C. 1. Wale
Mander College Bedford, England
Salt Cryorcopy
S a l t cryoscopy has recently become a standard method for determining the molecular weight and stability constants of complex and condensed ions ( I ) , hut it is not yet widely known. The purpose of this article is to provide an introduction to the subject, and to describe a simple student experiment. Salt cryoscopy is not, as one might suppose, the use of molten salts but rather the use of concentrated aqueous salt solutions. According to the phase rule, two types'of fixed point may be found-transition point of hydrated salts and eutectics. Both are depressed by a second solute, and both may be used for molecular weight determination-the most widely nsed system being the sodium sulfate decabydrate transition. Apart from supercooling, simple aqueous cryoscopy is not very satisfactory with ionic compounds. The activity coefficients vary with the ionic strength, so that the van't Hoff factor is critically dependent upon the total concentration. In turn, this means that extrapolation to infinite dilution is not very accurate. Moreover, the calculated molecular weight is the average of all ions present. It is thus particularly difficult to measure the molecular weight of a highly charged ion, since the osmotic effect of the large ion is swamped by the smaller counterions present. These diiculties are largely overcome by salt cryoscopy. One is working in a medium of high and virtually constant ionic strength, which in the case of sodium sulfate is 3.5 M. If one adds, say, sodium chloride, to a first approximation the sodium ion has no effect, and the effect of the chloride is proportional to the concentration, i.e., the activity of a common ion is zero, and a foreign ion proportional to concentration. For accurate work it is still necessary to extrapolate to infinite dilution, as in simple aqueous cryoscopy, but the variation with concentration is both smaller, and linear instead of square root. Although the method has only become important recently, the phenomena have been known for some time. The first reference to the effect of foreign salts on the sodium sulfate transition is 1895 (t);van't Hoff (3) in 1898 measured the effect of sodium chloride. Hoenen's (4) classic paper of 1913 on the thermodynamics of phase transformations included the sodium sulfate transition. References (5-7) give more recent theoretical studies. The method is particularly valuable for elementary and intermediate students. The experiment described here is simpler, easier, and much quicker than conventional cryoscopy, whether using water or organic solvents. At this level it is best to treat it as the depression of freezing of sodium sulfate crystals. The usual graphical theory depending upon Raoult's law and vapor pressure curves can be given.
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DEPRESSION Figure 1. Freezing point depression versus molecular depression for various solh Redrawn from Dormois 18).
Experimental
Salt cryoscopy was put on a sound experimental footing as a method of molecular weight determination (8) by Darmois (see Fig. 1). Results depend upon Type of thermometer and scale of working Rate of cooling Efficiency of stirring
The last is the most important. Unless all solid is fully fluidized very erratic results will be obtained. It is more a case of efficiency of stirring than high power input. Jennings or Electrothermal microstirrers are suitable for the student experiment. Rate of cooling must not he too fast, but need not be controlled accurately unless results of very high precision are required. The author found a cooling of lo in about 8 or 10 min very satisfactory, and this could be easily achieved with a thick walled centrifuge cup simply clamped in the open lab. Thin-walled vessels gave a higher - and were too sensitive to - rate of cooling, ;rafts. The scale of working is related to the immersion required by the thermometer, and this reacts back on the stirring. The ordinary standard Beckmann thermometers need an immersion of 150 mm or so, for which something like 250 ml of solution is required. At this scale stirring problems have started to become acute, and the whole thing is unwieldy. It is necessary to use a semimicro or micro Beckmann. Several makes of small differential thermometers exist, and the author has nsed the Elliott (cat G.S. 10083) with great success. This thermometer strictly speaking is supVolume 48, Number 8 , August 1971
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posed to have an immersion of 75 mm, but tests showed there is no difference down to about 55 mm. Shorter immersion caused error. Thermistors may be used but are too responsive to transient fluctuations, owing to their very low thermal capacity. They should be put in a small glass tube with a little mercury to act both as conductor and also as a "thermal buffer." Apparatus Required 100-ml thick-walled centrifuge cup Semimicra Beckmann, or thermistor in pocket Semimicro stirrer, with "screw propeller" Thermostat set a t about 34'IsoC Beaker or hasin calibrated to hold 55-60 g of anhydrous sodium sulfate
A rigid support is required, either a "chemi-frame," or an ordinary stand clamped to the bench with a G clamp, or equivalent (see Fig. 2). Figure 2.
Clamp centrifuge cup a t a convenient height and insert stirrer s t an angle. Pipet 50 ml of water a t 341/~0Cfrom flask in tbermostat, leaving room for thermometer bulb. Fill barin with powdered anhydrous sodium sulfate (if lumpy, crush first in pestle and mortar). Start stirrer slowly. Pour sodium sulfate on to a piece of folded glazed paper, then slowly into water. Inorease stirrer speed to keep well mixed, increasing speed as salt is poured in. Leave stirring for about 10 min, then add one crystal of sodium sulfate deoabydrate as seed. Insert thermometer. If a mercury Beckmann is used, it should not be inserted immediately after adding sodium sulfate, otherwise the mercury may rise into the reservoir and upset the calibration. Read a t approximately 2-min intemds until the reading is steady for a t least 5 min. Remove stirrer and thermometer. Clean with soft tissue. Weigh 1.8 g (10 mmoles) of A.R. glucose into a clean cup. Repeat experiment, taking care not to add sodium sulfate until all glucose har dissolved. Calculate K from depression of transition point, basing calculation on 50 g water. Repeat experiment again using unknown (without extrapolation). Calculate molecular weight.
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Journal of Chemical Education
Photograph courtesy of K.
Notes The error without extrapolation is about 1 or 2%, i.e., very similar to other colligative methods for neutral substances, and very much better than for ionic compounds. In particular, i values for binary salts are usually just above 2 in contrast to values about 1.8 for simple cryoscopy. Suitable salts for elementary work are CuS06.5Hn0,KCI, NaC1.
Literature Cited (1) Rosson'l, F. J. C.. AND R 0 9 8 0 ~ ~H. 1 . 6..J . Phys. Chem., 63, 1041 (1959). (2) Lowarrrmz, R.,Zeit. phys. Chem., 18, 70 (1895). (3) TAN'T HOPP. I. H.. A N D S A ~ N D E R A.BW., , Sitz Ber. Akad. Berlin, 387
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(4) HOENEN, P., Z. phm. Chem.. 83. 51H (1913). R.,"Changement de phaaes," Sooiete de Chimie Physique, Paria, (5) HAASE, 1952. (6) Tonrw R.S.,J. Inore. Nuclear Chem., 19,348 (1961). (7) Fsnamomz-P~NIE, R.,m n P n u ~I., E., J . Chem. Soc., 1974 (1967). (8) Dhn~ora,E.,"Activit4 des solutions slectrolytiques:' Herman, 1943.
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The experimental opparotur.
Whitbread, Bedfordrhire County Photographer.
Method
