In the Laboratory 258 Jo urna l o f Chemic a l Educ a tio n • Vol. 77 No. 2 February 2000 • JChemEd.chem.wisc.edu A Low -Cost Device for Automatic Photometric Titrations Fábio R. P. Rocha Departamento de Química, Universidade Federal de São Carlos, São Carlos, SP, Brazil; frprocha@cena.usp.br Boaventura F. Reis* Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, PO Box 96, 13400-970 – Piracicaba, SP, Brazil; *reis@cena.usp.br Electronics is an important topic in chemistry courses, since most instruments employed by chemists involve electronic components in their construction and operation. Understand- ing of some fundamental concepts helps to comprehend how modern instruments work, enabling students to recognize their advantages and limitations. Frequently, new devices nec- essary for characterizing chemical systems need to be devel- oped by chemists themselves and some knowledge of elec- tronics allows chemists to get additional information from an expert. The importance of teaching basic concepts in elec- tronics is evident in modern analytical chemistry. Nevertheless, teaching electronics is troublesome in chemistry courses, and several authors have suggested valuable ways to incorporate these topics in instrumental analysis courses (1–4). In this work, we propose the construction of a simple and inexpensive de- vice, a photometric titrator, that is useful to demonstrate some basic concepts in electronics, showing its importance to mod- ern analytical chemistry. Some simple instruments frequently used in analytical labs, such as spectrophotometers, can sometimes be seen as “black boxes” by the students. It is therefore useful to know some basic electronics, as common electronic devices can be employed to build simple instruments that can be used for analytical measurements. In addition, some important classical procedures (titrimetry, for example) might become more interesting by association with simple electronic circuits. T his can be adequately stressed by means of demonstrations or laboratory experiments. Volumetric procedures (titrations) are among the oldest methods employed in analytical chemistry. T heir uses include determination of physical chemistry parameters, such as equi- librium constants, and determination of concentrations by acid–base, redox, precipitation, or complexation reactions. Several procedures based on this kind of analysis are consid- ered as standard or reference methods for the determination of a large number of substances (5, 6 ). Titrations are based on the change of a physical property of the solution due to titrant addition. Concentrations are determined from the stoichiometry of the involved reactions and from the volume (or mass) of the titrant. The end point can be detected visually or by employing a sensor that responds to changes in the monitored property. Changes in the color of the solution are among the most frequently used strategies to find the end point of titrations. These changes can be followed visually or through photometric measurements. T he latter is more suitable because it is less dependent on the perception of the analyst and the color of the samples has less influence. T he fundamentals and some applications of photometric/spectrophotometric titrations have been presented (7–10 ). However, special apparatus is necessary to perform ti- trations using conventional spectrophotometers, since it is necessary to stir the solution and to measure absorbance (or transmittance) after each addition of the titrant. Sometimes, these devices are not available in analytical teaching labs. Another drawback to performing spectrophotometric titrations is the time spent to complete the analysis, which can make the procedure tedious, especially when a large number of samples needs to be analyzed. An alternative is the use of automatic procedures to perform volumetric analysis. According to definitions recommended by IUPAC (11), a procedure is considered automatic when at least one operation is controlled by a feedback system without human intervention. Feedback is defined as the use of the output of a device to modify the operation of an analytical instrument. In this sense, a titrator could be considered automatic if it is able to stop the addition of the titrant when the end point has been attained or to add an additional amount of the titrant if the change in the monitored property does not persist. Experiments with automatic equipment are described in some didactic articles (12–14 ), but automation is seldom employed according to IUPAC definitions. T his work proposes the construction of a simple low- cost apparatus (about US $150) to implement photometric titrations without a commercial spectrophotometer. The titrator is assembled employing a buret, a three-way solenoid valve, and electronic devices available commercially. It permits control of the addition of the titrant by switching the solenoid valve on or off until the end point is attained, as detected by a change in the color of the solution due to the presence of an indicator. A simple electronic circuit, which can be assembled by the students themselves, is employed to control the system. The assembly of the circuit is valuable to introduce to the students some fundamental topics in electronics that are useful in modern analytical chemistry. An automatic titration can be performed without control of a computer and terminology related to automation in analytical chemistry can be discussed. T he experiment is also useful to emphasize the utility of the classical methods, such as volumetric analysis, that remain in use in laboratories of chemical analysis. Construction of the Titrator Apparatus A three-way solenoid valve (model 161T 031N, manu- factured and supplied by NResearch) was employed in the construction of the photometric titrator. This is an electro- mechanical device usually employed to control flowing liquids or gases (15 ). Inside, these devices have a metallic moving