edited by JOE RICH Blackhawk Christian School
Inventory Control Construction of a Photoelectric Colorimeter and Application to Students' Experiments. Tsulomu Matsuo KobeTechmcal College Ma~kDdaiTarurn#-kuKobe 655. Japan Aklhlko Muromatsu, Kazuko Katayama, and Minako Morl Facuky of Education Kobe Un~vers~ty NadaXu Tsurukabuto Kobe, 657. Japan In recent years the functions of chemical measuring instruments have advanced remarkahly. Though they have heen miniaturized, they are high-performance instruments owing to the introduction of the electronic circuit andmicrocomputer technology. The chemical measuring instruments for students have also progressed. Now students use the same instruments in use a t commercial companies or lahoratories. Since the commercial measuring instruments are developed for rapidmeasurement and easy operation, the measurement is performed hy only pressing some keys on the instruments. However, these instruments are unsuitable for beginning students who are learning the principles of measurement. The history of research and development of chepical instrumentation took some time before high-level instruments were produced. They have been improved step by step from the early stages to the modem advanced type we now use. Therefore, we must give students the instruments that will help them understand the principles of measurement, and also, we must leave room for improvement and innovation on the student's part. Therefore, the instruments for students must be different from the commercial type. Many teachers are acquainted with the SPEC 20. We have developed the photoelectric colorirneter (PEC) ( I ) , which can he assemhled by the students themselves. The PEC can do much the same work as the SPEC 20, but a t a greatly reduced cost. The level of its measuring technology is from an earlier period; however, i t is sufficient for the student's experiments. Formerly, the handmade instrument could not compare with the commercial ones in function and accuracy. But, with the new electronic circuit elements and electronic parts, the function and accuracy of handmade instruments are comparable to commercial ones. Since the development of the spectrophotometer, the PEC is not frequently used. But the fundamentals of the spectrophotometer were based on the PEC. Therefore, beginners must get used to the PEC first, so that they will understand the principle of absorptiometric method. For example, one of the attractive points of chemistry for the students is the varied and beautiful colors of the solutions. The students are impressed by the coloration after the mixing of colorless solutions. The PEC is an instrument that examines quantitatively the color change in the solutions. I t can be used for quantitative analysis and also used for the verification of various chemical principles in student's experiments.
Fort Wayne. IN
incident light and transmitted light, respectively, f is the molar absorptivity, b is the cell path length, and c is the concentration of the solution. Though the assembled PEC can measure the transmitted linht intensitv, it cannot measure the incident light intensit;owing to the optical system of the assembled PIX. Thus Lamhert-Beer's law cannot he applied an it is. Then, by independently applying LambenBeer'slaw to the solution and thesolvent systems, theabsorbance with the use of PEC can be derived as fnlluws: A = log (I'lI) = ebe
(2)
where I' denotes the transmitted light intensity for the solvent and I denotes that for the solution. Accordingly, by measuring the values of I and P, the absorbance of the solution can be derived: that is, with the measurement of the light intensity for the solventand test solution, the absorbance can be determined. In the case of the assembled PEC, I and I' are measuredas the output voltages, which are proportional to each light intensity. Then the absorbance is given hy A = log (E'lm = rbe
(3)
where E' is the output voltage for the solvent and E is for the test solution. This explanation of the principle of measurement with the assemhled PEC must be given to the students before they use it. Construction of PEC The PEC was designed to help in the understandingof the prineiple of spectrophotometricmethod and to he assembled by the student with ease. Special attention was paid so that it would be able to he assembled without special ability by student. Modern commercial electronic parts were sufficiently used and were put to new use in the PEC. By using the light-intensity sensor and OP amplifier (3), the teacher or student can assemble it and ohtain the same expected quality. . . The cost was also taken into account. The side view of assemhled PEC is shown inFieure 1.EachDart of PEC is described below. Part A in Figure 1 is thipowrr supdly and amplifier for the lighr-intensity sensor; it sits in a aluminum box ('251) X 150 X 60 mm). H is theoptical system consistingof n lnrnp,a cell, and a light-intensity sensor, and these can be seen hy removing the side cover. The principle of the absorptiometrie method is easily
Principle of Colorimetry with PEC
According t o LamhertiBeer's law (2),the absorhance is expressed as A = log UdI)= cbe
