272 TITRIMETRIC METHODS 



4. After standing 2-4 lir., add a drop of water and centrifuge for 

 at least 5 min. at about 5000 R.P.M. 



5. Transfer the supernatant quantitatively, with washing of the 

 precipitate, to a porcelain titration dish (Fig. 44, page 169) using 

 a capillary pipette. 



6. Evaporate to a small vol. just short of dryness, and while 

 still warm, add 1 ml. zinc uranyl acetate reagent. Stir and warm for 

 about 5 min. 



7. Cover with a Petri dish and let stand 12-24 hr. for complete 

 crystallization. If reagent crystallizes during this period, place a 

 container of water under the Petri dish to prevent evaporation from 

 the sample. 



8. Suck off the supernatant through a sintered-glass filter stick 

 covered with fine asbestos (Fig. 61). Wash the porcelain dish and 

 precipitate thoroughly with the alcoholic wash soln. 



9. Remove the asbestos to the porcelain dish with a drop of 5% 

 sulfuric acid, which is also used to wash the end of the filter. Rinse 

 with a little water. 



10. Insert the cadmium spiral, warm and stir for 5 min.; then 

 remove and thoroughly wash the cadmium with water. 



11. After cooling for 5 min., titrate the reduced uranium with 

 eerie sulfate using one of the indicators. 



12. Run a blank determination on the reagents by repeating the 

 entire procedure without a sodium sample. 



CALCIUM 



Two methods for the microestimation of calcium have been given 

 by Siwe (1935b) and Lindner and Kirk (1937a) have described still 

 another. All the methods employ precipitation of calcium as the 

 oxalate. In one of Siwe's methods the precipitate is dissolved in acid, 

 an excess of permanganate is added followed by potassium iodide, 

 and the iodine liberated by the excess permanganate is titrated with 

 thiosulfate. In the other method the oxalate is converted to carbon- 

 ate by heating, an excess of acid is added, and the excess titrated 

 with alkali. The Lindner and Kirk procedure uses an excess of eerie 

 sulfate to react with the oxalate dissolved in acid, followed by titra- 

 tion of the excess with ferrous ammonium sulfate. Kirk and Tomp- 

 kins (1941) compared oxalate titrations by different methods and 



