In the Laboratory
An Apparatus for Fast Dilution V. P. Boiko Institute of Macromolecular Chemistry, National Academy of Sciences, 48 Kharkov Shaussee, Kiev, 253160, Ukraine Dilution is widely used in analytical chemistry for calibration curve plotting and in physical chemistry for determining intrinsic values of many physical properties. This routine procedure, usually carried out in volumetric flasks, is rather time-consuming and tedious. It may take the major portion of laboratory training time, to the detriment of basic measurements. This inconvenience may be avoided by using the apparatus for fast dilution, which we have named a “diluter-meter”, or “dilumeter”. The apparatus illustrated in Figure 1 consists of a glass dilution vessel with a cap. For measurements of temperature-dependent properties, the vessel can have a jacket for circulating a heat-transport liquid. The cap can have one or several conical ground sockets or can be simply a rubber stopper with holes for setting up necessary units. The simplest model of a fast-dilution apparatus contains a buret with a stopcock, for dosage of a solvent, connected to a cap with a socket–cone joint, a siphon to remove excess solution from the vessel, and a tube to create excess pressure in the vessel. The siphon’s upper end is above and its lower end is below the bottom of the vessel. When dilution is performed in an inert atmosphere, holes are provided in the vessel–cap joint. They are aligned during solvent delivery, connecting the vessel with the atmosphere to prevent spontaneous filling of the siphon and discharge of solution. In the measurement position they are displaced to make the vessel airtight. The apparatus operates in the following way. A quantity of an operating solution is placed in the vessel (or a solution is prepared in the vessel itself) and thermostated when necessary. The portion above the siphon’s upper end is removed by creating a pulse of pressure through the tube (9, in the figures). Efflux occurs spontaneously, based on the level difference between the upper and lower ends of the siphon. The volume of solution that will remain in the vessel is known precisely from previous determination by an appropriate method (e.g., by weighing water at a fixed temperature). Then a volume of solution that depends on the desired degree of dilution is added to the vessel from the buret. After mixing and maintaining the constant temperature, excess solution is removed in the same way as before. The first portion of solution, which rinses the siphon, is discarded; the rest is used for measurements. The cycle with addition of constant or variable volume is repeated until the required degree of dilution is achieved. When solvent is added in equal volumes, the concentration cn after n operations is
1. Dilution occurs quickly and with a minimum of labor. 2. Dilutions and measurements may be carried out in the same vessel, reducing the requirements for glassware and its washing. Because of that, accuracy is considerably improved, especially at very high dilutions. 3. Dilution is not limited by the capacity of the vessel and may be conducted to any desired low concentration. 4. The apparatus makes it easy to prepare mixtures of two or more liquids of variable composition—for example, to measure the relationship between composition and properties. 5. The degree of dilution is easily regulated by changing the amount of added solvent and/or the height of the siphon above the bottom of the vessel. 6. Simultaneous measurements of two or more physical properties of the same solution are possible through use of several gauges.
cn = c0[v 0/(v0 + v)]n where c0 is the initial concentration of solution and v0 and v are the volume of the initial solution after removing its excess through the siphon and the volume after addition from the buret, respectively. A version of such an apparatus for measuring limiting equivalent conductivity is shown in Figure 2. Here the solution conductivity is measured by means of electrodes that are sealed into the cap. An version of the apparatus, for determining intrinsic viscosity of a polymer solution, is illustrated in ref 2. The apparatus described here has a number of advantages over other modes of dilution:
Figure 1. Fast-dilution apparatus. 1: dilution vessel; 3: cap; 4: buret; 5: stopcock; 6: siphon; 7: upper end of siphon; 8: lower end of siphon; 9: tube for creating excess pressure in vessel; 10: conical ground sockets; 10-11: socket–cone joint; 12: holes in the vessel–cap joint.
Figure 2. Fast-dilution apparatus for measuring the electrical conductivity of a solution. 2: heating– cooling jacket; 13: electrodes; other parts as indicated in Figure 1.
Vol. 74 No. 4 April 1997 • Journal of Chemical Education
439
In the Laboratory The versions of the apparatus for measurements of limiting equivalent conductivity (1) and polymer intrinsic viscosity (2) have demonstrated considerable savings of time and labor and substantial increase of accuracy. One can expect that owing to the above features the described apparatus will be highly useful in laboratory training courses.
440
Literature Cited 1. Boiko V. P.; Boiko T. J.-V. Pribory i Technika Eksperimenta, Engl. Transl. 1992, 725. 2. Boiko, V. P. Polymer Sci. Ser. B 1995, 37, 1225.
Journal of Chemical Education • Vol. 74 No. 4 April 1997
