280 TITRIMETRIC METHODS 



formic acid in ether. Transfer the ether extracts back to the digestion 

 flask and completely evaporate the ether over warm water. Digest 

 again after the addition of 0.5 ml. sulfuric acid and a drop or two 

 each of the nitric and perchloric acids. Cool the clear digest and add 

 5 ml. water. 



If too little copper is present to interfere, omit this step. (The 

 separation of copper is not necessary for blood analysis.) 



3. Add 1 ml. 50% potassium thiocyanate to the diluted digest, 

 bubble carbon dioxide through the soln., open stopcock H, and deliver 

 the titanous sulfate until the last trace of pink just disappears. To 

 control the rate of delivery adjust the height of the liquid column in 

 the tube leading from A to B, and carry out the last of the titration 

 with C closed and B open. 



PHOSPHORUS 



A method for the measurement of phosphorus in the range of 0.5- 

 10.0 jxg. was given by Lindner and Kirk (1937b). The method con- 

 sists of conversion of the phosphorus to phosphate by ashing, precipi- 

 tation as ammonium phosphomolybdate, isolation of the precipitate 

 by filtration through a sintered-glass filter stick with an asbestos 

 mat, solution of the precipitate in a measured excess of sodium hy- 

 droxide, addition of an equivalent amount of acid, and finally titra- 

 tion of the excess acid with alkali to a phenolphthalein end point. 

 The end point is not sharp and the precision of the method leaves 

 much to be desired. 



In an early review, Glick ( 1935a) reported that, with Linderstr0m- 

 Lang and Holter, he had developed a method for inorganic phosphate 

 capable of determining 5 /xg. phosphorus with an error of less than 

 1%, and 1 )ug. with an error of about 5.5%. In this method the phos- 

 phate was precipitated as magnesium ammonium phosphate, the pre- 

 cipitate was centrifuged, washed with alcohol and then acetone, 

 and dissolved in an excess of standard hydrochloric acid. The excess 

 acid was titrated in an acetone medium with ammonium acetate 

 using naphthyl red indicator. Phosphoric acid does not ionize in ace- 

 tone ; therefore only the free hydrochloric acid is titrated. The acetate 

 ion is used like an alkali in this case since it will react with hydrogen 

 ions to form acetic acid which, in acetone, does not ionize. 



Both of these titrimetric methods lack the ease and simplicity 



