162 METHODS FOR THE ANALYSIS OF BLOOD 



the filter, the suction is increased and after five or ten minutes 

 the mat is quite dry. 



The crucible is returned to the beaker and 10 c.c. of 0.2 N 

 hydrochloric acid are run into the crucible and allowed to percolate 

 through. The beaker is covered by a piece of rubber dam held 

 in place by a rubber band or put in a desiccator over water, in 

 order to prevent evaporation. After standing several hours or 

 over night the asbestos is thoroughly stirred up in the acid and 

 the whole suspension transferred to a tube and centrifuged. 

 An aliquot portion of the clear supernatant fluid (usually 6 c.c.) 

 is pipetted off and used for the colorimetric determination. 



Colorimetric Comparison. Small Nessler tubes approximately 

 120 mm. long and 10 mm. internal diameter are used. They are 

 graduated at 10 c.c. and the bores of the tube should be such that 

 the graduation on each tube should be within 2 mm. of the gradu- 

 ation on any other tube in the set. Round-bottomed tubes may 

 be used, but those with flat bottoms are preferable. Aliquot por- 

 tions of the oxalate solution are measured into the tubes. Por- 

 tions of a standard calcium oxalate solution are measured into other 

 tubes. Then to each tube are added 2 c.c. of ferric thiocyanate 

 solution accurately measured with an Ostwald pipette. Each 

 tube is filled to the mark with 0.2 N hydrochloric acid, and the 

 contents are mixed by inverting several times, the ends of the 

 tubes being closed with a clean rubber stopper. 



Color comparisons are made by looking lengthwise through the 

 tubes against a dull white background. Readings are first made 

 against standard oxalate solutions differing from each other by 

 0.5 c.c., interpolating when necessary. Extra standards may then 

 be made up to obtain more exact results. 



Calculation. The calculation is simple, as, for example, 2 

 c.c. of serum were used, and a 6 c.c. aliquot portion of the oxalate 

 solution was taken. The color in the comparison tube, after ' 

 adding thiocyanate and diluting to the mark, was found to match 

 that corresponding to 6.3 c.c. of the standard oxalate solution; 

 then 



6.3X.02XV-X 1 f a =10.5 mg. per 100 c.c. of serum. 



It is necessary to run blank determinations on the reagents 

 used and to make allowance for any blank found to be present. 

 Usually a blank amounting to more than 0.1 c.c. of the standard 



