274 TITRIMETRIC METHODS 



a reaction tube, add a very small drop of ammonium hydroxide, and 

 drop in a mixing "flea." 



2. Warm to about 50-60°, mix well, and add 50 ix\. ammonium 

 oxalate soln. Let stand 2-6 hr. 



3. Centrifuge, remove the supernatant with a pipette, and wash 

 the precipitate twice with 100 lA. portions of ice-cold water. 



4. Centrifuge 15-30 min. and pipette off the supernatant. 



5. Dissolve the precipitate in 25 fxl. of the nitric acid by stirring 

 and heating for 30 sec. in boiling water. 



6. Add 50 /xl. permanganate soln., and after 1-1.5 min. add 25 /xl. 

 iodide soln. 



7. Titrate the iodine liberated with thiosulfate until the yellow 

 color is almost gone. Then add a very small amount of starch soln. 

 and continue the titration until the blue color just disappears. 



8. Run a control by titrating as above but omitting oxalate from 

 the mixture. 



NOTE : It has been the experience of the Carlsberg Laboratory workers that 

 a serious loss of iodine can occur by evaporation in iodometiic methods 

 unless a Hquid seal is placed across the lumen of the reaction tube. There- 

 fore it would be advisable to follow the technique given on page 311, 

 steps 4-5. 



Siwe Acidimetric Method for Calcium 



SPECIAL REAGENTS 



Ammonium Hydroxide (cone). 

 5% Ammonium Oxalate. 

 0.01 N Hydrochloric Acid. 

 0.01 N Sodium Hydroxide. 

 1% Phenolphthalein. 



PROCEDURE 



1-4. Same as steps in the preceding permanganate method. Use 

 a heat-resistant glass reaction tube. 



5. Heat on a sand bath or in an oven at 550-600°, not exceeding 

 the latter, for 30 min. 



6. After cooling, pipette in 50 /xl. 0.01 N hydrochloric acid and 

 heat for 30 sec. in boiling water. 



7. Add a very small amount of phenolphthalein and titrate with 

 0.01 A'' sodium hydroxide to the first pink color. 



