THE MELTING AND PREEZING POINT OF SODIUM CHLORIDE1

BY JOHN BRIGHT BERGUSON

The melting or freezing point of sodium chloride is usually relegated into the category of a secondary thermometric point. However, there is much to commend the use of this point to workers in chemical laboratories where the pure salt and suitable containers2 are readily obtained. For this reason, we desire to place on record some determinations of this point, which we were led to make by a somewhat unsatisfactory calibration of a thermo-element a t this point.

Thermo-couple and Calibration A platinum : platinum-rhodium thermo-element was used.

The wires were 0.5 mrn in diameter and from pre-war German stock.

The electrical measuring apparatus consisted of a Wil- liams double potentiometer, set up essentially as given by White.3 The galvanometer was of such sensitivity, that a reading of 50 divisions on the scale corresponded to 10 micro- volts.

A properly insulated alundum tube, wound with nichrome ,wire, served as a heating unit. Inside the alundum tube a large silica test-tube was placed and the crucible containing the metals were so placed in this test-tube as to be in the re- gion of uniform temperature in the furnace.

Crucibles turned from Acheson graphite rods were used as containers for the cadmiun, zinc, antimony, copper-silver and copper samples. Lids and graphite powder were also prepared from the same rods.4

Contribution from the Chemical Department of the University of Toronto. Nickel crucibles have been recommended. Bur. Standards, Circ., 66,

10 (1917). a W. P. White: Jour. Am. Chem. SOC., 36, 1880 (1914).

Day and Sosman: Am. Jour. Sci., 29, 125 (1910). Bur. Standards, Circ., 66, 11 (1917); 7, 5 (1920).

Melting and Freezing Po in t of S o d i u m Chloride 627 ,

Freezing point

2506.4 3429.6 3458 5539.1

7106.5 7 108

-

10574 10575.5

-

The silver point was determined by the wire method; the bare element was placed in a clean silica test-tube like that in which the crucibles were placed but free from carbonaceous material and the tube evacuated. Silica tubing was used both to insulate the parts of the element and also to protect it when necessary.

The cadiurn, zinc and antimony were from a stock of Kahlbaum's best pre-war chemicals. The copper consisted of carefully selected bright chips of electrolytic copper from the same source. The silver was obtained from the mint at Ottawa through the courtesy of the Director and was certified 999.9 fine.

The results obtained at these fixed points are given in Table I. For comparison, the values given by Adamsl for a standard element are given and also the corresponding tem- peratures in degrees centigrade upon the thermodynamic scale.

TABLE 1 Values obtained in the calibration a t the fixed points

Melting point

2509.4 3430.5 3428.3 5541.7

7123.2 7121.4

9161 9159

10586.5 10Fj85.4

Material

Cadmium Zinc

Antimony Copper-silver Eutectic

Sample 66sT, Silver

Silver Vacuum Wire Method

Copper

Standard element

2503 3430

5530

7102.1

-

- -

9111 10634 -

remper- ature

leg. cent.

320.9 410.4

630

779

-

-

- 960.2

1082.8 , -

The results given in the previous table would indicate that our element gave slightly greater E. M. I?. values a t tem-

L. H. Adams: Jour. Am. Chem. SOC., 36,6B (1914). The value for the copper-silver eutectic is not given by him but is easily derived from his table as- suming the value 779' given by the Bureau of Standards.

628 Johvt Bright Ferguson

peratures above that of the zinc point, than did the standard element. The results cannot all be considered of equal weight since the experimental errors would tend to lower the copper value (oxidation) whereas in the case of the silver value they would raise it (wire method). The copper-silver eutectic value may be low if other than the eutectic composition be used.l The freezing points'taken by us with this material were sharp, showing the slight under-cool with subsequent slow heating mentioned by Waidner and Burgess. In one case prior to the break, the cooling was a t the rate of 10 microvolts per minute; a t the break the temperature first rose 1 micro- volt in two minutes, remained constant 1 minute and then fell 1 microvolt in three minutes (1 pv = 0.09 degrees). This break would seem to leave nothing to be desired. However, in view of the work of Waidner and Burgess, we think that the melting point determinations probably correspond closer to the true eutectic temperature than do our freezing determi- nations.

A deviation curve,2 for calibration purposes, was drawn by plotting the differences in microvolts between our values at the fixed points and those of the standard element, against the temperatures in degrees centigrade. The differences chosen were: zinc 0, antimony 10, copper-silver eutectic 20, silver. 49, and copper 52. The curve was drawn so as to lie midway between the two latter values. This curve indicates that our element gives a reading 22 microvolts higher than the standard element at 800 degrees and this deviation appears to be as great a deviation as can be consistently deduced from our results.

Sodium Chloride Determinations The sodium chloride determinations were carried out in

the silica test-tube used for the silver determinations. The salt was first melted in a platinum crucible over a blast-lamp, care being taken to avoid the reduction of the salt. The

~

I Waidner and Burgess: Bur. Standards, Bull., 62,149 (1909). 2 R. B. Sosman: Am. Jour. Sci., (4) 30, 7 (1910).

Melting and Freezing Po in t of Sodium Chloride 629

4ve:age in C

803.1 -

- -

-

802.7

503.2

803.1 -

bare thermo-element was then allowed to freeze in place and the crucible and element transferred to the silica test-tube in the resistance furnace. The crucible held about 15 cc and was, unless otherwise stated, filled three quarters full of the molten salt. The freezing point of the salt was taken as the highest temperature to which the salt heated up after the slight under- cool. The melting point was taken as the upper break on the melting curve.

The results are given in Table 11. The temperatures given were calculated from the deviation curve and are in degrees centigrade on the thermo-dynamic scale.

TABLE I1 Melting and Freezing Point Determinations for Sodium Chloride

Remarks

Element in centre of charge

Small charge ele- ment in surface

Element at side near top

Element in centre of charge

Element in centre of charge

Element in centre of charge I

Material

7389.3 7388.8 -

7387.8

7382.8

7386.1

7388.8

7391.1 7391.3

Kahlbaum's purest salt from pre- war stock

803.4 -

- -

-

803.1

803.4

803.6 -

J. T. Baker c. P. Squibb's c. P. Merck C. P.

Melting point

Micro- Average volts in " C

Freezing point I Micro- volts

7386.4 7387.3

7384.6 7379.2

7376.2

7381.6

7387.2

7387.4 7385.5

Disoussion of Results These results would indicate that the pure salt freezes

a t 803" f 1" C. The generally accepted value is 801" C. The values given by the following workers1 also support a higher value than that now accepted: Plato, 804.1; Arndt,

1 Landolt-Bornstein, Roth: Physikalisch-chemische Tabellen, page 222 (1912).

630 John Bright Ferguson

805; Sardonini, 806; and Menge, 803. Griffiths’ value1 of 801, obtained with a salt 99.98% pure, made for dairy pur- poses, would seem also to favor a higher value since such salt could hardly be of the same purity as our best reagent chem- icals.

Toronto, Canada

1 Ezer Griffiths: “Methods of Measuring Temperatures,” page 75.