each of three colorimeter tubes containing 6 ml. of water and 

 of two standards, respectively. Mix the contents of all the tubes and 

 transfer the unknowns to colorimeter tubes. The blanks should be 

 golden yellow, and the solns. with 10-70 microgram per cent of zinc 

 should vary from orange to cherry-red. Make colorimetric measure- 

 ments immediately. With the Evelyn colorimeter use Filter 565. The 

 calibration curve is linear, but at least one standard should be run 

 every time an unknown is run since variations may occur even 

 though the reagent is prepared the same way every time. The blank 

 should be found to be negligible, since measurements carried out on 

 redistilled water substituted for the sample gave maximum values of 

 only ±0.6 microgram per cent of zinc. 



12. Since the inside of the very curved meniscus is used for 

 measurement of samples, make a vol. correction by the addition of 

 0.005 fA. for each meniscus. From the corrected vol., calculate the 

 final dilution (in a corrected sample of 0.2 lA. vol. the dilution is 

 30,000 times), and apply the following relation: 



Na concn. _ 23.00 Zn concn. in ^ilnfi-nn 

 in sample " QdM ^ final soln. ^ a^ution 