\
10 (1)
where A is the absorhance, loand I are the intensity of the
8
Figure 1. Side view of assembled photaelechic colorlmeter (PEC); A,, Power supply and amplitlw: 6, optical system. Volume 66 Number 4
April 1989
329
understood by observing the light passing through the test solution. 1is a lamo with the~~~. small lens 12.2 V.. 240 mAl~for the lieht . source. 2 is the diaphragm of the plastic plate (center radii, 1.O-3.0 mm). The radius was chosen to give the highest absorbance according u, the concentration of thesolution. Thesmall radiusgiver high sensitivity for quantitative analysis hut the stability of output voltage hecomes critical. 3 is a filter of colored cellophane, which can he exchanged according to the color of the test solution. 4 is a plastic cell (1 = 10, w = 10, h = 45 mm). This cell is same size as far the spectrophotometer but venr chean. 5 is a diaohraem . .. (3. mm) for the lieht-intensitv sensor. It shades the stray light. fi h a light.inten*ity sensor ( I S , SiemensTFA 1001W).7 is a cell stand of the silicon rubber stopper, thesameshape as the bottom of theceil that fits into it.The plny of the cell must he minimized for the accurate measurement of the absorbance. 8 is a coarse control knob for the sensing of light intensi. ty. 9 is a fine control. 10 is the power switch. On assembling the PEC, electronic parts themselves were effectively used, and only the metallic parts supportingthe lamp and LIS needed some treatments. The lamn must be keot aoart from the cell (3-4cm) in order toohmin the light beam.7'h;l.l~ is better located close to the cell. The lamp, the cell, and 1.1s must be aligned to acquire maximum light intensity. The critical adjustment of their mutual position was performed by measuring the light intensity. The adjusted beam Light produces high sensitivity in the measurement of ahsorhance. The electronic circuit diagram of the amplifier for measuring the light intensity is shown in Figure 2. The LIS is an IC consisting of a photodiode and amplifier. When it receives the light, a current proportional to the Light intensity is ohtained (1Lx = 5 FA, 0-40000 L 3 . If R, is selected accurately to be 200 0, a 1 mV potential corresponds to 1 Lx. As the output potential of LIS is low (0300 mV) at usual measurements of ahsorhance, it must be applied to the amplifier so as to measure with the usual voltage meter (&3 V). The first step of the amplifier turns aver the polarity of output of LIS and operates as a buffer amplifier. The second step amplifiesit up to 15 times. At usual measurements the output voltage for pure water is adjusted to 3.00 V with the come and fine control knobs. For the power supply of the amplifier, a switching regulated power supply (ELCO, KMC-13, f12 V, +5 V) was used. Any power supply is applicable if the capacity (f12 V, 0.1 A; +5 V, 0.5 A) is fulfilled.The power supply for the lamp must be regulated to avoid the drift of the light intensity. A voltage variable regulator circuit (0-5 V) shown in Figure 3 was used, and thevoltage is accurately adjusted to 2.20 V by VR,. ~~
~
.~~
Experiments as Teaching Activities in Using PEC Electric Characteristics of PEC As described above, the output voltage of PEC must he proportional to the transmitted light intensity. T h e correlation between the output voltage and light intensity was examined hefore the exoeriments took olace. T h e lieht intensity was changed hy placing some colored cellophane in the PEC celland theoutout voltare wasmeasured with adieital multimeter (SOAR ME-530). I\ standard voltage meter73 V full scale) was also used simultaneouslv to measure the tendency of the ahsorhance change. he light intensity was determined from the R1 potential (1mV = 1 Lx), and the output voltage was the voltage after amplification. The result is shown in Figure 4, and i t is found that the output voltaee is ~rooortionalto the lieht intensitv. Therefore. ~.the abso;bance c k he determined from the measurement of the output voltages for the solution and the solvent with PEC. ~d
~~~
~
Calibration Graph T h e calibration eraoh for Cu2+ was obtained to confirm the possibility of q;aniitative analysis hy using PEC. All the chemicals used for the exoeriments were of analvtical made. I n order t o get a high sensitivity for C U ~ +&,I ex&s of ammonia was added t o Cu2+ solution t o form ammine complexes. AO.l mol/LCuS04 stock solution, which isntandardized by 0.1 mol/l. EIYI'A soluti(m was used. T h e ahsorhance was calculated from the output voltages for pure water and test solution by eq 3,Thestraight calihrationgraphshown in the Figure 5 was obtained with a red filter, and this indicates that the PEC can be applied to the quantitative analysis of the colored solution. As the deviation from the straight line heginsat about 5 X lo-' mol/L, this concentration is considered to he the lower limit of the auantitative analysis of Cu2+. Thus the sensitivity of PEC & considered to be relatively high. Similarly, the straight calibration graphs for NiZf and Fe(I1)-o-~henanthrolinewere ohtained. Therefore, i t has been proven that PEC can he used for various experiments including the coloring of the solution.
