204 CAPILLARY TUBE COLORIMETRY 



to saturate the soln. Shake well and filter through quantitative 

 paper before using. 



Magnesium Uranium Acetate Reagent. According to Blanchetiere 

 ( 1923) : Dissolve 100 g. uranium acetate in 60 g. glacial acetic 

 acid and enough water to make 1 1. Dissolve 333 g. of magnesium 

 acetate in 60 g. glacial acetic acid and enough water to make 1 1. 

 Combine equal vol. of the two solns. Filter the reagent through 

 quantitative filter paper before use. 



Diphenylthiocarbazone solution. Prepare immediately before use 

 by shaking 100 mg. of the compound {Eastman Kodak) in 5 ml. 

 of the sodium hydroxide soln. for 3 min. in a glass-stoppered ves- 

 sel the ground surfaces of which are thinly coated with paraffin. 

 Filter off the excess reagent on quantitative filter paper which 

 has been washed in redistilled water and dried before use. Dilute 

 1 vol. of the filtrate with 4 vol. of the sodium hydroxide soln. A 

 considerable variation in the quality of the compound from one 

 lot to the next has been observed. 



Zinc Standards. Prepare pure sodium zinc uranium acetate by pre- 

 cipitating the sodium of pure sodium chloride with the zinc ura- 

 nium acetate reagent. Dissolve 0.235 g. of the triple salt in redis- 

 tilled water and make up to 1 1. This stock soln. contains 1 mg. zinc 

 per 100 ml. and it will keep for years in a Pyrex bottle in the dark. 

 Frequently prepare dilute standards containing from 10 to 70 

 microgram per cent of zinc by diluting the stock soln. with re- 

 distilled water. 



PREPARATION OF FILTERS 



1. Cut 3 cm. lengths of capillary tubing, 0.6 mm. internal di- 

 ameter. Cut ends squarely or the funnel openings to be made later 

 will be off center. 



2. Partially seal one end of each piece of tubing by twirling in a 

 microflame. Use a microscope to observe the result. The opening 

 should be funnel shaped, about 0.1 mm. at the top and less at the 

 bottom (Fig. 76). 



3. Prepare paper pulp by teasing apart the filter paper in re- 

 distilled water and drying at 105°. Bits teased off the dried pulp are 

 placed in the filter tubes and pushed down into the funnel end by a 

 thin capillary tube about 6 cm. long sealed at one end. Pack each bit 



