CX,-The Properties of Ammonium Nitrate. Part II. Ammonium Nitrate and Water.

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1 THE PROPERTIES OF AMMON~UM NITRATE. PART CX,-The Properties of Ammonium Nitrate. Part II. Ammonium Nitrate and Water. By IDA L. MILLICAN, ALFRED FRANCIS JOSEPH, and THOMAS MARTIN LOW-RY. IN the preceding paper of this series it was shown that the freezing point of ammonium nitrate can be raised by careful purification and drying to 169.6". The freezing point is, however, extremely sensitive to the influence of traces of moisture, and it appeared desirable, therefore, to make quantitative experiments to determine the magnitude of this effect. In view, moreover, of the fact that the freezing point of the salt lies within the range of easy experimental observation, a unique opportunity presented itself of plotting a complete equilibrium diagram for a soluble salt and water, a diagram which, in this instance, has the special interest of including four transition temperatures. Previous determinations of the solubility of ammonium nitrate between 0" and 100" have been made by Schwarz (Preisschrift, Gottingen, 1892, p. 42), and between 12" and 40" by Miiller and Kaufmann (2. physikal. Chem., 1903, 42, 499). On the aqueous side of the diagram, measurements of the freezing point of solutions of the salt have been made by de Coppet (Ann. Chim. phys., 1872, [iv], 25, 544), who has also located the eutectic point at " and 41.2 per cent. NH,*NO, (2. physikal. Chern., 1897, 22, 240). E x P E R I M E N T A L. First Series of Measurements: 90" to 170". In the first series of measurements, the observations were made with mixtures of ammonium nitrate and water enclosed in sealed tubes, in order to avoid loss of the small quantities of water, which ranged only from 0 to 10 per cent. of the weight of ammonium nitrate. The tubes were suspended in a large beaker of liquid paraffin heated on a sand-bath. The liquid was stirred and its temperature read by means of a long standard thermometer. The temperature was noted at which most of the nitrate had melted, and then that at which the last traces were just disappearing; the mean of these two temperatures wm taken w the temperature of dissohtion. The bath was then allowed to cool until crystals reappeared; this point waa recorded a~ the temperature of crystallisation. The mean of the temperatures of dissolution and crystallisation in a series of observations was taken as the initial freezing

2 960 MILLICAN, JOSEPH, AND LOWRY : point of the mixture. A correction for the exposed stem of the thermometer was made after the mean value of the initial freezing point had been determined. The method does not give results of a very high order of accuracy, but it is believed that the data are correct within 1". The numerical results are set out in Table I, and may be summarised as follows : Water ,/, Freezingpoint " Change of freezing point for 1 per cent. of water " - FIG. 1. Equilibrium diagram for ammonium nitrate and water. It will be seen that there is a very abrupt change in the magnitude of the effect produced by the addition of 1 per cent. of water, which at first lowers the freezing point by ll", but afterwards only by about 5". When plotted out as in Fig. 1, the two curves (one of which appears to be accurately rectilinear) intersect accurately at the transition temperature of 125".

3 THE PROPERTIES OF AMMONIUM NITRATE. PART II. 961 TABLE I. Initial Freezing Point of Mixtures of Ammonium Nitrate and Water (First Series). Temper- Water. Temperature of ature of crystallis- Per cent. dissolution. ation " 119" O* ot T * Dried over sulphuric acid. Initial freezing point " Mean. Uncorrected. 120" Mean. Corrected. 122" t Dried over phosphoric oxide. Second Series of Measurements : 6" to 136". In the second series of experiments, the initial freezing point of mixtures of ammonium nitrate and water was determined in a similar manner to that used for measuring the molecular depression of the freezing point in the Beckmann apparatus. The apparatus consisted of a tube 6 inches by 1 inch fitted with a rubber stopper carrying two thermometers and a stirrer ; the stirrer fitted tightly in the rubber stopper and was lubricated by glycerol. The tube was immersed in a well-stirred bath of water or oil, the temperature of which was read to 0.1' by means of a standard thermometer ; auxiliary thermometers for reading the temperature of the exposed thread of mercury were provided both inside and outside the tube. The ammonium nitrate was recrystallised three times, and dried during three or four days, until it no longer caked when allowed to cool in a stoppered bottle. About 20 grams were introduced into the tube, which was then corked and weighed; distilled water was added and the tube again weighed after its temperature had risen to that of the room. The rubber stopper with the thermometers and stirrer was then inserted and the tube immersed in a

4 962 THE PROPERTIES OF AMMONIUM NITRATE. PART TI. View Online bath, which was heated to a temperature about 6" above the melting point of the mixture, and then allowed to cool slowly while the mixture was stirred. The maximum temperature reached after crystallisation had started was recorded and the average taken of a series of concordant readings. It was found that a small quantity of water distilled off from the mixture and condensed on the side of the tube. An attempt was made to stop this by providing an additional stopper fitting like a piston immediately above the mixture, but this was not satisfactory. The moisture was therefore collected, with suitable precautions, on a weighed filter-paper, and a correction made for the water lost by the mixture, usually about 0.05 gram. A fresh mixture was made for each determination of the freezing point. The results obt,ained are recorded in Table 11. TABLE 11. Initial Freezing Point of Mixtures of Ammonium Nitrate and Water (Second Series). Per cent. of Per cent. of ammonium ammonium nitrate in Initial freezing point. nitrate in Initial freezing point. final mix- (Uncor- final mix- (Uncorture. rected.) Corrected. ture. rec ted. ) Corrected " 6-2" " 72.4" : The second series of measurements extends over a range of temperatures which includes three transition points. The break at 125", recorded in the first series of measurements, is accurately reproduced in the second series by drawing a straight line through the freezing point of the dry salt at 170" and the experimental observation at 136". There is, however, no sign of a break at 84", although it is unlikely that either the y- or &form of the salt should be suppressed in such a long series of measurements; it would therefore appear that the solubilities of the y- and 6-forms of the salt are not only identical at 84" but exhibit practically the same temperature coefficient at this temperature. The curve is, however, remarkable in that it also passes without a break through the transition temperature at 32". In this case there is clear evidence that the freezing points are those of 7-ammonium nitrate,

5 THE PROPERTIES OF AMMONIUM NITRATE. PART III. 963 persisting in a metastable form at temperatures below the normal transition point, since in these lower ranges of temperatures several determinations of the solubility of the salt are available, and a single measurement which we made of the composition of a solution saturated at the melting point of ice agrees closely with the value, 54 per cent. of NH,*NO,, given by Miiller and Kaufmann, whereas our main freezing-point curve would intersect the line of zero temperature at a point corresponding with 57 per cent. of NH4*N0,. The remainder of the equilibrium diagram can be plotted from data already available, so as to give a set of curves covering the whole range from 0 to 100 per cent. of ammonium nitrate. The eutectic temperature at - 17" is, however, so close to the transition temperature at - 16" that it is quite likely that this eutectic is for the common rhombic form of the salt rather than for the low-temperature tetragonal form. Summary. 1. The influence of water on the freezing point of ammonium nitrate has been detetmined from 170" to 0". It is therefore possible, with the help of earlier data, to plot a complete equilibrium diagram for ammonium nitrate and water. 2. The freezing-point curve shows it strongly-marked break at 125" and a small break at 32", but appears to pass smoothly through the transition point at 84". 3. Measurements have been made down to 6" of the solubility of the y-form of the salt, which is only stable between 32" and 84". THE CHEMICAL LABORATORY, THE UNIVERSITY, CAMBRIDGE. [Received, March 301h,