Using the Electrician's Multimeter in the Chemistry Teaching Laboratory Part 1. Colorimetry and Thermometry Experiments Roberto T. Andres and Fortunato Sevilla Ill Research Center for the Natural Sciences, University of Santo Tornas, Espana, Manila 1008, Philippines The multimeter is an essential piece of equipment on the workbench of an electrician. It is a universal instrument, useful not only for measuring electrical quantities, such as voltage, current, and resistance, but also for testing electrical and electronic circuits. The multimeter could also be a verv useful instrument for the chemistry laboratory bench. kontemporary electronics and instrumentation have reached a level of a ~ ~ l i cation that allows the measurement, direct or indire% of chemical auantities. These measurements involve the encoding of ehemical information into electrical signals. This trausduction can he carried out through a variety of physical and chemical principles, and for most cases, the reskltant electrical quantity could be measured using a multim.....
In this paper, the versatility of the multimeter in the chemistry teaching laboratory will be demonstrated. Lowcost., simble measurine that use a multimeter will - svstems " be presented, and their application in the teaching of chemical principles will be described. These experiments involve a clear-box approach, rather than a black-box approach, to chemical measuring instruments.
.
Colorimetry Colorimetry involves the measurement of the concentration of a chemical species through the amount of light absorbed at certain wavelengths. The extent of absorption of light is measured through the intensity of the radiation transmitted by the sample solution. A multimeter can perform colorimetric measurements when coupled with a photodetector, such as a light-depen-
- plastic cap Rubber Stopper Test Tube
u
9V Battery
Figure 1. Diagram of the colorimeter assembly. The appropiate LED Ired. areen. or vellow) is connected to a 9-V Banerv bv alliaator clios. ihe'r&istoi in series with the bitterv, ,iotects ihe ~ - ll'kfll ~ . cbnnected a ode oy m i ng me current in tne clrcdit. The corresponolng -DR. wh ch IS d renly across from the LED. IS also connected lo me mJIt meter leads tnro~ghall galor cl ps. ~
~~
514
~~~~
~~
~~
Journal of Chemical Education
m.
dent resistor. The schematic diamam of an easily constructed colorirneter assembly is srown in Figure 1.i t consists of a cell holder made from a 9.0-cm-long PVC tube (outside diameter: 2.5 em) on which light-emitting diodes (LED'S)and light-dependent resistors (LDR's) are set opposite each other. The LED supplies radiation of certain wavelengths and is powered by a 9-V battery. The LDR is a semiconductor whose resistance depends on the intensity of the radiation striking its surface. A multimeter is used to measure the r e s i s t i c e of the LDR. An ordinary test tube is used as the sample cell. It is fitted into the cell holder, which optically isolates it. This simple setup allows a highly precise and generally accurate chemical analysis. It is adequate for a number of experiments on chemical equilibrium, chemical kinetics, stoichiometry, and quantitative analysis. I t is adaptable for experiments in clinical chemistry, environmental analysis, industrial monitoring, and other applications in chemistry. A wide range of experiments can be devised around it, a few of which are described below. Calibration Curves
Plastic Base
~
Figure 2. Calibration curve obtained foraqueous solutions of copper sulfate,using a red LED. R is the multimeter reading expressed in
~
Photometric analvsis reauires the use of calibration curves that relate tce concehation of a chemical species with a function of the radiation intensitv ( I ) . This curve is constructed by preparing several standard solutions and determinine the colorirneter resnonse. A linear relationship exists 6etween the molar concentration and the loga-
r
1
'
3.2
4.2
5.2
6.2
7.2
0.6
8.2
PH +
Bromocresoi green
-A-
0
1
2
3.1 mole ratio
3
4
5
mole ratio (PhenanthrolineIFe) Figure 4. Plots of the logarithm of the multimeter reading, in M, against the mole ratio of phenanthroline to Fe(lll).
