IODISE .lKD P O T A Y d I r l I IODIDE
Temperature-composition data for the system iodine and potassiunl iotlidr in the absence of m t r r wrrr first reported by Xbegg and Hamburger' in a paper on the formation of solid polyiodides in benzene containing iodinr and an alkali-metal iodide. They allon.rtl inixturrs of the two components to cool slowly from a moderately high temperature and determined the thermal arrests in the usual way. The materials were placctl in a narrow tube open a t the top to receive a thermometer which was hrld in plaer by nicans of an asbestos plug to prevent excessive loss of iodine. The n-hole was then heated on a sand-bath and subsequently permitted to cool. stirring being accornplished by means of a loop of platinum Jvirr. potassium iodide was found to depress thc freezing point of iodine anti the first eutectic point n.ith iodine as one of t h e solid phases was estiniated to be at 80.j"antl about 2 1 mole percent KI (the niolecular formula of iodine being taken as 1 2 ) . For mixtures, however, which contained more potassium iotlitle than tht, eutectic, Abegg antl Hamburger could detect only a single teniprrnture arrest; this was in the neighborhood of 80'. Since they assumed that these mixtures (up to about 50 mole percent IiI) w r r coniplrtcly rnrltetl brfow they wrre allowed t o cool, thry rlrtelv the conclusion that the arrests representrd points on an extremcly flat liquidus curve showing the prcwnce of an unstable polyioclide. Furthermore, since thrir thermal (lata appearrd t o indicate a slight trinperature niaxi~nuniin the nupposetl liquiclus curvc. anti as they had already reached the conclusion, from their exprriincnts with tlie benzene solutions, that a heptaiotlide IiIj was the only compound brtwcen iodine antl potassiuni iotiitlr, Abrgg ant1 Hamburger frlt assured that they wpre dealing with the golyioditle KI; antl that this melted congruently at about 82. j". T h e second eutectic betwrcn the heptaiotiitle antl IiI roulcl not be determined with certainty but it was piaced tentatively near 80' antl j o niolr percent KI. From the second eutectic the liquidus curve for potassium iodide was supposed to rice steeply to the melting point (680') of the latter salt, and, because of its steepnrss. it x i s impossible t o locate it by means of the method of thermal aiialysis. The teinperaturc-composition diagram according to dbegg and Hamburger is given in Fig. I . Shortly after the appearance of the 11-ork of hbegg antl Hamburger: Parsons? took occasion to criticize their interpretation of the thermal data. Parsons pointed out that what Abegg and Hamburger took to be the liquidus curve of a solid polyioditle-i.e. the curve R I I C of Fig. I-might well be t h e "flat" representing the frerzing of a two-liquid-layer system, which! becausc, of the extreme opacity of mixtures of iodine anti potassium iodide, had escaped .
-. l
2. anorg. Chem., 50, 403 (1906). J. Phys. Chem., 11, 659 (1907).
I O D I X E .%SD POT.%SSIUM I O D I D E
22jI
detection. This suggestion would also account for the apparent absence of upper temperature arrests, since it is well known that such arrests are small when a second liquid phase is separating. Parsons then performed an interesting, even though inconclusive, experiment. He prepared a mixture containing three parts of iodine and one part of potassium iodide, heated this above its “melting point’’ (82’) and shook it thoroughly after sealing the glass tube which served as the container. The tube was then allowed t o stand for some time at 8 j oso that the two liquid phases, if present, might separate into distinct layers; thereupon the tube
I30
, ,
I
A
MELT
110
90
70
k MOLE PERCENT K I 1
I
20
I
I
40
I
60
FIO.I The System KI-I? according t o .Allegg and Hamhurger.
was suddenly chilled antl the contents frozen. dnalysie of the top and bottom portions shon.ed that marked segregation had occurred, the top being rich in potassium iotiitle and the bottom rich in iodine. Parsons drew from this experiment the conclusion t h a t the mixture at 8;’ was not really homogeneons antl believed that this discovery was at least indicative of the presence of txyo liquid phases. He ac!mitted frankly, hoivever, that he was unable to detect two layers by dircct obsrrvation. but this he ascribed to the extremely opaque Later on. I
