506 PRINCIPLES OF CHEMISTRY 



iodide is prepared by adding 1 part of common (yellow) dry phosphorus 

 to 10 parts of dry iodine in a glass flask. On shaking the flask, 

 union proceeds quietly between them (light and heat being evolved), 

 and when the mass of iodide of phosphorus which is formed has 

 cooled, water is added drop by drop (from a tap funnel) and hydrogen 

 iodide is evolved directly without the aid of heat. These methods of 

 preparation will be at once understood when it is remembered (p. 468) 

 that phosphorus chloride gives hydrogen chloride- with water. It is 

 exactly the same here the oxygen of the water passes over to the 

 phosphorus, and the hydrogen to the iodine, thus, PI 3 + 3H 2 O 

 = PH 3 3 + SHI." 



In a gaseous form hydrobromic and hydriodic acids are closely 

 analogous to hydrochloric acid ; they are liquefied by pressure and cold, 

 they fume in the air, form solutions and hydrates, of constant boiling 

 point, and react on metals, oxides and salts, &c. 78 Only the relatively 



into combination ; at a higher temperature, 78 p.c. to 79 p.c. , and at a strong heat 

 about 82 p.c. 



77 But generally more phosphorus is taken than is required for -the formation of 

 PI 3 , because otherwise a portion of the iodine distils over. If Jess than one-tenth part 

 of iodine be taken, much phosphonium iodide, PHjI, is formed. This proportion, 

 was established by Gay-Lussac and Kolbe. Hydriodic acid is also prepared in many 

 other ways. Bannoff dissolves two parts of iodine in one part of a previously prepared 

 strong (sp. gr. T67) solution of hydriodic acid, and pours it on to red phosphorus in a 

 retort. Personne takes a mixture of fifteen parts of water, ten of iodine, and one of red 

 phosphorus, which, when heated, disengages hydriodic acid mixed with iodine vapour ; the 

 latter is removed by passing it over moist phosphorus (Note 76). It must be remembered 

 however that reverse reaction (Oppenheim) may take place between the hydriodic acid 

 and phosphorus, in which the compounds PH*! and PI 2 are formed. 



It should be observed that the reaction between phosphorus, .iodine and water 

 must be carried out in the above proportions and with caution, as they may react with 

 explosion. With red phosphorus the reaction proceeds quietly^but nevertheless requires 

 care. 



L. Meyer showed that with an excess of iodine the reaction proceeds without the 

 formation of bye-products (PB^I), according to the equation P + 51 + 4H 2 O = PH 5 O 4 + 6HL 

 For this purpose 100 grams of iodine and 10 grams of water are placed in a retort, and a 

 paste of 5 grams of red phosphoru* and 10 grams of water is added little by little (at first 

 with great care). The hydriodic acid may be obtained free from iodine by directing the 

 neck of the retort upwards and causing the gas to pass through a shallow layer of water 

 (respecting the formation of HI, see also Note ^5). 



78 The specific. gravities of their solutions as deduced by me on the, basis of Topeb'e 

 and Berthelot's determinations for 15/4 are as foUows : 



10 20 80 40 50 60p.c t . 



HBr 1-071 1-156 T258 1'874 T505 T650 



HI 1-075 1-164 1-267 T899 T567 1'769 



Hydrobromic acid forms two hydrates, HBr,2H 2 O and HBr,H 8 O, which have been 

 studied by Roozeboom with as much completeness as the hydrate of hydrochloric acid 

 (Chapter X. Note 87). 



With metallic silver, solutions of hydriodic acid give hydrogen with great ease, 

 forming silver iodide. Mercury, lead, and other metals act in a similar manner. 



