200 CAPILLARY TUBE COLORIMETRY 



Zinc Sulfate-Sodium Hydroxide Supernatanls 



1. For frog plasma chlorides a 3.0 mm. column of plasma is 

 drawn in 1 cm. from the end of a 10-12 cm. capillary tube followed 

 by separate columns of 6.0 mm. of 0.1 N sodium hydroxide and 2.1 

 cm. of 0.64% zinc sulfate (ZnS04.7H20, freed from excess acid by 

 three recrystallizations from w^ater) . 



2. Draw in the columns so that at least 2 cm. from the end of 

 the tube is empty, break off the portion of the tube containing the 

 liquids, and seal both ends in a flame. 



3. Place in the centrifuge with the plasma uppermost and mix the 

 liquids by four centrifugations. 



4. Immerse the tube for 30 sec. in water at 90-95° and centrifuge 

 once for 5 min. 



5. Cut off the tube above the fluid and then cut off the part con- 

 taining the protein precipitate. The protein-free fluid is then ready 

 for analysis. 



CHLORIDE 



By the use of s?/m-diphenylcarbazide ( Cazeneuve reagent) West- 

 fall, Findley, and Richards ( 1934) increased the sensitivity of 

 Isaac's (1922) method for the determination of chloride and then 

 adapted it to capillary tube colorimetry. Their procedure allows 

 chloride determination in a fraction of a fA. of liquid containing 1 /xg 

 or less of sodium chloride with an average error of under 3.0%. The 

 principle of the method is that dry silver chromate will react with 

 chloride to precipitate silver chloride and leave the chromate ion, 

 which can be estimated by its yellow color. However, a much more 

 intense purple-red color will develop in the presence of diphenyl- 

 carbazide. For other methods see pages 224 and 281. 



Westfall, Findley, and Richards Method for Chlorides 



SPECIAL REAGENTS 



Potassium Chromate Standards. Dissolve 3.321 g. of pure dry 

 potassium chromate (corresponding to 2.00 g. sodium chloride) in 

 1 1. distilled water. Dilute to prepare standard solns. in the range 

 10-70 milligram per cent sodium chloride at 2.5 milligram per 

 cent intervals. 