Bmmothymol blue
Figure 3. Plot of pH versus multimeter readings, in M, for 0.05% solutions of bromocresol green and bromothymol blue in 20% ethanol. Readings were taken using a red LED. ritlun of the colorimeter response. The calibration curve obtained for solutions of CuS04 is shown in Figure 2. The performance of this assembly is comparable to that of the Spectronic 20, especially when a digital multimeter is used. Satisfactory results have been obtained for colorless substances, such as glucose, cholesterol, and phosphate, which were derivatized into colored species. Chemical Equilibrium The equilibrium constant for systems involving colored species could he evaluated colorimetrically.The results obtained for the ionization equilibrium of acid-base indicators are illustrated in Figure 3. Buffer solutions of varying pH were prepared, and 1mL of the indicator solution was added to 5 mL of the buffer. The intensitv of the color of the solutions was determined using the colorimeter, and the colorimeter readings were dotted arrainst the OH of the solution. The inflection pointor the midpoint of 'the steep portion of the sigmoid curve gives the pK of the sample indicator (I). The experimentally determined values were not significantly different from those reported in the handbook (21, attesting to the adequacy of the simple setup used.
Chemical Kinetics Colorimetry also provides a convenient technique for monitoring the rate of a reaction that involves a color change (I). Determination of the color intensity of the reaction mixture during the progress of the reaction could provide data on the kinetics of the reaction. The effect of concentration, temperature, and other factors could be studied through the colorimeter assembly. The simplicity of the procedure used allows the collection of much data that could yield information on the rate and mechanism of the reaction. The results obtained for the reaction between acetone and iodine ions are shown in Figure 5. Enzyme reactions, such as starch hydrolysis, could also be investigated with satisfactory results using this colorimeter setup. Thermometry
Thermometry or temperature measurement is another common laboratory operation that can be conveniently carried out using the multimeter. The usual technique for temperature measurement involves a glass thermometer, which is based on the thermal expansion of a liquid (usually mercury) in a capillary tube. A temperature scale is devised through the height of the liquid column in the tube.
Stoichiometry Some ions combine to form a colored complex. The combining ratio of the reactants can be determined through the intensity of the color generated by the reaction (1).A mole-ratio method could be used: Varying amounts of one reactant are added to a fixed amount of the other reactant. and the color of the reaction mixture is measured. Acontinuous variation method could also he used: Several reaction ~ - mixtures containing different ratios of the two reactants are prepared, and the amount of the colored product is determined. Figure 4 shows the results obtained for the reaction between iron(I1) and l,lO-phenanthroline using the moleratio method. The results obtained with the instrumentation setup were consistent with the theoretical values. ~~
~~
1 0
time (min)
4.0
Figure 5. Plot of the kinetic data for the reaction of iodine with acetone. Readings, expressed in kn,were taken using a green LED. Volume 70 Number 6 June 1993
515
!
An electronic alternative for the laboratory measurement of temperature uses a thermistor coupled with a multimeter. The thermistor is a semiconductor whose resistance is a function of temperature. Thus, temperature Plastic Tube measurement could be carried out through the determination of resistance of the thermistor, which functions as the transduction system. Figure 6 shows the schematic diagram of an elecThermistor tronic thermometer based on a Lco per ~~~b~~~~ thermistor. The relation of the resistance of the thermistor to Buk the temperature is shown in a semi-log plot in Figure 7. This electronic thermometer ~i~~~~ 6. ~i~~~~~ of the construction of a simple offers several advantages. It is electronic thermometer. not as fragile as the glass thermometer. Readings can be easily made without the difficulty encountered using the fine scales of a glass thermometer. Furthermore, remote measurement could be carried out with the multimeter far from the measurement system. The small dimens~onsof the thermistor also make it useful in sites that cannot be reached by an ordinary thermometer. CalorimetricMeasurements The determination of the heat of solutions and heat of reactions require the measurement of temperature change accompanying the process. This measurement can be carried out under adiabatic conditions using a styropor calorimeter. The first part of this experiment involves the detennination of the heat ca~acitv " of the calorimeter svstem (3).A measured quantity of warm water is initially placed in the calorimeter, and its temperature is determined. Then a known amount of cold water, whose temperature has been previously determined, is added. The equilibrium temperature change for the mixture is recorded and used to calculate the heat capacitv - .of the calorimeter. In the second part of the experiment the mixing process (e.g., sulfuric acid and water; sodium thiosulfate and A
Mole ratio (NaOHIHBO,) Figure 8. a. Plot of the temperature change versus the mole ratio of (a)sodium hydroxide to HCI. b.Plot of the temperature change versus the mole ratio of (b) sodium hydroxide to sulfuric acid in a continwater) or neutralization process (e.g., HC1 and NaOH) is carried out in the calorimeter. The change in t e m p e r a h is measured. When the heat capacity of the calorimeter system and the specific heats of the solutions are known, the heat of reaction can be calculated. Stoichiometry
- 0 . 1
0
20
40
60
80
100
Temperature (C) Figure 7. Calibration curve for the electronic thermometer. R is the multimeter reading expressed inn.
