M. van Swaay and R. F. Lolley' Kansas State University Manhattan, Kansas
A Simple Constant-flow Device for Use in Titrimetric Analysis
Devices for the delivery of titrants and other liquids a t constant rate have been in use for many years. The design may be based on positive displacement or on restricted flow. I n all cases a source of energy is required to drive the device, usually in the form of a smaU electric motor. In restricted flow devices, gravitational forces may be utilized; the energy then is introduced as potential energy of the liquid itself. It will be clear that with gravity feed devices, the rate of flow must necessarily decrease with time. Because of this inherent limitation, the use of gravity feed devices has been restricted to applications where extreme constancy is not required or where the economical advantages of the simple construction outweigh all other factors. In view of the tendency to introduce students to instrumental methods of analysis a t an early stage of their education, a need was felt for a constanLnt-flowdevice with precision comparable to that of the common buret, but simple enough to allow its use in introductory courses.
' Present address: Department of Chemistry, Purdue University, Lafayette, Indiana.
Design and Construction
Figure 1 is a diagram of the construction. The titrant is separated from the water overflow system by a thin sleeve of polyethylene film,2 which is virtually impermeable to commonly used titrants. It is folded over the edge of the glass body a t the top and stretched over the ground glass joint a t the bottom to hold it in place. Dimensions are not critical and may be adapted to fit the size of sleeving available. A ' / w joint was used for the units described, which had a titrant capacity of about 250 ml. Because of the flexibility of the film, pressures on both sides will be equal a t aU times. It should be noted that proper operation of the device is not contingent on equal densities of the titrant and the water. At equilibrium, the pressure on the titrant a t any point below the overflow level of the water will be equal to the pressure in the water a t the same height, and will be independent of the density of the titrant. Although the oontrolbmg pressure a t the restriction is the sum of the hydrostatic and hydrodynamic pressures, the latter may be made very small by suitable ' A sample of polyethylene sleeving was made available through the courtesy of S. Riekes and Sons, Wichita, Kansas.
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design of the overflow system. The dynamic pressure is largely due to resistance to the flow of water in the body of the device and a t the overflow edge. Placement of the water inlet port close to the overflow edge minimizes the first factor. The resistance of the overflow edge depends on the surface tension of the water and on the shape of the edge, and cannot be made negligibly small. Consequently the pressure, and hence the rate of titrant flow, will increase slightly with increasing water flow rate. Thus, fine control of the titrant flow may be achieved by adjustment of the water flow.
O.loC/hour. Even the seasonal fluctuations were found to be less than 2'C. Performance
To test the performance of the device, two units were set up and filled with 0.1 M acid and base, respectively. Their flow rates were then adjusted for equivalent flow of the two reagents. The outflowing solutions were mixed in a small flow cell, and the pH of the cell solution was recorded as a function of time. A typical result is shown in Figure 2. At the points marked a and b, single drops of acid, and base, respectively, were added
Figure 2. Differential l e d of two constant-flow devices Motched flows of acid and base are mired in o Row cell, and the pH of the resulting d u lion is recorded os o function of time. Pdntr o and b indieole odditions of single drops of acid ond barn, respectively, to the Row cell
manually to the solution in the flow cell. From a comparison of the magnitudes of the random fluctuations and the deliberately introduced disturbances, it can he shown that the variations in relative flow were snialler than 5 pl. Discussion
Flgvre 1.
Cut-owoy view of conrtont-flow device.
The rate of titrant flow can be altered either by varying the resistance of the restriction, or by adjusting the height of the delivery tip. When the restriction consists of a length of capillary tubing mounted with a sliding fit as shown, adjustment of its position allows one to vary the flow rate over a range of slightly less than 2: 1. Thus a set of four capillaries will cover all flow rates in a range of 10: 1. The viscosity of dilute aqueous solutions, and hence their flow rate, changes about 1% per C O . It was found in practice, however, that the jacket of tap water surrounding the titrant acted as an effective thermostat, the temperature of which seldom varied more than
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It is interesting to note that the titrant is in open contact with the atmosphere a t all times, and that the device may be refilled during operation. The volume of titrant solution added displaces an equal volume of water, giving rise to increased water flow a t the overflow edge and through the drain connections. If the rate of refilling is kept within reasonable limits, the extra water can be disposed of without overloadmg the overflow edge or the drain connection, and the system will not be disturbed. The titrant may also be isolated from the atmosphere and kept under inert gas. No precautions are necessary concerning the amount of gas enclosed with the titrant, and even under inert gas the device can be filled simply via a stopcock, without venting of the gas already present. Various refinements might be suggested, such as the use of a needle valve as the restriction and a shut-off valve a t the delivery tip. A capillary restriction was used in the present design for its extreme simplicity, and because a capillary restriction, with its relatively large cross section, shows little tendency to clog with solid impurities.
