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THEODORE W‘. RICHARDS AND VICTOR YNGVE.
[CONTRIBUTION FROM THE WOLCOTT GIBBSMEMORIAL LABORATORY OF HARVARD UNIVERSITY. ]
THE SOLUBILITY OF SODIUM SULFATE AS A MEANS OF DETERMINING TEMPERATURES. BY
THEODORE W. RICHARDS AND VICTORYNCVE Received hTovember 18, 1917.
The accurate fixing of temperatures is becoming more and more important in view of the increasing exactness of physico-chemical measurements. Among the definite points employed, those involving a single component, such as the melting point of ice and the boiling point of water under atmospheric pressure, are most common.1 Quadruple points, or more strictly speaking, bicomponent transitions under definite pressure, such as the transition temperatures of hydrated salts, also serve excellently.2 Other methods employing two components, such as the freezing points of dilute aqueous Solutions, or the floating equilibrium of a known float in a solution whose density is changed by adding an easily determined solute until an exact equilibrium is obtained at the temperature in q ~ e s t i o nor , ~a thermochemical method6 may be used in emergencies. Of course, any definite property of material possessing a large temperature coefficient could be used for this purpose. If the solubility of salts has not been used before (we can find no reference to its use), the reason has been, perhaps, a doubt concerning the certainty of obtaining exact saturation, and the lack of adequate data. Indeed, nearly, if not quite all, of the published results of this kind are not of the order of accuracy required. In order that a salt may serve as a means of fixing temperatures with precision through the determination of its solubility at any point in question, it must possess a large temperature coefficient of solubility; its solutions must be easily saturated, but not easily supersaturated; it must be readily determinable quantitatively, and must not be difficult to obtain in a high degree of purity. Many substances possess three of these requiremeats. With regard to definiteness of saturation, we were guided by van’t Roff’s experience that univalent metals are better than bivalentss 1 For a convenient list of recently found values of a number of such points, see Bureau of Standards, Bull. 35, Ed. z (1915). 2 This method was first proposed in 1866, but the proposal was forgotten and buried. Apparently it was first applied practically in 1898. For references see Richards and Fiske, Tais JOURNAL, 36, 486 (191p);also p. 89 of this number. 3 Richards and Jackson, Pruc. Am. Acad. Arts Sci., 41, 451 (1906). 4 Richards and Shipley, THISJOCTNAL, 36, 1 - 1 0 (1914). Richards and Thorvaldson, THISJOURNAL, 37, 81 (1915). One of the authors remembers having heard orally from van’t Hoff that the latter came to this conclusion in the course of his work on the Stassfurt salts; a published reference to this coiiclusion has not yet been found.
SODIUM SULFATE AS MEANS OF DETERMINING TEMPERATURES.
165
Sodium sulfate seemed to possess the necessary qualities in as happy conjunction as any, and it was accordingly chosen fondetailed study. Historical. The solubility of sodium sulfate has been determined by many investigators. The determinations which are chosen as best by LandoltBornstein-Roth between the temperatures of 15' and 25' are those of Loewel' and the Earl of Berkeley.2 Loewel's thermometer was not described in detail, but Lord Berkeley's had been standardized a t Kew, and was doubtless better, although rendered somewhat doubtful by a slight uncertainty in the ice point. Neither of these series of measurements which are given below, is precise enough for the object in hand. TABLE I.-EARLIER DATACONCERNING THE SOLUBILITYOF Na,SO,.IoHgO
IN WATER.
NstSOd in 100 g. HzO.
Temp. IS .OO
Loewe1. 13.20
Berkeley.
IS e65
...
14 so7
16.80 19.40
..,
I8 .oo 20.00 24.90 25 .oo
...
28.00
... ...
27.67
...
Purification of Materials. In most of our determinations the purest water was used, prepared by redistilling ordinary distilled water, first from alkaline permanganate, and then from a trace of sulfuric acid with a block tin condenser. Ordinary laboratory steam-distilled water was used in a few trials, and gave exactly the same result for the solubility of sodium sulfate as did the purest water. "Chemically pure" sodium sulfate was four times recrystallized in porcelain and drained in the centrifuge, a portion of the thrice recrystallized product being reserved for comparison. No difference could be found in the solubility of the two specimens; hence further crystallization was deemed unnecessary. Evidence of purity is also afforded by the fact that such material almost always gives a constant transition temperature, even after many further crystallization^.^ Our experience thus shows that both water and sodium sulfate are easily obtained in a state pure enough for the purpose in hand. The Experimental Procedure for Determining Solubility. "%e method of procedure was chosen so that it might combine simplicity and ease of manipulation with the requisite degree of accuracy. Coarsely granular crystals, about I to 3 mm. in diameter, were placed 1
Loewel, Ann. chim. phys., [3] 49, 50 (1857). Trans., ( A ) 2 0 3 , 209 (1904). Richards and Wells, Proc. A m . Acad. Arts Sci., 38,435 (1902).
* Berkeley,Phil. 8
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THEODORE W. RICHARDS AND VICTOR YNGVE.
in a wide test tube 3 cm. in diameter and 15 cm.long. Water was added t o fill the tube to within 3 cm. from the top and a thoroughly cleaned rubber stopper was fitted to the opening. Thin wide rubber tubing was drawn over the rubber stopper and the lip of the test tube in order to keep the lip of the test tube dry. Finally the stopper was wired in place in order to prevent the possibility of its accidental dislodgement. The test tube was then secured by rubber bands to a strip of brass soldered at right angles to a revolving axle, and rotated in a thermostat which was maintained constant in temperature within 0.001'. This method attains saturation much more quickly than that which depends upon stirring the solution above resting crystals. The equilibrium is, of course, established only on the surface between the two phases, and the speed of its attainment is accelerated by promoting diffusion and convection immediately on the surface. The rotation was a t the rate of forty revolutions per minute, a speed which gave ample stirring and yet was not violent enough to produce fine particles which would give the solubility an uncertain value. It is well known that the solution tension of a fine powder presenting sharply curved surfaces has a higher value than the flat surface of the same substance.' In due time the rotation was stopped. The upper part of the test tube was brought to the surface of the bath by raising the whole rotating apparatus, and the wire and protecting rubber tube were carefully removed. The stopper was then withdrawn and two successive portions of the solution immediately removed by means of a filtering pipet, which consisted a t first of an ordinary pipet with an attached rubber tube connecting successively extra tips loosely plugged with pledgets of cotton wool.2 Before attaching the filtering device the pipet was provided with a large rubber stopper which closed the test tube whep the pipet was lowered into it. A notch in this stopper providing inlet for air to displace the solution with drawn from the test tube. The pipet was kept a t a proper temperature by enclosing it is a dry receptacle or sheath imrners
