//. Gruener — Iodometric Determination of Nitrates. 45 



the hydriodic acid may occur at the boiling point of the 

 stronger acid. According to Berthelot* aqueous solutions of 

 hydriodic acid under pressure begin to dissociate at 275° or 

 even under. The temperature of the retort at the end of an 

 experiment was taken and found to be far beyond the range of 

 a thermometer reading to 360°. It is evident, then, that under 

 the conditions there should be opportunity for dissociation. 

 Four experiments in which potassium iodide was treated with 

 phosphoric acid, specific gravity 1*7, undiluted, gave of libe- 

 rated iodine caught in arsenious acid *0030 grm. -0044 grm. 

 •0049 grm. -0030 grm. of iodine, corresponding to '0008 grm. 

 •0012 grm., '0013 grm., -0008 grm. of potassium nitrate respec- 

 tively. These results would account only in part for the 

 errors noted but, they are in the same direction. Chapmanf 

 has found that concentrated hydriodic acid acting upon organic 

 nitrates and nitrites reduces them not alone to nitric oxide but 

 even in part to ammonia. The conditions of the experiments 

 in Series 2 approximate to concentration of the hydriodic acid, 

 and the reaction that Chapman found probably takes place here 

 to a limited extent. Concentration of the hydriodic acid then 

 must be avoided, both to eliminate dissociation and excessive 

 reduction of the nitrate. 



Therefore the plan was adopted of diluting the solution, so that 

 the temperature and concentration should be effective in decom- 

 posing the nitrate, but should not bring about the complications 

 just mentioned. A blank experiment made thus, 18 cm 3 of a 

 solution of phosphoric acid of specific gravity 1/39, being used, 

 gave of liberated iodine only -0004 grm. corresponding to '0001 

 grm. of potassium nitrate. So that under these conditions we 

 may be assured that dissociation occurs if at all to only a slight 

 extent. In the following experiments, therefore, the nitrate is 

 decomposed in presence of dilute phosphoric acid, the dilution 

 and the quantity taken being included in Table II. 



The iodine of 41 was collected in potassium iodide and titra- 

 ted with thiosulphate. Series 3 requires little special comment. 

 The experiments ran along smoothly, the manipulation was 

 easy, the care required in watching very little and the results 

 are fairly satisfactory. The causes leading to the high results 

 of Series 2 seem entirely eliminated by the dilution. Series 4 

 is a direct continuation of Series 3, differing in the single point 

 that larger amounts of nitrate are handled. These results with 

 some exceptions are decidedly low. It appears that with a fair 

 percentage of water present, as the amounts of nitrate grew 

 larger, the error became a negative one. The explanation for 

 this seemed to be that the last portions of nitric acid must have 



* Ann. de Chim. et Phys., (IY), xx, p. 392. 



f Journ. of the Chem. Soc, vol. xx, 1867, p. 166. 



