T H E T E J I P E R A T U R E O F MAXIMVTRI REFRACTIVITY O F SOME AQUEOUS SOLUTIOSS BY S O R A GREGG-YILSON A S D ROBERT TIXIGHT
The phenomenon of the maximum density of water at 4°C is generally explained on the assumption that ice molecules are produced in solution at a temperature somewhat above the freezing point. Since ice is less dense than water its presence will counteract the increasing density of water due to the lowering of the temperature. At 4°C the two effects will balance each other, and between 4°C and 0°C the rapid formation of ice molecules will more than overcome the normal contraction, thus causing an expansion t o take place between 4°C and the freezing point. The existence of the point of maximum density is thus seen t o be due to the fact that the change of density on freezing is in the opposite direction to the normal change which accompanies fall of temperature in the liquid state. R-ith most physical properties the change on freezing and the change with fall of temperature in the liquid are in the same direction. For example, water increases in viscosity and in surface tension with fall of temperature, and the viscosity and surface tension of ice are greater than that of water. S o w the mean refractive index of ice (1.31) is less than that of water (1.331, and the refractive index of water increases with fall of temperature. lye may therefore expect t o find a temperature of maximum refractivity (t.m.r.) corresponding to the temperature of maximum density (t.m.d.). But ns the change of refractive index of water on freezing is only about 1.5C; compared with a change of 8. j'; in density, we should expect a greater concentration of ice molecules, i.e. a lower temperature, to be necessary a t the t.m.r. than at the t.m.d. Jamin' by means of his interferometer, found the t.m.r. of water to be very close to the freezing point. Pulfrich' using the instrument designed by himself, and employing the natural cold of a winter night, found the t.m.r. of water to be between -1°C and -2°C. These would seem to be the only recorded observations of the phenomenon. It has long been known that the presence of a solute lowers the t.m.d. of water, the lowering being in general proportional to the concentration of the dissolved substance. An attempt has now been made to determine if a similar effect is to be found in the case of the t.m.r. Ezperimental. Both the Pulfrich refractometer and the Jamin interferometer are difficult instruments to control a t temperatures lower than that of the room. It was therefore decided to employ a Zeiss dipping refractometer which is slightly more sensitive than the Pulfrich instrument and was found to be readily workable at temperatures near the freezing point of water. 'Chem. Reviews, 43, 1191( 1 8 5 8 ) . * W e d . Ann., 34,332 (1888).
NORA GREGG-WILSOS AKD ROBERT WRIGHT
3012
The liquid to be investigated was placed in a weighing bottle and attached to the prism of the instrument by means of a rubber connexion, the instrument was then placed in the cooling bath and kept at a definite temperature for quarter of an hour, after which a reading was taken. S o difficulty was found in holding the bath at a steady temperature by the addition of ice or if neccssary of a small amount of common salt. The temperature of the bath was then lowered, and after equilibrium had again been reached, a second reading was taken, and so on. At temperatures near zero the field of view became obscured on account of the condensation of moisture on the back of the prism inside the instrument. This film of moisture made accurate readings impossible, but it was effectively prevented by the simple expedient of placing a granule of calcium chloride-wrapped in tin foil-on the upper surface of the prism before attaching the latter to the instrument. The presence of the granule decreases slightly the intensity of the light passing into the instrument, but does not interfere otherwise with the field of view. It was hoped that it would haye been possible to obtain readings on both sides of the t .m.r., and to observe decreasing refractive index with decreasing temperature. This was only found possible in the case of water itself. K i t h all solutions the t .m.r. was considerably below the freezing point, and freezing took place before a decrease in refractive index could be observed. It was therefore decided to obtain the t.m.r. by means of an indirect graphic method. The temperature coefficient of refractive index of an aqueous solution decreases with fall of temperature, till at the t.m.r. its value is equal to zero. The value of the coefficient was therefore determined at a series of tempertitures, plotted against the temperature, and the curve exterpolated to find the temperature a t which thP coefficient was of zero value. This temperature is obviously the t.m.r. I n practice it was found most expedient to obtain readings of the refractive index at about 4' intervals of temperature, and hence calculate the temperature coefficient for the average temperature of each interval, carrying the series of readings to as low a temperature as possible. Unfortunately the exterpolated value of the t.m.r. is by no means definite. There is moreover an additional error caused by the slight change of refractive index of the prism with fall of temperature. This change in the prism is however, according to the manufacturers of the instrument, of very small value, and may probably be safely neglected in comparison with the much greater error due to exterpolation. As an example of the method we may consider a set of results obtained with water:dR/dT T' dT dR T R I6 ' 15.15 ,200
Ij.0
3 4
.1 .5
.16j
12.j
-0
15.8j 16.35 16.85
.5
1j.Ij
1
.3
.LZj .o; j
9.0
3 O
I-lo I Io
2
j.0
30'3
T E M P E R A T U R E O F MAXIMGM REFRACTIVITY
The first two columns give the temperatures and instrument readings, the temperature differences and instrument reading differences are given in the third and fourth columns, and the temperature coefficients and the corresponding average temperatures are in the last two columns. If the temperature coefficient dR/dT is plotted against the average temperature we get an approximately straight line, which on exterpolation t o zero value for the coefficient indicates that the t.m.r. lies between oo and - I O C , Fig. I .
