Jan., 1956
ISOPIESTIC STUDIESOF DICARBOXYLIC ACIDSOLUTIONS
41
ISOPIESTIC STUDIES OF AQUEOUS DICARBOXYLIC ACID SOLUTIONS BY MANSELDAVIES AND D. K. THOMAS The Edward Davies Chemical Laboratories, University College of Wales, Aberystwyth, Wales Received June 8 , 10.56
A precise and convenient volumetricform of the isopiessc method of studying the vapor pressure lowering of aqueous solutions at various temperatures is described. Using sulfuric acid solutions as standards the solvent vapor pressure lowering and activities of both solvent and solute for aqueous malonic, methylsuccinic and gjutaric acids are determined and the activities of maleic, succinic and malic acid are calculated from previously published data. Over the concentration ranges measured (c 0.5 m to 3 m ) most of the solutes are essentially normal but glutaric acid shows marked departures whose moleculrtr interpretation is considered.
The isopiestic method has been widely used for determination of solvent vapor pressures over solutions of non-volatile solutes, and in the gravimetric form developed by Robinson and Sinclair' and by Scatchard, et it is capable of yielding activity data of an accuracy comparable with that obtained by e.m.f. and freezing point measurements. It also offers the advantage of being readily operated over a range of temperature. More recently the volumetric form has been describeda in which attainment of equilibrium is assessed by the constancy in solution volumes, and this is not markedly inferior to the gravimetric procedure. It is a convenient and reasonably rapid arrangement for the volumetric procedure which we wish to describe.
Experimental E uilibration Procedure.-Our arrangement was based
on t i a t of Lassettre and Dickinson" with modifications to allow as rapid equilibration as possible, ease in following small changes in volume, and greater efficiency in the simultaneous control of up to eight pairs of solution whose changes in composition on equilibration at various temperatures could be followed directly. The Pyrex equilibrators consisted of two hemispherical compartments of diameter ca. 4 cm. joined to form a short bent-sausage shape by a 3.5 cm. wide tube (Fig. la): the g.ap between the bulbs was ca. 0.5 cm. X 3 cm., and from their centers there extended measuring limbs of total length 111 cm. having end compartments 1.6 cm. diameter, 3.5 CIP. long joined to the stem which was of diameter 0.7 cm. (yig. lb). About half-way along each stem was a fine etched line from which the meniscus position in the stem could be measured by a cathetometer reading to 10.001 cm. An olpen limb (Z, Fig. l a ) allowed an appropriate volume (ca. 6 ml.) of each solution being carefully introduced into the soparate compartments from a pipet with a long fine stem. I11 the equilibrating position (Fig. l a ) these formed shallow pools between which free passage of vapor was facilitated by the wide connection and further promoted by gentlt rocking and previous evacuation. On turning through 90 the solutions drained into the measuring limbs (Fig. lb), the narrower stem section ensuring adequate accuracy in measuring volume changes, as 0.001 ml. corres onded to 0.003 cm. in meniscus position. Measurement o f t h e meniscus height for varying weights of mercury in these limbs provided an accurate calibration of the corresponding volumes. For aqueous solutions, the reproducibility of drainage from the compartments into the measuring limbs was one of the limiting factors in our accuracy. To attain the requisite cleanliness of the glass it was found best to allow alcoholic potash to stand in the equilibrator for several hours followed by successive washings with distilled water. After the two solutions had been introduced without splashing, the equilibrator was turned into the measuring position and the solutions frozen ( -20") before removing
the air with a rotar oil pump. A screw-clip on the connection to Z (Fig. l a f w a s then closed and the solutions allowed to melt, releasing their dissolved air, refrozen and the evacuation re eated. This time Z was sealed off with a hand-torch. gests, in which the evacuation was continued for much longer periods than were normally used and the change in weight of the equilibrator and contents determined, showed that negligible concentration changes were produced by the evacuation. The equilibrator was clamped in a stand in which i t could be turned precisely, without being touched, from position a to b and the whole placed in the thermostat: all subsequent observations were made on it without moving its stand in the thermostat. This operation was completed before the frozen solutions had warmed up so as to prevent condensation of vapor on any part of the vessel not subsequently reached by the solutions in their transfer from the compartments to the measuring limbs.
Lib ..... ,cy; .... . .. ........
0
.. .-.-..-. .
.....
la
lb
Fig. 1.
Temperature control during equilibration is the most obvious factor limiting the accuracy of the isopiestic method. For dilute aqueous solutions at 25" a temperature difference of 0.002' causes a solvent vapor pressure change of 0.003 mm., whilst an ideal 1 m solution has a vapor pressure lowering of only ca. 0.4 mm. Our thermostat consisted primarily of a slate tank 20" X 20" X 18" surrounded by 1.5" of glasswool insulation held in a pressed board box and having a plate-glass window, in which water could be rapidly circulated through inlets in the base connected to a rotary pump. Heating was by four 60-watt units whose positions were chosen to give maximum uniformity of temperature, and control was effected from a long mercurytoluene regulator constructed in a planar serpentine form. The equilibrator stands were mounted in two glass tanks (8" X 11" X 14") placed on a light metal platform givoted at its center so that the whole could be rocked f 1 0 by an eccentrically-driven rigid coupling: the whole assembly being immersed in the main thermostat. The rocking served to stir gently the solutions and to promote vapor transfer as well as to stimulate further the water circulation in the thermostat. An essential improvement was found in tapping the main water circulation throu h the pump to provide controlled streams to the bottom of the immersed glass tanks via a long copper spiral In the outer bath. .As a result, the temperature within the glass tanks varied neither with position nor with time (up to periods of 100 hours) by more than h0.002 a t 25': at 45' with a t
