232 CirV'ETTE COLORIMETRY 



The principle of this method, which was originally proposed by 

 Rappaport (1935), is iodometric measurement of the excess hypo- 

 bromite remaining after reaction with ammonia according to the 

 equation: 3 NaOBr + 2 NH3 -^ N2 + 3 H2O + 3 NaBr. The pro- 

 cedure of Levy and Palmer can be used for total nitrogen in the 

 range 500-5 /xg. or, if the microvolumetric techniques of Linder- 

 str0m-Lang and Holter are used, down to 0.5 [xg. After digestion in 

 small test tubes the material is diluted with water, made alkaline 

 with a neutralizing reagent, treated with an excess of hypobromite, 

 and this excess is measured by the addition of potassium iodide and 

 titration with thiosulfate. The entire treatment can be carried out in 

 one small tube. 



As an improvement on the diffusion procedure of Bentley and 

 Kirk (1936), Tompkins and Kirk (1942) described a specially con- 

 structed unit for both digestion and diffusion designed for the meas- 

 urement of samples containing 0.5-20 /xg. nitrogen. A probable error 

 of not more than about 1% was claimed by the authors, but this was 

 challenged by Hawes and Skavinski (1942), who stated that the 

 diffusion time employed by Tompkins and Kirk is sufficient to trans- 

 fer only about 90% of the ammonia in samples containing less than 

 10/xg. nitrogen. 



Hawes and Skavinski ( 1942) described another modification of 

 the diffusion cell technique. They employed a test tube for both di- 

 gestion and diffusion. A small helix of platinum wire, which was 

 sealed to a glass tube held in a rubber stopper, was used to hold a 

 drop of the ammonia-absorbing solution. AIM primary sodium 

 phosphate solution, rather than the commonly used saturated boric 

 acid solution, was employed to absorb the ammonia because of the 

 greater absorption capacity of the former. The volume of phosphate 

 solution need not be measured ; it is sufficient merely to dip the helix 

 into the stock solution. After the absorption is complete, the helix is 

 immersed in water and the solution is titrated with standard acid 

 to an end point between pH 4.3 and 4.7. Electrometric titration is 

 advantageous but an indicator may be used; in the latter event a 

 bromocresol green-methyl red mixture is superior to the methylene 

 blue-methyl red combination. As used by Hawes and Skavinski, the 

 method enables the determination of from 10 to 100 fig. nitrogen. 



Russell ( 1944) , by utilization of the Conway-Byrne diffusion cell 

 for the collection of ammonia, refined the phenol-hypochlorite 