I
rc,wlnEr?ffrupE
FIG.I
Results. The t.m.r. for water has been found by a series of measurements to be in the neighbourhood of -0. j°C. As already stated this was the only case in which it was found possible to cool the liquid below the t.m.r., and find a decrease of refractive index with decrease of temperature. The values obtained at the different temperatures were as follows:- -0 T. IO3 6' 2 O -z0 -3O -4 O
11.1.
1,33370
1.33389
1.33424
1.33124
1.33416
1,33397
1,33385
TABLE I Temperatures of Maximum Refractivities of Aqueous Solutions (The figures indicate degrees below zero)
s )2 H sj4
Si 2 Sa
Chloride
Bromide
Iodide
Xitrate
4.2 2.1
2 . 1
2 . 1
2.0
3.5 3.9 3.0 3.0
6.5
4.2
2.8 3.0
4.3 3.8 2.3
3.7 3.8 2.9
5.5 5.3 3.8
6.8 3.5
2 . 5
2.3
3.5
2.j
,
i .1
3.5 3.4 6.9
7.5
6.0
6.3
11.2
9.8 3.5 3.6
j . 0
6.8
4.0
4.3
10.9 7.0
3.9
4.2
6.6
6.5 7.5 3.5 3 .o
j.8
8.3 8.5
10.0
8.1 3.5
4.5
9.8 6.2
4.2
4.2
5.4
NORA GREGG-WILSOS A S D ROBERT WRIGHT
30'4
TABLE I (Continued) Chloride 4 5 3 3
Bromide
9 9 7 0
2
5.1
2.8 Acetate
Formate
5.3 Propionate
0 4 4 4
4 3 4 6
4.0
4.7
4.5
4
8
2
2
2 . 5
2
.0
I T
2.6
2.;
12
6
I2 0
6 4 6 0
5 0
4 0
8.6 8.1 3 i
is
4 s
7.8 8.0 3.5 3.3
x/s S/Z
3.0
6 6 3 3
H
Sa
Nitrate 7.6 8.1
5
3 0
Sulphate s,/2
Iodide 5.5 j.8
4 9 4 3
S/4
'
4.2
The values of the t.m.r. for a number of solutions of S/z and S / 4 strengths are given in Table I, and the lowerings of the t.m.r. caused by the presence of some solutes in S / 4 solution are compared with the corresponding lowerings of the t.m.d. in Table 11. The figures for the t.m.d. lowerings are taken from a former paper by one of u9.l TABLE I1 Lowering of T.1I.R. S / i Solutions e1 Br I SO:
H Li Sa Ti SHd
1.6 1.9 3.1 2 . 7
3.5 3.3
2.5 3,1 3.8 3.8
2.8
2.2
2.4
I
.6
2.1
3.0 2.5
6.3 5.3 4.7
Lowring of T.1I.D. S / 4 Solutions
H L, Sa Ii KH4
CI 1.3
Br I.?
I 2
2
KO, 3.1
1.4
1.9
2 3
3.1
3.1 2.8
3.; 3.2 2.3
4 0
5.0 4.5 3.6
I
.8
3.7 2.7
I t will be seen that for any given solute the lowcrings of the t.m.r. increase with concentration, but it is not certain that the two are directly proportional t o each other. I n comparing the density with the refractivity effects it must be remembered that the former are capable of much more accurate determination. Physical Chemistry DepaTlment, Glasgoz rnioersit y. .Ifarch 31, 1931,
J. Chem. SOC.,115, 119(1919)
