408 Standard for Determination of Ammonia [April-July 



precipitate is formed, care being taken not to pass this point. Then. 

 150 gm, of potassium hydroxid, dissolved in 250 c.c. of water, are 

 added to the iodide sol. and the mixture is then diluted to i liter. 

 In nesslerizing, a portion of this sol. is diluted five times and 35 c.c. 

 are used. 



There are conditions which influence greatly the ease with which 

 nesslerized Solutions may be matched. Even in following closely 

 the same procedure there are frequently striking variations. There 

 may be times when the readings, in large numbers, do not vary more 

 than o. I mm. ; at other times, under apparently identical conditions, 

 it has been necessary to average readings with a maximum Variation 

 of as much as i mm. This irregularity led the authors to study the 

 limits of errors in nesslerizing with the Duboscq colorimeter and 

 also to seek a more stable Standard. 



A number of yellow and red Solutions were tried singly and in 

 combination. At the Suggestion of Mr. A. M. Buswell, of the De- 

 partment of Sanitary Engineering, Columbia University, the papers 

 of Jackson and Richards were referred to.^ These authors give us 

 a Standard now used extensively in sanitary laboratories, consisting 

 of mixtures of Solutions of potassium platinic chlor id and cobalt 

 chlorid in hydrochloric acid. These colors did not match our Stand- 

 ard made from an ammonium sulfate sol. containing i mg. per c.c, 

 but the reading of the papers suggested chloroplatinic acid, which 

 was tried. A 21 percent sol. of this acid just matches our Standard 

 ammonium sulfate sol. to which the Nessler reagent has been added 

 and allowed to stand for 20 min. It was found, however, that this 

 expensive Solution could be replaced by a five percent sol. of 

 chlorplatinic acid to which was added one-half its vol. of hydro- 

 chloric acid sol. containing 1.2 percent of cobaltic chlorid. 



The readings are as easily discerned in using this Standard as in 

 the case of ammonium sulfate sol. to which Nessler reagent has been 

 added. In this case, as in the other referred to above, there are 

 conditions not yet determined under which it is difficult to get read- 

 ings within the limits of 0.2 mm. This difficulty may be due to the 

 formation of molecular aggregates that induce irregulär transmis- 

 sion of the light, as is suggested by the illusive opalescence which 



2 Jackson: Tech Quart. 1900, xiii, p. 320; Richards and Miller; Ibid., 1904, 

 xvii, p. 277. 



