276 TITRIMETRIC METHODS 



PROCEDURE 



1. If sample must be ashed, place in a small platinum crucible 

 and heat in a micro furnace not exceeding 450°. Dissolve ash in a 

 drop of 0.1 A'^ hydrochloric acid and transfer the liquid to a porcelain 

 titration dish (Fig. 44, page 169) using a pipette. Rinse crucible and 

 pipette with water to make the transfer quantitative (see page 165) . 



2. Warm the acidified soln. of the sample in the titration dish, in- 

 sert a thread stirrer, and add, with mixing, a vol. of 4% oxalate 

 soln. equal to that of the sample. 



3. Add 0.1 ml. saturated sodium acetate and continue heating for 

 5 min., adding a little water to keep up the vol. 



4. Let stand overnight protected from dust. 



5. Filter the calcium oxalate with a micro external filter stick 

 (1.5 mm, diameter, and covered with a thin layer of asbestos). Use 

 about 30 drops of wash soln. to rinse the dish and the precipitate on 

 the filter (see page 177). 



6. Transfer asbestos pad to titration dish by using a drop of 6 A^" 

 sulfuric acid. Wash the filter with a few drops of the sulfuric acid 

 followed by a few drops of water. 



7. Heat the dish for 2 min. while stirring to dissolve the oxalate, 

 and then let cool for 5 min. 



8. Add a measured excess of standard eerie sulfate, stir for 3 min. 

 and add 5 /xl. of the phenanthroline indicator. Titrate the excess with 

 ferrous ammonium sulfate. 



9. Run a control on the reagents alone. 



IRON 



Kirk and Bentley ( 1936) developed a micro method for the esti- 

 mation of iron which involves the reduction of the iron by cadmium 

 amalgam, addition of a measured excess of eerie sulfate, and titra- 

 tion with ferrous ammonium sulfate using phenanthroline ferrous 

 sulfate as the indicator. The method is adapted to the measurement 

 of 2-15 fig. iron. With blood or simple solutions a mean error o( 

 1-2%, and a maximum error of 4%, has been reported. 



Ramsay (1944) reported that in his laboratory all reductions 

 with pure metals and amalgams gave blanks amounting to several 

 micrograms of iron. Accordingly, he employed a titanometric method. 

 Small amounts of copper have been found to interfere with the 