Fiwe 2. Electronic circun diagram of ampilfler for measuring light intenshy; LIS, llgMlntenshysensor,TFAlOOlW, OP = pA741, VR, = coarsesense; VR2 = finesense: VR. = gain.
0 Flgve 3. Elemnic circuit diagram ofwltage vsrlable regulator fa lamp; VR, = voltage adjuster.
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Journal of Chemical Education
~
50
100
150
200
Light Intensity / Lx Figure 4. Conelation between ilght i.ntenslty and output voltage.
250
I
I
I
I
0.1
0.2
0.3
I
I
0.4 0.5
[Cult / ([Cult
I
I
I
I
0.6 0.7 0.8 0.9 CEOTAIt)
~ l g v 5. e Callbratlm graph for C U ~ + (ammonia mettmd); filter = red.
Figure 6. Comlnwxrs variation memod for Cu(1lkEDTA system: finer = red.
Composition of Complexes
sured. A platinum plate (1.0 X 4.5 cm) and a carbon rod (core of pencil 3.5 an in length) are used for the cathode and anode, respectively. A variable, regulated power supply was used, and the voltage was manually adjusted to obtain a constant current (9.5 mA). A 0.1 m o l b CuSOl solution was electrolyzed, and its ahsorhance was measured. With the electrolysis proceeding, the absorbance of the solution is gradually decreased, as the line in Figure 8 shows. The concentration of Cu2+ under electrolysis was determined hy the calibration graph for Cu2+.From the variation of the concentration of Cu2+by electrolysis, the Faraday constant is determined as 94330 C. This value deviates from 96500 C by only 2.3%. When both of the electrodes are substituted by copper plates, the concentration of the solution is not changed by electrolysis. This fact indicates that anodic stripping has
If the compositions of the complexes are experimentally determined, the students can try to estimate the coordination number of the metal ions and ligands and the structure of the complexes. Then the composition of Cu(I1)-EDTA was determined hy continuous method (3) at a total molar concentration of0.01 mol/L. The result is shown in Figure 6, and it is found that the composition ratio of Cu(II)-EDTA is 1:l judging from the molar ratio of the absorbance peak ([Cu]J[[Cujt + [EDTAItJ= 0.5, t = total concentration). This compositionratio is the basis of the calculation for the chelatometric titration by using the standard EDTA solution. The composition of Cu(I1)-NTA (NTA = nitrilotriacetic acid) complex was also examined by the molar ratio method (3). Figure 7 represents the absorhance of CuW-NTAvarying with NTA concentration ([Cu2+]t= mol/L). The absorbance is gradually increased with increasing [NTA], but it reached a plateau at the ratio of [Cu]J[NTA]t = 1. This indicates the composition of Cu(I1)-NTA is in the ratio of 1:l. In addition, the compositions of Cu(NH3)aZ+, Ni(NHs)s2+, Co(I1)-EDTA, and Fe(I1)-phenanthroline complexes were determined, and the coordination numbers NiZ+, and Fez+were estimated. Since the coordinaof CU~+, tion numbers of those metal ions were estimated by using the PEC, the student's interest in the "second atomic valence" would he increased. Determinationof Faraday Constant
When the solution containing the electrolytes is electrolyzed, the concentration of the solution will he changed. The change of the concentration owing to electrolysis is measured with the PEC and the Faraday constant is determined. The electrolytic current is limited by the area of the electrode surface. It is necessary to minimize the volume of the electrolytic solution in order to obtain the sufficient concentration change at limited Coulomb number. Therefore, the cell for PEC is employed for the electrolytic cell. The volume of the PEC cell is very small (4.5 em3),and the absorbance of the solution under electrolysis can be simultaneously mea-
I
I
0
2
I 4
I I I I 10 12 1.4 16 [ N T A I / X 10" r n o l / l I
I
6
8
Flgue 7.Molar ratlo mmhod for Cu(ll~NTA~,system: flner = red.