516
Journal of Chemical Education
Practically all chemical reactions are accompanied by heat changes. The heat involved in a reaction depends on the amount of reactants. It is therefore possible to investigate the stoichiometry of the reaction by observing the heat effectsaccompanyingthe reaction (4). A thermometric study of the reaction stoichiometry can be carried out using an electronic thermometer. A continuous variation scheme could be used: A series of solutions of varying mole fractions of the reactants are used, and the temperature changes are measured. The results obtained for the reaction between NaOH and HCI and between NaOH and HzSOa are depicted in Figure 8. The maximum temperature variation occurs at the stoichiometricratio.
Square of HCI concentration
.-
0
1
2
3
4
Figure 10. A kinetic plot for the reaction of Mg with hydrochloric acid. The rate was calculated as the reciprocal of the time required forthe temperature of the reaction mixture to change 2 OC. Concentration was ex~ressedin mol/L.
lime (min) Figure 9. Cooling curves obtained from (A) pure naphthalene and (6) a mixture of naphthalene and benzoic acid, obtained using the electronic thermometer. Phase Equilibrium Phase eauilibria can be investigated through thermal analysis. '&is method involves the determinaGon of coolinp curves for the test system (3).From such curves, the temperature at which a phase change starts and ends can be determined, and the temperatures a t which transformations and transitions oecur can be identified. The appearance of a new phase is indicated by a change in the rate of change of the temperature of the system, and the occurrence of an equilibrium state is manifested by a constant temperature. The electronic thermometer based on a multimeter can be used to carry out a thermal analysis. Figure 9 shows the cooling curves obtained for a melt c o n t a i n i i pure naphthalene (curve A) a mixture of naphthalene and benzoic acid (curveB)
rate constant. order of reaction. rate eauation. or reaction mechanisms. Fimre 10 shows a kinetic k lot for the reaction between Mg and HC1. Several mixtures containing different coucentrations of HC1 and a constant amount of Mg were prepared. The time required to reach a given temperature change (e.g., 2 'C) is then recorded for each mixture. The straight line obtained by plotting the square of the acid concentration against the reaction time indicates that the reaction is second-order with respect to HC1. Conclusion Many laboratory experiments for the introductory chemi s t course ~ can be carried out using colorimetric and thermometric systems based on a mdtimeter. The measuring systems involve components that are readily obtained from electronic shops and then easily constructed by the students. This approach enhances the students'understanding of the chemical principles. It also helps them to appreciate the working principle of chemical instrumentation and the benefits of modern technology
These cooling curves can be used to illustrate the effect of a solute on the solidification behavior of a liquid. These curves could also be used for an experiment on mlligative properties.
Acknowledgment The authors gratefully acknowledge the financial support of the United Nations Educational, Scientific, and Cultural Organization (UNESCO) and the assistance of Paolo Quiton in the thermometric experiments.
Chemical Kinetics
Literature Clted
Measurement of heat changes can also be used to study the kinetics of a chemical reaction. The electronic thermometer and the calorimeter assembly described can be used to obtain kinetic data suitable for determining the
.
.
.
.
ety: London. 19%. 4. Mahoney, D.W.at al. J. C h m . Edue. 1981,58,730.
.
.
Volume 70 Number 6 June 1993
517
