An Inexpensive Solid-Solution Mixer for Initiating Moderately Fast Reactions Study of the kinetics of aquation of compounds with half-lives SoLID-yxu''~ MIXER of seconds is difficult to achieve with conventional speetrophotometers and standard spectrophotometer cells. Many undergraduate and some graduate laboratories are not equipped with expensive stopped-flow or flow-mixing equipment which are normally employed in the study of fast and moderately fast reactions. Clearly, the ability to study moderately fast reactions with low-cost equipment would allow the types of kinetics experiments available in the undergraduate laboratory to be expanded, and would substantially compress the time required for completion of experiments. For example, base hydrolysis reactions of cohalt(II1) complexes in various concentrations of hydroxide ion are amenable to study when the relatively fast rates can "he measured. A simple and inexpensive syringe type of apparatus for mixing a powdered solid directly with pre-thermostated aqueous solutions has been developed. The apparatus (see figure) can be readily constructed by a competent workshop technician entirely from plastic, apart from a Teflon' tap and a standard glass sinter. The powdered compound is placed on the glass s i n k (porosity 1or 2, ground to fit in the base of the detachable holder), and the unit assembled with the tap closed. Sufficient solution to nearly fill a standard spectrophotometer cell is placed in the syringe, and the syringe piston forced in to produce a slight positive pressure. If required, thermostated water can he circulated about the syringe body. The apparatus can be mounted with clamps on a stand above the cell compartment of a spectrophotometer, and a short flexible plastic tube attached and fed through a small hole in the compartment lid directly into the top of a standard cell mounted in the cell compartment. Reaction is initiated by opening the tap and pressing down gently on the syringe piston, then immediately switching on the chart recorder of the spectrophotometer and following the reaction a t a preselected wavelength. Alternatively, the apparatus can he hand-held above the in situ cell, the solution injected, the compartment lid quickly placed in position, and the reaction monitored as usual. In either case, the reaction can be monitored within seconds, thus allowing determination of relatively fast rates. Further, with sparingly soluble compounds, any undissolved solid is retained on the glass frit and not injected into the cell. However, in most cases, complete dissolution can be achieved. This apparatus has been successfully used a t the research level for following aquation reactions of metal complexes with labile leaving groups where half-lives as short as 5 s have been determined (Dixon, N. E., Jackson, W. G., Laneaster, M. J., Lawrance, G.A., and Sargeson, A. M., Inorg. Chem.,20,470(1980)).As described,the apparatus is suitable for aqueous and alcohol solvents; built entirely of the more expensive Teflon@,it would he able to accept d l nouaqueous solvents. Repetitive experiments can he conveniently performed, since the detachable holder and glass frit can be washed with water, then alcohol, and dried rapidly by a stream of compressed air.
k,.-w"",
-
Geoffrey A. Lawrance Research School of Chemistry me Australian National University P.O. Box 4, Canberra 2600, Australia
Volume 60 Number 8
August 1983
663
