The System NaCl–AlCl₃

Abstract

The system NaCl–AlCl₃ has been restudied by DTA, visual observation, and x-ray diffraction powder techniques for identification of crystalline phases. It was confirmed that the system contains one intermediate compound NaAlCl₄ with an incongruent mp of 153 ± 0.5 °C and a region of liquid immiseibility extending from 80.25 to 99.6 mol percent AlCl₃ at 191.3 °C, the monotectic temperature.

1. Introduction

A summary of the pertinent data in previous studies [1–5]¹ of the system is given in table 1. In addition, Boud [6] in 1904 reported compound compositions of 3NaCl · AlCl₃, NaCl · AlCl₃, and NaCl · 2AlCl₃. The compound NaAlCl₄ with an incongruent melting point at ~ 153 °C seems well established and its crystal structure has been determined [7J, using a Weissenberg camera. It is orthorhombic (space group P2₁2₁2₁), with Z = 4 and d (x-ray) = 2.00 g/cm³. However, the existence of additional compounds and the occurrence of a stable two liquid region of immiseibility are questionable.

Table I.

Phase studies in the system NaCl–AlCl₃

Investigator	Methods of Study	Incong. mp of NaAlCl4	Location of NaAlCl4-AlCl3 Eutectic		Two Liquid Region		Notes
					Extent Mol % AlCl3	Temp.
		°C	Mol % AlCl3	Temp. °C
Kendall, et al. (1923) [1]	Visual observation in sealed tubes	152	~ 61	115	82.0 - > 99.8	193.5	Compound XNaCl · YAlCl3 under two liquids.
Plotnikov and Shvartsman (1936) [2]	Heating and cooling curves automatically reocorded; sealed		61	108			Only studied be· tween 50.2 and 80.8 mol % AlCl3.
Shvartsman (1940) [3]	Heating and cooling curves automatically reocorded, sealed	152	61	108	80.8 – 100	190.0
Cretien and Lous (1943) [4]	Cooling curves in N2 atm; Chem. analysis	155	65	105	None detected
Midorikawa (1955) [5]	Cooling curves	154	62	112			Only studied between 48.1 and 70.7 mol % AlCl3.
Present study	DTA; visual observation in sealed tubes; x ray	153 ±0.5	61.4	107.2	80.25 – 99.6	19l.3

2. Experimental Procedure

The general nature of the diagram was first established with a commercial DTA apparatus², using a microholder with Al₂O₃, as a reference material and a 10°/min heating rate. The final diagram (fig. 1) was refined by determining the liquidus and solidus curves by visual observation. The chlorides of aluminum and sodium were obtained as Reagent Grade. The NaCl was analyzed to be 99.7 ± 0.1 percent pure. The AlCl₃ was resublimed at 135–150 °C under vacuum from molten NaCl · AlCl₃ mixtures containing Al-metal strips [8]. The final purity of AlCl₃ was analyzed to be 99.8 ± 0.1 percent. Atomic absorption analysis for Na confirmed a content of < 0.1 percent Na in the AlCl₃. To prevent hydration of AlCl₃, various mixtures of NaCl and AlCl₃ were prepared in glove bags flushed with anhydrous N₂ and sealed under vacuum in Pyrex tubes. Compositions not forming two liquids, i.e., those below 80 mol percent AlCl₃, were subjected to three cycles of grinding and sealing in the glove box, followed by heating to promote homogeneity. Samples which had been reground several times appeared pure white in color; but when they were heated to the appropriate invariant temperature of either 153 or 107 °C, they turned sharply to a gray hue.

Figure 1. The system NaCl–AlCl₃.

o – Visual observation.

O – DTA.

X – X-ray examination at room temperature.

The two liquid region of the diagram was studied visually by introducing the sealed sample into a nichrome-wound Pyrex tube furnace (40 mm O.D.) insulated by an outer Pyrex tube. Sealed samples with 80 mol. percent AlCl₃ and less were studied visually in a silicone oil bath, stirred with a magnetic stirrer. Heat control was accurate enough to permit heating and cooling rates of 6 °C per hour or less. The liquidus temperatures were determined by observing under a magnifying glass the disappearance of tiny crystals approximately 0.5 mm in their largest dimension. Several determinations were averaged, and all temperatures are estimated accurate to ±0.5 °C. The Chromel-Alumel thermocouple was calibrated using pure Sn (mp 231.89 °C) and the normal freezing and boiling points of water. At times a calibrated Hg thermometer was used also in the oil bath.

The solid phases were identified by x-ray diffraction powder techniques, using a dry mount with Mg(ClO₄)₂ as desiccant. All compositions could be interpreted in terms of the components NaCl and AlCl₃, and one intermediate compound NaAlCl₄. The x-ray powder pattern of NaAlCl₄ was indexed on the basis of an orthorhombic cell [7], with a = 10.33 Å, b = 9.91 Å, c = 6.18 Å, within a few hundreths Ångstroms of the dimensions reported by Baenziger [7] and Semenenko et al. [9]. Small irregular variations in interplanar spacings with different compositions are attributed to hydration effects rather than to an indication of slight solid solubility of AlCl₃ in NaAlCl₄.

A two liquid region, as reported by Kendall et al. [1], and Shvartsman [3], was found to exist between a composition of 80.25 ± 0.25 and 99.6 mol percent AlCl₃ at 191.3 °C. Two liquids were observed visually to form in several samples containing 80.5 mol percent AlCl₃ or greater (fig. 1). In repeated experiments, two liquids were never observed in samples of 80.0 mol percent AlCl₃ or less, i.e., 79.5, 77.9, and 75 percent. Atomic absorption analysis for Na in the upper liquid segregated at ~ 192 °C revealed the composition to be 0.4 mol percent NaCl. The composition of the AlCl₃-rich liquid, therefore, is 99.6 mol percent AlCl₃.

Several attempts were made to study the closure of the two-liquid dome. Samples of composition 80.6 and 90.2 mol percent AlCl₃ were heated in sealed glass tubes to form two liquids. After heating at a rate of 5 °C/day and reaching temperatures of above 210 °C, the tubes burst. The triple point of AlCl₃ is reported [10] at 193.3 °C and 2.33 atm.

3. Conclusion

In conclusion, a restudy of the system NaCl–AlCl₃ has shown it to contain only one intermediate compound and a definite region of liquid immiscibility. Also, temperatures and compositions of invariant points have been closely defined.