270 TITRIMETRIC METHODS 



5. Cap the tube with a short piece of clean rubber tubing plugged 

 at one end with a glass ball. The rubber should have no rough 

 surfaces which might rub off and contaminate the precipitate with 

 particles. Centrifuge for 5 min. at 2000 R.P.M. (radius 14 cm.). 



6. Draw off the supernatant liquid with the capillary tube 

 arrangement leaving less than 5 ju.1. covering the bottom of the tul)e 

 to a depth of about 1 mm. 



7. Repeat the washing process and dry the precipitate by heating 

 at 95° for 30 min. 



8. Add 45 [A. boiling hot phosphate buffer, pH 6.98, with a hand 

 pipette to dissolve the precipitate. Cap the tube and set aside for 

 6-12 hr. to insure complete soln. 



9. Add 15 ^1. 2 N potassium iodide with a hand pipette, introduce 

 a stirring "flea," cap the tube, and mix the contents. 



10. After 30 min. and up to 24 hr., titrate the liquid with 0.02 N 

 thiosulfate to the disappearance of the rose color. Carry out the 

 titration in the green light obtained by passing the light from an 

 electric bulb fixed on the back of the titration stand through a cell 

 filled with the green-light filter soln. Compare the end point color 

 to that of the tube of water placed beside the titration tube. 



SODIUM 



Lindner and Kirk ( 1938) described a method for the determina- 

 tion of sodium which can be applied to small samples containing 

 0.13-4.13 /xg. sodium. The standard error, on the average, is reported 

 as a few tenths of a per cent. The method depends on the precipita- 

 tion of sodium as sodium zinc uranyl acetate, isolation of the pre- 

 cipitate, reduction of the uranium with cadmium, and titration of 

 the reduced uranium with eerie sulfate. The procedure involves 

 several quantitative transfers, not of aliquots, but of total material. 

 Hence scrupulous care must be exercised to avoid loss at each of 

 these steps (see page 165). 



The procedure of Clark et at. (1942) might be adapted to the re- 

 quired micro level for histochemical work. In their procedure, the 

 sodium zinc uranyl acetate is simply dissolved and titrated with 

 sodium hydroxide using phenolphthalein as indicator. Each sodium 

 atom in the sodium zinc uranyl acetate is equivalent to nine of the 

 alkali molecules. 



