D. D. VAN SLYKE 



Cholak and Hubbard (36). The latter describe the appHcation of the 

 procedure to determination of minute amounts of cadmium in blood 

 and urine, and compare it with polarographic and photometric pro- 

 cedures for this purpose. An apparatus devised by the American 

 Cyanamid Company, not yet in general production, determines 

 potassium in serum in a few minutes with an error not over =*=5%. 



Micro Diffusion Analysis 



Conway (41) has devised a simple chamber for the determi- 

 nation, primarily, of ammonia, but applicable also to estimation of 

 other volatile substances that can be set free by quantitative reactions 

 and transferred by diffusion at atmospheric pressure to absorbing 

 solutions in which the diffused substance can be measured, by titration, 

 photometry, or otherwise. The apparatus consists of a flat, cylin- 

 drical dish, of 60-mm. diameter and 10 mm. high (inner measure- 

 ments), from the inner bottom of which rises a ring of 33-mm. diameter 

 and 5 mm. high. The top of the dish is ground accurately flat, so that 

 when lubricated with vaseline or other proper material it can be closed 

 gas tight by a flat glass cover. The chamber consists, therefore, of an 

 outer ring, about 60 mm. wide, surrounding an inner low cylinder; 

 when the chamber is covered a free space of 5 mm. is left open between 

 the covering plate and the wall about the inner cylinder, permitting 

 free diffusion of volatile substances from the outer compartment to the 

 inner. To determine ammonia, the solution containing it is alka- 

 linized in the outer compartment, and acid is placed in the inner 

 compartment. In one or more hours, depending on temperature 

 and the other conditions, the ammonia from the former diffuses through 

 the air space of the chamber into the acid, where it can be measured 

 in amounts of a few micrograms. Conway applied this procedure to 

 determination of the minute amounts of ammonia in blood, to esti- 

 mation of the ammonia formed by micro Kjeldahl nitrogen digestion, 

 and of the ammonia formed by decomposition of urea with urease; 

 also to chloride and bromide, which were oxidized to chlorine and 

 bromine and diffused into potassium iodide solution for iodometric 

 titration. Conway also applied the diffusion chamber to the determi- 

 nation of carbon dioxide, which was caused to diffuse into a barium 

 hydroxide solution, where the excess alkali was titrated. Borsook (40) 



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