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Number 4
April 1989
331
occurred and the total concentration of Cu2+in the solution is kept constant. By the experiment of&ctrolysis with the PEC.atudentsareable u, learn that the concentration of the solution is changed while electrolysis is proceeding. ~~~~
~~~~~~~
~~
Reaction Rate Constant and Activation Energy for Color Fading Reaction of Phenolphthalein Reaction rate and activation energy are taught at the senior high school level, but there are only a few experiments (4) to determine the activation energy. Then, as a teaching activity, the reaction rate constant and the activation energy for the color fading reaction of phenolphthalein are determined by using the PEC. The phenolphthalein changes to violetred in basic solution, and then it gradually becomes discolored in strong basic solution with the passage of time. This color fading reaction is expressed as follows: R2-
+ OH-
-
R3-
(4)
where R2- denotes phenolphthalein at a basic state, and R3denotes it at a strong basic state. If [OH-] is sufficiently large, this reaction can be regarded as following pseudofirst-order kinetics. As the absorbance is proportional to the concentration by Lambert-Beer's law, the following relationship is derived for the first-order kinetics (5,6).
constant. The reaction rate constant can be also determined from the half-life time ( 7 ) by the following relationship: Then the reaction rate constants at 1, 12, and 22 O C were elucidated by measuring the half-life time by fading color. From the slope of the log k vs. 1IT plot (Arrhenius plot (6)) (Fig. l l ) , the activation energy can he evaluated asE. = 12.4 kcal/mol. Thus the PEC can be used for the experiment determining the reaction rate constant and the activation energy. Conclusion
Since there are manv excellent commercial chemical measuring instruments, we do not need to pay close attention to assemblina them. But we can now assemble a comvarable one and obtain high quality by using the new el&tronic
-
15
" 0 X
where A is the absorbance at the time t, k is the reaction rate constant of reaction 4, and A. is the initial absorbance for R2-. One drop of the phenolphthalein solution (1%) was added to 6 mL of 1.0 molL NaOH solution, and then the absorbance was measured at 10-s intervals. The result is shown in Figure 9. The absorbance is gradually decreased with the elapsed time, and the absorbance reaches zero after 150 s. The correlation between the time ( t )and the logarithmic value of the absorhance (log A) is shown in Figure 10. The reaction rate constant can he elucidated from the slope of the line (k = 0.020 s-1). Then the activation energy was determined from the variation of the reaction rate constant with the temperature. The correlation between the reaction rate constant and the. activation energy is expressed as follows (5):
\
U
"C 0
n L.
", n 0
4
5 -
0
50 Time /
100
150
s
Flgure 9. Abscibance varying wim fading color. One drop of phenolphthdein solution (1%) Is added10 1.0 mol/L NaOH solution; filter =yellow.
where k is the rate constant, R is the gas constant (8.31 Jouleldeg mol), T is the absolute temperature, and Cis the
-
, , 30
0
5
10
15
20
25
Coulomb Flgue 8. CarelatIan between a b s o m c e and Coulomb number. Elemolysis d 0.1 mol/LCuSO,solutlon; filter =red.
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Journal of Chemical Education
0
50
Time /
100
s
F i g m 10. Conelation between time and logarithmic value of absorbance Same system as shown in Figure 9; finer = yellow.
narts. On assembline the measurine instrument, imaeination is encouraged i d thinking becomes flexible, and-also the enjoyment of learning chemistry is increased. I t has been found that the assembled PEC has higher quality in colorime t w than was expected, and manv experiments for teaching concepts have bken peiformed. AS the straight calibration eraphs were obtained for some metalions, the determination of many other colored solutions is possible with the assembled PEC. All the experiments described here can be used by senior high school students or undergraduates a t the university. The students can assemble the PEC by themselves. From the exneriments with the assembled PEC. the students unders'tand experimen~allythe principle of the absorptiometric method, and also rreativity will be enhanced. Acknowledgment
The authors are grateful to Satomi Murakami and Keiitsu Saito, Faculty of Education Kobe University, for helpful advice with this work. Literature Cited 1. Matsu0.T. Kagoku Kyouikn 1984.32.430. 2. Harris, W. E.; Kratochvil. B. An Infmduetionto Chemical Analysis; Saundera: Philadelphia, 1982;p 378. 3. Ewing, G.W. Insfrumanfd Wfhods of Chornieol Anolyaia: MeGraw-Hill; New York,
Figure 11. Arrheniu~plot for color fading reaction. Same system as shown in Figure 9.
Volume 66
Number 4
A ~ r i 1989 l
333
