The major issue for the general chemistry laboratory has been, and continues to be, whether laboratory time should be spent in training students in the use of apparatus, instruments, and standard procedures of analysis, or in having students learn how to ask and find answers for testable scientific questions, and how to obtain and interpret reliable data.
Use of the glass memhrane electrode to measure hvdrogen ion activity of solutions is important to both theorkical and experimental branches of chemistry, and experiments using this electrode are found in most undergraduate programs. With the design of electrodes which can selectively measure the activities of other ions, experiments which can introduce students to the theoretical and practical aspects of such electrodes would be quite useful. Recently several procedures, which can he adapted for student use, have been published describing the construction of ion selective electrodes (1-4). These electrodes are less expensive and in some cases more stable than the commerciallv available mcdels. In addition, the construction by the student of the electrode itself is a useful exercise in develooine an understanding of electrode design and of the fuktron of each part of the cell in the measurement of ion activities. Too oftemthese aspects escape students using commercial electrodes.
The Ion Selective ElectrodeSeveral types of ion selective electrodes have been constructed (5). All contain a material which will interact with some selectivity with an ion whose activity is to he determined in solution. One such electrode which is auite convenient for student preparation involves incorporating a liquid ion-exchange material into a hinder (polyvinylchloride, PVC, silicone rubber, etc.) to form an ion selective membrane. The PVC matrix is relatively inert, stable, inexpensive, simple to use, and, therefore, recommended for student electrodes. The resulting membrane can be attached to a tube made of anv inert material. The tube is then filled with a solution containing the ion of interest, and an internal reference electrode is inserted. A half-cell assemblv is thus constructed which can respond to external changes in activitv of the ion of interest. The liauid ion-exchaneer within-the membrane provides sites whiih selectively interact with the ions from the internal reference and external sample solutions. The process involved is a form of ion-exchange in which ions outside the memhrane are in equilibrium with the surface layers of liquid ion-exchanger. A dynamic equilibrium occurs which involves the two surface layers of the memhrane and a very small percentage of ionsis a constant whose value depends upon the internal and external reference electrodes, 2.3RTIF = 59.6 mV at 25'C; Z represents the charge on the ion of interest; a A -is the activity of the ion of interest in the test solution. The electrode assembly is first calibrated using test solutions of known ion activity. (These standards need to approximate as closely as possible the ion activity of the unknown sample solutions.) A plot of E versus log a*-can he made to be used as a working curve for the determination of unknown ion activities. If there are nonideal membrane effects, then the slope of such a plot will deviate from the value of 59.61 Z predicted by eqn. (1). These memhrane effects will not be eliminated by replacing ion concentrations hy activities. ...
This paper describes a set of inquiry-based experiments designed to help students develop an understanding of basic chemical concepts within the framework of studying the properties and reactivity of metals and metal ions. The students perform these experiments before the concepts are discussed in class, with the emphasize on the construction of meaning from observation, measurement, and data analysis. The set includes: Are All Pennies the Same?, which introduces students to the differences between extensive and intensive quantities. How Much is Enough?, examines the quantitative aspects of the reaction between metals and hydrogen ions leading to the concept of the mole and stoichiometric calculations. How Much is Too Much?, which identifies and distinguishes between limiting reagents, excess reagents, spectator ions and reactant ions. What is an Active Metal?, provides the observations needed to sort common metals into an activity series and How Active are the Active Metals?, allows students to develop a quantitative model of metal activity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.